Question 22M.2.sl.TZ2.4e(i)
| Date | May 2022 | Marks available | [Maximum mark: 3] | Reference code | 22M.2.sl.TZ2.4e(i) |
| Level | sl | Paper | 2 | Time zone | TZ2 |
| Command term | Calculate | Question number | e(i) | Adapted from | N/A |
Carbon forms many compounds.
Chlorine reacts with methane.
CH4 (g) + Cl2 (g) → CH3Cl (g) + HCl (g)
Calculate the enthalpy change of the reaction, ΔH, using section 11 of the data booklet.
[3]
bond breaking: C–H + Cl–Cl / 414 «kJ mol–1» + 242 «kJ mol–1»/656 «kJ»
OR
bond breaking: 4C–H + Cl–Cl / 4 × 414 «kJ mol–1» + 242 «kJ mol–1» / 1898 «kJ» ✔
bond forming: «C–Cl + H–Cl / 324 kJ mol–1 + 431 kJ mol–1» / 755 «kJ»
OR
bond forming: «3C–H + C–Cl + H–Cl / 3 × 414 «kJ mol–1» + 324 «kJ mol–1» + 431 kJ mol–1» / 1997 «kJ» ✔
«ΔH = bond breaking – bond forming = 656 kJ – 755 kJ» = –99 «kJ» ✔
Award [3] for correct final answer.
Award [2 max] for 99 «kJ».
This was the best answered question on the paper. Candidates identified the bonds and used bond enthalpies to calculate the enthalpy of reaction accurately. The most common mistakes were reversing the signs of bonds broken and bonds formed, assuming two Cl-Cl bonds were broken and using an incorrect value of bond enthalpy for one of the bonds.
