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AS · Practice questions · Enthalpy changes and definitions

Define it exactly.

Six original Cambridge-style questions on enthalpy changes: exothermic and endothermic, activation energy, standard conditions and writing the standard enthalpy change equations.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

One mole, standard states, right sign.

01
Recall
[2 marks]

Define standard enthalpy change of formation.

  • The enthalpy change when one mole of a compound is formed from its elements in their standard states, ✓
  • under standard conditions (298 K and 101 kPa). ✓
02
Recall
[2 marks]

Define standard enthalpy change of combustion, and write the equation for methane.

  • The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions. ✓
  • CH4(g) + 2O2(g) → CO2(g) + 2H2O(l). ✓
03
Analysis
[3 marks]

Sketch and label a reaction pathway diagram for an endothermic reaction, showing ΔH and the activation energy.

  • Axes: enthalpy against progress of reaction; the products line is drawn above the reactants line. ✓
  • ΔH: arrow pointing up from reactants to products, labelled positive. ✓
  • Ea: arrow from the reactants up to the top of the hump. ✓
04
Recall
[2 marks]

Write equations, with state symbols, for the standard enthalpy change of formation of (a) ammonia and (b) sodium chloride.

  • (a) ½N2(g) + &frac32;H2(g) → NH3(g). ✓
  • (b) Na(s) + ½Cl2(g) → NaCl(s). ✓
05
Explain
[2 marks]

Explain why the standard enthalpy change of formation of an element is zero.

  • Forming an element in its standard state from itself involves no chemical change, ✓
  • so no enthalpy change occurs. ✓
06
Explain
[2 marks]

The enthalpy change of neutralisation of ethanoic acid with sodium hydroxide is −55.2 kJ mol−1, less exothermic than for hydrochloric acid (−57.1 kJ mol−1). Suggest why.

  • Ethanoic acid is a weak acid and is only partly ionised. ✓
  • Some energy is used to ionise the acid fully as H+ ions are removed, so less heat is released overall. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.