CAIE IGCSE Chemistry Enthalpy Change from Bond Energies

Practise calculating reaction enthalpy by totalling bonds broken and formed, preserving stoichiometric multipliers and the final positive or negative sign.

Exam points

count every bond from the balanced equation and total the energy required to break them
total energy released by bonds formed and calculate broken minus formed
retain the sign when classifying the result or rearrange the energy balance for an unknown bond

[Maximum number: 4]

The chemical equation for the complete combustion of ethanol, $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}$ , is shown.

\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}+3 \mathrm{O}_{2} \rightarrow 2 \mathrm{CO}_{2}+3 \mathrm{H}_{2} \mathrm{O}

The energy released when one mole of ethanol undergoes complete combustion is 1280 kJ .
Part of the energy level diagram for this reaction is shown.

Figure for Question 5.1.8—Enthalpy change of a reaction using question 1 — CAIE IGCSE Chemistry

The chemical equation for the complete combustion of methanol, $\mathrm{CH}_{3} \mathrm{OH}$ , is shown.

2 \mathrm{CH}_{3} \mathrm{OH}+3 \mathrm{O}_{2} \rightarrow 2 \mathrm{CO}_{2}+4 \mathrm{H}_{2} \mathrm{O}

The equation can be represented as shown.

Use the bond energies in the table to determine the energy change, $\Delta H$ , for the complete combustion of one mole of methanol.

Table for Question 5.1.8—Enthalpy change of a reaction using question 1 — CAIE IGCSE Chemistry

- energy needed to break bonds kJ
- energy released when bonds are formed kJ
- energy change, $\Delta H$ , for the complete combustion of one mole of methanol kJ/mol

650 kJ / mol

M1 bonds broken

$2 \times((3 \times 410)+360+460)+(3 \times 500)$ $2 \times(1230+360+460)+1500$ $2 \times 2050+1500$

4100+1500=5600

M2 bonds formed

$(2 \times(2 \times 805))+(4 \times(2 \times 460))$ $2 \times 1610+4 \times 920$

3220+3680=6900

M3 = M1 - M2

energy change of reaction =5600-6900=-1300

M4 = M3 / 2

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5.1.8—Enthalpy change of a reaction using question 1