DP Chemistry (first assessment 2025)
Question 21N.2.SL.TZ0.7
| Date | November 2021 | Marks available | [Maximum mark: 5] | Reference code | 21N.2.SL.TZ0.7 |
| Level | SL | Paper | 2 | Time zone | TZ0 |
| Command term | Determine, Formulate | Question number | 7 | Adapted from | N/A |
7.
[Maximum mark: 5]
21N.2.SL.TZ0.7
Alkanes undergo combustion and substitution.
(a)
Determine the molar enthalpy of combustion of an alkane if 8.75 × 10−4 moles are burned, raising the temperature of 20.0 g of water by 57.3 °C.
[2]
Markscheme
«q = mcΔT = 20.0 g × 4.18 J g−1 °C−1 × 57.3 °C =» 4790 «J» ✔
«» –5470 «kJ mol–1» ✔
Award [2] for correct final answer.
Accept answers in the range –5470 to –5480 «kJ mol−1».
Accept correct answer in any units, e.g. –5.47 «MJ mol−1» or 5.47 x 106 «J mol−1».
(b)
Formulate equations for the two propagation steps and one termination step in the formation of chloroethane from ethane.
[3]
Markscheme
Cl· + C2H6 → ·C2H5 + HCl ✔
·C2H5 + Cl2 → Cl· + C2H5Cl ✔
·C2H5 + Cl· → C2H5Cl
OR
Cl· + Cl· → Cl2
OR
·C2H5 + ·C2H5 → C4H10 ✔
Do not penalize incorrectly placed radical sign, eg C2H5·.
Syllabus sections
Reactivity 1. What drives chemical reactions? » Reactivity 1.1—Measuring enthalpy changes » Reactivity 1.1.4—The standard enthalpy change for a chemical reaction, ΔH⦵, refers to the heat transferred at constant pressure under standard conditions and states. It can be determined from the change in temperature of a pure substance. Apply the equations Q = mcΔT and ΔH = − Qn in the calculation of the enthalpy change of a reaction.
Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.3—Electron sharing reactions » Reactivity 3.3.3—Radicals take part in substitution reactions with alkanes, producing a mixture of products. Explain, using equations, the propagation and termination steps in the reactions between alkanes and halogens.