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Edexcel A-Level Chemistry AS 6.10 Mean Bond Enthalpies From Enthalpy Changes of Reaction Questions

Use marked Edexcel IAL Chemistry questions to count bonds, rearrange energy balances, and calculate unknown mean bond enthalpies from atomisation and reaction data.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
AS

Exam points

  • Count reactant and product bonds using molecular structures and equation coefficients.
  • Rearrange bonds broken minus bonds formed to calculate an unknown bond enthalpy.
  • Divide the total bond energy by bond multiplicity to report the mean bond enthalpy.

Edexcel A-Level Chemistry AS 6.10 Mean Bond Enthalpies From Enthalpy Changes of Reaction Questions question 1

[Maximum number: 3]

Fuels

Fuels burn in oxygen to release a lot of energy.
Many hydrocarbons and alcohols are used as fuels. During complete combustion, they produce carbon dioxide and water.

Petrol contains 2,2,4-trimethylpentane, an isomer of octane, that promotes smooth combustion.

Figure for Question Edexcel A-Level Chemistry AS 6.10 Mean Bond Enthalpies From Enthalpy Changes of Reaction Questions question 1 — Edexcel A-Level Chemistry AS

2,2,4-trimethylpentane

Alcohols, such as methanol and ethanol, can be used as fuels either on their own or as additives in petrol.

The enthalpy changes for the conversion of four compounds in the gas phase into their constituent atoms are shown.

H2O( g)→2H( g)+O( g)ΔrH=+928 kJ mol−1CH4( g)→C( g)+4H( g)ΔrH=+1740 kJ mol−1CH3OH( g)→C( g)+4H( g)+O( g)ΔrH=+2105 kJ mol−1C2H5OH( g)→2C( g)+6H( g)+O( g)ΔrH=+3322 kJ mol−1\begin{aligned} \mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) & \rightarrow 2 \mathrm{H}(\mathrm{~g})+\mathrm{O}(\mathrm{~g}) & \Delta_{\mathrm{r}} H & =+928 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \mathrm{CH}_{4}(\mathrm{~g}) & \rightarrow \mathrm{C}(\mathrm{~g})+4 \mathrm{H}(\mathrm{~g}) & \Delta_{\mathrm{r}} H & =+1740 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \mathrm{CH}_{3} \mathrm{OH}(\mathrm{~g}) & \rightarrow \mathrm{C}(\mathrm{~g})+4 \mathrm{H}(\mathrm{~g})+\mathrm{O}(\mathrm{~g}) & \Delta_{\mathrm{r}} H & =+2105 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{~g}) & \rightarrow 2 \mathrm{C}(\mathrm{~g})+6 \mathrm{H}(\mathrm{~g})+\mathrm{O}(\mathrm{~g}) & \Delta_{\mathrm{r}} H & =+3322 \mathrm{~kJ} \mathrm{~mol}^{-1} \end{aligned}

Calculate the bond enthalpy of the C-C bond, in kJmol−1\mathrm{kJ} \mathrm{mol}^{-1}.
You must show your working.

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