AQA A Level Chemistry – Equilibria Questions and Answers

AQA A Level Chemistry – Equilibria Questions and Answers What are the two characteristics of a system is in dynamic equilibrium? 1. both forward and backward reactions proceed at equal rate 2. the concentrations of the reactants and products remain constant What is Le Chatlier's principle used for? It is a tool to help predict the effects of temp, pressure and concentration on the position of equilibrium What happens to the position of equilibrium if temperature is increased? The equilibrium will shift to oppose the change by moving in the endothermic direction (reduce the temperature by absorbing heat). If the position of equilibrium moves in the exothermic direction, what has happened to the temperature? The temperature has decreased (position of equilibrium moves to oppose the change, by generating heat energy) What will happen to the position of equilibrium if pressure is increased (in gaseous system)? Equilibrium will shift in the direction with the fewer moles of gas to oppose (reducing the pressure) In the production of ammonia from nitrogen and hydrogen, does high or low pressure increase the yield? High pressure (N₂ + 3H₂ →2NH₃) will increase the yield of ammonia. The equilibrium will move the the right, with fewer gas molecules, to oppose the change) What effect does a catalyst have on the equilibrium? NO EFFECT on the position of an equilibrium Speeds up the rate of the forward and backward reactions equally I₂ ₊ 2OH⁻ ⇌ I⁻ + IO⁻ + H₂O What will be the effect if increasing the concentration of OH⁻ ions? The position of equilibrium will shift to oppose the change (remove OH⁻ ions) and will move to the right. Why are the temperature and pressure used in the Haber process (N₂+3H₂⇌2NH₃) considered to be a compromise? High pressure / Low temperature move equilibrium to right. Both have compromise value as very high pressures are expensive (special equipment needed to handle safely) and low temperatures slow the reaction rate. What is the equation for the heating of hydrated copper sulfate? CuSO₄.5H₂O ⇌ CuSO₄ + 5H₂O What colour is anhydrous copper sulfate? white How can the position of the following reaction be moved to the right? Cu(H₂O)₆²⁺ + 4Cl⁻ ⇌ CuCl₄²⁻ + 6H₂O Add concentrated hydrochloric acid What is the equation for the equilibrium constant Kc for the reaction: 2A + 3B ⇌ 2C + D? [C]²[D] / [A]²[B]³ What are the units of Kc for the reaction: A + 3B ⇌ 2C + D units for Kc: mol⁻¹dm³ What effect does an increase in temperature have on the Kc of an exothermic reaction? Kc will decrease. Equilibrium will shift in the endothermic direction (backwards) to oppose the change. How does increasing the concentration of iodine affect Kc for the reaction: I₂ ₊ 2OH⁻ ⇌ I⁻ + IO⁻ + H₂O It DOES not affect it. Position of equilibrium will move to the right, and all concentrations will be adjusted but Kc is a constant. How is the partial pressure of a substance calculated in a gaseous mixture? (mole fraction) x (total pressure of gas) What is the equation for the mole fraction? No. of moles of a gas / total moles of gas present What is the equation for equilibrium constant Kp for the reaction: 2SO₂ + O₂ ⇌ 2SO₃? p(SO₃)² / p(SO₂²)p(O₂) What effect does an increase pressure have on the value of Kp? NO EFFECT Value of Kp stays constant. What change causes Kp of an endothermic reaction to decrease? A decrease in temperature (system opposes change and moves in exothermic direction, which is to the left) What is the only variable that has an effect on the value of Kp or Kc? Temperature How are the values of Kp for the following two equations related? 1.2SO₂ + O₂ ⇌ 2SO₃ and 2. SO₂ + ¹/₂O₂ ⇌ SO₃ Kp(1) = (Kp(2) )² What happens to the position of equilibrium of the following reaction when the temperature is increased? NH₃ + HCl ⇌ NH₄Cl Moves the the left (system opposes change by moving in the endothermic direction, which is the direction where bonds are broken) If 2 moles of SO₂ is heated with 1 mole of O₂ until equilibrium is reached and at equilibrium, the moles of SO₂ are 1.2, calculate Kp. At eqm: 1.2moles SO₂, 0.6 mole O₂, 0.8 moles SO₃. Ptotal = 2.6. Kp = (0.8²x2.6)/(1.2²x0.6) = 1.925.