CAIE A-Level Chemistry 5.1.5 Bond Energies and Reaction Enthalpy

Practise counting bonds and calculating reaction enthalpy as energy to break bonds minus energy released on bond formation.

Exam points

count every changed bond with the equation stoichiometry before substituting energy values
calculate ΔHr as the sum of bond energies broken minus the sum of bond energies formed
retain the sign and state that values based on average bond energies are estimates

[Maximum number: 2]

Hydrogen halides are compounds formed when halogens (Group 17 elements) react with hydrogen. The bond polarity of the hydrogen halides decreases from HF to HI.

Some relevant data are shown in the table.

Table for Question 5.1.5—Bond energies and ΔHr question 1 — CAIE A-Level Chemistry AS

Hydrogen chloride undergoes a reversible reaction with oxygen.

4 \mathrm{HCl}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{Cl}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g})

The reaction is carried out at $400^{\circ} \mathrm{C}$ in the presence of a copper(II) chloride catalyst.

Use the data in the table to calculate the overall enthalpy change of reaction.

enthalpy change of reaction = $\mathrm{kJ} \mathrm{mol}^{-1}$

$\Delta_{\mathrm r}H=\Delta_{\mathrm r}H_{\mathrm{products}}-\Delta_{\mathrm r}H_{\mathrm{reactants}}=2\times(-242)-4\times(-92)$ =-116 (sign and answer)

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5.1.5—Bond energies and ΔHr question 1