Edexcel IGCSE Chemistry 3.4 Bond energy calculations

Practise converting heat energy change into molar enthalpy change using masses, Mr values, moles, units and the correct sign.

Exam points

calculate moles from mass and Mr before dividing Q by amount of substance
convert J to kJ and give ΔH in kJ/mol with the correct sign
apply negative signs for exothermic combustion, neutralisation or displacement data

[Maximum number: 4]

A student investigates the reaction between zinc and copper(II) sulfate solution.
This is the equation for the reaction.

\mathrm{Zn}(\mathrm{~s})+\mathrm{CuSO}_{4}(\mathrm{aq}) \rightarrow \mathrm{ZnSO}_{4}(\mathrm{aq})+\mathrm{Cu}(\mathrm{~s})

This is the student's method.
Step 1 add $25.0 \mathrm{~cm}^{3}$ of copper(II) sulfate solution to a glass beaker
Step 2 record the temperature of the solution
Step 3 add between 5 g and 6 g of zinc powder (an excess) and stir the mixture
Step 4 record the highest temperature reached

The student repeats the experiment and calculates Q as 3800 J.

The solution used contains 6.38 g of copper(II) sulfate.
Calculate the molar enthalpy change, $\Delta H$ , in kJ / mol.
Include a sign in your answer.
[for $\mathrm{CuSO}_{4}, M_{\mathrm{r}}=159.5$ ]

- moles of $\ce{CuSO4}$ = 6.38 / 159.5 = 0.04 mol
- 3800 / 0.04 = 95000
- divide by 1000 = 95 kJ
- molar enthalpy change = -95 kJ/mol

Allow:
- ECF from moles
- ECF from energy per mole

Marking guidance:
- Correct answer -95 scores 4.
- ECF -79 scores 3 if using the energy change from 10(b)(i).

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3.4 Bond energy calculations question 1