Enthalpy change, ΔH

AS Level Chemistry · Chemical energetics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Chemical reactions transfer energy because bonds are broken and new bonds are made. Breaking bonds takes energy in; making bonds gives energy out. The overall enthalpy change, ΔH, is the difference between the two.

In an exothermic reaction more energy is released than absorbed: ΔH is negative and the surroundings get warmer. In an endothermic reaction ΔH is positive. On a reaction pathway diagram, ΔH is the gap between reactants and products, and the activation energy is the rise from the reactants to the top of the curve.

You can find ΔH from bond energies, or from an experiment in which the reaction heats or cools a known mass of water or solution.

Rules to remember

Common mistake

Students forget the sign, or use the mass of the fuel or solid as m. A temperature rise means ΔH is negative, and m is the mass of the water or solution that changed temperature.

Worked example

Bond energies in kJ mol−1: N≡N 944, H–H 436, N–H 388. What is the enthalpy change for N2(g) + 3H2(g) → 2NH3(g)?

  1. Bonds broken: 1 N≡N and 3 H–H = 944 + (3 × 436) = 944 + 1308 = 2252 kJ.
  2. Bonds formed: 2 NH3 contain 6 N–H bonds = 6 × 388 = 2328 kJ.
  3. ΔH = bonds broken − bonds formed = 2252 − 2328 = −76 kJ mol−1.

Answer: ΔH = −76 kJ mol−1 (exothermic)

Practice questions

Try each one, then open the answer.

1. The standard enthalpy change of neutralisation is the enthalpy change when

  1. A
    one mole of acid reacts completely with an alkali
  2. B
    one mole of salt is formed from an acid and an alkali
  3. C
    one mole of hydrogen ions is added to excess water
  4. D
    one mole of water is formed by the reaction of an acid with an alkali under standard conditions
Show answer

Answer: D. ΔHneut is defined per mole of water formed. For example, one mole of H2SO4 reacting with NaOH forms two moles of water, releasing about twice ΔHneut.

2. Which statement about the reaction pathway diagram for an endothermic reaction is correct?

  1. A
    the products are at lower enthalpy than the reactants
  2. B
    the activation energy is measured from the products to the top of the curve
  3. C
    the products are at higher enthalpy than the reactants and ΔH is positive
  4. D
    ΔH is equal to the activation energy
Show answer

Answer: C. Endothermic means energy is absorbed, so the products sit above the reactants and ΔH is positive. The activation energy is always measured from the reactants up to the peak.

3. Which equation represents the standard enthalpy change of combustion of methanol?

  1. A
    2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(l)
  2. B
    CH3OH(l) + 1½O2(g) → CO2(g) + 2H2O(l)
  3. C
    CH3OH(l) + O2(g) → CO(g) + 2H2O(l)
  4. D
    C(s) + 2H2(g) + ½O2(g) → CH3OH(l)
Show answer

Answer: B. ΔHc refers to one mole of the substance burning completely in oxygen under standard conditions, with water as a liquid. The equation making CO is incomplete combustion, and the one from the elements is formation.

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