Equilibrium and Reaction Rates
(answers at the end)
1. What is meant by the term Dynamic Equilibrium?2. Briefly explain Le Chatelier's Principle.
3. List the factors that affect the extent of equilibrium in a chemical system.
4. Sketch a properly labelled potential energy diagram for an exothermic reaction. Show how the addition of a catalyst will affect the graph.
5. Describe the effect of the addition of a catalyst to a chemical system at equilibrium.
6. Sketch a graph showing how the concentrations of the reactants and products of a typical chemical reaction vary with time during the course of a chemical reaction. Assume no products are present at the start of the reaction.
7. What is meant when we say that chemical reactions are reversible?
8. Here are some reactions and their equilbrium constants:
a) 2CH4(g) ---> 6C2H6(g) + H2(g) / Kc = 9.5 x 10-13
b) CH3OH(g) + H2(g) ---> 6CH4(g) + H2O(g) / Kc = 3.6 x 1020
c) C2H4(g) + H2O(g) --> 6C2H5OH(g) / Kc = 8.2 x 103
Arrange these reactions in order of their increasing tendency to go toward completion.
9. How will the equilibrium:
CH4(g) + H2O(g) + heat 6CS2(g) + 4H2(g)
be affect by the following:
- the addition of CH4(g)
- the addition of H2(g)
- the removal of CS2(g)
- a decrease in the volume of the container
- an increase in temperature
11. What is meant by the rate of a chemical reaction?
12. In terms of reaction rates, what is an explosion?
13. Given the following reaction:
CO(g) + 2H2(g) --> 6CH3OH(g) / H = -18kJ
how will the ammount of CH3OH(g)presenmt at equilibrium be affected by the following?
- adding CO(g)
- removing H2(g)
- decreasing the volume of the container
- adding a catlyst
- increasing the temperature
15. Persons who have been submerged in very cold water and who are believed to have drowned sometimes can be revived. On the other hand, persons who have been submerged in warmer water for the same length of time have died. Explain this is terms of factors that affect the rates of chemical reactions.
16. How must the exponents in the equibilbirum constant expression be determined?
17. What is the rate-determining step?
18. What does a catalytic converter do in the exhaust system of an automobile? Why can't leaded gasoline be used in cars equipped with catalytic converters?
19. Define transition state and activated complex.
20. Consider the following steps in a reaction mechanism:
C2H4Br- + I- --> C2H4Br- + IBr / slow
C2H4Br- --> C2H4 + Br- / fast
IBr + I- --> Br + I2 / fast
I2 + I- --> I3- / fast
- What is the overall rate of this reaction?
- Write the overall reaction for this mechanism.
- What effect would the addition of I2 have on the reaction? Why?
- What effect would the addition of I- have on the reaction? Why?
Na2S2O3(aq) + 2HCl(aq) --> 2NaCl(aq) + CO2(g) + 1/8S8(s) + H2O(l)
- List TWO cahnges that could be made to this reaction system to increase the reaction rate.
- Suggest an experimental technique that could be used to monitor the rate of reaction.
23. Give TWO reasons why a soft drink will go flat if the bottle is opened and placed in a sunny spot.
24. Briefly describe an experiment you could perform to demonstrate how changes in the surface area of reactants can alter reaction rates.
25. Give ONE reason why CH4(g) will react more rapidly than C3H8(g) in the combustion reaction with oxygen.
26. A saturated sugar solution and a solution of acetic acid are each in a state of dynamic equilibrium. What two features do these equilibria have in common and how are they different?
27. Consider the following equations representing twp different reactions:
Reaction 1: / C2H4(g) + 3O2(g) --> 2CO2(g) + 2H2O(g)
Reaction 2: / SO4I-(aq) + I-(aq) --> SO42- + I2(g)
Explain, in terms of the reacting species, why reaction #2 is faster than reaction #1.
28. Equal masses of zinc with approximately the same surface area were placed in two test tubes, A and B. 10mL of 3.0 mol/L HCL(aq) were added to both test tubes. In addition, test tube A received 5 drops of CuSO4(aq). The reaction in test tube A proceeded faster than that in test tube B.
- Use the concept of sctivation energy to explain why the reaction proceeded faster in test tube A than in test tube B.
29. How would the solubility of SO2(g) in water be affected by an increase in pressure of SO2(g)?
Answers for:Equilibrium and Reaction Rates
1. Dynamic Equilibrium means that at the macroscopic level there appears to be no changes but at the microscopic level the reactants continue to form products as the products decompose to form reactants.2. Le Chatelier's Principle states that when a change is introduced to a system at equilibrium the system will adjust (try to resist) the change.
3. a) Change in the concentration of Reactant:
Increase reactant / - shift right
Decrease reactant / - shift left
Remove reactant / - shift left
Remove product / - shift right
b) Change volume or pressure (applies to gases only):
Increase pressure (decrease volume) / - shift to side with least number of moles
Decrease pressure (increase volume) / - shift to the side with the greater number of moles
c) Temperature:
Heat as reactant / - increase, shift right
- decrease, shift left
Heat as Product / - increase, shift left
- decrease, shift right
d) Effects of a catalyst:
No effect on equilibrium
4. See diagran below:
5. The catalyst lowers the Activation Energy (Ea) so the reaction will occur faster. The catalyst has no effect on the overall equilbrium.
6. See diagram below:
7. A reaction can proceed to form products and, under the proper conditions, the products can react to form reactants.
8. b, c, a - higher number of products favoured
9. a) shift right, increase in [products], decrease in [reactants]
b) shift left, increase in [reactants], decrease in [products]
c) shift right, increase in [H2], decrease in [reactants]
d) shift left, increase in [reactants], decrease in [products]
e) shift right, decrease in [reactants], increase in [products].
10. a) Nature of Reactants:
- bond strength
- shape
b) Concentration of Reactants:
The greater the concentration, the faster the reaction rate (the number of collisions per second is higher).
c) Temperature:
The higher the temperature, the more collisions per second and the faster the reaction rate.
d) Catalyst:
Increases the reaction rate
e) Surface area:
The larger the surface area, the faster the reactions become. There is more surface area exposed.
11. Reaction rate is the measure of how fast a reactant is used in a chemical reaction or how fast the products are formed. The number of collisions per second.
12. An explosion is a very fast reaction rate.
13. a) increase [CH3OH]
b) decrease [CH3OH]
c) decrease [CH3OH]
d) No effect e) decrease [CH3OH]
Exothermic reaction
14. The concentration of the hardener added to the mixture.
15. In the colder water the rates of chemical reactions are slowed down. The brain will not use O2 as fast and can prevent brain damage.
16. Keq= / [products]x / where x = cooefficient
[reactants]x
17. The rate-determining step is the slowest step of a reaction and has the highest Activation Energy.
18. Converts CO into CO2 at a lower temperature than normal. The lead reacts with the chemical to clog the catalytic converter.
19. Transition State -
an unstable compound formed between two steps of a multi-step chemical reaction
Activated Complex -
is located at the top of an energy barrier (activation energy). It is an unstable grouping of reactant molecules that has bonds in the form of being broken and bonds that are in the process of forming. It has a very short life span because of its high Potential Energy.
20. a) Slow
b) 2Cr2H4Br- + I- ---> C2H4 + 2Br- + I3-
c) No effect - not in rate determining step
d) Speed up reaction - is in rate determining step.
21. a) increase in temperature
increase in concentration of reactants
b) the amount of CO2 produced could be measured
photo gate - light will not pass through and measure how fast it happens
22. For a reaction to occur there must be enough energy in the collision to break bonds and reform new ones. There must be sufficient Activation Energy for the reaction to occur. The reaction will also be dependent on the slowest step of the reaction, the rate determining step. If the increase in the concentration is not added to the rate determining step then it will have no effect on the overall reaction because the reaction can only go as fast as the slowest step.
23. A soft drink will go flat because the temperature is increased and the pressure above the liquid is decreased.
24. Get two solids, for instance calcium carbonate to be reacted with hydrochloric acid. Take a chunk of calcium carbonate and react it with the hydrochloric acid and then use the same amount of calcium carbonate as the chunk and grind it up into a powder. React this with the acid. The powder should react faster than the chunk.
25. Smaller molecules react faster than larger molecules.
26. The two features they have in common is that the reactants are converting into products and at the same rate, the products are converting into reactants. On the macroscopic scale, there appears to be no reaction. On the microscopic scale there is.
27. Reaction 2 is faster than reaction 1 because ionic substances in solutions react at very fast reaction rates.
28. a) Test tube "A" had a catalyst added to it which would cause the reaction rate to go faster. The addition of a catalyst causes the reaction rate to go faster due to lowering the activation energy.
b) So that the surface area would not be a factor in the reaction rate because surface area will affect the rate of the reaction.
29. Increasing the pressure of a gas above the liquid will increase the solubility of the gas in the liquid to increase.