Chemistry 12 Equilibrium Review

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Part 1 Relationships

1. Describe the changes in reactant and product concentration as equilibrium is approached.

The [reactant] decreases and the [product] increases.

2. Describe the changes in the forward and the reverse rates as equilibrium is approached.

The forward rate decreases and the reverse rate increases.

3. State three conditions that are necessary to achieve equilibrium.

Closed system. Constant temperature. Low enough Ea so that a reaction is possible.

4. Asuming all three conditions are present, describe what would happen if only reactants are placed in a container.

Reactants would be converted into products and equilibrium will be achieved.

5. Asuming all three conditions are present, describe what would happen if only products are placed in a container.

Products will be converted into reactants and the same equilibrium will be achieved.

6. Describe the relationship between the size of the equilibrium constant, large, small, or about 1, and the relative amounts of reactants or products.

Large Keq means the products are favoured (more products than reactants).

Small Keq means the reactants are favoured (more reactants than products).

A Keq of about 1 means there are roughly the same amount of reactants as products.

7. Describe each of the following: dynamic equilibrium (the forward and reverse rates are equal and continue), LeChatelier's principle (when a system in equilibrium is subjected to stress it will shift in a direction that will apose the stress) , Ktrial (initial concentration are evaluated with the equilibrium expression to determine if the system is at equilibrium or not and to determine the direction to equilibrium), enthalpy ( potential energy or chemical bond energy), entropy ( disorder or randomness) and macroscopic property ( observable property like colour or mass).

8. Describe the effect of temperature on the equilibrium constant for an exothermic and endothermic reaction.Exothermic: increasing the temperature will decrease the Keq and decreasing the temperature will increase the Keq.

Endothermic: increasing the temperature will increase the Keq and decreasing the temperature will decrease the Keq.

9. Describe the effect of changing the temperature, pressure , volume, concentration or adding a catalyst on the value of the equilibrium constant. The only factor that will change the equilibrium constant is the temperature. All others have no effect on the Keq.

10. What is the only variable that will change the value of the equilibrium constant. Temperature.

11. What are the phases of the compounds that are not included in the equilibrium expression. (s) and (l)

12. Write the equilibrium expession for:

4A(g) + 3B(aq) <------> 2C(l) + 3D(s) Keq = 1/[A]4[B]3

13. Pick the best Keq for each of the reactions below.

Keq = 100 Keq = 0.01 Keq = 1.0

a) Zn(s) + 2HCl(aq) ------> ZnCl2(aq) + H2(g) + 152kJ Keq = 100

b) 3C(s) + 3H2(g) ------> C3H6(g) H = +20.4 kJ Keq = 0.01

c) 2Pb(NO3)2(s) + 597 kJ ------> 2PbO(s) +4NO2(g) + O2(aq) Keq = 1.0

14. For each reaction in equilibrium describe the shift for the following changes: increasing temperature, increasing pressure, decreasing volume, adding a gaseous product and removing an aqueous reactant.

a) Zn(s) + 2HCl(aq) <------> ZnCl2(aq) + H2(g) + 152kJ

b) A(aq) + 6B(g) <------> 2C(g) + 4D(g) H= +56kJ

 

increasing temperature

increasing pressure

decreasing volume

adding a gaseous product

removing an aqueous reactant

a)

left

left

left

left

left

b)

right

no shift

no shift

left

left

15. Zn(s) + 2HCl(aq) <------> ZnCl2(aq) + H2(g) + 152kJ

Give three ways to increase the yield of the reaction. Increase [HCl]. Decrease the temperature. Remove the products. Lower the pressure.

Give five ways to increase the rate of the reaction. Increase [HCL]. Increase the temperature. Increase the surface area of the Zn. Add a catalyst. Agitate.

16. A(aq) + 6B(g) <------> 2C(g) + 2D(g) H= +56kJ

Give three ways to increase the yield of the reaction. Increase the temperature. Increase the pressure. Continually add reactants and remove products.

Give five ways to increase the rate of the reaction. Increase [A] or [B]. Increase the temperature. Add a catalyst. Agitate.

17. What is equal at equilibrium? The forward rate is equal to the reverse rate.

18. What is constant at eqilibrium? The concentration of reactants and products.

19. Which reaction has the greatest yield? Why?

a) Keq = 8.0 x 10-12 b) Keq = 7.0 x 10-11 ( greater yield due to a larger Keq)

20. Which reaction has the smallest yield?

a) Keq = 1.0 x 10-15 ( smaller yield due to a smaller Keq) b) Keq = 9.0 x 10-15

21. Which has the greater entropy?

a) H2O(s) b) H2O(l) c) H2O(g)

22. Which has the greater enthalpy?

a) H2O(s) b) H2O(l) c) H2O(g)

Hint: Consider H2O(s) ----> H2O(l) Draw a potential energy diagram. Which side is higher?

23. Review your kinetics test. There will be 5 question on this test from kinetics.

24. Consider the following equilibrium system:

SO3(g) + NO(g) <----> NO2(g) + SO2(g)

a) Describe what happens to the forward and reverse reaction rate immediately after adding SO3(g)

forward rate increases reverse rate is constant

b) Describe what happens to the forward and reverse reaction rate immediately after removing NO2(g)

forward rate is constant reverse rate is decreases

c) Describe what happens to the forward and reverse reaction rate immediately after adding a catalyst .

forward rate increases reverse rate increases

d) Describe what happens to the forward and reverse reaction rate after a new equilibrium has formed compared to the original equilibrium after removing NO2(g)

forward rate decreases reverse rate is decreases

e) Describe what happens to the forward and reverse reaction rate after a new equilibrium has formed compared to the original equilibrium after adding SO3(g)

forward rate increases reverse rate increases

f) Describe what happens to the forward and reverse reaction rate after a new equilibrium has formed compared to the original equilibrium after adding a catalyst .

forward rate increases reverse rate increases

g) Describe what happens to the forward and reverse reaction rate after a new equilibrium has formed compared to the original equilibrium after decreasing the volume of the container.

Both increase

h) Describe what happens to the reactant and product concentrations after a new equilibrium has formed compared to the original equilibrium after decreasing the volume of the container.

Both increase

 

Part 2 Calculations

1. SO3(g) + NO(g) <----> NO2(g) + SO2(g)

[SO3] = 0.400M [NO] = 0.480M [NO2] = 0.600M [SO2] = 0.450M

Keq = 0.800 at 100C

a) Show by calulation that this reaction mixture is not at equilibrium at 100C.Ktrial=1.4 Keq=.800 Not at equilibrium.

b) What will happen to [SO3] and [SO2] as the system moves to equilibrium? [SO3] increases and [SO2] decreases.

2. Consider the equilibrium below:

Co(H2O)6+2(aq) + 2Cl-1(aq) <-----> Co(H2O)6Cl2(aq) + 2H2O(l)

(pink blue)

If the colour of the equilibrium mixture is pink at 5C and blue at 60C, is the reaction endo or exothermic?

3. SO3(g) + H2O(g) <----> H2SO4(l)

[SO3] = 0.400M [ H2O] = 0.480M [H2SO4] = 0.600M

Calculate the value of the equilibrium constant. Keq=5.21

4. 2SO2(g) + O2(g) <-----> 2SO3(g)

4.00 moles of SO2 and 5.00 moles O2 are placed in a 2.00 L container at 200C and allowed to reach equilibrium. If the equilibrium concentration of O2 is 2.00M, calculate the Keq. Keq=.500

5. 2SO2(g) + O2(g) <-----> 2SO3(g)

If at equilibrium [O2] =.500M and [SO3] = .400M and the equilibrium constant is the same as question 4 at 200C , calculate the [SO2]. [SO2]= .800M

6. Consider the following equilibrium:

2NO2(g) <-----> N2O4(g)

If 2.00 moles of NO2 are placed in a 1.00 L flask and allowed to react. At equilibrium 1.80M NO2 are present. Calculate the Keq. Keq = .031

7. SO3(g) + NO(g) <----> NO2(g) + SO2(g) Keq = 0.800 at 100C

If 4.00 moles of each reactant is placed in a 2.00L container, calculate all equilibrium concentrations at 100C. [SO3]=[ NO]= 1.06 M [SO2]=[ NO2]= .944 M

8. Keq = 0.0183 at 150C for : 2HI(g) <----> H2(g) + I2(g)

If 3.00 moles of HI is placed in a 5.00 L container and allowed to establish equilibrium, what are all equilibrium concentrations? [H2]=[I2]=.0639M [HI]=.472M

Challenge Question

9. Consider the following equilibrium in a 5.00 L container:

CO (g) + H2O (g) <---------> CO2(g) + H2 (g)

At equilibrium, there is 1.0 mole of CO , 3.0 moles of H2O, 3.0 moles CO2, and 3.0 moles of H2. If 2.0 moles of CO are now added, find the equilibrium [CO2].

Keq = 3.0 [CO2] = 0.76 M

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