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11 Topic Chemical Equilibrium Unit 39 An introduction to chemical equilibrium Unit 40 Factors affecting chemical equilibrium systems Key C o ncepts An introduction to chemical equilibrium • Irreversible and reversible reactions • Characteristics of a system in dynamic equilibrium • Calculations involving equilibrium constant, Kc Chemical Equilibrium Factors affecting chemical equilibrium systems • Le Chatelier’s principle • Effect of concentration changes • Effect of pressure changes • Effect of temperature changes • Applying principles of reaction rates and equilibria to industrial processes Topic 11 Chemical Equilibrium Unit 39 An introduction to chemical equilibrium Unit 39 An introduction to chemical equilibrium 39.1 – 39.13 Summary 39.1 Irreversible and reversible reactions 39.2 Equilibrium 39.3 Chemical equilibrium for a reversible reaction 39.4 The importance of a closed system 39.5 39.6 1 A dynamic equilibrium is reached when the forward and backward reactions occur at the same rate. 2 Characteristics of a system in dynamic equilibrium: • Equilibrium can only be established in a closed system with constant temperature and pressure. Equilibrium established from either direction of a reaction • Once equilibrium is reached, the composition of the system remains constant. Effect of changing conditions on chemical equilibrium systems • The reaction has not stopped, rate of forward reaction = rate of backward reaction. 39.7 Characteristics of a system in dynamic equilibrium 39.8 The equilibrium constant 39.9 The equilibrium law 39.10 Calculating equilibrium constants 39.11 What does the equilibrium constant tell us? 39.12 Equilibrium systems involving components in more than one state 39.13 Determining the equilibrium constant for an esterification reaction experimentally • Reactants and products will both be present in equilibrium mixture. • Equilibrium can be reached from either direction. • If the conditions (temperature, pressure and concentration) are changed, the equilibrium established may be affected. 3 For a reversible reaction: aA + bB equilibrium constant Kc = cC + dD [C]ceqm [D]deqm [A]aeqm [B]beqm 4 At a given temperature, the equilibrium constant, Kc, for a reaction always has the same value. 5 The expression for the reaction quotient, Qc, is defined in the same way as the equilibrium constant except that the concentrations of reactants and products can be taken at any moment of the reaction (not necessarily the concentrations at equilibrium). Qc is not a constant. • If Qc = Kc, the system is at equilibrium. • If Qc < Kc, a net forward reaction occurs until equilibrium is reached. • If Qc > Kc, a net backward reaction occurs until equilibrium is reached. Exam tips ♦ Questions often give the percentage dissociation of a substance and ask students to calculate the equilibrium constant. ♦ When calculating Kc, remember that the concentration of a solid is constant. e.g. NH4HS(s) NH3(g) + H2S(g) Kc = [NH3(g)][H2S(g)] ♦ DO NOT confuse the terms ‘reaction quotient’ and ‘equilibrium constant’. Topic 11 Chemical Equilibrium Unit 39 An introduction to chemical equilibrium ♦ Questions may ask students to decide the direction in which the reaction will proceed to achieve equilibrium based on the value of Qc. e.g. 3 –1 The equilibrium constant, Kc, for the following reaction is 0.200 dm mol at 873 K. CO(g) + Cl2(g) COCl2(g) A mixture of 2.00 moles of CO(g), 1.00 mole of Cl2(g) and 0.500 mole 3 of COCl2(g) is introduced into an evacuated vessel of 5.00 dm kept at 873 K. = )( ) –1 = 1.25 dm mol Hydrogen is manufactured by steam reforming of natural gas, which involves the following reaction: Ni(s) CO(g) + 3H2(g) In a simulation study of the manufacturing process, CH4(g) and H 2O(g) are allowed to react in the presence of nickel as catalyst in a closed container kept at a constant temperature at 1 000 K. The initial concentrations of CH4(g) and H2O(g) are 0.0120 mol dm–3. When equilibrium is attained, the concentration of CH4(g) is 0.00694 mol dm–3. a) Calculate the equilibrium constant, Kc, for the reaction at 1 000 K. (4 marks) b)Give TWO advantages of using H2(g) as a source of energy. (2 marks) Answer a) (0.00506 mol dm–3)(0.0152 mol dm–3)3 (0.00694 mol dm–3)(0.00694 mol dm–3) = = 3.69 x 10 –4 2 –6 mol dm ∴ the equilibrium constant, Kc, for the reaction at 1 000 K is 3.69 x 10–4 mol2 dm–6. CH4(g) + H2O(g) CO(g) + 3H2(g) Initial concentration (mol dm–3) 0.0120 0.0120 0 0 Equilibrium concentration (mol dm–3) 0.00694 0.00694 0.00506 0.0152 (1) (1) Hydrogen has a small molar mass. The ratio of energy output per unit mass of H2(g) is higher than that of other fuels. (1) ➤Instead of the concentrations of the species involved, questions may give the number of moles of each species. So, remember to use concentrations when calculating the equilibrium constant, Kc. Example CH4(g) + H2O(g) Remarks* Remarks Qc > Kc ∴ reaction will proceed to the left to achieve equilibrium. (1) ➤Given the equilibrium constant, Kc, students should be able to calculate the equilibrium concentrations, and vice versa. 0.500 mol dm–3 5.00 2.00 1.00 mol dm–3 mol dm–3 5.00 5.00 3 [CO(g)][H2(g)] [CH4(g)][H2O(g)] [COCl2(g)] [CO(g)][Cl2(g)] ( Kc= b)The combustion of H2(g) gives only water, which will not cause pollution to our environment. (1) Calculate the reaction quotient, Qc, of the system at the start of the reaction. Then decide the direction in which the reaction will proceed to achieve equilibrium. Qc = 3 (1) Topic 11 Chemical Equilibrium Unit 40 Factors affecting chemical equilibrium systems Unit 40 Factors affecting chemical equilibrium systems 40.1 – 40.9 Summary 40.1 Position of equilibrium 40.2 Effect of changing conditions on systems in equilibrium 40.3 The Haber process 40.4 The effect of concentration changes on chemical equilibrium system 40.5 1 Le Chatelier’s principle states that if the condition of a system in equilibrium is changed, the position of equilibrium will shift so as to reduce that change. 2 The following is a summary of effect of concentration changes on a chemical equilibrium system. Predicting the shift in equilibrium position using the reaction quotient, Qc Action on equilibrium system Rates of forward and backward reactions Direction of net reaction Attainment Shift of of position of Value of Kc equilibrium equilibrium Increasing the concentration of a reactant rate of forward reaction increases a net forward reaction occurs faster to the right Decreasing the concentration of a reactant rate of forward a net backward reaction decreases reaction occurs slower to the left 40.6 The effect of pressure changes on chemical equilibrium systems 40.7 The effect of temperature changes on chemical equilibrium systems 40.8 Applying principles of reaction rates and equilibria to industrial processes Increasing the concentration of a product rate of backward a net backward reaction increases reaction occurs faster to the left 40.9 Linking equilibria together Decreasing the concentration of a product rate of backward a net forward reaction decreases reaction occurs slower to the right no change 3 The following is a summary of effect of pressure changes on a chemical equilibrium system. aA(g) + bB(g) cC(g) + dD(g) Pressure (volume) change on equilibrium system Rates of forward and backward reactions a + b > c + d rates of both reactions increase, but the forward reaction rate increases more a net forward reaction occurs a + b = c + d same increase in rates of both reactions no net reaction a + b < c + d rates of both reactions increase, but the backward reaction rate increases more a net backward reaction occurs to the left a + b > c + d rates of both reactions decrease, but the forward reaction rate decreases more a net backward reaction occurs to the left a + b = c + d same decrease in rates of both reactions no net reaction a + b < c + d rates of both reactions decrease, but the backward reaction rate decreases more a net forward reaction occurs Increase in pressure (decrease in volume) Decrease in pressure (increase in volume) Direction Attainment Shift of of net of position of Value of Kc reaction equilibrium equilibrium to the right faster slower no change no change to the right no change no change Topic 11 Chemical Equilibrium 10 Unit 40 Factors affecting chemical equilibrium systems 4 The following is a summary of effect of temperature changes on a chemical equilibrium system. Temperature Rates of change on forward and equilibrium backward system reactions Type of forward reaction Direction of net reaction Increase in temperature rates of both a net backward exothermic reactions reaction occurs increase, but a net forward not to the endothermic reaction occurs same extent Decrease in temperature rates of both a net forward exothermic reactions reaction occurs decrease, but a net backward not to the endothermic reaction occurs same extent Attainment Shift of of position of equilibrium equilibrium Value of Kc to left decrease to right increase to right increase to left decrease faster slower Answer a) Initial concentration (mol dm–3) Equilibrium concentration (mol dm–3) Kc= CH2=CH2(g) + H2O(g) 1.64 2.00 ♦ Instead of pressure change, questions may ask the effect of volume change on an equilibrium system. 1.00 – (1.64 x 5.00%) 1.64 x 5.00% 2.00 2.00 = 0.779 = 0.459 = 0.0410 [CH3CH2OH(g)] [CH2=CH2(g)][H2O(g)] (1) (1) 0.0410 mol dm (0.779 mol dm–3)(0.459 mol dm–3) = = 0.115 dm mol 3 (1) –1 (1) (1) When the temperature is decreased, the system will respond by raising the temperature. (1) As the forward reaction is exothermic, the system will undergo a net forward reaction. (1) The position of equilibrium will shift to the right. Consider the following equilibrium system: c) Any two of the following: CaCO3(s) • Cool the gaseous mixture emerged from the reaction chamber to separate the ethanol. Allow the unreacted ethene and steam to pass back to the reaction chamber so as to increase the percentage of conversion of ethene to ethanol.(1) • Use excess steam. The position of equilibrium will shift to the right. • Increase the pressure of the system. The position of equilibrium will shift to the right. (1) CaO(s) + CO2(g) ∆H > 0 Kc = [CO2(g)] As the value of Kc depends only on temperature, adding CaCO3(s) to the system will NOT affect the concentration of CO2(g). Example Remarks* Remarks Ethanol is manufactured by catalytic hydration of ethene: 0 –3 ♦ The value of Kc depends only on temperature. e.g. CH3CH2OH(g) 1.00 2.00 1.64 x (100 – 5.00)% 2.00 b)Kc will increase. Exam tips 11 CH2=CH2(g) + H2O(g) CH3CH2OH(g) ➤Questions often ask the ways of increasing the yield of a product when a chemical process is put into industrial practice. ∆H < 0 In a simulation study of the manufacturing process, 1.60 moles of CH2=CH2(g) and 1.00 mole of H2O(g) are mixed in a 2.00 dm3 container to react in the presence of a catalyst at 573 K and 60 atmospheres. When equilibrium is attained, 5.00% of ethene is converted into ethanol. a) Calculate the equilibrium constant, Kc, under the above conditions. (4 marks) b)State, with explanation, the effect of a decrease in temperature of the system on Kc. (3 marks) c) In practice, the conversion of ethene to ethanol can be made much higher than 5.00%. Suggest TWO ways to increase the conversion of ethene into ethanol. (2 marks) e.g. Ammonia is manufactured by the Haber process: N2(g) + 3H2(g) Fe(s) 2NH3(g) ∆H < 0 Suggest TWO ways to increase the yield of ammonia when the process is put into industrial process. – Increase the pressure of the system. – Separate ammonia by liquefaction and pass the unreacted N2(g) and H2(g) back into the reaction chamber. (1)