Topic11 Chemical Equilibrium

Transcription

1 Topic11 Chemical Equilibrium Unit 39 An troduction to chemical Unit 40 Factors affectg chemical

2 Key C o ncepts An troduction to chemical Irreversible and reversible Characteristics of a dynamic Calculations volvg constant, K c Chemical Equilibrium Factors affectg chemical Le Chatelier s prciple Effect of changes Effect of pressure changes Effect of temperature changes Applyg prciples of rates and equilibria to dustrial processes

3 Topic 11 Chemical Equilibrium Unit 39 An troduction to chemical Unit 39 An troduction to chemical 39.1 Irreversible and reversible 39.2 Equilibrium 39.3 Chemical for a reversible 39.4 The importance of a closed 39.5 Equilibrium established from either direction of a 39.6 Effect of changg conditions on chemical 39.7 Characteristics of a dynamic 39.8 The constant 39.9 The law Calculatg constants What does the constant tell us? Equilibrium volvg components more than one state Determg the constant for an esterification experimentally Summary 1 A dynamic is reached when the forward and backward occur at the same rate. 2 Characteristics of a dynamic : Equilibrium can only be established a closed with constant temperature and pressure. Once is reached, the composition of the remas constant. The has not stopped, rate of forward = rate of backward. Reactants and products will both be present mixture. Equilibrium can be reached from either direction. If the conditions (temperature, pressure and ) are changed, the established may be affected. 3 For a reversible : aa + bb cc + dd constant K c = [C] c eqm[d] d eqm [A] a eqm[b] b eqm 4 At a given temperature, the constant, K c, for a always has the same value. 5 The expression for the quotient, Q c, is defed the same way as the constant except that the s of reactants and products can be taken at any moment of the (not necessarily the s at ). Q c is not a constant. If Q c = K c, the is at. If Q c < K c, until is reached. If Q c > K c, until is reached. Exam tips Questions often give the percentage dissociation of a substance and ask students to calculate the constant. When calculatg K c, remember that the of a solid is constant. NH 4 HS(s) NH 3 (g) + H 2 S(g) K c = [NH 3 (g)][h 2 S(g)] DO NOT confuse the terms quotient and constant.

4 Topic 11 Chemical Equilibrium Unit 39 An troduction to chemical Questions may ask students to decide the direction which the will proceed to achieve based on the value of Q c. The constant, K c, for the followg is dm 3 mol 1 at 873 K. CO(g) + Cl 2 (g) COCl 2 (g) A mixture of moles of CO(g), 1.00 mole of Cl 2 (g) and mole of COCl 2 (g) is troduced to an evacuated vessel of 5.00 dm 3 kept at 873 K. Calculate the quotient, Q c, of the at the start of the. Then decide the direction which the will proceed to achieve. [COCl Q c = 2 (g)] [CO(g)][Cl 2 (g)] mol dm 3 = mol mol ( 5.00 dm 3)( 5.00 dm 3) = 1.25 dm 3 mol 1 Q c > K c will proceed to achieve. K c = [CO(g)][H 2 (g)] 3 [CH 4 (g)][h 2 O(g)] = ( mol dm 3 )( mol dm 3 ) 3 ( mol dm 3 )( mol dm 3 ) = 3.69 x 10 4 mol 2 dm 6 the constant, K c, for the at K is 3.69 x 10 4 mol 2 dm 6. b) The combustion of H 2 (g) gives only water, which will not cause pollution to our environment. Hydrogen has a small molar mass. The ratio of energy output per unit mass of H 2 (g) is higher than that of other fuels. Remarks* Given the constant, K c, students should be able to calculate the s, and vice versa. Instead of the s of the species volved, questions may give the number of moles of each species. So, remember to use s when calculatg the constant, K c. Example Hydrogen is manufactured by steam reformg of natural gas, which volves the followg : Ni(s) CH 4 (g) + H 2 O(g) CO(g) + 3H 2 (g) In a simulation study of the manufacturg process, CH 4 (g) and H 2 O(g) are allowed to react the presence of nickel as catalyst a closed contaer kept at a constant temperature at K. The itial s of CH 4 (g) and H 2 O(g) are mol dm 3. When is attaed, the of CH 4 (g) is mol dm 3. a) Calculate the constant, K c, for the at K. (4 marks) b) Give TWO advantages of usg H 2 (g) as a source of energy. (2 marks) Answer a) CH 4 (g) + H 2 O(g) CO(g) + 3H 2 (g) Initial (mol dm 3 ) Equilibrium (mol dm 3 )

5 Topic 11 Chemical Equilibrium Unit 40 Factors affectg chemical Unit 40 Factors affectg chemical 40.1 Position of 40.2 Effect of changg conditions on 40.3 The Haber process 40.4 The effect of changes on chemical 40.5 Predictg the shift position usg the quotient, Q c 40.6 The effect of pressure changes on chemical 40.7 The effect of temperature changes on chemical 40.8 Applyg prciples of rates and equilibria to dustrial processes 40.9 Lkg equilibria together Summary 1 Le Chatelier s prciple states that if the condition of a is changed, the position of will shift so as to reduce that change. 2 The followg is a summary of effect of changes on a chemical. Action on Increasg the of a reactant Decreasg the of a reactant Increasg the of a product Decreasg the of a product Rates of forward and backward rate of forward creases rate of forward decreases rate of backward creases rate of backward decreases Direction of net Attament of Shift of position of Value of K c 3 The followg is a summary of effect of pressure changes on a chemical. aa(g) + bb(g) cc(g) + dd(g) Pressure (volume) change on Rates of forward and backward Direction of net Attament of Shift of position of Value of K c Increase pressure (decrease volume) a + b > a + b = a + b < crease, but the forward rate creases more same crease crease, but the backward rate creases more forward no net backward Decrease pressure (crease volume) a + b > a + b = a + b < decrease, but the forward rate decreases more same decrease decrease, but the backward rate decreases more backward no net forward

6 10 Topic 11 Chemical Equilibrium Unit 40 Factors affectg chemical 11 4 The followg is a summary of effect of temperature changes on a chemical. Temperature change on Increase temperature Decrease temperature Exam tips Example Rates of forward and backward crease, but not to the same extent decrease, but not to the same extent Type of forward exothermic endothermic exothermic endothermic Direction of net Attament of Shift of position of to left to right to right to left Value of K c decrease crease crease decrease Instead of pressure change, questions may ask the effect of volume change on an. The value of K c depends only on temperature. Consider the followg : CaCO 3 (s) CaO(s) + CO 2 (g) H > 0 K c = [CO 2 (g)] As the value of K c depends only on temperature, addg CaCO 3 (s) to the will NOT affect the of CO 2 (g). Ethanol is manufactured by catalytic hydration of ethene: CH 2 =CH 2 (g) + H 2 O(g) CH 3 CH 2 OH(g) H < 0 In a simulation study of the manufacturg process, 1.60 moles of CH 2 =CH 2 (g) and 1.00 mole of H 2 O(g) are mixed a dm 3 contaer to react the presence of a catalyst at 573 K and 60 atmospheres. When is attaed, 5.00% of ethene is converted to ethanol. a) Calculate the constant, K c, under the above conditions. (4 marks) b) State, with explanation, the effect of a decrease temperature of the on K c. (3 marks) c) In practice, the conversion of ethene to ethanol can be made much higher than 5.00%. Suggest TWO ways to crease the conversion of ethene to ethanol. (2 marks) Answer a) CH 2 =CH 2 (g) + H 2 O(g) CH 3 CH 2 OH(g) Initial (mol dm 3 ) x ( )% (1.64 x 5.00%) x 5.00% Equilibrium (mol dm 3 ) = = = [CH K c = 3 CH 2 OH(g)] [CH 2 =CH 2 (g)][h 2 O(g)] mol dm 3 = (0.779 mol dm 3 )(0.459 mol dm 3 ) = dm 3 mol 1 b) K c will crease. When the temperature is decreased, the will respond by raisg the temperature. As the forward is exothermic, the will undergo. The position of will shift. c) Any two of the followg: Cool the gaseous mixture emerged from the chamber to separate the ethanol. Allow the unreacted ethene and steam to pass back to the chamber so as to crease the percentage of conversion of ethene to ethanol. Use excess steam. The position of will shift. Increase the pressure of the. The position of will shift to the right. Remarks* Questions often ask the ways of creasg the yield of a product when a chemical process is put to dustrial practice. Ammonia is manufactured by the Haber process: Fe(s) N 2 (g) + 3H 2 (g) 2NH 3 (g) H < 0 Suggest TWO ways to crease the yield of ammonia when the process is put to dustrial process. Increase the pressure of the. Separate ammonia by liquefaction and pass the unreacted N 2 (g) and H 2 (g) back to the chamber.