unit 14 for issuu march 10 2012

Unit 14

How do I recognize and deal with a reversible chemical reaction?

The problem with chemical reactions is that the reverse reaction can, and usually does occur. To put it another way, for the reaction AB, while A ⇌ is being converted to B, B is often being converted back to A. And what good is that?! This is equilibrium, a balance between forward and reverse reactions. In a sense, this means that a chemical reaction is never “done”; it never stops; it simply gets to a point where the rate of the forward reaction is equal to the rate of the reverse

reaction. Fortunately it is usually an easy matter to destroy this reversibility, this equilibrium. For example a reaction can be irreversibly driven to completion by removing the products as they are formed.

Note that this doesn’t necessarily mean the reaction will now happen quickly: equilibrium and rate are two separate aspects of a chemical reaction…and just what is the relationship between equilibrium and rate? What does it mean to have something that can go forwards and backwards at the same time, at different speeds? Will the product ever form? Will the product ever stay formed?

In practice, reversible reactions can give lousy yields of product, and chemists are always trying to come up with a way to drive the reaction to completion.

The graph below shows the typical result of a reversible chemical reaction. Dinitrogen tetroxide is decomposing into nitrogen dioxide, but since the reaction is reversible, the reaction never goes to completion.

In this unit we will learn how to calculate the concentrations of reactants and products at equilibrium, and we will use several methods to adjust the equilibrium in the direction we prefer. ∏

Name: __________________________ Period: _____

equilibrium lab 1

Paper Clip Equilibrium Activity To demonstrate the characteristics of a reversible chemical reaction, imagine the reaction A + B ďƒ C below:

A

C

B

This is an example of a _______________ reaction. The reverse reaction is an example of a _________ reaction. At your instructors prompts, make as many C molecules as you can in 15 seconds. Then see how many you can take apart in 15 seconds. Record your answers in the box.

Summary: C molecules made in 15 seconds (forward reaction): __________ C molecules decomposed in 15 seconds (reverse reaction): ________ At this rate, it would take ____ minutes for this reversible reaction to go to completion.

6. Before returning the materials to the front of the classroom, be certain the composition of the paper clips in the pile are the same as when your received them.

Summary Question: You have just simulated a reversible chemical reaction. In actual practice, the rate of a chemical reaction at constant temperature and pressure slows down over time until there is no change in the ratio of product to reactants. Explain this using collision theory.

↔



D

  

D



↔

D

         

Name: _______________________

Period: _____

equilibrium lab 2

Perfume Lab Introduction: Esters may be prepared through the reaction of a carboxylic acid RCO2H with an Alcohol (R’OH), using a small amount of sulfuric acid as a catalyst.

RCO2H carboxylic acid

+

R’OH + alcohol

H2SO4



sulfuric acid

RCO2R’ ester

+

H2O water

+

H2SO4

sulfuric acid

Esters often have strong pleasant aromas. Carefully guarded mixtures of esters create expensive perfumes including Chanel #5, Aramis (for men) and others, some of which sell for hundreds of dollars per bottle. In this lab each student will create his own ester, and we will then share them to make perfumes.

For this chemical reaction, all of these reactants and products remain in solution. Therefore this reaction is reversible, and yields for this reaction can be low. In this experiment we will investigate the equilibrium mixture for this mixture after 24 hours.

Materials: Carboxylic acids listed on board Alcohols listed on board Sulfuric Acid (to be distributed by instructor) as a catalyst .

Chemical Reaction Procedure: Mix 0.1 moles of your carboxylic acid, 0.1 moles of ethanol, and 5 drops of sulfuric acid. The calculations below will help make sure you are using the right amounts. Heat but do not boil on a hot plate for 20 minutes then store covered overnight.

Calculations: My carboxylic acid has a formula of _____, therefore one mole has a mass of ______g, and 0.1 mole has a mass of ______g.

My alcohol has a formula of ______, therefore one mole has a mass of ______g, and 0.1 mole has a mass of ______g.

Workup The following day, carefully neutralize the mixture with a measured amount of baking soda (NaHCO3).

This reaction required ____g of baking soda for neutralization.

Calculation: Sodium bicarbonate has a molecular formula of NaHCO3. Therefore one mole of NaHCO3 has a mass of ____g and 0.1 mole has a mass of ____g. Since ____ g of sodium bicarbonate reacted, this is ____moles of sodium bicarbonate. Therefore it reacted with ____moles of my carboxylic acid. Based on this we estimate that the reaction is ____% complete.

All of the substances in the mixture are water soluble, except the fragrant ester you have produced.

Bottle and artistically label the ester you have created.

If time permits, combine small amounts of your perfume with those made by others to create your own perfume.

Results: 1. Based on our workup, our reaction created ___ g of ester after ____ hours for a ____ % yield.

I would describe the odor of our ester produced as __________ I would describe the odor of our perfume as _______..

Questions

1. Show a balanced chemical equation for the reaction of acetic acid with baking soda.

3. Based on chemical equilibrium, indicate three ways the yield of this reaction could be improved.

Name: _____________________________________

Date: ______

Period: _____

Science and Technology Posters 100 Points Introduction: Choose a poster on a topic of your choice.

Topic: Each group of two will present a poster on any approved topic that is titled: The Chemistry of ____________________ Choose something that you are personally interested in. Possible topics include The Chemistry of : 1. A rose

19. Scopolamine

2. Explosives

20. Mouthwash

3. DNA

21. flavonoids

4. skin cream

22. Cellular phones

5. chocolate

23. Reverse osmosis

6. dirt

24. artificial blood

7. car tires

25. hydrofluoric acid

8. the space shuttle

26. chemical warfare

rocket engine

agents

9. A battery

27. organ transplants

10. Hybrid vehicles

28. the bliss molecule

11. nuclear power

29. pain

12. Nuclear warheads

30. anabolic steroids

13. The Connecticut

31. mucous

river

32. energy drinks

14. The ozone layer

33. really smelly gases

15. Liquid crystals

34. combinatorial

16. A baseball

chemistry

17. carbon

35. dynamite

18. Coca-cola

1

Scoring Rubric 1. These posters are purely informational, not research-based. The goal is to instruct the reader in a logical, succinct, and interesting way. No experiments are necessary.

2. These posters should reflect the fact that we are near to completion of a full year high school level chemistry course. Try to get as deep as you can into your subject.

3. There should be several chemical structures included in your poster (2 minimum).

4. There should be a properly cited reference section for your poster. Include trusted scientific sources wherever possible. Include enough details in your citation that anyone could easily retrieve that source.

5. Include numerous images in your poster (2 minimum). Cite the source below the image if it is not original.

All posters must be typed. Your instructor will provide details.

Name: ____________________________________ Period: _____

equilibrium worksheet 1

Writing Equilibrium Concentration Expressions Directions: Write the equilibrium constant expression for each of the equations illustrated below.

These all follow the format:

for aA + bB  cC +dD

Keq 

[C]c [D]d [A]a [B]b

Example: write the equilibrium constant expression for the gas-phase synthesis of ethane (C2H6) from the elements.

Solution: First, we write the balanced chemical equation: 2C(g) + 3H2 ↔ C2H6 (g) Then we use the format above to write the equilibrium constant expression:

Keq 

[C2H6 ]

[C]2 [H2 ]3

1. At 1405 K, hydrogen sulfide, also called rotten egg gas because of its bad odor, decomposes to form hydrogen and a diatomic sulfur molecule, S2.

2H2S(g) ↔ 2H2(g) + S2(g) Write the equilibrium constant expression for this reversible reaction.

2. Methanol, a formula-1 race car fuel, can be made from carbon monoxide and hydrogen gas: CO(g) + 2H2 (g) ↔ CH3OH(g) Write the equilibrium constant for this reversible reaction.

3. Write the balanced reaction for the combustion of hydrogen at 200 OC, and show that this is a reversible reaction.

Write the equilibrium constant for this reversible reaction.

4. Write a balanced reaction for the combustion of methane at room temperature. Be sure to include the physical states of the reactants and products.

Write the equilibrium constant for this reversible reaction.

Name: _______________________

Date: ______Period: _____ eauilibrium worksheet 2

Calculating Equilibrium Concentrations

Directions: Write the equilibrium constant expression for each of the equations illustrated below and solve for the missing value.

These all may be solved using the equilibrium constant expression: for aA + bB ↔ cC +dD

Keq 

[C]c [D]d [A]a [B]b

And then plugging in the given data and solving for the unknown.

Example: For the reaction of carbon monoxide with oxygen to form carbon dioxide, determine the equilibrium concentration of carbon dioxide when the concentration of carbon monoxide is 0.8 moles/liter, the concentration of oxygen is 2.1 moles/liter, and the equilibrium constant is 225.

Solution: We begin by writing a balanced chemical, equation for the reaction: 2CO + O2 ↔ 2CO2 We then write the equilibrium constant expression and plug in the numbers given:

Keq 

[CO 2 ]2

[CO]2 [O2 ]

; 225 

[CO 2 ]2

[0.8]2[2.1]

Finally, we solve for the concentration of carbon dioxide:

[CO 2]  225(0.8)2 (2.1)  17.4

The concentration of carbon dioxide is 17.4 moles/liter

1. Lead sulfide may be prepared under high pressure by the reaction of lead with elemental sulfur: Pb(g) +S(g) ↔ (PbS(g)

What is the value of the equilibrium constant (Keq) if [Pb] = 0.30 mol/L and [S] = 0.184 mol/L, and [PbS] is 2.00 mol/L?

How far has this reaction progressed?

A. Unfortunately, it is still mostly reactants B. This reaction is mostly products

2. Methanol can be prepared from carbon monoxide and hydrogen: CO(g) + 2H2 (g) ↔ CH3OH(g) Calculate these equilibrium constants: a. Keq when all substances have a concentration of 1 mol/L

b. Keq when all substances have a concentration of 2 mol/L

C. Keq when all substances have a concentration of 3 mol/L

d. For each reaction indicate if the reaction is mostly products, or mostly starting material.

3. For the combustion of methanol, determine the concentration of methanol given the following data: Keq = 0.32 [O2] = 2 mol/liter [CO2] = 4 mol/liter [H2O] = 5 mol/liter

Name: ____________________________ Period: _____

equilibrium worksheet 3

Le Chatelier’s Principle Henri Le Chatelier came up with a cryptic quote for explaining what causes chemical equilibrium, and what to do about it:

"Placing a stress on an equilibrium causes the equilibrium to shift so as to relieve the stress" What he was referring to were some common things one can do to modify a chemical reaction and the net result: Add reactant: reaction moves forward () Add product: Reaction moves backward (reverse;  Add temperature: Moves forward if endothermic (positive DH) Add pressure: moves toward the fewer number of moles.

Remember, liquids and solids are considered to be outside of the reaction mixture – don’t count them when adding up moles.

Example: For the aqueous reaction of table salt with magnesium sulfide, the standard enthalpy of formation is +22.6 kJ/mol. Predict the equilibrium shift if the temperature is increased, if the pressure is increased, or if sodium sulfide is added to the reaction mixture.

Solution: We begin by writing a balanced chemical equation:

2NaCl (aq) + MgS (aq) ↔ Na2S (aq) + MgCl2 (aq) DHo = +22.6 kJ/mol Note that in this case 3 moles of reactants form 2 moles of products, and that the standard enthalpy of formation indicates this reaction is endothermic. Using this information and the tips at the top of this worksheet, we can conclude Increasing temperature will shift the equilibrium forward () since this reaction needs heat Increasing pressure will shift the equilibrium forward ( ) since the product has fewer

moles Adding sodium sulfide is like adding water to a fire, and shift the equilibrium backwards ( )

1. For the following reaction 5 CO(g) + I2O5(s)  I2(g) + 5 CO2(g)

DHo = -1175 kJ/mol

for each change listed, predict the equilibrium shift and the effect on the indicated quantity.

Direction of Shift

Change (a) (b) (c) (d) (e)

( ; ; or no change)

decrease in volume raise temperature addition of I2O5(s) addition of CO2(g) removal of I2(g)

Effect on Quantity

Effect (increase, decrease, or no change)

amount of CO (g) amount of CO(g) amount of CO(g) amount of I2O5(s) amount of CO2(g)

2. Consider the following equilibrium system in a closed container: Ni(s) + 4 CO(g)  Ni(CO)4(g)

DHo = - 161 kJ

In which direction will the equilibrium shift in response to each change, and what will be the effect on the indicated quantity?

Direction Effect on Effect (increase, decrease, Change of Shift Quantity ( ; ; or no change)

(a) (b) (c) (d) (e) (f) (g)

add Ni(s) raise temperature add CO(g) remove Ni(CO)4(g) decrease in volume lower temperature remove CO(g)

or no change)

Ni(CO)4(g) Keq amount of Ni(s) CO(g) Ni(CO)4(g) CO(g) Keq

3. For the conversion of oxygen (O2) to ozone (O3), predict the equilibrium shifts from the following changes:

Change (a) (b) (c) (d) (e) (f) (g)

add Ni(s) raise temperature add CO(g) remove Ni(CO)4(g) Apply a vacuum lower temperature remove CO(g)

Direction of Shift

( ; ; or no change)

Name__________________________ Period________

equilibrium worksheet 4

Equilibrium Review Worksheet 1.

What is the best way to drive a reversible reaction to completion?

If you were watching a chemical reaction, list three observations that would indicate that the reaction is not subject to equilibrium and can only move forward.

2.

3.

4.

Write the gas equilibrium constant (Kc) for each of the following chemical reactions.

5) CS2(g) + H2 (g)  CH4 (g) + H2 (g)

8)

6)

Ni (s)

+

7)

HgO(s) 

CO(g)



Ni(CO)4 (g)

Hg

(l)

+

O2(g)

In your own words, paraphrase Le Chatelier's Principle.

9) Balance the following reaction: ___N2 (g) + ___H2 (g)  ___NH3 (g) DH= -386 KJ/mole 10. Known as the Born-Haber Process, this is an example of a __________ reaction.

Predict the direction the equilibrium will shift if: 11) N2 is added?

12) H2 is removed?

13) NH3 is added?

14) NH3 is removed?

15) the volume of the container is increased?

16) the pressure is increased by adding Argon gas?

17) the reaction is cooled?

18) equal number of moles of H2 and NH3 are added?

The equilibrium constant for the following reaction is 5.0 at 400 C. CO (g) + H2O(g)  CO2 (g) + H2 (g) Determine the direction of the reaction if the following amount (in moles) of each compound is placed in a 1.0 L flask.

CO (g) H2O (g) CO2 (g) H2 (g) 19.

0.50 0.40 0.80 0.90 20.

0.01 0.02 0.03 0.04 21.

1.22 1.22 2.78 2.78

22.

At a particular temperature a 2.0 L flask contains 2.0 mol H2S, 0.40 mol H2, and 0.80 mol S2.

Calculate Keq at this temperature for the reaction: H2 (g) + S2 (g)  H2S (g)

23) Balance the following conversion of methane into the monomer ethylene, used to make the polymer polyethylene: ___CH4 (g)  ___H2C2 (g) + ___H2(g) The initial concentration of CH4 is 0.0300 M and the equilibrium concentration of H2C2 is 0.01375 M: 24) calculate the equilibrium concentrations of CH4 and H2; 25) Determine the numerical value of Keq. 26) At a particular temperature, 8.0 mol NO2 is placed into a 1.0 L container and the NO2 dissociates by the reaction (which needs balancing): ___NO2(g)  + ___O2(g) ____NO (g) 27. At equilibrium, the concentration of NO is 2.0 M. Calculate Keq for this reaction.

28. At a certain temperature, 4.0 mol NH3 is introduced into a 2.0 L container, and the NH3 partially dissociates by the reaction (please balance it): ___NH3 (g)  ___N2 (g) + ___H2(g) At equilibrium, 2.0 mol NH3 remains. What is the value of Keq for this reaction?

How to Ace the Equilibrium Exam

Howtoaceitunit18

In our previous unit we investigated the rate of chemical reactions- how fast do they go? In this equilibrium unit we point out that even if a reaction is going fast, it might not be going very far overall if the reverse reaction is also occurring. This is the big idea behind chemical equilibrium, the condition where the rate of a forward reaction is equal to the rate of the reverse reaction.

We can write the equilibrium constant expression and from this we can determine if we are getting anywhere or whether the reaction is standing still. Generally speaking, if we mix chemicals together we would like them to go forward, and this will happen if the value of the equilibrium constant (Keq) is greater than one. Note that Keq is only true at a specific temperature, and it says nothing about the rate of a reaction- only the direction.

A nice benefit of the equilibrium constant expression is that it can also tell you what the concentration of a reactant is, given enough information.

Since chemical equilibrium can prevent a reaction from going to completion, it would be nice to know how we can destroy it, or at least get things moving forward. Simple. To destroy chemical equilibrium, one must remove the product as it is formed- this makes the reverse reaction impossible. This is accomplished by having the product precipitate, for example by precipitating as a solid. As a general rule, this is why we omit liquids and solids from our equilibrium constant expression. In practice, it is easy to observe a precipitate. Examples include the gaseous precipitate we observe when we mix baking soda and vinegar, or the solids that crash out of solution during many double replacement reactions. These reactions can only move forward, since collisions between products to form reactants are no longer possible.

There are several other ways one can adjust chemical equilibrium. Known as Le Chatelier’s Principle, the direction of a reaction after a stress is applied may be summarized: Le Chatelier’s Principle Adding reactant:  Adding product:  Heating:  if endothermic Pressurizing:  if there are fewer moles of product Each of these may be reversed; for example cooling an endothermic reaction will favor the reverse reaction.

Imagine going on a trip. It’s nice to know in what direction you are going, and how long it will take.

These last two units have shown us just that for a chemical reaction. In the next unit we can apply these navigational skills to the study of acids and bases.

To ace this exam you should know: 1. What is chemical equilibrium?

2. What is a synonym for equilibrium?

3. What is the best way to destroy chemical equilibrium?

4. What does it mean if the rate of a forward chemical reaction a. Is faster than the reverse reaction b. Is the same as the reverse reaction? c. Is slower than the reverse reaction?

5. Please balance the reaction below and write the chemical equilibrium expression: ___Fe3O4(s) + ___H2(g)  ___Fe(s) + ___H2O (g) Keq =

6. Please determine the direction of the reaction given the following data: C2H4(g) + a. 1M b. 1.0520M

H2 (g)  2M 3.0400M

C2H6(g) DH = +32kJ/mol 3M Direction of reaction:______ 3.1909M Direction of reaction:______

7. For the reaction below the rate of the forward reaction is equal to the rate of the reverse reaction. Therefore, Keq = ____. Determine the concentration of ethane (C2H6) in the mixture: C2H4(g) + H2 (g)  C2H6(g) DH = +32kJ/mol 2M 4M ?

8. Please determine the direction of the following hypothetical reversible reaction: 4A(g) + 7B(g) + 13C +D (l)  9E (g) + 3F (g)+ 2G (g) Concentrations (M): 1.06 2.12 1.42 3 2.10 1.44 3.26

9 (L1 only). Please determine the concentration of G in the following reaction if it is at equilibrium.

Concentrations (M):

4A(g) + 7B(g) + 13C +D (l) ďƒ 9E (g) + 3F (g)+ 2G (g) 1.06 2.12 1.42 3 2.10 1.44 ?

9. List five ways to help the following reaction move forward: C2H4(g) + H2 (g) ďƒ C2H6(g) DH = +32kJ/mol

1.

2.

3.

4.

5.

10. In our next unit we will be studying acids and bases. Write a balanced chemical equation for the reaction of hydrochloric acid with sodium hydroxide to form table salt and water:

a. Can you move this reaction forward by pressurizing it? b. If the standard enthalpy of formation for this reaction is 0.004KJ/Mol, can you move it forward by heating it? c. What is the only product that might precipitate from this reaction at room temperature? d. Why would it be a big deal if that product did precipitate? e. Would it be a good idea to add water to this reaction? f. This is a segue into the next unit: If this reaction used 10 grams of sodium hydroxide and ten grams hydrochloric acid, would it result in a neutral, acidic, or basic solution (assuming a complete reaction)?