Equilibrium Practice Problems


Write the equilibrium expression for questions 1-5.

1. The reaction between propane and oxygen to form carbon dioxide and water vapour:

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

2. The reaction between nitrogen gas and oxygen gas at high temperatures: N2(g) + O2(g)  2NO(g)

3. The reaction between hydrogen gas and oxygen gas to form water vapour: 2H2(g)  + O2(g)  2H2O(g)

4. The reduction-oxidation equilibrium of iron and iodine ions in aqueous solution:

2Fe3+(aq) + 2I(aq)2Fe2+(aq) + I2(aq)  Note: You will learn about reduction-oxidation reactions in Electrochem.

5. The oxidation of ammonia: 4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)


6. The following reaction took place in a sealed flask at 250C. PCl5(g) PCl3(g) + Cl2(g) At equilibrium, the gases in the flask had the following concentrations: [PCl5] = 1.2 10−2 M, [PCl3] = 1.5 10−2 M, and

[Cl2] = 1.5 10−2 M. Calculate the value of Kc at 250C.

7. Iodine and bromine react to form iodine monobromide: I2(g) + Br2(g)  2IBr(g)  At 250C, an equilibrium mixture in a 2.0 L flask contained 0.024 mol of I2(g), 0.050 mol of Br2(g), and 0.38 mol of IBr(g). What is the value of Kc for the reaction at 250C?

8. At high temperatures, carbon dioxide gas decomposes into carbon monoxide and oxygen gas. At equilibrium, the gases have the following concentrations: [CO2(g)  ] = 1.2 mol/L, [CO(g)] = 0.35 mol/L, and [O2(g)] = 0.15 mol/L. Determine Kc at the temperature of the reaction.

9. Hydrogen sulfide is a pungent, poisonous gas. At 1400 K, an equilibrium mixture was found to contain 0.013 mol/L hydrogen, 0.18 mol/L hydrogen sulfide, and an undetermined amount of sulfur in the form of S2 (g). If the value of Kc is 2.40 10−4 at this temperature, what concentration of S2(g) is present at equilibrium?

10. Methane, ethyne, and hydrogen form the following equilibrium mixture. 2CH4(g) C2H2(g) + 3H2(g)

While studying this reaction mixture, a chemist analyzed a 4.0 L sealed flask at 1700C. The chemist found 0.46 mol of CH4(g) , 0.64 mol of C2H2(g), and 0.92 mol of H2(g). Calculate the value of Kc at 1700C.

11. At a certain temperature, hydrogen fluoride gas dissociates. 2HF(g) H2(g) + F2(g)

At equilibrium in a 1.0 L reaction vessel, the mixture of gases contained 0.045 mol of H2(g), 0.045 mol of F2(g), and 0.022 mol of HF(g). What is the value of Kc?

12. At 25C, the following reaction takes place. I2(g) + Cl2(g) 2ICl(g) A chemist determined that a 10 L container contained these amounts of gases at equilibrium: I2 = 0.15 mol, Cl2(g) = 0.15 mol, and ICl = 1.4 mol. What is the value of Kc for the reaction at 25C?


13. A chemist was studying the following reaction. SO2(g) + NO2(g) NO(g) + SO3(g)   In a 1.0 L container, the chemist added 1.7 10−1 mol of SO2(g) to 1.1 10−1 mol of NO2(g) . At equilibrium, the concentration of SO3(g) was found to be 0.089 mol/L. What is the value of Kc for the reaction at this temperature?

14. Phosgene, COCl2(g), is an extremely toxic gas. It was used during World War I. Today it is used to manufacture pesticides, pharmaceuticals, dyes, and polymers. It is prepared by mixing carbon monoxide

and chlorine gas. CO(g) + Cl2(g)  COCl2(g)   0.055 mol of CO(g) and 0.072 mol of Cl2(g) are placed in a 5.0 L container. At a certain temperature, the equilibrium constant is 20. What are the equilibrium concentrations of the mixture?

15. Hydrogen bromide decomposes at 700 K. 2HBr(g) H2(g)  + Br2(g)  Kc = 4.2 10−9

0.090 mol of HBr is placed in a 2.0 L reaction vessel and heated to 700 K. What is the equilibrium concentration of each gas?


16. 2.50 mol of oxygen and 2.50 mol of fluorine gas are placed in a 2.00 L glass container at room temperature. The container is heated to 400 K and the following equilibrium is established: O2(g) + 2F2(g)  2OF2(g)

If 37.2% of the fluorine reacts, what is the value of the equilibrium constant?

17. When 0.846 mol/L of dinitrogen tetrachloride gas is placed in a closed reaction vessel, 73.5% of it decomposes into dinitrogen dichloride gas and chlorine gas. Calculate the equilibrium constant.

18. When 3.00 mol/L of phosphorus heptabromide gas is initially placed in a sealed 3.00 L container at 295 K, it is the only gas present. Upon heating the container to 500 K and maintaining the temperature, an equilibrium is established and phosphorus tribromide gas and bromine vapour are observed products. Calculate the equilibrium constant, given the fact that 85.2% of the PBr7(g) reacts.

19. Nitrogen gas is relatively unreactive at room temperature, but combines with oxygen at higher temperatures:

N2(g) + O2(g) 2NO(g) When equilibrium is attained at a temperature of 1800 K, 5.3% of nitrogen will react with oxygen. What is the value of Kc at this temperature?


20. For the reaction H2(g)  + I2(g)  2HI(g), the value of Kc is 25.0 at 1100 K and 8.0 102 at room temperature, 300 K. Which temperature favours the dissociation of HI(g) into its component gases?

21. Three reactions, and their equilibrium constants, are given below. Arrange them in the order of their tendency to form products.

I. N2(g) + O2(g)  2NO(g)   Kc = 4.7 10−31            II. 2NO(g) + O2(g) 2NO2(g)    Kc = 1.8 10−6

III. N2O4(g) 2NO2(g)    Kc = 0.025

22. Identify each reaction as essentially going to completion or not taking place.

(a) N2(g) + 3Cl2(g) 2NCl3(g) Kc = 3.0 1011             (b) 2CH4(g) C2H6(g) + H2(g) Kc = 9.5 10−13

(c) 2NO(g) + 2CO(g) N2(g)  + 2CO2(g)  Kc = 2.2 1059

23. Most metal ions combine with other ions in solution. For example, in aqueous ammonia, silver(I) ions are at equilibrium with different complex ions. [Ag(H2O) 2]+(aq)  +  2NH3(aq)  →  [Ag(NH3)2] +(aq)  +  2H2O(l)

At room temperature, Kc for this reaction is 1 107. Which of the two silver complex ions is more stable? Explain your reasoning.

24. Consider the following reaction. H2(g) + Cl2(g) 2HCl(g) Kc = 2.4 1033 at 25C   HCl(g) is placed in a reaction vessel. To what extent do you expect the equilibrium mixture to dissociate into H2(g) and Cl2(g) ?


25. The following equation represents the equilibrium reaction for the dissociation of phosgene gas.

COCl2(g) CO(g) + Cl2(g)  At 100C, the value of Kc for this reaction is 2.2 10−8. The initial concentration of COCl2(g) in a closed container at 100C is 1.5 M. What are the equilibrium concentrations of CO(g) and Cl2(g)?

26. Hydrogen sulfide is a poisonous gas with a characteristic, offensive odour. It dissociates at 1400C, with Kc equal to 2.4 10−4.    H2S(g) 2H2(g) + S2(g)  4.0 mol of H2S is placed in a 3.0 L container. What is the equilibrium concentration of H2(g) at 1400C?

27. At a certain temperature, the value of Kc for the following reaction is 3.3 10−12.

2NCl3(g)   N2(g) + 3Cl2(g)   A certain amount of nitrogen trichloride, NCl3(g), is put in a 1.0 L reaction vessel at this temperature. At equilibrium, 4.6 10−4 mol of N2(g) is present. What amount of NCl3(g) was put in the reaction vessel?

28. At a certain temperature, the value of Kc for the following reaction is 4.2 10−8.

N2(g) + O2(g) 2NO(g)       0.45 mol of N2(g) and 0.26 mol of O2(g) are put in a 6.0 L reaction vessel. What is the equilibrium concentration of NO(g) at this temperature?

29. At a particular temperature, Kc for the decomposition of carbon dioxide gas is 2.0 10−6.

2CO2(g) 2CO(g) + O2(g)  3.0 mol of CO2 is put in a 5.0 L container. Calculate the equilibrium concentration of each gas.


30. The following reaction takes place inside a cylinder with a movable piston. 2NO2(g) N2O4(g)  At room temperature, the equilibrium concentrations inside the cylinder are [NO2] = 0.0206 M and [N2O4] = 0.0724 M.

(a) Calculate the value of Kc.

(b) Calculate the concentration of each gas at the moment that the piston is used to halve the volume of the reacting mixture. Assume that the temperature remains constant.

(c) Determine the value of Qc when the volume is halved.

(d) Predict the direction in which the reaction will proceed to re-establish equilibrium.

31. Ethyl acetate is an ester that can be synthesized by reacting ethanoic acid (acetic acid) with ethanol. At room temperature, the equilibrium constant for this reaction is 2.2. 

CH3COOH(l) + CH3CH2OH(l) CH3COOCH2CH3 (l)+ H2O(l)  Various samples were analyzed. The concentrations are given in the table below. Decide whether each sample is at equilibrium. If it is not at equilibrium, predict the direction in which the reaction will proceed to establish equilibrium.






























32. In the past, methanol was obtained by heating wood without allowing the wood to burn. The products were collected, and methanol (sometimes called wood alcohol) was separated by distillation. Today methanol is manufactured by reacting carbon monoxide with hydrogen gas. CO(g) + 2H2(g) CH3OH(g)

At 210C, Kc for this reaction is 14.5. A gaseous mixture at 210C contains the following concentrations of gases: [CO] = 0.25 mol/L, [H2] = 0.15 mol/L, and [CH3OH] = 0.36 mol/L. What will be the direction of the reaction if the gaseous mixture reaches equilibrium?


33. Consider the following reaction. 2HI(g) H2(g) + I2(g) + 52 kJ  In which direction does the equilibrium shift if there is an increase in temperature?

34. A decrease in the pressure of each equilibrium system below is caused by increasing the volume of the reaction container. In which direction does the equilibrium shift?

(a) CO2(g) + H2(g) CO(g) + H2O(g)                                         (b) 2NO2 (g) N2O4(g)

(c) 2CO2(g) 2CO(g) + O2(g)                                                  (d) CH4(g) + 2H2S(g) CS2(g) + 4H2(g)

35. The following reaction is exothermic. 2NO(g) + 2H2(g) N2(g) + 2H2O(g)

In which direction does the equilibrium shift as a result of each change?

(a) removing the hydrogen gas

(b) increasing the pressure of gases in the reaction vessel by decreasing the volume

(c) increasing the pressure of gases in the reaction vessel by pumping in argon gas while keeping the volume of the vessel constant

(d) increasing the temperature

(e) using a catalyst

36. In question 35, which changes affect the value of Kc? Which changes do not affect the value of Kc?

37. Toluene, C7H8, is an important organic solvent. It is made industrially from methyl cyclohexane.

C7H14(g) C7H8(g) + 3H2(g) The forward reaction is endothermic. State three different changes to an equilibrium mixture of these reacting gases that would shift the equilibrium toward greater production of toluene.