IB Chemistry Topic 7 SL Equilibrium

(b) (reaction is) endothermic; Kc increases with (increasing) temperature; forward reaction favoured/heat used up/OWTTE; 3

(b) The following equilibrium is established at 1700°C. CO2(g) + H2(g) -->H2O(g) CO(g) for the equilibrium reaction is determined at two different temperatures. At 850°C, Kc = 1.1 whereas at 1700°C, Kc = 4.9. On the basis of these Kc values explain whether the reaction is exothermic or endothermic.

(c) high pressure expensive/greater cost of operating at high pressure/reinforced pipes etc. needed; lower temperature - greater yield, but lowers rate; 2 Do not award a mark just for the word "compromise".

(c) In practice, typical conditions used in the Haber process are a temperature of 500 °C and a pressure of 200 atmospheres. Explain why these conditions are used rather than those that give the highest yield.

(d) Kc = (ignore units); 1

(d) Write the equilibrium constant expression, Kc, for the production of ammonia.

D

1. I2(g) + 3Cl2(g) --> 2ICl3(g) What is the equilibrium constant expression for the reaction above? A. Kc = B. Kc = C. Kc = D. Kc =

B

10. The manufacture of sulfur trioxide can be represented by the equation below. 2SO2(g) + O2(g) 2SO3(g) ∆Hο = -197 kJ mol-1 What happens when a catalyst is added to an equilibrium mixture from this reaction? A. The rate of the forward reaction increases and that of the reverse reaction decreases. B. The rates of both forward and reverse reactions increase. C. The value of ∆Hο increases. D. The yield of sulfur trioxide increases.

A

13. The equation for a reversible reaction used in industry to convert methane to hydrogen is shown below. CH4(g) + H2O(g) CO(g) + 3H2(g) ΔHӨ = +210 kJ Which statement is always correct about this reaction when equilibrium has been reached? A. The concentrations of methane and carbon monoxide are equal. B. The rate of the forward reaction is greater than the rate of the reverse reaction. C. The amount of hydrogen is three times the amount of methane. D. The value of ΔHӨ for the reverse reaction is -210 kJ.

C

16. Iron(III) ions react with thiocyanate ions as follows. Fe3+(aq) + CNS-(aq) Fe(CNS)2+(aq) What are the units of the equilibrium constant, Kc, for the reaction? A. mol dm-3 B. mol2 dm-6 C. mol-1 dm3 D. mol-2 dm6

B

2. 2SO2(g) + O2 (g) -->2SO3(g) ∆Hο = -200 kJ According to the above information, what temperature and pressure conditions produce the greatest amount of SO3? Temperature Pressure A. low low B. low high C. high high D. high low

C

23. The sequence of diagrams represents the system as time passes for a gas phase reaction in which reactant X is converted to product Y. Which statement is correct? A. At t = 5 days the rate of the forward reaction is greater than the rate of the backward reaction. B. At t = 7 seconds the reaction has reached completion. C. At t = 10 minutes the system has reached a state of equilibrium. D. At t = 5 days the rate of the forward reaction is less than the rate of the backward reaction.

(a) two curves - one labelled "forward" starting up high up y-axis and one labelled "reverse" starting from zero; curves merge and become horizontal; No penalty for failing to label axes. forward reaction: highest concentration, thus rate high to begin with; as reaction proceeds, concentrations decrease, so does rate; reverse reaction: zero rate initially/at t = 0 (since no products present); rate increases as concentration of products increases; equilibrium established when rate of forward reaction = rate of reverse reaction; 7

28. (a) The following equilibrium is established at 1700°C. CO2(g) + H2(g) -->H2O(g) CO(g) If only carbon dioxide gas and hydrogen gas are present initially, sketch on a graph a line representing rate against time for (i) the forward reaction and (ii) the reverse reaction until shortly after equilibrium is established. Explain the shape of each line.

B

3. Which statement(s) is/are true for a mixture of ice and water at equilibrium? I. The rates of melting and freezing are equal. II. The amounts of ice and water are equal. III. The same position of equilibrium can be reached by cooling water and heating ice. A. I only B. I and III only C. II only D. III only

D

5. Which statement concerning a chemical reaction at equilibrium is not correct? A. The concentrations of reactants and products remain constant. B. Equilibrium can be approached from both directions. C. The rate of the forward reaction equals the rate of the reverse reaction. D. All reaction stops.

A

6. In the reaction below N2(g) + 3H2(g) --> 2NH3(g) ∆H = -92 kJ which of the following changes will increase the amount of ammonia at equilibrium? I. Increasing the pressure II. Increasing the temperature III. Adding a catalyst A. I only B. II only C. I and II only D. II and III only

B

8. What will happen if CO2(g) is allowed to escape from the following reaction mixture at equilibrium? CO2(g) + H2O(l) -->H+(aq) + HCO3-(aq) A. The pH will decrease. B. The pH will increase. C. The pH will remain constant. D. The pH will become zero.

C

9. Which statements are correct for a reaction at equilibrium? I. The forward and reverse reactions both continue. II. The rates of the forward and reverse reactions are equal. III. The concentrations of reactants and products are equal. A. I and II only B. I and III only C. II and III only D. I, II and III