The questions are on the chemical equilibrium test. Take it under real conditions before reading the mark scheme.
Quick answer: At chemical equilibrium the forward and reverse reaction rates are equal, so concentrations stay constant. This mark scheme gives the model answers and the marks for the 8 questions of the AP Chemistry test on chemical equilibrium, from writing K to temperature and K, with the equivalent answers to accept.
Model answers and marking guides follow. Award method marks for correct expressions even if the arithmetic slips, and accept answers to two or three significant figures. A correct comparison of Q and K is required for full marks on direction questions.
Level: AP Chemistry · Curriculum: AP Chem Unit 7
Question 1 Writing K (2 marks)
(a) Kc = [NO₂]²/([NO]²[O₂]).
(b) Kc = [CO₂].
Marking: 1 mark each; solids must be omitted in (b).
Question 2 Reaction quotient (3 marks)
(a) Q = (0.10)²/(0.50 × 0.20³) = 0.010/0.0040 = 2.5.
(b) Q > K (2.5 > 0.50), so the reverse reaction runs.
Marking: 1 for the expression, 1 for the value, 1 for the direction with reason.
Question 3 ICE table (3 marks)
HI −2x, H₂ +x, I₂ +x: x²/(0.80 − 2x)² = 1/64, so x/(0.80 − 2x) = 1/8, 8x = 0.80 − 2x and x = 0.080.
[H₂] = [I₂] = 0.080 M; [HI] = 0.80 − 0.16 = 0.64 M.
Marking: 1 for the ICE changes, 1 for solving, 1 for all three concentrations.
Question 4 Le Chatelier (2 marks)
(a) To the right (endothermic direction).
(b) To the left (towards 1 mol of gas instead of 2).
Marking: 1 mark each.
Question 5 Catalyst and inert gas (2 marks)
A catalyst lowers the activation energy of both reactions by the same amount, so both rates increase equally and Q, K and the position are unchanged. Argon at constant volume does not change any partial pressure or concentration, so Q stays equal to K.
Marking: 1 for each explanation.
Question 6 Kp and Kc (3 marks)
(a) Kp = (0.60 × 0.60)/0.40 = 0.90.
(b) Δn = +1, so Kc = Kp/(RT) = 0.90/(0.08206 × 500) = 0.90/41.03 = 0.022.
Marking: 1 for Kp; 1 for using Δn = +1; 1 for the answer.
Question 7 Reading a disturbance (3 marks)
(a) Ammonia is removed (its concentration drops suddenly from 0.40 to 0.20 M).
(b) The shift is to the right: [NH₃] rises again while [N₂] and [H₂] fall.
(c) K does not change (temperature is constant).
Marking: 1 mark each.
Question 8 Temperature and K (2 marks)
K is smaller at 500 K. Raising the temperature favours the endothermic reverse reaction, so the ratio of products to reactants at equilibrium decreases.
Marking: 1 for smaller, 1 for the reasoning.
