Endothermic reactions

2 marks · Hard

A reaction releases 212 kJ of heat when 7.0 mol of a reactant are completely used up. Calculate the enthalpy change, , in kJ/mol for this reaction. State the sign of and what it shows about the direction of energy transfer.

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−30 kJ/mol; negative sign; energy is released to surroundings

Also accept: −30.3 kJ/mol, energy released to surroundings; Negative ΔH, exothermic reaction

Common mistakes: +30 kJ/mol (wrong sign); Missing sign or wrong direction described

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Marking points

M1 · 1 mark · Correct method: divides heat released by number of moles ()

A1 · 1 mark · Correct answer: −30 kJ/mol, negative sign explained as energy released to surroundings

Worked solution

ΔH = q / n = (−212 kJ) / 7.0 mol = −30.3 kJ/mol

The negative sign shows that energy is released to the surroundings (the reaction is exothermic).