Exothermic reactions

2 marks · Easy

For an exothermic reaction, the reactants have an energy of 90 kJ/mol and the products have an energy of 55 kJ/mol.

Calculate the energy change (ΔH) for this reaction. State whether it is exothermic or endothermic.

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ΔH = -35 kJ/mol; exothermic kJ/mol (±0)

Must show: ΔH, negative sign, correct units, identified as exothermic

Common mistakes: ΔH positive; Forgetting negative sign

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Marking points

M1 · 1 mark · States or calculates ΔH = products energy - reactants energy = 55 - 90 = -35 kJ/mol.

A1 · 1 mark · Identifies this as exothermic (negative ΔH).

Worked solution

ΔH = energy of products - energy of reactants = 55 kJ/mol - 90 kJ/mol = -35 kJ/mol

Since ΔH is negative, the reaction is exothermic.