GCSE Rates of Reaction

GCSE Chemistry Workbook Rates of Chemical Reactions

Rates of Chemical Reactions

4.6.1 Rate of reaction 4.6.1.1 Calculating rates of reactions Content

Key opportunities for skills development

The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product formed over time:

MS 1a

f reactant used mean rate o f reaction = quantit y o time taken f product f ormed mean rate o f reaction = quantit y otime taken

Recognise and use expressions in decimal form.

MS 1c

Use ratios, fractions and The quantity of reactant or product can be measured by the mass in percentages. grams or by a volume in cm3.

MS 1d The units of rate of reaction may be given as g/s or cm3/s.

For the Higher Tier, students are also required to use quantity of reactants in terms of moles and units for rate of reaction in mol/s.

Make estimates of the results of simple calculations.

Students should be able to:

MS 4a

• calculate the mean rate of a reaction from given information about the quantity of a reactant used or the quantity of a product formed and the time taken • draw, and interpret, graphs showing the quantity of product formed or quantity of reactant used up against time • draw tangents to the curves on these graphs and use the slope of the tangent as a measure of the rate of reaction • (HT only) calculate the gradient of a tangent to the curve on these graphs as a measure of rate of reaction at a specific time.

Translate information between graphical and numeric form.

MS 4b Drawing and interpreting appropriate graphs from data to determine rate of reaction.

MS 4c Plot two variables from experimental or other data.

MS 4d Determine the slope and intercept of a linear graph.

MS 4e Draw and use the slope of a tangent to a curve as a measure of rate of change.

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GCSE Chemistry 8462. GCSE exams June 2018 onwards. Version 1.0 21 April 2016 Rates of Chemical Reactions

4.6.1.2 Factors which affect the rates of chemical reactions Content

Key opportunities for skills development

Factors which affect the rates of chemical reactions include: the concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature and the presence of catalysts.

Students should be able to recall how changing these factors affects the rate of chemical reactions.

This topic offers opportunities for practical work and investigations in addition to required practical 5.

Required practical 5: investigate how changes in concentration affect the rates of reactions by a method involving measuring the volume of a gas produced and a method involving a change in colour or turbidity.

This should be an investigation involving developing a hypothesis.

AT skills covered by this practical activity: 1, 3, 5 and 6.

This practical activity also provides opportunities to develop WS and MS. Details of all skills are given in Key opportunities for skills development (page 106).

4.6.1.3 Collision theory and activation energy Content

Key opportunities for skills development

Collision theory explains how various factors affect rates of reactions. According to this theory, chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.

Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.

Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction.

WS 1.2

Students should be able to : • predict and explain using collision theory the effects of changing conditions of concentration, pressure and temperature on the rate of a reaction

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Rates of Chemical Reactions

Content

Key opportunities for skills development

• predict and explain the effects of changes in the size of pieces of MS 5c a reacting solid in terms of surface area to volume ratio MS 1c • use simple ideas about proportionality when using collision theory to explain the effect of a factor on the rate of a reaction.

4.6.1.4 Catalysts Content

Key opportunities for skills development

Catalysts change the rate of chemical reactions but are not used up AT 5 during the reaction. Different reactions need different catalysts.

An opportunity to Enzymes act as catalysts in biological systems. investigate the catalytic Catalysts increase the rate of reaction by providing a different effect of adding different pathway for the reaction that has a lower activation energy. metal salts to a reaction such as the decomposition A reaction profile for a catalysed reaction can be drawn in the of hydrogen peroxide. following form:

Students should be able to identify catalysts in reactions from their effect on the rate of reaction and because they are not included in the chemical equation for the reaction.

Students should be able to explain catalytic action in terms of activation energy.

Students do not need to know the names of catalysts other than those specified in the subject content.

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Rates of Chemical Reactions

Helping yourself Read the section on Atomic structure and the Periodic table in your textbook Read the relevant sections of these websites bbc.co.uk/schools/gcsebitesize/chemistry gcsescience.com docbrown.info revisioncentre.co.uk youtube for video tutorials other teachers!

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Rates of Chemical Reactions

Rates of Reaction The Rate Some reactions are fast, some reactions are slow.

Rusting takes years

Baking takes hours

Explosions take milliseconds

How quickly something reacts is known as the speed of the reaction or the ..................

The Rate is a measure of the change that happens in a single unit of time.

For instance Concorde flew at a rate of 2000 kilometres per hour, the petrol pump pumps at a rate of 50 litres per minute, a machine can print newspapers at a rate of 10 copies per second.

The quicker the rate, the ......................... time something takes to happen. We say that rate is ......................... proportional to time. !"#$ !

1 !"#$ !"#$%

Reactants and Product The rate of a reaction .......................... throughout a reaction.

It is greatest at the ...................... of the reaction when there is the ........................ concentration reactant.

At the end of the reaction it has obviously finished and so the rate is ..................... because there is are more reactants to react.

Between the quick rate at the start and the end of the reaction the rate slowly .........................

In the reaction A Ă B at the start of the reaction we have all reactant and no product. At the end of reaction we have all product no reactant.

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Measuring the Rate of Reaction To measure the rate of a reaction we can either: i)

Measure the amount of ........................ used up per unit time e.g. measuring the mass loss in the reaction between marble chips and hydrochloric acid

ii)

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Measure the amount of ...................... produced per unit time

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Rates of Chemical Reactions

e.g. measuring the amount of gas produced in the reaction between marble chips and hydrochloric acid.

Different methods of measurement will be deployed depending on what the products or the reactants are. We then plot this measurement against time on a graph.

Three types of Rate There are three different rates of reaction which are helpful to us: 1. The Instantaneous Rate of Reaction is calculated at a specific point as on the graphs below.

If we plot a graph of how the products or reactants changed over time, the instantaneous rate of reaction is the ............of that graph.

We can calculate the gradient from the formula

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!"#$%&'( =

!"#$ (! − !"#$) !"# (! − !"#$)

2. The ................ Rate of Reaction is the speed of the reaction as a ................. and is calculated in this way: !"#$%&# !"#$ !" !"#$%&'( =

!"!#$ !"#$%& !" !"# !"!#$ !"#$ !"# !"#$%&'(

3. The .................. Rate of Reaction is the rate of reaction at the very start of the reaction before there are any significant changes to the concentrations of reactants. It found by calculating the gradient at the steepest point on the curve.

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Rates of Chemical Reactions

How to Calculate Rates of Reactions

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Collision Theory How do reactions occur?

Reactants are compounds, specific arrangements of atoms. When they react the atoms ....................... themselves to form new combinations of atoms which are the products. e.g.

!"#$%&'() + ℎ!"#$%ℎ!"#$% !"!" → !"#$%&'(! !ℎ!"#$%& + ℎ!"#$%&' !" ! + 2!"# !" → !"#!! !" + !! (!) a) This rearrangement of atoms can only occur when the compounds are in ........... b) There are also strong bonds between the atoms in the reactants. For a rearrangement of atoms we need to .............. those bonds.

This can only happen when the .................. with ....................... energy to ................. the bonds otherwise they just bounce off each other.

The amount of energy required to break the bonds in the reactants will be ........................ for every reaction. c) We also find that the reactants need to collide with each other in the correct ....................... also.

So we can say that reactions occur when atoms or molecules:

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i)

.................. with each other

ii)

with .............................. energy

iii)

In the right ......................................

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If they do not collide, they will not react. If they don’t collide with enough energy, they won’t react either.

Increasing the rate of reaction will therefore mean either: i)

increasing the ........................... of collisions

ii)

increasing the ...................... of the atoms or molecules

iii)

increasing both

Providing a different .................. pathway to get to the same products This is called COLLISION THEORY and is essential to our understanding of Chemical iv)

reactions.

In any sample particles are moving at very great speeds and colliding millions of times per second. Most collisions therefore are unsuccessful.

Energy and Temperature The energy of collisions is proportional to the .................................

Temperature is a measure of the average ....................... energy of the particles in a sample. As a rule of thumb, increasing the temperature of a compound by 10°C results in a doubling of its kinetic energy.

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If we made a histogram and counted the number of particles that have specific energies in a sample at a specific temperature and did that for all the energies we get a graph that looks like the one above. If we then just look at the outlines of these we find that there is a large variation in kinetic energies within a sample.

The range of energies are represented by this ................................ distribution. Notice the shape changes at different temperatures. Increasing the temperatures causes the mean (highest point) to move to the right but the breadth of the break to broaden so there is a broader range of energies within a sample.

All the lines meet at the origin as zero particles have zero energy. The lines never meet the x-axis as there is the possibility that one particle might have almost infinite energy!

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Rates of Chemical Reactions

Activation Energy The activation energy is the .............................. energy required for a particle to react successfully when it collides.

We can represent it on our Maxwell-Boltzmann distribution. Any particle that collides with energy greater than the activation energy will result in a successful reaction.

This is represented by the ............. region. This means at only the particles in the red region will be successful when they collide. The majority of particles are therefore .................................... when they collide.

Notice how increasing the temperature increased the number of particles with energy ...................... than the activation energy as the shape of distribution changes. A 10°C increase in temperature results in a doubling of this are on the graph. This means a ........................... of the rate of reaction.

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Factors affecting the Rate of Reaction The rate of a reaction is dependent on a few factors as these will either increase the number of ................... or increase the ........................ of the atoms or molecules and therefore the collisions. i) Concentration ii)

Pressure

iii)

Temperature

iv)

Surface Area

v)

Catalysts

Changing any of the above will affect the .................. at which a reaction progresses but might not affect the ....................... of product that is produced in the end. i)

Effect of changing Concentration on the Rate Concentration is the number of particles per unit .....................

Therefore increasing the concentration increases the number of ................

Increasing the number of particles increases the number of ......................

So increasing the concentration ................... the rate of reaction. Therefore decreasing concentration ......................... the rate of reaction.

ii)

Effect of changing Pressure on the Rate Pressure is the effect of particles of a gas colliding with the walls of its container.

So increasing the pressure means ..................... the number of collisions.

Increasing the number of collisions means increasing the ........................ of a reaction. Therefore increasing the pressure ............................ the rate of reaction.

So decreasing the pressure means decreasing the rate of reaction

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iii)

Effect of changing Temperature of the Rate Temperature a measure the average ...................... energy of particles So increasing the temperature ................... the energy of collisions So increasing the temperature ....................... the rate of reaction Therefore decreasing the temperature ......................... the rate of reaction

iv)

Effect of changing Surface Area on the Rate Collisions occur on the surface a substance.

So increasing the surface of a substance ........................ the number of collisions that can take place with that substance.

So increasing the Surface Area .............................. the rate of reaction.

Therefore decreasing the Surface Area ....................... the rate of reaction.

The smaller the sizes of all the particles, the .................... total surface area. A large sphere has the largest surface area but dividing it up increases the total surface are considerably.

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v)

Effect of a Catalyst on the Rate

A Catalyst is a substance that changes the ............. of a reaction without being...................itself.

It is always ..................... at the end of the reaction.

It does this by providing an ................... reaction pathway with a .................... activation energy so the reaction proceeds much more quickly.

The Activation Energy is the ...................... energy required for a collision to be ..................

Lowering the activation energy results in ................. sucessful collisions.

Therefore decreasing the activation energy increases the rate of reaction.

A higher activation energy means a ....................... rate of reaction.

It’s the energy difference between going over a hill and going through the tunnel to get to the same place. The tunnel is always quicker.

Catalysts are very ................... to a reaction and can only be found through trial and error.

They are mainly made from transition metals and transition metal compounds. In the human body they are known as ....................

They can be either .......................... (in the same state as the reactants) or ....................... (different state from the reactants).

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Catalysts Definition: A Catalyst is a ……………………………………….. that ………………………. the rate of a reaction without being used up.

Key features: 1. It ………………… up a reaction.

2. It is always ………………… at the end of the reaction.

3. It can be used …………………………… again.

4. The higher ………………. ……… of catalyst the more it speeds up the reaction.

5. They are ………………. to a reaction.

6. It does this by providing an ………………………. reaction pathway with a …………………… activation energy so the reaction proceeds much more quickly.

The Activation Energy is the ………………… energy required for a reaction to be successful.

Lowering the activation energy means ………………. collisions are successful.

Therefore decreasing the activation energy ……………………………. the rate of reaction.

It’s like the energy difference between going over a hill and going through the tunnel to get to the same place. The tunnel is always quicker.

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Catalysts are very …………………. to a reaction and can only be found through trial and error.

They are mainly made from transition metals and transition metal compounds. In the human body they are known as ………………………..

They can be either homogeneous (in the same state as the reactants) or heterogenous (different state from the reactants).

The lower activation energies can be represented on an energy profile diagram. The particles have to overcome a smaller energy barrier than before.

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Collision Theory In the boxes below, draw diagrams to show how each factor is affecting the rate of the reaction. In the box labelled ‘Explanation’, explain what is happening. REMEMBER! Whenever you see a “Rate of Reaction” question in an exam you must talk about COLLISIONS and PARTICLES Increase Temperature

Explanation

Concentration

Increase Concentration

Explanation

Surface Area

Increase Surface Area

Explanation

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Effect of concentration on the rate of reaction – 1 Rates of Chemical Reactions

I4.1

Billy and Connie were investigating the reaction between magnesium and hydrochloric acid. A diagram of the apparatus they used is shown below. magnesium ! hydrochloric acid Mg !

2HCl

magnesium chloride ! hydrogen MgCl2 !

"2 rubber tubing

They carried out the experiment using dilute acid and then repeated it using more concentrated acid. In each experiment the acid was in excess.

The graph below shows their results.

syringe

dilute hydrochloric acid

I

magnesium

60 50 40 volume of gas (cm3)

A

30 B

20 10 0

0

10

20

30

40 50 time (s)

60

70

80

90

1 Write down two things that Billy and Connie must do to make it a fair test.

2 Look at the graph. a) Which line shows the results for the dilute acid? b) Which line shows the results for the more concentrated acid? c) Explain your answers for parts a) and b). d) How long did the reaction take with dilute acid? e) How long did the reaction take with the more concentrated acid?

3 Billy and Connie carried out the reaction again with another acid (they used the same amount of magnesium as before). The acid had a concentration in between the other two acids. Sketch a copy of the graph into your book and draw a line on your graph to show the results for this acid. Label your line ‘C’.

4 a) Why was the same volume of gas produced in each experiment? b) What would Billy and Connie have to do to produce more hydrogen gas?

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I6.1

Effect of temperature on the rate of reaction Rates of Chemical Reactions

Saheeda carried out an experiment to study the rate of reaction between magnesium and hydrochloric acid. She added a 3 cm strip of magnesium to 25 cm3 of acid. magnesium ! hydrochloric acid

magnesium chloride ! hydrogen

She carried out her first experiment at 25 °C. 1 The diagram shows some of the apparatus. Copy and complete the diagram to show how Saheeda could collect the gas and measure the volume. Label the additional pieces of apparatus.

dilute hydrochloric acid volume of hydrogen (cm3)

rubber connector

50 40 30 20

25°C

I

10 0

0

2

4

6

8

10

12

time (mins)

magnesium metal

2 The graph shows Saheeda’s results when the acid was at 25 °C. a) Saheeda heated up the acid to 35 °C and repeated the experiment. Explain why the reaction went faster. b) Sketch the graph (no graph paper required) and add a line on the graph to show the results for the reaction at 35 °C. Label your line ‘35 °C’.

3 a) What was the total volume of hydrogen collected at 25 °C? b) Does changing the temperature affect the volume of hydrogen collected? c) Write down one way you could increase the volume of hydrogen collected.

4 Explain, in terms of kinetic energy and activation energy, how increasing temperature changes the rate of reaction.

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I7.1

Effect of surface area on the rate of reaction Rates of Chemical Reactions

1 Jane and Derek carried out an experiment to study the reaction between marble chips and hydrochloric acid. They wanted to see what effect changing the surface area of the marble chips had on the rate of reaction.

They used 5 g of marble and 25 cm3 of dilute acid.

They used the ‘loss in mass’ method. Here are their results using two different sizes of marble chips, A and B. a) b) c) d)

Time (min)

0

1

2

3

4

5

6

7

Loss in mass (g) A

0

0.46 0.84 1.10 1.22 1.38 1.38 1.38

Loss in mass (g) B

0

0.36 0.68 0.92 1.09 1.24 1.38 1.38

Plot a graph of these results. Label the curves A and B. Which has the largest surface area, chips A or chips B? Use the results to explain the difference between chips A and B. Jane and Derek repeated the experiment using 5 g of marble powder (C) and 25 cm3 of the same dilute acid. The results are shown below.

Time (min) Loss in mass (g) C

0 0

1 0.15

2 1.10

3 1.34

4 1.49

5 1.49

6 1.49

7 1.49

i) Plot the graph of these results on your graph for A and B. Label the curve C. ii) Look at the results for C. Describe two examples of results that are unexpected and one instance where the results show an expected trend. Try to explain how the unexpected results could have been obtained. e) The chemical name for marble is calcium carbonate. Other metal carbonates react in a similar way. Write word and balanced symbol equations for the reaction between zinc carbonate and hydrochloric acid. You may find these formulae useful: ZnCO3, ZnCl2.

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I8.1

Catalysts

Rates of Chemical Reactions

1 State whether each of these sentences is true or false and rewrite the false statements correctly. a) A catalyst speeds up a chemical reaction. b) A catalyst gets used up during a reaction. c) A lot of catalyst is needed for a reaction. d) One catalyst may be used for different reactions.

2 a) Match these catalysts with their uses (platinum has two uses). platinum

making margarine

nickel

making nitric acid

platinum

making ammonia

iron

used in catalytic converters in car exhausts

b) Why is it important to use catalysts in industry?

3 Hydrogen peroxide solutions decompose to form water and oxygen gas. hydrogen peroxide

water + oxygen

Fred and Uzma investigated the effect of a catalyst on this reaction. They both started with a small solid lump of catalyst. Fred used his lump as it was, but Uzma crushed her lump into powder. a) Write a balanced symbol equation for this reaction. The formula for hydrogen peroxide is H2O2. b) How would you show that oxygen is produced when hydrogen peroxide decomposes? c) What difference would you expect in their results? Briefly explain your answer. d) Describe how Fred and Uzma could show that their catalyst is not used up in the reaction.

4 The concentration of hydrogen peroxide is described in an odd way. The units of concentration are described as ‘volumes’ and they refer to the ratio of the volume of oxygen produced to the volume of solution. For example, 5 cm3 of ‘10 volume’ hydrogen peroxide will produce 50 cm3 of oxygen when decomposed. a) Draw a diagram of the apparatus you could use to measure the concentration, in ‘volumes’, of a solution of hydrogen peroxide. State which measurements you would have to make. b) What volume of oxygen could be produced from 15 cm3 of ‘20 volume’ hydrogen peroxide?

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Rates of Chemical Reactions

Different types of rates graphs

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Rates of Reaction Graphs 1. The graphs below represent studies of the reaction between marble chips and hydrochloric acid. In each case the amount of acid was in excess. a. Draw a second line on the graph to represent the reaction with double the concentration of acid.

b. Draw a second line on the graph to represent a reaction using powdered marble chips.

c. Draw a line on the graph to represent the reaction when conducted at a higher temperature

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d. Draw a line on the graph to represent the reaction when more water was added to the acid.

e. Draw a line on the graph to represent the reaction when half the amount of marble chips was added to the reaction.

f.

Draw a line on the graph to represent the reaction when twice as much marble chips was added to the reaction.

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Activation Energy 1. Finish the following sentences Bond breaking is an …………………………….. process.

Bond making is an …………………………………. process.

Bond making ……………………………..energy to the ………………………………

Bond breaking ………………………….. energy from the………………………………

2. What is the definition of the activation energy?

3. What is the enthalpy change?

4. Why don’t hydrogen and oxygen gas simply react as soon as they are mixed together?

5. What does the spark that start a reaction do in relation to the bonds?

6.

Below is the energy profile diagram for the reaction between magnesium and hydrochloric acid. a. Label the activation energy and the enthalpy change of reaction on the energy profile diagram below.

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Rates of Chemical Reactions

b. Is this an endothermic or an exothermic reaction? How can you tell?

7. Below is the energy profile diagram for the reaction between calcium oxide and water. a. Label the energy profile diagram below with the activation energy and the enthalpy change of reaction.

b. Is this an exothermic or endothermic reaction?

8. Draw the energy profile for an endothermic reaction labelling the activation energy and the enthalpy change

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Catalysis 1. What is the definition of a catalyst?

2. Name four features of a catalyst.

3. What does a catalyst do to a reaction?

4. How does it do it?

5.

Will a catalyst for one reaction necessarily work for another reaction?

6. On the energy profile below, draw a new energy profile for the catalysed reaction.

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Factors affecting the Rate of Reaction 1.

The monomer chloroethene is made from ethene in a two-stage process, (a)

The first stage is to convert ethene to 1,2-dichloroethane.

2C2H4(g) + 4HC1(g) + O2(g) ethene

2C2H4Cl2(g) + 2H2O(g) 1,2-dichloroethane

State and explain the effect of increasing the pressure on: (i)

the yield of 1,2-dichloroethane; ........................................................................................................................... ...........................................................................................................................

(2)

(ii)

the rate of reaction. ........................................................................................................................... ...........................................................................................................................

(2)

(b)

In the second stage 1,2-dichloroethane is converted into chloroethene.

C2H4C12 → C2H3C1 + HCl This reaction is a thermal decomposition.

Suggest what would need to be done to decompose 1,2-dichloroethane. ...................................................................................................................................... ......................................................................................................................................

(1) (Total 5 marks)

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2.

This label was on a bottle of stain remover.

When ‘Simply Amazing’ is mixed with water a reaction takes place which produces bubbles of oxygen gas. (i)

Suggest a method that you could use to measure how quickly this reaction takes place. ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... .....................................................................................................................................

(2)

(ii)

Read the instructions on the label and then suggest how increasing the temperature of the water affects the rate of this reaction. ..................................................................................................................................... .....................................................................................................................................

(1)

(iii)

Suggest one other way in which the rate of a reaction can be changed. ..................................................................................................................................... .....................................................................................................................................

(1) (Total 4 marks)

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3.

This article appeared in a newspaper.

(a)

The balanced chemical equation shows the reaction between steel and hydrochloric acid.

Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g) (i)

Which metal in steel reacted with the hydrochloric acid? ...........................................................................................................................

(1)

(ii)

The gas released was described as explosive. Explain why. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ...........................................................................................................................

(3)

(b)

In the factory hydrogen chloride is manufactured by reacting hydrogen with chlorine.

Hydrochloric acid is formed when hydrogen chloride forms a solution in water. (i)

Water was sprayed on the steel and hydrochloric acid. This slowed the rate of reaction. Explain why. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ...........................................................................................................................

(2)

(ii)

It would have been better to neutralise the acid with an alkali rather than to just add water. Hydrochloric acid can be neutralised by reaction with sodium hydroxide.

Complete the ionic equation for the neutralisation reaction. (aq) +

(aq) → H2O(l) (2)

(iii)

In the factory the acid leak was neutralised with slaked lime, Ca(OH)2, and not sodium hydroxide, NaOH. Suggest why. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ...........................................................................................................................

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Rates of Chemical Reactions

...........................................................................................................................

(2) (Total 10 marks)

4.

This item appeared in the Wolverhampton Express and Star on October 31st, 1997.

Read the passage and answer the questions that follow.

Fumes scare at factory Workers were forced to flee a factory after a chemical alert.

The building was evacuated when a toxic gas filled the factory.

It happened when nitric acid spilled on to the floor and mixed with magnesium metal powder. (a)

The equation which represents the reaction between magnesium and nitric acid is: Mg(s) + 4HNO3(aq) → Mg(NO3)2(aq) + 2H2O(1) + 2NO2(g) Give the formula of the toxic gas that was produced. ....................................................................................................................................

(1)

(b)

Explain, in terms of particles, how the toxic gas was able to fill the factory quickly. .................................................................................................................................... .................................................................................................................................... ....................................................................................................................................

(2)

(c)

The reaction of nitric acid with magnesium metal powder is more dangerous than if the acid had fallen on to the same mass of magnesium bars. Explain why. .................................................................................................................................... .................................................................................................................................... ....................................................................................................................................

(1)

(d)

(i)

Water was sprayed on to the magnesium and nitric acid to slow down the reaction.

Explain, in terms of particles, why the reaction would slow down. .......................................................................................................................... ..........................................................................................................................

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Rates of Chemical Reactions

..........................................................................................................................

(2)

(ii)

Explain why it is better to add alkali, rather than just add water to the spillage. .......................................................................................................................... ..........................................................................................................................

(1) (Total 7 marks)

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Think creatively

1. Create a colourful poster explaining the basics of collision theory and what factors affect the rate of reaction and why.

2. Come up with a creative way of explaining Le Chatelier’s principle and a memorable way to remember which way equilibrium will shift.

3. Research how a heterogeneous catalysts works and write a beginners guide to catalysts.

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Taking it further

Why not do an internet search for Fritz Haber?

What was he famous for and find out some interesting things about his life.

Why not look in an A-level textbook to see how we measure how far a reaction goes?

Why not watch some Royal Institute lectures that demonstrate lots of fast, explosive reactions? Or watch some Brainiac to see some more?

Explosions occur every action film. How many can you name in 5 minutes?

The Haber process is a very important equilibrium process. Watch a clip on youtube about it. Can you name any other economically important reactions where we have to shift the equilibrium to make it more viable?

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Mr Singh

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Name : _________________________________________

(C3) Calculating rates

Rate of reaction is a number !

You might calculate it so that you can compare the rate at different points in the reaction.

►Measuring rate of reaction (I) : calculation Rate = amount of change in the “dependent variable” time measured for

This leads to the question : why is the rate so high (34 cm3/min) at the start, but so much lower by the 4th minute (2 cm3/min)……….…?

►Measuring rate of reaction (II) : Tangent-method on a graph This method is useful if you’re given some practical results in the form of a graph and asked to calculate the rate of reaction …………

If you’ve got a curve (which many rates graphs are), it’s hard to measure the “gradient” of the curve.

One way is to draw a tangent - a straight line (drawn with a ruler) that touches the curve at a particular point. Using the steps below, this can help you to calculate the gradient of that curve and the gradient is the rate of reaction at that point….

Tangent Straight line just touching the curve

QUESTIONS Q1] Zinc metal reacts with hydrochloric acid to release the gas hydrogen.

Zn (s) + 2HCl (aq)  ZnCl2 (aq) + H2 (g) This release of gas can be measured to monitor how quickly a reaction is proceeding.

In a reaction between 1.5g of zinc powder and 50mls of 2.0M hydrochloric acid at room temperature, the following results were obtained:

a) What was the dependent variable in this experiment ? [1]

b) On the graph, label the following points on the curve with the appropriate letter: A – point where reaction is fastest & B – point where no more gas is formed (reaction has finished) [2] c i) Sketch a 2nd curve (starting at zero-zero) on the same axes to show the reaction involving 1.5g of zinc granules (lumps) and 50mls of 2.0M hydrochloric acid at room temperature. [2]

c

ii) Suggest why both reactions (represented by the 2 curves on the graph) would eventually produce the same total volume of gas ? [1]

d i) Draw a “tangent” on the original curve (using a ruler) to help you to calculate the initial rate of reaction [1] d ii) Complete the “triangle” (by adding 2 sides to the tangent on the graph)and use the data to calculate the initial rate of reaction ? [2]

Rate of reaction = Change in Y-axis reading Change in X-axis reading

d iii) What are the units for the rate of reaction ? [1]

e) Using ideas about particles and successful collisions per second, suggest in your own words why both of the graph lines above curve, rather than carrying on in a straight lines ? [2]

f) In this reaction, it was planned so that the acid would be in “excess” – it wouldn’t all be used up in the reaction (some would be left over, unreacted, at the end) – suggest why ?

[1] hint : good idea to use the phrase “limiting reactant” here !

g) Suggest an independent variable that you could change to make the first reaction (1.5g of zinc powder and 50mls of 2.0M hydrochloric acid ) go slower , other than using lumps of zinc rather than powder, and explain why it would slow down……..?

[3]

Q2] A reaction was carried out between a different metal (magnesium ribbon) and Hydrochloric acid.

The following results were collected and recorded, by reading off the volume of gas in the syringe every 10 seconds for 1 minute 30 seconds:

0 10 20 30 40 50 60 70 80 90 Time (secs) Volume of hydrogen 0 18 30 40 48 53 57 58 58 58 gas collected in syringe (cm3) a) Why is it important to record the volume of gas result at time 0 (zero) seconds ?

[1]

b) Which measuring device (gas syringe or stopwatch) had the highest resolution ? Explain

c) Using the data in the table, calculate the rate of reaction for: i) the first 10 second period of the reaction (including units) ? [2]

ii) the 2nd 10 second period of the reaction ? [1]

iii) the 5th 10 second period of the reaction ? [1]

d) Calculate the “Average” rate for this reaction (including units) ? [1]

e) Why is the average rate of reaction less “useful” than the rate of reaction at each 10 second interval ? [1]

[2]

3] In the reaction in Q1, Zinc metal reacts with hydrochloric acid.

Zn (s) + 2HCl (aq) ďƒ¨ ZnCl2 (aq) + H2 (g) A different dependent variable was measured to monitor the rate of the reaction, resulting in the following graph of the results.

a) Which dependent variable might have been measured during this reaction ?

[1]

b) Suggest how it was measured in the experiment. [2]

c) Suggest how this new method of measuring the rate of reaction might lead to results with a higher resolution than in the original experiment in Q1. [2]

C3 Rates calculations How did I do ?

How can I improve ?

Grade

evidence

How can I achieve this ?

(Q ?)

Achieved ? (teacher)

 / X

A*

CALCULATE rate by drawing a tangent on a curve and using it to calculate rate of reaction.

EXPLAIN (using particles) why the rate changes with changing temperature or concentration CALCULATE the rate of reaction using data (at a specific point in a reaction) and record result to appropriate number of d.p.

EXPLAIN (in own words) that rate depends on frequency of successful of collisions (per sec) between particles EXPLAIN (in own words) that rate depends on energy of collision between particles EVALUATE & EXPLAIN resolution of data

USE information to work out when reaction has finished

C

DRAW sketch graph to show how rate changes with changing surface area of reactant RECOGNISE that when the “limiting reactant” is used up, the reaction stops UNDERSTAND that rate of reaction depends on number of successful collisions between particles per second Recognise & use the idea that amount of product formed is related to amount of reactant used UNDERSTAND common units used for rate of reaction (and work out what they will be given primary data) EVALUATE value of average rate of reaction versus interval rate of reaction

Grade achieved = _____________ To improve, I need to :

1d i & ii 1g 2c i, ii, iii

1e 1g 1g (if temp) 2b + 3c

1b & 1ci 1c i 1f 1e & 1g 1 c ii 1 d iii 2e

Target grade = ________________

needs more work (student)

Rates of Chemical Reactions

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Rates of Chemical Reactions

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Rates of Chemical Reactions

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Mr Singh

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Rates of Chemical Reactions

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LABCOATS MUST BE

Effect of concentration on rate of reaction Hazard

HIGHLY FLAMMABLE

Control Measure Wear eye protection Avoid looking at the flame if ignited

Substance Magnesium metal (ribbon)

Wear eye protection

2M Hydrochloric Acid

IRRITANT

Method 1. Measure out a 15cm length of magnesium ribbon and clean it up with some emery paper.

2. Cut the ribbon into five equal pieces of 3cms length.

3. Using a measuring cylinder, measure out 50cm3 of 1M hydrochloric acid and pour it into a 100 cm3 beaker.

4. Drop a 3cms strip of magnesium into the acid and start the timer.

5. Stop timing when the strip has completely dissolved into the acid.

6. Record the time in seconds in the results table below.

7. Wash out the beaker with water and repeat the experiment with the other solutions of acid (1.25M, 1.5M, 1.75M and 2M).

Results Concentration of HCl (M)

Time for ribbon to dissolve (s)

Rate (1/time) s-1

1.0 1.25 1.5 1.75 2.0

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Mr Singh

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Rates of Chemical Reactions

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Graph

Plot a graph of the results obtained. Put the concentration on the x-axis and the time on the y-axis.

Draw a best fit curve for the results.

Questions

1. Which concentration of acid reacts the fastest with the magnesium? [1]

2. An increase in the concentration of the acid causes the rate of reaction to be faster. Explain why, using the collision theory.

[3]

3. Write a word equation with state symbols for the reaction.

[2]

4. Write a balanced equation for the reaction.

[2]

5. Name the substances that will be present in the mixture after the reaction has finished. [3]

6. Which reactant is in excess? How do you know? [2]

7. The concentration of the acid is varied but all other possible variables are held constant to make the experiment a fair test. Name two of these. [2]

8. Using your graph, what would be the time taken to fully the magnesium if a 1.4M solution of the acid was used?

[2]

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Mr Singh

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Rates of Chemical Reactions

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LABCOATS MUST BE WORN

Effect of temperature on the rate of reaction

Hazard

Control Measure Wear eye protection

Substance 2M Hydrochloric acid

Wear eye protection Avoid looking at the flame if ignited

Magnesium metal (ribbon)

IRRITANT

HIGHLY FLAMMABLE

Method

1. Take some magnesium ribbon and cut a 5 cm strip from it. Clean it up with emery paper.

2. Take a 250 cm3 beaker. Using a measuring cylinder, place 50 cm3 of 2M hydrochloric acid into the beaker.

3. Stir the acid with a thermometer and then measure the temperature of the acid.

4. Add the magnesium strip to the acid and start the timer. Measure how long it takes for the magnesium to totally dissolve in the acid.

5. Cut another 5 cm strip of magnesium and clean it with emery paper.

6. Now wash out the beaker and add another 50 cm3 of 2M hydrochloric acid to it. Place the beaker on top of a tripod and gauze.

7. Using a Bunsen flame, heat up the acid to 30oC. Take the beaker off the gauze and place it on a heatproof mat.

8. Add the magnesium strip and time how long it takes to dissolve the metal.

9. Repeat the experiment, using fresh acid and metal using temperatures of 40oC, 50oC, 60oC and 70oC. Results

Write your results in the table below:

Temperature(oC) Time for magnesium to dissolve(s)

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Mr Singh

Rate of reaction (1/t) (s-1)

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Rates of Chemical Reactions

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LABCOATS MUST BE WORN

[3]

Graph

Draw a xy scatter graph with temperature of acid on the x-axis and rate of reaction on the yaxis. [5]

Questions

1. What happened to the rate of reaction as the temperature increased?

[1]

2. Why does the rate get affected in this way if the temperature increases? Use collision theory to explain this.

[4]

3. Why is the concentration of acid kept constant in all five runs?

[2]

4. Write a word equation with state symbols for the reaction of magnesium and hydrochloric acid.

[2]

5. Write a balanced equation for the reaction.

[2]

6. Give two other factors that must be held constant in all five runs so that it is a fair test.

[2]

7. Why was the magnesium rubbed over with emery paper?

[1]

8. Use the graph, to work out what temperature rise is needed to get a doubling of the rate of reaction.

[2]

9. What would happen to the rate of reaction if it was carried out a low temperature of 10oC. Explain your answer, using collision theory.

[3]

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Rates of Chemical Reactions

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10. Give two major sources of error and describe how you would modify the practical to reduce these errors. [4]

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Mr Singh

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Rates of Chemical Reactions

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The Effect of Particle Size on a reaction rate Hazard

Control Measure Wear eye protection

Substance Calcium carbonate (marble) chips

Wear eye protection

2M Hydrochloric Acid

LOW HAZARD

IRRITANT

Introduction Marble is a form of calcium carbonate. It reacts with dilute hydrochloric acid, giving off carbon dioxide. In this experiment the effect of the size of the marble chips on the rate of formation of carbon dioxide is investigated.

Method 1. Put 40 cm3 of 2 mol/ dm3 hydrochloric acid in a 250 cm3 conical flask and place a loose plug of cotton wool in the neck of the flask.

2. Weigh out 20g of large marble chips on a folded piece of paper.

3. Put the flask and the paper on the pan of a balance and press the tare button.

4. Without taking the flask off the balance, remove the cotton wool plug and add the marble chips to the acid and start timing as quickly as possible.

5. Replace the cotton wool plug and put the paper back on the balance.

6. Read off the loss in mass of the apparatus at regular intervals until the reaction stops.

7. Repeat the procedure using the same amount of acid with 20 g of small marble chips.

Questions 1. What is the purpose of the cotton wool plug?

[2]

2. Why does the apparatus lose mass during the experiment?

[1]

3. Why must the volume and concentration of the acid be the same in both parts of the experiment?

[2]

4. Record the measurements in a table with the following headings [3] e.g

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WEAR EYE PROTECTION WORN Time(mins)

Loss in mass with large marble chips(g)

LABCOATS MUST BE

Loss in mass with small marble chips(g)

5. Plot the results on graph paper. Show the loss in mass on the vertical (y) axis and time on the horizontal (x) axis. Plot both sets of results on the same axes. Draw a line or a curve through each set of points. Label the two lines clearly to show which is which.

[5]

6. Why is the final loss in mass approximately the same in both parts of the experiments?

[2]

7. Which react faster -large chips or small chips? How can you work this out from your graph ?

[2]

8. Which has the larger surface area -20 g of small chips or 20 g of large chips?

How does this explain your answer to Question 7 ?

[2]

9. Which of the reactants was in excess In this experiment -the marble or the acid ?

How can you tell?

[2]

10. Why does the reaction gradually slow down and stop in both parts of the experiment?

[4]