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Multiple-choice questions : 1. Which of the following statements concerning dynamic equilibrium is/are correct? (1) A reversible reaction stops when equilibrium is reached. (2) At equilibrium, the concentrations of reactants and products are constant. (3) At equilibrium, the concentrations of reactants and products are the same. A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only □ 2. Which of the following statements concerning the chemical equilibrium below is/are correct? CaCO3(s) CaO(s) + CO2(g) (1) The above chemical equilibrium can only be established in a sealed container. (2) The reaction occurs spontaneously at room conditions. (3) Equilibrium can be reached from the backward direction of the reaction. A. (1) only B. C. D. (2) only (1) and (3) only (2) and (3) only □ 3. Which of the following statements concerning the effect of catalysts on the reversible reactions is correct? A. Catalysts increase either the rate of forward reaction or the rate of backward reaction of reversible reactions. B. Catalysts shorten the time for the reversible reactions to reach equilibrium C. D. by increasing the rate of forward reaction and decreasing the rate of backward reaction. Catalysts shorten the time for the reversible reactions to reach equilibrium by increasing both the rates of forward and backward reactions. Catalysts increase the yield of products for reversible reactions. □ 1 4. When a small amount of N2O4(l) is introduced into a sealed container at a certain temperature, the following reaction takes place: N2O4(g) 2NO2(g) Which of the following graphs correctly shows the changes in concentrations of N2O4 and NO2 with time until the reaction reaches equilibrium? Concentration B. Concentration A. N2O4 NO2 N2O4 NO2 Time Concentration D. Concentration C. Time N2O4 NO2 N2O4 NO2 Time Time □ 5. Equal numbers of moles of I2(g) and H2(g) are introduced in a sealed container at a certain temperature. The following reaction occurs: I2(g) + H2(g) 2HI(g) The reaction mixture is allowed to reach chemical equilibrium. Which of the following statements concerning the above chemical equilibrium is/are INCORRECT? A. The rate of combination between I2(g) molecules and H2(g) molecules is equal to the rate of decomposition of HI(g) molecules. B. The colour intensity of the reaction mixture remains constant. C. The concentrations of I2(g) and H2(g) are always the same. D. When 1 mol of HI(g) decomposes, 1 mol of I2(g) and 1mol of H2(g) combine at the same time. □ 2 6. In dynamic equilibrium, which of the following properties of a reaction mixture remain constant? (1) Concentrations of all involved species (2) Colour intensity of the reaction mixture (3) Rates of reactions in both forward and backward directions A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) □ 7. Hydrogen sulphide decomposes to give hydrogen gas and sulphur gas: 2H2S(g) 2H2(g) +S2(g) Which of the following equilibrium constant expressions for the reaction is correct? A. Kc B. Kc C. Kc D. Kc 2 [H 2S(g)] eqm 2 [S 2 (g)] eqm [H 2 (g)] eqm [H 2S(g)] eqm [H 2 (g)] eqm [S2 (g)] eqm 2 [H 2 (g)] eqm [S 2 (g)] eqm 2 [H 2S(g)] eqm [H 2 (g)] eqm [S2 (g)] eqm [H 2S(g)] eqm □ 8. What is the unit of the equilibrium constant for the following equilibrium system? 2NO2(g) N2O4(g) A. B. C. D. mol dm3 mol1 dm3 mol2 dm6 mol2 dm6 □ 3 9. What is the unit of the equilibrium constant for the following heterogeneous equilibrium? Fe3O4(s) + 4H2(g) A. no unit B. C. D. 3Fe(s) + 4H2O(g) mol2 dm6 mol4 dm12 mol1 dm3 □ 10. Which of the following is the balanced chemical equation that gives the equilibrium constant expression shown below? Kc = 4 [HCl(g )]eqm 2 [SiCl 4 (g )]eqm [H 2 (g )]eqm A. SiCl4(g) + 2H2(g) 4HCl(g) B. C. D. SiCl4(g) + 2H2(g) Si(s) + 4HCl(g) Si(s) + 4HCl(g) SiCl4(g) + 2H2(g) 4HCl(g) SiCl4(g) + 2H2(g) □ 11. Consider the following reaction: H2(g) + I2(g) 2HI(g) Suppose the equilibrium constant of the given reaction at 298 K is Kc. Which is the correct expression of equilibrium constant, Kc’, for the following reaction at 298 K? 2HI(g) H2(g) + I2(g) A. Kc’ = Kc 1 B. Kc’ = Kc C. Kc’ = Kc2 D. Kc’ = K c □ 4 12. Which is the correct expression of the equilibrium constant, Kc, for the following reaction? 2CrO42(aq) + 2H+(aq) Cr2O72(aq) + H2O(l) 2 [Cr2O7 (aq)]eqm[H 2O(l)] eqm A. Kc = B. Kc = [CrO 4 (aq)]2 eqm[H (aq)]2 eqm C. Kc = [Cr2O7 (aq)]eqm D. Kc = [CrO 4 2 (aq)] 2 eqm[H (aq)] 2 eqm 2 [Cr2O7 2 (aq)]eqm[H 2 O(l)] eqm 2 [CrO 4 2 (aq)]2 eqm[H (aq)] 2 eqm 2 [CrO 4 (aq)]2 eqm[H (aq)] 2 eqm [Cr2O7 2 (aq)]eqm □ 13. The value of Kc for the reaction H2(g) + CO2(g) H2O(g) + CO(g) is 1.45 at 1100C. What is the value of Kc for the reaction H2O(g) + CO(g) CO2(g) at the same temperature? A. 0.69 B. 0.83 C. D. H2(g) + 1.20 1.45 □ 14. Which of the following is the expression for the equilibrium constant, Kc, for the reaction: CaCO3(s) CaO(s) + CO2(g)? [CaO(s)] eqm[CO 2 (g)] eqm A. Kc = [CaCO 3 (s)] eqm [CaCO3 (s)] eqm [CaO(s)] eqm[CO 2 (g)] eqm B. Kc = C. Kc = [CO2(g)] 1 Kc = [CO 2 (g)] eqm D. □ 5 15. Which of the following statements concerning the magnitude of equilibrium constant, Kc, is correct? A. It is affected by the change in pressure. B. If Kc is a positive value, the forward rate is faster than the backward rate. C. It indicates how far the equilibrium reaches. D. The larger the Kc value, the faster the reaction. □ 16. The equilibrium constants of four different equilibrium systems are shown below. Equilibrium system I II 5 Kc 3.2 × 10 4.7 × 10 III 17 IV 17 3.2 × 10 4.7 × 105 Which of the equilibrium systems favours the reactants most? A. B. C. D. I II III IV □ 17. Consider the following reaction: 2SO2(g) + O2(g) 2SO3(g) At one equilibrium position, [SO2(g)]eqm, [O2(g)]eqm and [SO3(g)]eqm are 5.33 × 103 M, 3.70 × 103 M and 5.33 × 103 M respectively. Which of the following set of concentrations ([SO2(g)]eqm, [O2(g)]eqm and [SO3(g)]eqm respectively) constitutes another equilibrium position at the same temperature? A. B. C. D. 4.72 × 102 M, 1.013 × 105 M and 2.47 × 103 M 3.68 × 103 M, 5.284 × 105 M and 3.09 × 103 M 5.99 × 102 M, 1.221 × 105 M and 8.12 × 103 M 7.21 × 103 M, 4.592 × 105 M and 5.83 × 105 M □ 18. Consider the following equilibrium: C(s) + CO2(g) 2CO(g) The equilibrium mixture was found to contain 0.42 mol dm3 of CO2(g) and 0.82 mol dm3 of CO(g) at a certain temperature. What was the equilibrium constant at that temperaure? A. 1.95 B. C. D. 1.60 mol dm3 0.625 mol1 dm3 0.512 □ 6 19. At a certain temperature, the equilibrium constant, Kc, for the reaction C2H5CO2H(l) + C2H5OH(l) C2H5CO2C2H5(l) + H2O(l) is 4. When 1 mol dm3 of propanoic acid (C2H5CO2H) reacted with 1 mol dm3 of ethanol (C2H5OH) in the presence of concentrated sulphuric acid and the reaction mixture was allowed to reach equilibrium, what was the equilibrium concentration of propanoic acid in the mixture? A. B. C. D. 0.33 mol dm3 0.67 mol dm3 1 mol dm3 2 mol dm3 □ dm3 dm3 20. In Haber process, 0.60 mol of nitrogen is mixed with 1.80 mol of hydrogen in a sealed container at a certain temperature to give ammonia. N2(g) + 3H2(g) 2NH3(g) At equilibrium, 30% of the original nitrogen is consumed. What is Kc for this reaction? A. B. C. D. 0.154 mol2 dm6 0.265 mol2 dm6 0.680 mol2 dm6 1.470 mol2 dm6 □ 21. It is known that hydroxyapatite, Ca5(PO4)3OH, is present in our tooth enamel. It undergoes continuous reversible reactions with the minerals supplied from saliva. The following equation shows the reaction: Ca5(PO4)3OH(s) 5Ca2+(aq) + 3PO43(aq) + OH(aq) At equilibrium, the concentration of OH(aq) is 2 × 104 mol dm3. What is the value of Kc? A. B. C. 2 × 104 mol dm3 1.20 × 1010 mol3 dm9 2.16 × 1025 mol9 dm27 D. 4.32 × 1029 mol9 dm27 □ 7 22. Consider the following equilibrium: SO2Cl2(g) SO2(g) + Cl2(g) In an experiment, 13.5 g of SO2Cl2(g) was introduced into a 4 dm3 container and allowed to reach equilibrium with SO2(g) and Cl2(g). At equilibrium, 0.069 mol of Cl2(g) was found in the reaction mixture. What is the equilibrium constant of the reaction? A. B. C. D. 0.0384 mol dm3 0.0768 mol dm3 2.23 mol dm3 3.34 mol dm3 □ 23. Chemical A ionizes in water and reaches an equilibrium: A(aq) 2B(aq). The equilibrium constant for the ionization is 180 mol dm–3. Originally there is 0.97 M of A. What is the equilibrium concentration of B? A. B. C. D. 0.02 mol dm3 0.07 mol dm3 0.95 mol dm3 1.90 mol dm3 □ 24. Consider the following chemical equilibrium: Cr2O72(aq) + H2O(l) 2CrO42(aq) + 2H+(aq) Which of the following statement(s) is/are correct? (1) Both Cr2O72(aq) and CrO42(aq) are present in the reaction mixture. (2) The solution becomes orange when dilute NaOH(aq) is added to the mixture. (3) The solution becomes yellow when dilute H2SO4(aq) is added to the mixture. A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only □ 8 25. Consider the following equilibrium reaction: N2(g) + 3H2(g) 2NH3(g) When extra nitrogen gas is added to the closed container, which of the following changes would happen on the value of Kc at 298 K? A. The value of Kc increases. B. The value of Kc decreases. C. The value of Kc remains unchanged. D. The change cannot be determined. □ 26. Consider the following equilibrium reaction: 2CrO42(aq) + 2H+(aq) Cr2O72(aq) + H2O(l) The colour of the solution turns yellow by adding (1) dilute hydrochloric acid. (2) sodium carbonate solution. (3) A. B. C. D. sodium sulphite solution. (1) only (2) only (1) and (3) only (2) and (3) only □ 27. Consider the following system which is in equilibrium at a certain temperature: 2CO(g) + O2(g) 2CO2(g) If some CO(g) is added to the system, which of the following statements is correct? A. There is a change in concentrations for the reactants only. B. There is a change in concentration for the product only. C. There is a change in concentrations for both reactants and products. D. There is no change in concentrations for both reactants and products. □ 28. Which of the following statements are correct if an equilibrium system is heated? (1) (2) (3) A. B. C. D. The rate of forward reaction is increased. The rate of backward reaction is increased. Kc is increased. (1) and (2) only (1) and (3) only (2) and (3) only (1), (2) and (3) □ 9 29. When there is a decrease in temperature, what are the changes in Kc value and equilibrium position for the following equilibrium reaction? CH4(g) + 4NO(g) 2N2(g) + CO2(g) + 2H2O(g) Kc value Change in equilibrium position A. increases to the right B. increases to the left C. decreases to the right D. decreases to the left ΔH = ve □ 30. For the following reaction: Cu(OH)2(s) Cu2+(aq) + 2OH(aq) ΔH = +ve 2+ The equilibrium concentration of Cu (aq) would be increased by (1) increasing the temperature. (2) adding a few drops of dilute hydrochloric acid. (3) adding a few drops of dilute sodium hydroxide solution. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) □ 31. Consider the following equilibrium reaction: 4HCl(g) + O2(g) 2Cl2(g) + 2H2O(g) ΔH = +ve Which of the following changes will cause a shift in the equilibrium position of the above reaction? (1) Adding Cl2(g) (2) Removing O2(g) (3) Adding a catalyst A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) □ 10 32. The following graph shows the change in concentrations of the reactant and products with time for the reversible reaction: C(aq) A(aq) + B(aq) ΔH = +ve Concentration (mol dm3) A(aq) B(aq) C(aq) Time Which of the following changes is done to the system at time t? A. Adding a small amount of C(aq) B. Removing some of C(aq) C. Increase in temperature D. Decrease in temperature □ 33. Consider the following equilibrium system at constant pressure. 2SO2(g) + O2(g) 2SO3(g) ΔH = ve Which of the following statements is/are correct? (1) An increase in temperature favours the backward reaction. (2) An increase in concentration of SO3(g) shifts the equilibrium position to the right. (3) An increase in temperature decreases the value of Kc. A. (1) only B. (2) only C. (1) and (3) only D. (2) and (3) only □ 11 34. When there is an increase in temperature, what are the changes in Kc value and equilibrium position for the following equilibrium reaction? I2(g) + C5H8(g) C5H6(g) + 2HI(g) ΔH = +ve Kc value Change in equilibrium position A. Increases Shifts to the left B. Increases Shifts to the right C. Decreases Shifts to the left D. Decreases Shifts to the right □ Each question below consists of two separate statements. Decide whether each of the two statements is true or false; if both are true, then decide whether or not the second statement is a correct explanation of the first statement. Then select one option from A to D according to the following table: A. Both statements are true and the 2nd statement is a correct explanation of the 1st statement. B. Both statements are true and the 2nd statement is NOT a correct explanation of the 1st statement. C. The 1st statement is false but the 2nd statement is true. D. Both statements are false. 35. When a reaction reaches equilibrium, there are no observable changes of the reaction mixture. The rates of forward and backward reactions are the same at equilibrium. □ 36. Equilibrium constant does not depend on the concentrations of reactants and products. Equilibrium constant is dependent on temperature. □ 37. The larger the equilibrium constant, the higher is the reaction rate. If the equilibrium constant is larger than 1, the product side is favoured. □ 12 Conventional questions : 1. It is known that weak acids ionize slightly in water. For example, CH3COOH(aq) CH3COO(aq) + H+(aq) (a) State the chemical species that can be found in the reaction mixture at equilibrium. (b) (i) State TWO reactions that are taking place at equilibrium. (ii) What can you say about the rates of these two reactions at equilibrium? (c) Explain why 1 M ethanoic acid has a higher pH value than 1 M hydrochloric acid. (d) In an experiment, a student accidentally added a small amount of HCl to the reaction mixture in (a). He found that the pH value of the reaction mixture did not change much. Explain briefly. [8M] 13 2. When 0.4 mol of H2(g) and 0.3 mol of I2(g) are mixed in a 1 L closed vessels, they react to form HI(g). After 3 minutes, the reaction mixture reaches equilibrium and the vessel contains 0.5 mol of HI(g). (a) Write an equation to represent this state of the equilibirum. (b) (i) (ii) Calculate the concentrations of H2(g) and I2(g) at equilibrium respectively. Hence, sketch the changes in concentrations of H2(g), I2(g) and HI(g) against time until the reaction mixture reaches equilibrium. [11M] 14 3. In an experiment, 1.00 × 103 mol of PCl5(g) was introduced into a 250 cm3 container and allowed to reach equilibrium with PCl3(g) and Cl2(g) at a certain temperature. 9.65 × 104 mol of Cl2(g) was found in the equilibrium mixture. (a) Write a balanced chemical equation for the reaction. (b) Write the expression for the equilibrium constant for the reaction. (c) Calculate the equilibrium constant for the reaction. [8M] 15 4. At 250C, the equilibrium constant, Kc, for the following equilibrium PCl5(g) PCl3(g) + Cl2(g) is 3.8 × 102 mol dm3. In an experiment, 0.55 mol of PCl3(g) and 0.55 mol of Cl2(g) were mixed in a 2.5 dm3 container and allowed to reach equilibrium with PCl5(g). (a) Calculate the initial concentrations of PCl3(g) and Cl2(g) respectively. (b) Calculate the equilibrium concentrations of PCl5(g), PCl3(g) and Cl2(g) in the reaction mixture. [7M] 16 5. Consider the following equilibrium: CO2(g) + H2(g) CO(g) + H2O(g) (a) State whether the equilibrium is homogeneous or heterogeneous. Explain briefly. (b) In an experiment, 0.10 mol of CO2(g) and 0.12 mol of H2(g) were introduced into a 1 dm3 container and allowed to reach equilibrium with CO(g) and H2O(g). The equilibrium constant of the reaction is 0.63 at a certain temperature. Determine the equilibrium concentrations of all reactants and products at this temperature. [10M] 17 6. Given: Reaction N2(g) + O2(g) + Br2(g) 2NOBr(g) 2NO(g) N2(g) + O2(g) 2NO(g) + Br2(g) 2NOBr(g) (a) (i) Equilibrium constant Kc Kc1 Kc2 Write the expression for the equilibrium constant, Kc1. (ii) Write the expression for the equilibrium constant, Kc2. (iii) Hence, write the expression for the equilibrium constant, Kc, in terms of Kc1 and Kc2. (b) Calculate Kc if Kc1 and Kc2 are 2.4 × 1018 and 1.4 mol1 dm3 respectively. [5M] 7. Consider the following equilibrium: CH4(g) + H2O(g) CO(g) + 3H2(g) ΔH = +x kJ According to the above information, a student made the following statements. Comment on each of his statements. (a) At equilibrium, the amount of carbon monoxide must be equal to the amount of methane. (b) The enthalpy change for the reverse reaction is x kJ. (c) At equilibrium, the rate of forward reaction is higher than the rate of backward reaction. [5M] 18 8. In an experiment, bromine was dissolved in water to form bromine water. Br2(aq) + H2O(l) H+(aq) + Br(aq) + HOBr(aq) The pale yellow colour of bromine water is due to bromine, as the products are essentially colourless. Deduce the colour change when a few drops of the following solutions are added to bromine water. (a) Concentrated hydrochloric acid (b) Concentrated potassium hydroxide (c) Silver nitrate solution [6M] 19 9. Consider the following equilibrium system: 2SO2(g) + O2(g) 2SO3(g) ΔH = ve If the pressure of the system is kept constant, predict and explain the change in equilibrium position for the reaction when (a) the temperature of the system is increased. (b) the concentration of SO2(g) is decreased. (c) the concentration of SO3(g) is decreased. [6M] 10. Consider the following equilibrium system: H2O(g) + CO(g) H2(g) + CO2(g) ΔH = ve According to Le Châtelier’s Principle, fill in the following table for the disturbance to the equilibrium system: Addition of H2O(g) Addition of H2(g) Change in temperature Effect on [CO2(g)] Effect on [CO(g)] Effect on equilibrium position Effect on equilibrium constant [12M] 20 11. For the following equilibrium reactions, state whether, and in which direction, the equilibrium is shifted by the change applied. In each case, explain your answer. (a) 3O2(g) 2O3(g) volume is increased (b) 3Fe(s) + 4H2O(l) (c) NH4Cl(s) (d) 2NO(g) Fe3O4(s) + 4H2(g) NH3(g) + HCl(g) pressure is decreased N2(g) + O2(g) (e) BiCl3(aq) + H2O(l) H2(g) is removed volume is decreased BiOCl(s) + 2HCl(aq) NaOH(aq) is added [10M] 21 12. The following equilibrium was originally established in a 1 dm3 container at a certain temperature: CO2(g) + H2(g) CO(g) + H2O(g) The equilibrium concentrations of of CO2(g), H2(g), CO(g) and H2O(g) are 0.0517 mol dm3, 0.0717 mol dm3, 0.0483 mol dm3 and 0.0483 mol dm3 respectively. (a) If 0.02 mol dm3 of CO2(g) is added to the equilibrium system, determine the new equilibrium concentrations of all species in the mixture. (b) State the effect on the equilibrium position if the reaction mixture is introduced into a 2 dm3 container. Explain briefly. [9M] 22 Answers : Multiple-choice questions : 1 2 3 4 5 6 7 8 9 10 B C C A D D C B A B 11 12 13 14 15 16 17 18 19 20 B C A C C B A B A A 21 22 23 24 25 26 27 28 29 30 D A D A C B C A A A 31 32 33 34 35 36 37 38 39 40 A C C B A B C Conventional questions : 1. (a) CH3COOH(aq) ions, CH3COO(aq) ions and H+(aq) ions [1] (b) (i) The two reactions are: CH3COOH(aq) CH3COO(aq) + H+(aq) [1] CH3COO(aq) + H+(aq) CH3COOH(aq) [1] (ii) The rates of these two reactions are the same at equilibrium. [1] (c) Ethanoic acid is a weak acid that ionizes slightly in water while hydrochloric acid is a strong acid that ionizes completely in water. [1] Therefore, the concentration of H+(aq) in hydrochloric acid is higher and the pH value is lower. [1] (d) The newly added small amount of H+ reacted with CH3COO(aq) in the reaction mixture, forming CH3COOH(aq). [1] Therefore, the H+ concentration which determines the pH value did not change much. [1] 23 2. (a) H2(g) + I2(g) (b) (i) 2HI(g) [1] 0.4 mol = 0.4 mol L1 1L The initial concentration of H2(g) = The initial concentration of I2(g) = 0.3 mol = 0.3 mol L1 1L The concentration of HI(g) at equilibirum = 0.5 mol = 0.5 mol L1 [1] 1L According to the equation, the mole ratio of H2 : I2 : HI = 1 : 1 : 2 0.5 mol No. of moles of H2(g) and I2(g) reacted = = 0.25 mol [1] 2 ∴ the concentration of H2(g) at equilibrium = 0.4 mol L1 0.25 mol L1 = 0.15 mol L1 [1] the concentration of I2(g) at equilibrium = 0.3 mol L1 0.25 mol L1 = 0.05 mol L1 [1] (ii) Concentration HI H2 I2 Time Correct drawing [3] Correct labelling [3] 3. (a) PCl5(g) (b) Kc = PCl3(g) + Cl2(g) [1] [PCl3 (g )]eqm [Cl2 (g )]eqm [PCl5 (g )]eqm [1] (c) Initial concentration of PCl5(g) = 1.00 103 mol = 4 × 103 mol dm3 [1] 0.25 dm3 Equilibrium concentration of Cl2(g) = 9.65 104 mol = 3.86 × 103 mol dm3 [1] 3 0.25 dm Consider the equilibrium, 24 Concentr ation (mol dm3 ) Initial Change Equilibriu m Kc = PCl5(g) PCl3(g) 4 × 103 3.86 × 103 4 × 103 3.86 × 103 = 1.4 × 104 [1] + Cl2(g) 0 +3.86 × 103 0 + 3.86 × 103 = 3.86 × 103 [1] 0 +3.86 × 103 0 + 3.86 × 103 = 3.86 × 103 [1] [3.86 103 mol dm 3 ][3.86 103 mol dm 3 ] [1.4 10 4 mol dm 3 ] = 0.106 mol dm3 [1] 4. (a) Initial concentration of PCl3(g) = Initial concentration of Cl2(g) = 0.55 mol = 0.22 mol dm3 [1] 2.5 dm 3 0.55 mol = 0.22 mol dm3 [1] 2.5 dm 3 (b) Let x mol dm3 be the change in concentration of PCl3(g). Concentration (mol dm3) Initial Change Equilibrium PCl3(g) + Cl2(g) 0.22 x 0.22 x 0.22 x 0.22 x The equilibrium constant for PCl3(g) + Cl2(g) 1 = = Kc = 26.3 mol1 dm3 [1] Kc Kc = 26.3 = [PCl5 (g)]eqm [PCl3 (g)]eqm [Cl2 (g)]eqm PCl5(g) 0 +x 0+x = x PCl5(g) [1] x (0.22 x)(0.22 x) Solving for x, 26.3(0.22 x )(0.22 x ) = x x = 0.146 or x = 0.332 (rejected) ∴ the equilibrium concentration of PCl3(g) = 0.22 0.146 mol dm3 = 0.074 mol dm3 [1] the equilibrium concentration of Cl2(g) = 0.22 0.146 mol dm3 25 = 0.074 mol dm3 [1] the equilibrium concentration of PCl5(g) = 0.146 mol dm3 [1] 5. (a) It is a homogeneous equilibrium [1] because all the reactants and products are in the same phase. [1] 0.10 mol (b) Initial concentration of CO2(g) = = 0.10 mol dm3 [1] 1 dm3 0.12 mol = 0.12 mol dm3 [1] 1 dm3 Initial concentration of H2(g) = Let x mol dm3 be the change in concentration of CO2(g). Concentration (mol dm3) Initial Change Equilibrium Kc = CO2(g) 0.10 x 0.10 x [CO(g)]eqm [H 2O(g)]eqm [CO2 (g)]eqm [H 2 (g )]eqm + H2(g) H2O(g) 0.12 x 0.12 x 0 +x 0+x= x [1] x2 [1] (0.10 x)(0.12 x) Solving for x, 0.63 = 0.63(0.012 0.22x + x2) = x2 x = 0.0483 or x = 0.423 (rejected) ∴ equilibrium concentrations: [CO2(g)]eqm = 0.10 0.0483 mol dm3 = 0.0517 mol dm3 [1] [H2(g)]eqm = 0.12 0.0483 mol dm3 = 0.0717 mol dm3 [1] [CO(g)]eqm = 0.0483 mol dm3 [1] [H2O(g)]eqm = 0.0483 mol dm3 [1] 6. (a) (i) Kc1 = (ii) Kc2 = (iii) Kc = [ N 2 (g)]eqm [O2 (g)]eqm 2 [ NO(g)]eqm 2 [ NOBr(g )]eqm 2 [ NO(g )]eqm [Br2 (g )]eqm [1] [1] 2 [ NOBr(g)]eqm [ N 2 ]eqm [O2 (g)]eqm [Br2 (g)]eqm 26 [1] CO(g) + 0 +x 0+x= x = (b) Kc = K c2 [1] K c1 1.4 mol 1 dm3 = 5.83 × 1017 mol1 dm3 [1] 18 2.4 10 7. (a) It is incorrect [1] because the equilibrium concentration of each species is dependent on the equilibrium constant of the reaction. [1] (b) It is correct. [1] (c) It is incorrect [1] because the rates of forward and backward reactions are the same at equilibrium. [1] 8. (a) When concentrated hydrochloric acid is added, this increases the concentration of H+(aq), thus the equilibrium position of the system shifts to the left. [1] The colour of bromine water becomes darker. [1] (b) When concentrated potassium hydroxide is added, the OH(aq) removes H+(aq) from bromine water, thus the equilibrium position of the system shifts to the right. [1] The colour of bromine water becomes paler. [1] (c) When silver nitrate solution is added, Ag+(aq) removes Br(aq) from bromine water, the equilibrium position of the system shifts to the right. [1] The colour of bromine water becomes paler. [1] 9. (a) The equilibrium position will shift to the left. [1] Since the forward reaction is exothermic, [1] increasing the temperature shifts the equilibrium position to the left. (b) The equilibrium position will shift to the left. [1] According to Le Châtelier’s Principle, the equilibrium position will shift to the left producing more SO2(g). [1] (c) The equilibrium position will shift to the right. [1] According to Le Châtelier’s Principle, the equilibrium position will shift to the right producing more SO3(g). [1] 27 10. Addition of H2O(g) Addition of H2(g) Increases [1] Decreases [1] Effect on [CO2(g)] Decreases [1] Increases [1] Effect on [CO(g)] Shifts to the right [1] Shifts to the left [1] Effect on equilibrium position No change [1] No change [1] Change in temperature Decreases [1] Increases [1] Shifts to the left [1] Decreases [1] 11. (a) The equilibrium position shifts to the left. [1] Since an increase in volume can decrease the pressure, the equilibrium position shifts to the left, raising the number of gas molecules and bringing the pressure back up. [1] (b) The equilibrium position shifts to the right. [1] Since the concentration of hydrogen decreases, the equilibrium position shifts to the right to produce more hydrogen. [1] (c) The equilibrium position shifts to the right. [1] Since the pressure is decreased, the equilibrium position shifts to the right to produce more gases. [1] (d) There is no change in equilibrium position. [1] It is because the numbers of moles of gas molecules are equal on both sides of chemical equation. [1] (e) The equilibrium position shifts to the right. [1] Since sodium hydroxide removes HCl once it is formed, the concentration of HCl decreases. Thus the equilibrium position shifts to the right to produce more HCl. [1] 28 12. (a) The equilibrium constant, Kc = = [CO(g)]eqm [H 2O(g)]eqm [CO2 (g)]eqm [H 2 (g)]eqm [1] (0.0483 mol dm3 )(0.0483 mol dm3 ) (0.0517 mol dm 3 )(0.0717 mol dm 3 ) = 0.629 [1] dm3 Let x mol be the change in concentration of CO2(g). Concentr ation (mol CO2(g) + H2(g) CO(g) + 3 dm ) 0.0517 + 0.02 0.0717 0.0483 Initial = 0.0717 +x Change x x 0.0717 0.0483 Equilibri 0.0717 + um x x x 0.629 = H2O(g) 0.0483 +x 0.0483 + x (0.0483 x mol dm3 )(0.0483 x mol dm3 ) (0.0717 x mol dm3 )(0.0717 x mol dm3 ) Solving for x, x = 4.78 × 103 or x = 0.508 (rejected) new equilibrium concentrations [CO2(g)]eqm = 0.0717 4.78 × 103 mol dm3 = 0.0669 mol dm3 [1] [H2(g)]eqm = 0.0717 4.78 × 103 mol dm3 = 0.0669 mol dm3 [1] [CO(g)]eqm = 0.0483 + 4.78 × 103 mol dm3 = 0.0531 mol dm3 [1] [H2O(g)]eqm = 0.0483 + 4.78 × 103 mol dm3 = 0.0531 mol dm3 [1] (b) The equilibrium position will remain unchanged. [1] Since the numbers of moles of gas molecules are equal on both sides of reaction, [1] the change in volume does not affect the equilibrium position. [1] 29