DP Chemistry (first assessment 2025)
Question 21N.2.HL.TZ0.c(i)
Date | November 2021 | Marks available | [Maximum mark: 1] | Reference code | 21N.2.HL.TZ0.c(i) |
Level | HL | Paper | 2 | Time zone | TZ0 |
Command term | Calculate | Question number | c(i) | Adapted from | N/A |
c(i).
[Maximum mark: 1]
21N.2.HL.TZ0.c(i)
Calculate the standard enthalpy change (ΔH⦵) for the forward reaction in kJ mol−1.
ΔH⦵f PCl3 (g) = −306.4 kJ mol−1
ΔH⦵f PCl5 (g) = −398.9 kJ mol−1
[1]
Markscheme
«−398.9 kJ mol−1 − (−306.4 kJ mol−1) =» −92.5 «kJ mol−1» ✔

Syllabus sections
Reactivity 1. What drives chemical reactions? » Reactivity 1.2—Energy cycles in reactions » Reactivity 1.2.4—An application of Hess’s law uses enthalpy of formation data or enthalpy of combustion data to calculate the enthalpy change of a reaction. Calculate enthalpy changes of a reaction using ΔHf⦵ data or ΔHc⦵ data: ΔH⦵ = Σ ΔHf⦵products − Σ ΔHf⦵reactants ΔH⦵ = Σ ΔHc, ⦵reactants − Σ ΔHc. ⦵products