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CAIE A-Level Chemistry AS 5.2.2 Calculations Cycles and Relevant Energy Terms Questions

Practise calculating reaction enthalpy from enthalpies of formation, combustion and other energy terms using CAIE 9701 AS questions and mark schemes.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • Calculate enthalpy changes using Hess’s law, enthalpies of formation and combustion, and relevant energy terms.

CAIE A-Level Chemistry AS 5.2.2 Calculations Cycles and Relevant Energy Terms Questions question 1

[Maximum number: 1]

The enthalpy change for a reaction can be calculated from values of:
- enthalpies of formation, ΔHf⊖\Delta H_{\mathrm{f}}^{\ominus}
- enthalpies of combustion, ΔHc⊖\Delta H_{\mathrm{c}}^{\ominus}
- bond energies, E.

The enthalpy change of the reaction given =ΔHr⊖=\Delta H_{\mathrm{r}}^{\ominus}.

2C2H6( g)+3O2( g)→2CH4( g)+2CO2( g)+2H2O(l)2 \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})+3 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{CH}_{4}(\mathrm{~g})+2 \mathrm{CO}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

Which expression could be used to calculate ΔHr⊖\Delta H_{\mathrm{r}}^{\ominus} ?

A

ΔHce(C2H6( g))\Delta H_{\mathrm{c}}^{e}\left(\mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})\right)

B

2ΔHc⊖(C2H6( g))−2ΔHc⊖(CH4( g))2 \Delta H_{\mathrm{c}}^{\ominus}\left(\mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})\right)-2 \Delta H_{\mathrm{c}}^{\ominus}\left(\mathrm{CH}_{4}(\mathrm{~g})\right)

C

E(C−C)+2E(C−H)−4E(C=O)−4E(H−O)E(\mathrm{C}-\mathrm{C})+2 E(\mathrm{C}-\mathrm{H})-4 E(\mathrm{C}=\mathrm{O})-4 E(\mathrm{H}-\mathrm{O})

D

ΔHf⊖(CH4( g))+ΔHf⊖(CO2( g))+ΔHf⊖(H2O(l))−ΔHf⊖(C2H6( g))\Delta H_{\mathrm{f}}^{\ominus}\left(\mathrm{CH}_{4}(\mathrm{~g})\right)+\Delta H_{\mathrm{f}}^{\ominus}\left(\mathrm{CO}_{2}(\mathrm{~g})\right)+\Delta H_{\mathrm{f}}^{\ominus}\left(\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right)-\Delta H_{\mathrm{f}}^{\ominus}\left(\mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})\right)

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