Mr Singh
GCSE Chemistry Energy Changes
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Mr Singh
4.5.1 Exothermic and endothermic reactions 4.5.1.1 Energy transfer during exothermic and endothermic reactions Content
Key opportunities for skills development
Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place. If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.
AT 5 An opportunity to measure temperature changes when substances react or dissolve in water.
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases. Exothermic reactions include combustion, many oxidation reactions and neutralisation. Everyday uses of exothermic reactions include self-heating cans and hand warmers. An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases. Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate. Some sports injury packs are based on endothermic reactions. Students should be able to: • distinguish between exothermic and endothermic reactions on the basis of the temperature change of the surroundings • evaluate uses and applications of exothermic and endothermic reactions given appropriate information. Limited to measurement of temperature change. Calculation of energy changes or ΔH is not required. Required practical 4: investigate the variables that affect temperature changes in reacting solutions such as, eg acid plus metals, acid plus carbonates, neutralisations, displacement of metals. 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 and skills development (page 105).
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2
GCSE Chemistry 8462. GCSE exams June 2018 onwards. Version 1.0 21 April 2016 Mr Singh
4.5.1.2 Reaction profiles Content
Key opportunities for skills development
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. Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction. Students should be able to: • draw simple reaction profiles (energy level diagrams) for exothermic and endothermic reactions showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved line to show the energy as the reaction proceeds • use reaction profiles to identify reactions as exothermic or endothermic • explain that the activation energy is the energy needed for a reaction to occur.
4.5.1.3 The energy change of reactions (HT only) Key opportunities for skills development
Content
During a chemical reaction: • energy must be supplied to break bonds in the reactants • energy is released when bonds in the products are formed. The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies. The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction. In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds. In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds. Students should be able to calculate the energy transferred in chemical reactions using bond energies supplied.
MS 1a
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Mr Singh
4.5.2 Chemical cells and fuel cells (chemistry only) 4.5.2.1 Cells and batteries Content
Key opportunities for skills development
Cells contain chemicals which react to produce electricity.
AT6
The voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte.
Safe and careful use of liquids.
A simple cell can be made by connecting two different metals in contact with an electrolyte. Batteries consist of two or more cells connected together in series to provide a greater voltage. In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up. Alkaline batteries are non-rechargeable. Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied. Students should be able to interpret data for relative reactivity of different metals and evaluate the use of cells. Students do not need to know details of cells and batteries other than those specified.
4.5.2.2 Fuel cells Content
Key opportunities for skills development
Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a potential difference. The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water. Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries. Students should be able to: • evaluate the use of hydrogen fuel cells in comparison with rechargeable cells and batteries • (HT only) write the half equations for the electrode reactions in the hydrogen fuel cell.
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Mr Singh
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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Mr Singh
Energy changes All reactions involve an ............................. change. Exothermic Reactions Reactions which give out energy are ........................... reactions. Combustion An oxidation reaction in which heat is given out is combustion. Combustion is accompanied by a flame and burning. A substance which is oxidised with the release of energy is a fuel. !!! ! + !! ! → !!! ! + !! ! !
Heat given out
Respiration We use the combustion of food to supply our body with energy. Carbohydrates are oxidised to carbon dioxide and water with the release of energy. !! !!" !! ! + !! ! → !!! (!) + !! !(!)
Heat given out
Neutralisation When an acid neutralises an alkali, heat is given out. ! ! !" + !! ! (!") → !! !(!)
Heat given out
Hydration When anhydrous salts react with water to form hydrates, heat is given out. !"#!! ! + 5!! ! ! → !"#!! . 5!! !
Heat given out
In an exothermic reaction, the reactants have more energy than the products. As the reactions happens energy is given out in the form of heat and warms up the surroundings.
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Mr Singh
Chemical Energetics All reactions involved an .................... change. Whenever we are doing a study of a reaction we divide the universe into two. a) The ................................... is part of the universe which we are studying b) The ................................... is everything else in the universe.
If we are studying the change in energy for the neutralisation reaction between an acid and an alkali, !"# + !"#$ → !!! + !"#$ the ......................... would be the molecules which will be doing the reacting, i.e. just the acid and alkali molecules. The .............................. would therefore be everything else in the world: the water they’re both dissolved in, the class beaker they are in, the thermometer, the table it is resting on, the air it the floor, the walls….. around
During a reaction energy is transferred from one to the other. An exothermic reaction is when energy is transferred from the ............................. to the surroundings An ........................... reaction is when energy is transferred from the surrounds to the system.
We measure a change in energy by measuring the temperature of the surroundings using thermometer. If the thermometer goes up, it means the thermometer has absorbed heat from the system – exothermic reaction If the thermometer goes down it means heat has been taken away from the thermometer by the system- endothermic reaction. 8
Mr Singh
Energy change of reactions Chemical bonds are forces of attraction between the atoms or ion or molecules in a substance. To break these bonds energy must be ......................... in. When bonds are created, energy is ...................... out. In a chemical reaction bonds are ........... broken and made. e.g.
These bonds must be broken.
These bonds must be made
Energy must be taken in
Energy is given out
When the energy given out by making the new bonds is greater than the energy taken in to break the old bonds the reaction is exothermic. When the energy give out by making the new bonds is smaller than the energy taken to break the old bonds, the reaction is endothermic. Remember BENDOMEX. Break – Endo Make - Exo Using a table of average bond energies we can calculate the energy change of a reaction.
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Mr Singh
∆! = 1)
!"#$!%# !"!#$% !" !"# !"#$% !"#$%& +
!"#$!%# !"!#$% !" !"# !"#$% !"#$
Add up all the energy needed to break all the old bonds of reactants
2) Add up all the energy released with the reaction of the new bonds of products 3) Add using the above formula. Keep whatever sign comes from the calculation and this tells you whether the reaction is exo or endothermic. Worked example: Use bond energies to calculate the ΔH for the reaction !!$ ! + 2!! ! → !!! ! + 2!! !(!) Solution:
Bonds broken are:
4(C-H) bonds ;
energy = 4 x 412 = +1648KJ/mol
2(O=O) bonds;
energy = 2 x 496 = +992KJ/mol Total energy required = +2640 KJ/mol
Bonds made are
2(C-O) bonds;
energy = -2 x 743 = -1486 KJ/mol
2(H-O) bonds;
energy = -4 x 463 = - 1852 KJ/mol Total energy given out = -3338 KJ/mol
Heat of reaction = Energy to break bonds + Energy to make bonds = +2640 – 3338 = -698 KJ/mol
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Mr Singh
Cells and Batteries We use electrochemical cells to make electricity from …………………………………….. reactions. A redox reaction involves a transfer of …………………………………………. and moving electrons is a current. Every day devices that do this are called Electrochemical cells or more commonly batteries. A simple cell can be made by connecting two different metals in contact with an electrolyte.
This is to do with the difference in reactivity between the two metals. The bigger the difference, the higher the voltage produced by the cell. Multiple cells can be connected in series to create a battery.
There are three main types of cells: Types of Cells
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Mr Singh
1 Non-rechargeable cells - A primary cell may be used only once and is reaches exhaustion when the reaction reaches ………………………………….. Alkaline dry batteries are an example of this type. Alkaline dry cell Anode: !" ! + 2!! ! !" → !" !" ! ! + 2! ! Cathode: !"!! ! + 2!! ! ! + ! ! → !" !" ! ! + !!! (!") 2. Rechargeable cells - A secondary cell is ……………………………….. and when current is passed in the opposite direction to the current flow during discharge the original state of equilibrium is restored. Nickel-Cadmium cells and lithium ion and lithium polymer ions are examples of this type of battery. Nickel-Cadmium cell Anode: !" ! + 2!!! !" → !" !" ! ! + ! ! Cathode: !" !" ! (!) + ! ! → !" !" ! ! + !!! (!") Lead-acid cell Anode: !" ! + !!!!! !" → !"#!! ! + 2! ! Cathode: !"!! ! + 4!! !" + !!!!! !" + 2! ! → !"#!! ! + 2!! !(!)
3. Fuel cells Fuel cells are electrochemical cells that oxidize an external gaseous ………………………… such as …………………………… or ……………………………. to produce an electric current.
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Mr Singh
Fuel cells operate continually as long as ………………………………….. is supplied. Hydrogen fuel cells Hydrogen and oxygen produce ……………………………. and electricity. The reactants flow in and products flow out by the electrolyte remains unchanged. The electrolyte is a proton conducting solid membrane and the anode is platinum powder. The cathode is nickel based. In this type the hydrogen ions pass through the electrolyte from the anode to the cathode. Negative terminal
2!! ! ! + 2! ! ⇌ !! ! + 2!!!
!
Positive terminal: !! ! + !! ! ! + 2! ! ⇌ 2!!! (!") !
The more negative hydrogen provides the ……………………………... The overall cell equation is the half equations added together. !! ! + 2!!! !" → 2!! ! ! + 2! !
Overall
1 ! ! + 2!! ! ! + 2! ! → 2!!! (!") 2 ! ! !! ! + !! ! → !! !(!) !
Fuel Cell Vehicles Fuel cells are considered a possible alternative to the petrol combustion engine as they getting to a point of being competitive. Some considerable hurdles remain such as fuel storage and operating temperatures.
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Mr Singh
There advantage is that they are far less polluting to run and are much more energy efficient.
They use hydrogen rich fuels such as ………………………………….. There limitations are • Large scale storage and transportation. There is currently no infrastructure • The feasibility of storing a pressurized liquid • Adsorbers and adsorbers of hydrogen have a limited life span., • Fuel cell use toxic chemicals in their production. The use of hydrogen has to be accepted politically and by the general public. Logistical problems of handling and maintaining hydrogen systems Hydrogen is an energy carrier, not a source – it requires energy to make it that might not be carbon neutral.
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Mr Singh
Energy Profile Diagrams Draw the energy profile diagrams for the following reactions labelling the graphs with the reagents and products and showing whether the reaction is exothermic or endothermic. 1. !!! ! + !! ! → !!! ! + !! ! !
2. !! ! + !! ! → !!! !
3. !"!!! ! → !"# ! + !!! !
!"#$%& !"##$%$&'$ = −890.3 !"
!"#$%& !"##$%$&'$ = +89.1!"
!"#$%& !"##$%$&'$ = +236.1 !"
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Mr Singh
4. !! !! → !! + !! !
5. !"
!
!"#$%& !"##$%$&'$ = −1946 !"
+ !!! ! → !"#!! !"
!"#$%& !"##$%$&'$ = −418 !"
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Mr Singh
Activation Energy 1. Finish the following sentences Bond breaking is an …………………………….. process. Bond making is an …………………………………. process.
2. What is the definition of the activation energy?
3. Why don’t hydrogen and oxygen gas simply react as soon as they are mixed together?
4. What does the spark that start a reaction do in relation to the bonds?
5.
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.
b. Is this an endothermic or an exothermic reaction? How can you tell?
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Mr Singh
6. 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?
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Mr Singh
Energy change calculations bond H-H O=O C-C C=C C-H O-O H-Cl
bond energy (kJ/mol) 436 496 348 612 412 146 431
bond C=O H-O Br-Br C-Br S=O Cl-Cl
bond energy (kJ/mol) 743 463 193 276 435 242
1.) Use the bond energy data in the table to calculate the energy change in each reaction (make sure your sign is correct). 2.) Draw an energy diagram to show the relative position of the reactants and products in each one. O 2
O
S
+
O
O
O
1 2
O S
2
O
H
+
H
Cl
2
Cl
H
Cl
ii) 3
H
iii)
H
H
H
C
C
C
H
H
H
H
+
5 O
3
O
H 2
4
5
O
iv)
H
O
2
O H
H
O
+
C
O
+
O
O
H
H
H
C
C
C
H
Br
Br
4
O H
H
H
H
H
H
C
C
C
H
H
+
Br
Br
H
H
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Mr Singh
1.
Energy in Chemistry
(a) Define the term ‘endothermic’.
(1)
(b) Draw the labelled energy level diagram for an exothermic reaction
(3) 2. Methane(CH4) is a compound used as a fuel. The equation below shows that structural formula equation for its combustion reaction: H
H
H
C
H
+
2 O =O
→
O=C=O
+
2H-O-
H
The bond energies in kJ per mole of bonds are: C-H 435, O=O 497, C=O 803,
H-O
464
(a) Calculate the energy needed to break all the bonds in the reactant molecules.
(2)
(b) Calculate the energy released when all the bonds in the products molecules are formed.
(2)
(c) What is the overall heat change of the reaction(∆H)?
(1)
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Mr Singh
(d) Why is this reaction exothermic? Explain in terms of bond energies. (2)
3. One industrial process for making ethanol involves reacting ethene with stream.
a) Identify the catalyst and one other condition used for this reaction ……………………………………………………………………………… ……………………………………………………………………………… ………………………………………………………………………………(2)
b) The table shows the values of some average bond dissociation energies. Bond C-C Dissociation 348 Energy(kJ/mol)
C=C 612
C-H 412
C-O 360
O-H 463
Use these values to calculate: i) The energy required, in kJ.mol, to break the bonds in the reactants. (1)
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Mr Singh
ii)
iii)
The energy given out, in kJ.mol, when the bonds in the products are formed.
(1)
The enthalpy change, in kJ.mol, for his reaction.
(1)
22
© www.CHEMSHEETS.co.uk
Disadvantages
Advantages
Example uses
Comments & what happens when the chemicals are used up
Non-rechargeable cells
20-February-2017
Rechargeable cells
Chemsheets GCSE 1162
Fuel cells
COMPARING CELLS
CELLS Electrochemical cell
Electrolysis
V
Simple electrochemical cells can be made by placing two different metals connected by a wire in an electrolyte. The voltage of this cell can be measured and it can be used to power electrical components such as light bulbs.
1
In both electrochemical cells and electrolysis, how is current carried through the wires
……………………………………………………………………………………………………………………………………………………………………
the electrolyte 2
An electrolysis cell where two electrodes in an electrolyte are connected to a cell.
……………………………………………………………………………………………………………………………………………………………
Complete this table about the anode and cathode in each case. electrochemical cell electrode
anode
cathode
electrolysis anode
cathode
reduction or oxidation? positive or negative?
3
a In an electrochemical cell, is the electrode made from the more reactive metal the positive or negative electrode? …………………….……………………………………………………………………………………………………………………………………………………………
b Explain why the electrode made from the more reactive metal has this polarity. …………………….……………………………………………………………………………………………………………………………………………………………
c What would be the voltage of an electrochemical cell where both electrode are made of the same metal? …………………….……………………………………………………………………………………………………………………………………………………………
© www.CHEMSHEETS.co.uk
27-May-2018
Chemsheets GCSE 1282
4
A student made a series of cells using metals P, Q and R. The results are shown in the table. metal connected to the positive terminal of the voltmeter
metal connected to the negative terminal of the voltmeter
voltage (V)
P
Q
–0.37
P
R
+0.23
a Place the three metals in order of reactivity. most reactive
……………..
……………..
……………..
least reactive
b Predict the voltage when Q is connected to R, with Q connected to the positive terminal of the voltmeter. …………………….……………………………………………………………………………………………………………………………………………………………
5
Some electrochemical cells can be recharged if an external current is applied. What must happen in the cell for this to happen? …………………….…………………………………………………………………………………………………………………………………………………………… …………………….……………………………………………………………………………………………………………………………………………………………
6
A hydrogen fuel cell is an excellent cell that has many uses. Many scientists and engineers believe that most cars will be powered by hydrogen fuels cells in the near future. a Write an overall equation for the reaction in a hydrogen fuel cell. …………………….……………………………………………………………………………………………………………………………………………………………
b Complete the table about hydrogen fuel cells. negative electrode
positive electrode
half equation for reaction reduction or oxidation? anode or cathode?
Area
Strength To develop Area
Strength To develop Area
Strength To develop
Done with care and thoroughness
Understands polarity of cell electrodes
Predict voltage of cells
Shows suitable working
Knows link between reactivity & polarity
Knows what happens in recharging
How current is carried in wires
Explain reactivity & polarity link
Knows H2 fuel-cell overall reaction
How current is carried in electrolyte
Voltage of cell with same electrodes
Knows H2 fuel-cell half equations
Understands what anode/cathode are
Put metals in reactivity order with voltage
© www.CHEMSHEETS.co.uk
27-May-2018
Chemsheets GCSE 1282
Mr Singh
C7.6
Student activity Name .....................................................................
Class .................
Date .....................
Fuel cells Specification references
C5.2.2 Fuel cells
Aims This activity will help you to understand how a hydrogen fuel cell works. You will also find out about its applications, advantages, and disadvantages.
Learning outcomes After completing this activity, you should be able to:
describe a hydrogen fuel cell and explain how it produces electricity list the advantages and disadvantages of hydrogen fuel cells state some uses of hydrogen fuel cells and explain why they are an alternative to rechargeable cells and batteries
describe the reactions in fuel cells using balanced symbol and half equations
determine which half equation is oxidation and which is reduction in a hydrogen fuel cell.
Setting the scene Fuel cells are fed with an external source of fuel (e.g., hydrogen), plus oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a voltage. The electricity provided by the fuel cell can be used to run an electric vehicle or to power other equipment.
Task A fuel cell is made from an anode and a cathode with an electrolyte between them.
© Oxford University Press 2016: www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original.
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Mr Singh
C7.6
Student activity Name .....................................................................
Class .................
Date .....................
Below is a diagram of a hydrogen fuel cell with an acidic electrolyte:
1 2 3
Explain what is meant by a fuel cell. The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water. Write a balanced equation for this reaction. In groups of three or four: Discuss the advantages and disadvantages of using hydrogen fuel cells compared with conventional cells. Find out the main uses of hydrogen fuel cells. Compare an electrical car powered by a rechargeable battery with a car powered by a hydrogen fuel cell. Present your findings in a poster.
(1 mark) (1 mark)
(12 marks)
Higher Tier questions The following questions refer to hydrogen fuel cells. 1
2
Hydrogen gas is supplied as a fuel to the negative electrode (the anode), where it releases electrons to produce H ions. The electrons flow round the external circuit. a Deduce the half equation for this reaction. b Is this oxidation or reduction? Explain your answer.
(1 mark) (2 marks)
Oxygen is supplied at the positive electrode (the cathode), where it combines with H ions and the electrons from the external circuit to form water. a Deduce the half equation for this reaction. b Is this oxidation or reduction? Explain your answer.
(1 mark) (2 marks)
© Oxford University Press 2016: www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original.
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Mr Singh
C7.6
Student activity Name .....................................................................
Class .................
Combine the two half equations to deduce the overall equation. Explain how you worked out the overall equation. What type of reaction is this?
3
a b c
4
Some hydrogen fuel cells use an alkaline electrolyte rather than an acidic one. Hydrogen gas is supplied as a fuel to the negative electrode (the anode), where it reacts with OH– ions, releasing electrons and producing water. The oxygen is supplied at the positive electrode (the cathode), where it combines with water and the electrons from the external circuit to produce OH– ions. a Use this information to write half equations for the reactions at the anode and cathode. b Identify which reaction is oxidation and which is reduction. c Combine the two equations and show that it is the same as your answer to 3a.
© Oxford University Press 2016: www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original.
Date ..................... (1 mark) (1 mark) (1 mark)
(2 marks) (2 marks) (1 mark)
25
3
3
))))))))))))))))))))))))))))))))))))
Calculate the heat released in kJ/mol of fuel burned.
))))))))))))))))))))))))))))))))))))
Calculate the number of moles of fuel burned.
))))))))))))))))))))))))))))))))))))
Calculate the heat released in kJ/g of fuel burned.
))))))))))))))))))))))))))))))))))))
Calculate the heat released by burning 0.40 g of fuel in kJ.
9-July-2013
))))))))))))))))))))))))))))))))))))
Calculate the heat released in kJ/mol of glucose burned.
))))))))))))))))))))))))))))))))))))
Calculate the number of moles of glucose burned.
))))))))))))))))))))))))))))))))))))
Calculate the heat released in kJ/g of glucose burned.
))))))))))))))))))))))))))))))))))))
Calculate the heat released by burning 0.50 g of glucose in kJ.
© www.CHEMSHEETS.co.uk
d)
c)
b)
a)
2 In an experiment, 0.50 g of glucose (C6H12O6) was completely burned in air. The heat given off raised the temperature of 50 g of water by 36.1°C.
d)
c)
b)
a)
1 In an experiment, 0.40 g of butane (C4H10) was completely burned in air. The heat given off raised the temperature of 100 g of water by 54.2°C.
The specific heat capacity of water is 4.2 J/g/°C and the mass of 1 cm of water is 1 g.
H
H
H
C
H H
+
5 O O 3 O C
O
+
4 H
O
H
)))))))))))))))))))))))))))))))))
)))))))))))))))))))))))))))))))))
)))))))))))))))))))))))))))))))))
)))))))))))))))))))))))))))))))))
C
C
H
C
C
H
+
2 H
H
H
H
C
H
C
H H
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Chemsheets GCSE 050
)))))))))))))))))))))))))))))))))
)))))))))))))))))))))))))))))))))
b) Explain why this reaction is exothermic or endothermic in terms of bond breaking and making.
)))))))))))))))))))))))))))))))))
)))))))))))))))))))))))))))))))))
)))))))))))))))))))))))))))))))))
)))))))))))))))))))))))))))))))))
H
H
4 a) Calculate the energy change in this reaction. Bond energies are: (C≡C) = 837, (C-H) = 412, (H-H) = 436, (C-H) = 412, (C-C) = 348 kJ/mol
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b) Explain why this reaction is exothermic or endothermic in terms of bond breaking and making.
H
H
3 a) Calculate the energy change in this reaction. Bond energies are: (C-C) = 348, (C-H) = 412, (O=O) = 498, (C=O) = 743, (H-O) = 463 kJ/mol
Mr Singh
Mr Singh
Think creatively
1. Research types of batteries and how they work. 2. Compose a poem exploring how batteries have infiltrated our lives.
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Mr Singh
Taking it further
Why not read about batteries and thermochemistry in an A-level textbook? Why not do some internet research into future development of batteries? How have the changed the way we live and what are we hoping of them in the future? Where is the next promising breakthrough going to come from in this field?
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Energy in a Reaction Revision Questions 20 Marks 1.
The balanced equation for the combustion of ethane is shown using structural formulae.
2H
(a)
H
H
C
C
H
H
C +
7O
O
4O
C
O +
6H
O
H
Complete the table to show the number of bonds broken and made when two molecules of ethane react with seven molecules of oxygen. Type of bond
Number of bonds broken
Number of bonds made
C –– C C –– H O=O C=O H –– O (2)
(b)
The combustion of ethane is a strongly exothermic process. Draw a labelled energy level diagram showing the endothermic and exothermic parts of the overall reaction. Indicate the activation energy on the diagram. (4)
(c)
Explain, in terms of particles and the activation energy of a reaction, how a catalyst is able to increase the rate of reaction. .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... (2) (Total 8 marks)
2.
Read the information about energy changes and then answer the questions.
Bradfield College
29
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Mr Singh
A student did an experiment to find the energy change when hydrochloric acid reacts with sodium hydroxide. The equation which represents the reaction is: HCl + NaOH → NaCl + H2O The student used the apparatus shown in the diagram.
Thermometer
50cm3 of sodium hydroxide solution
Glass beaker 50cm3 of hydrochloric acid The student placed 50 cm3 of hydrochloric acid in a glass beaker and measured the temperature. The student then quickly added 50 cm3 of sodium hydroxide solution and stirred the mixture with the thermometer. The highest temperature was recorded. The student repeated the experiment, and calculated the temperature change each time.
Experiment 1
Experiment 2
Experiment 3
Experiment 4
Initial temperature in °C
19.0
22.0
19.2
19.0
Highest temperature in °C
26.2
29.0
26.0
23.5
7.2
7.0
6.8
4.5
Temperature change in °C
(a)
The biggest error in this experiment is heat loss. Suggest how the apparatus could be modified to reduce heat loss. ..................................................................................................................................... ..................................................................................................................................... (1)
(b)
Suggest why it is important to stir the chemicals thoroughly. .....................................................................................................................................
Bradfield College
30
2
Mr Singh
(1)
(c)
Which one of these experiments was probably carried out on a different day to the others? Explain your answer. ..................................................................................................................................... ..................................................................................................................................... (1)
(d)
Suggest why experiment 4 should not be used to calculate the average temperature change. ..................................................................................................................................... ..................................................................................................................................... (1)
(e)
Calculate the average temperature change from the first three experiments. ..................................................................................................................................... Answer = .............................. °C (1)
(f)
Use the following equation to calculate the energy change for this reaction. energy change in joules = 100 Ă— 4.2 Ă— average temperature change ..................................................................................................................................... Answer = .............................. J (1)
(g)
Which one of these energy level diagrams, A or B, represents the energy change for this reaction? Explain why.
Bradfield College
31
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Mr Singh
Diagram A
Diagram B NaCl + H2O
HCl + NaOH Energy
Energy NaCl + H2O
HCl + NaOH
..................................................................................................................................... ..................................................................................................................................... (1) (Total 7 marks)
3.
An airship caught fire when it was coming in to land in 1937. The airship was filled with hydrogen. A spark or flame ignited the hydrogen. The hydrogen reacted with oxygen in the air to produce water.
(a)
The equation for the reaction can be represented using structural formulae for the chemicals. 2H–H+O=O→2H–O–H Use the bond energies given in the table to help you to calculate the energy change for this reaction. Bond
Bond energy in kJ per mole
H–H
436
O=O
498
O–H
464
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Mr Singh
Energy change = ................................... kJ (3)
(b)
Explain, in terms of making and breaking bonds, why this reaction is exothermic. ..................................................................................................................................... ..................................................................................................................................... (1)
(c)
Use the energy level diagram for this reaction to help you to answer these questions.
(i)
The hydrogen did not burn until ignited by a spark or flame. Explain why. ........................................................................................................................... ........................................................................................................................... (1)
(ii)
Platinum, a transition metal, causes hydrogen to ignite without using a spark or flame. Explain why. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... (2) (Total 7 marks)
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Energe&cs Past Paper Ques&ons 1.
An airship caught fire when it was coming in to land in 1937. The airship was filled with hydrogen. A spark or flame ignited the hydrogen. The hydrogen reacted with oxygen in the air to produce water. (a)
The equaBon for the reacBon can be represented using structural formulae for the chemicals. 2 H – H + O = O → 2 H – O – H Use the bond energies given in the table to help you to calculate the energy change for this reacBon.
Bond
Bond energy in kJ per mole
H – H
436
O = O
498
O – H
464 ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... Energy change = ................................... kJ (3)
(b)
Explain, in terms of making and breaking bonds, why this reacBon is exothermic. ..................................................................................................................................... ..................................................................................................................................... (1)
(c)
Use the energy level diagram for this reacBon to help you to answer these quesBons. (i)
The hydrogen did not burn unBl ignited by a spark or flame. Explain why. ........................................................................................................................... ........................................................................................................................... (1)
2.
Methanol (CH3OH) can be made by reacBng methane (CH4) and oxygen (O2) in the presence of a plaBnum catalyst. The reacBon is exothermic. An equaBon that represents the reacBon is: 2CH4 (a)
+ O2
2CH3OH
The energy level diagram for this reacBon is given below.
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(i)
Use the diagram to explain how you know that this reacBon is exothermic. .......................................................................................................................... .......................................................................................................................... (1)
(ii)
Explain, in terms of the energy level diagram, how the plaBnum catalyst increases the rate of this reacBon. .......................................................................................................................... .......................................................................................................................... (1)
(b)
The equaBon can also be wriXen showing the structural formulae of the reactants and the product.
(i)
Bond
Use the bond energies given in the table to help you to calculate the energy change for this reacBon. Bond energy in kJ
C –– H
435
O O
=
498
C –– O
805
O –– H
464 .......................................................................................................................... .......................................................................................................................... .......................................................................................................................... .......................................................................................................................... .......................................................................................................................... Energy change = ...................................... kJ (3)
(ii)
In terms of the bond energies, explain why this reacBon is exothermic. .......................................................................................................................... .......................................................................................................................... (1)
3.
At room temperature, hydrogen peroxide decomposes very slowly to form water and oxygen. The decomposiBon is speeded up when a catalyst is added. (a)
The following equaBon represents the decomposiBon of hydrogen peroxide. The structural formulae of the chemicals involved are shown. 35
Mr Singh
Use the following informaBon about bond energies to answer this part of the quesBon. BOND
BOND ENERGY (kJ)
O = O
498
O – O
146
H – O
464
(i)
Calculate the energy needed to break all the bonds in the reactants. ............................................................................................................................ ............................................................................................................................ ....................... kJ (2)
(ii)
Calculate the energy released when new bonds are formed in the products. ............................................................................................................................ ............................................................................................................................ ....................... kJ (2)
(iii)
Calculate the energy change for this reacBon. ............................................................................................................................ ....................... kJ (1)
(iv)
Is the reacBon exothermic or endothermic? ............................................................................................................................ Explain why. ............................................................................................................................ ............................................................................................................................ (1)
(b)
(i)
What is meant by ‘acBvaBon energy’? ............................................................................................................................ ............................................................................................................................ (1)
(ii)
The energy level diagram for the decomposiBon of hydrogen peroxide into water and oxygen is shown below.
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Which energy change, A, B, C or D, is the acBvaBon energy? ....................... (1)
(iii)
Explain, in terms of energy, how a catalyst makes hydrogen peroxide decompose more quickly. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ (1)
4.
A student burned four fuels and compared the amounts of energy they produced. The student set up the apparatus as shown in the diagram.
The heat produced when each fuel was burned was used to raise the temperature of 100 g of water. The student noted the mass of fuel burned, the increase in temperature and whether the ame was smoky. The results are shown in the table. Mass of fuel burned (g)
Temperature increase ( C)
Type of ame
Ethanol
4
24
Not smoky
Methanol
3
9
Not smoky
Peanut oil
2
20
Smoky
Vegetable oil
1
15
Smoky
Fuel
(a)
The student suggested that the vegetable oil was the best fuel for producing heat. Explain why. ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... (2)
(b)
Suggest an environmental problem that could be caused when large amounts of vegetable oil are burned. Suggest how the problem could be overcome. ..................................................................................................................................... ..................................................................................................................................... 37
Mr Singh
..................................................................................................................................... ..................................................................................................................................... (2)
(c)
An energy level diagram for the burning of vegetable oil is shown below.
Which of the energy changes A, B or C: (i) represents the acBvaBon energy (ii)
............................... (1)
shows the amount of energy given out during the reacBon? ...............................
(1) (Total 6 marks)
5.
The reacBon between aluminium and iron oxide is used to weld together railway lines. 2Al(s) + Fe2O3(s) 2Fe(l) + Al2O3(s)
A simple, qualitaBve energy level diagram for this reacBon is shown.
Use the energy level diagram to: (i)
describe the idea of acBvaBon energy; ..................................................................................................................................... ..................................................................................................................................... (1)
(ii)
explain why the reacBon produces molten iron. ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... (2)
6.
Many hydrocarbons are used as fuels. An energy level diagram is shown for the combusBon of the hydrocarbon methane.
Describe and explain why the line rises and then falls to a lower level. ............................................................................................................................................... 38
Mr Singh
............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... (Total 4 marks)
7.
Some of the hydrogen and chlorine are reacted together to form hydrogen chloride. H2(g) + Cl2(g) 2HCl(g) Bond
Bond energy in kJ/mol
Cl–Cl
242
H–Cl
431
H–H
436
(i)
Use the bond energies to calculate the energy change for the formaBon of hydrogen chloride. ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... ..................................................................................................................................... Energy change = .................................. kJ/mol (3)
(ii)
Is this reacBon exothermic or endothermic? Explain your answer. ..................................................................................................................................... .....................................................................................................................................
8.
(2)
Hydrogen chloride is made by reacBng hydrogen with chlorine. H2(g) + Cl2(g) 2HCl(g) Bond
Bond energy in kJ
H – H
436
Cl – Cl
242
H – Cl
431
Is the reacBon between hydrogen and chlorine exothermic or endothermic? 39
Mr Singh
Use the bond energies to explain your answer. ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... (Total 3 marks)
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302P
Exothermic and Endothermic reactions All reactions take in and give out energy. A reaction that gives out more energy than it takes in is called an exothermic reaction. This means it gives out heat (it gets hot!). A reaction that takes in more energy than it gives out is called an endothermic reaction. This means it takes in heat (it gets cold!).
Method You are going to decide whether these experiments are exothermic or endothermic. Use a clean, dry test tube for each new experiment. Experiment
Observations
Teacher Demonstration Add 3 sodium hydroxide pellets to 5cm3 of water in a test tube. Measure the temperature change with a thermometer.
Add a spatula of zinc powder to 5cm3 of copper sulphate solution in a test tube. Measure the temperature change with a thermometer.
Add a spatula of ammonium carbonate to 5cm3 of ethanoic acid in a test tube. Measure the temperature change with a thermometer.
Add 2 spatula of potassium chloride to 5cm3 of water in a test tube. Measure the temperature change.
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1601P
Enthalpy Change of Combustion of Alcohols Introduction: Combustion is the reaction of a fuel with oxygen to produce heat. Different fuels give out different amounts of heat when burnt. In this experiment you are going to compare a number of alcohols. The most simple alcohol is methanol. The equation for its complete combustion is
CH3OH(l) + 1.5O2(g) Ă CO2(g) + 2H2O(l) method: 1.
Place 100 cm3 of water into the copper pot (calorimeter)
2.
Clamp the container above the burner so the bottom of the container sits 3 cm above the top of the burner
3.
Record the initial temperature of the water - to 1 d.p.
4.
Measure and record the initial mass of the burner and its lid
5.
Arrange the draught shield around the burner
6.
Remove the burner lid and light the burner
7.
Stir the water carefully using the thermometer
8.
When it has rise by approximately 30 oC extinguish the lamp by replacing the lid
9.
Record the final temperature of the water - to 1 d.p.
10. Measure and record the final mass of the burner and its lid 11. Repeat using different alcohol burners
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Theoretical Value (kJ mol-1 )
∆H (kJ mol-1)
Joules produced (J) = MW x T x 4.18 Energy Produced per mole = [(MW x T x 4.18)/1000] ÷ Moles
Rise in Temp (T)
Final Temp of Water
Initial Temp of Water
Mass of Water Heated (Mw)
Moles Burnt = Mass(MA )÷ RMM
Mass of Alcohol Burnt (MA)
Final Mass of Burner
Initial Mass of Burner
RMM
Molecular Formula
-715
CH3OH
Methanol
-1371
C2H5OH
Ethanol
-2010
C3H7OH
Propanol
-2673
C4H9OH
Butanol
1601P
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