The questions are on the thermochemistry test. Take it under real conditions before reading the mark scheme.
Quick answer: Thermochemistry measures heat changes in reactions. This mark scheme gives the model answers and the marks for the 8 questions of the AP Chemistry test on thermochemistry, from reading the sign to specific heat capacity, with the equivalent answers to accept.
Model answers and marking guides follow. Award credit for a correct method with an arithmetic slip, and for sign errors only when the reasoning about direction of heat flow is stated. Bond-enthalpy answers may differ slightly if the table is rounded differently.
Level: AP Chemistry · Curriculum: AP Chem Unit 6
Question 1 Reading the sign (2 marks)
The solution absorbs heat from its surroundings: the process is endothermic and ΔH is positive.
Marking: 1 for endothermic, 1 for positive.
Question 2 Calorimetry (3 marks)
(a) q = 150.0 × 4.18 × 13.5 = 8.46 × 10³ J.
(b) The temperature rises, so the reaction is exothermic: ΔH = −8.46/0.0400 = −212 kJ/mol.
Marking: 1 for ΔT = 13.5, 1 for q, 1 for the value with correct sign.
Question 3 Hess’s law (3 marks)
Add (1) and (2): 2 C + 2 O₂ → 2 CO₂, ΔH = −221 − 566 = −787 kJ. Divide by 2: ΔH = −393.5 kJ.
Marking: 1 for adding, 1 for cancelling CO, 1 for dividing by 2.
Question 4 Formation enthalpies (3 marks)
ΔH° = [4(90.3) + 6(−241.8)] − [4(−45.9) + 0] = (361.2 − 1450.8) + 183.6 = −906.0 kJ.
Marking: 1 for products minus reactants, 1 for substitution, 1 for the answer.
Question 5 Bond enthalpies (3 marks)
Broken: C–H + Cl–Cl = 413 + 243 = 656 kJ. Formed: C–Cl + H–Cl = 339 + 432 = 771 kJ. ΔH ≈ 656 − 771 = −115 kJ.
Marking: 1 for broken, 1 for formed, 1 for ΔH.
Question 6 Heat loss (2 marks)
Some of the heat escapes to the surroundings, so the water warms less than it should. A smaller ΔT gives a smaller q and a smaller magnitude of ΔH (−53 instead of −57).
Marking: 1 for the smaller temperature rise, 1 for the smaller magnitude of ΔH.
Question 7 Enthalpy levels (2 marks)
(a) A at 0 and C at −85: −85 kJ.
(b) C is at −85 and B at −120: −35 kJ.
Marking: 1 mark each.
Question 8 Specific heat capacity (2 marks)
q(water) = 100.0 × 4.18 × 3.5 = 1463 J. The metal cools by 76.5 °C: c = 1463/(45.0 × 76.5) = 0.425 J/(g °C).
Marking: 1 for heat gained by water, 1 for c with unit.
