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Enthalpy changes and enthalpy level diagramsEdexcel International A Level Chemistry: Revision notes

Section 1

Enthalpy change and its sign

Enthalpy change, ΔH, is the heat energy change at constant pressure. Its unit is kJ mol⁻¹.

  • Exothermic: heat is given out to the surroundings, the temperature of the surroundings rises, and ΔH is negative.
  • Endothermic: heat is taken in from the surroundings, the temperature falls, and ΔH is positive.

ΔH = H(products) – H(reactants).

Key termsenthalpy changeexothermicendothermic
Common mistake

Saying an exothermic reaction has 'a negative temperature change'. The surroundings get hotter; it is ΔH that is negative.

Section 2

Standard conditions

Enthalpy changes depend on temperature, pressure and the physical states of the substances, so values are compared under standard conditions: a pressure of 100 kPa, a stated temperature (usually 298 K), and each substance in its standard state (its physical state under these conditions).

Standard enthalpy changes are written with the symbol °, for example ΔH°, and the state symbols (s), (l), (g), (aq) must be included in equations. Elements in their standard states have ΔfH = 0.

Key termsstandard conditionsstandard state
Exam tip

Water's standard state at 298 K is liquid, so combustion equations should show H₂O(l).

Section 3

Enthalpy level diagrams

An enthalpy level diagram has enthalpy on the vertical axis. Reactants and products are drawn as horizontal levels, labelled with formulae and state symbols, joined by an arrow labelled with ΔH.

  • Exothermic: products below reactants, arrow points down, ΔH negative.
  • Endothermic: products above reactants, arrow points up, ΔH positive.

To interpret a diagram, read which level is higher, give the sign of ΔH and, for a given amount of reactant, the size of the energy change.

Key termsenthalpy level diagram
Common mistake

Drawing the arrow upwards for an exothermic reaction. For exothermic reactions the arrow points down to the lower level.

Section 4

Definitions of standard enthalpy changes

Each definition is 'under standard conditions' and the equation must match it exactly.

  • Reaction, ΔrH: the enthalpy change when the amounts of reactants shown in the equation react to give the products.
  • Formation, ΔfH: when 1 mol of a compound is formed from its elements in their standard states. Example: C(graphite) + O₂(g) → CO₂(g).
  • Combustion, ΔcH: when 1 mol of a substance is completely burned in oxygen. Example: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l).
  • Neutralisation, ΔneutH: when an acid and alkali react to form 1 mol of water. Example: H⁺(aq) + OH⁻(aq) → H₂O(l), about –57 kJ mol⁻¹ for strong acids and alkalis.
  • Atomisation, ΔatH: when 1 mol of gaseous atoms is formed from the element in its standard state. Example: ½Cl₂(g) → Cl(g).
Key termsstandard enthalpy change of formationstandard enthalpy change of combustionstandard enthalpy change of neutralisationstandard enthalpy change of atomisation
Common mistake

Removing the fractions from equations for formation and atomisation. They are needed to give exactly 1 mol of product, e.g. ½Cl₂(g) → Cl(g).

Section 5

Using the definitions in calculations

Energy change scales with the amount of substance: ΔH for a given mass = moles × ΔH per mole.

Worked example: ΔcH of methane is –890 kJ mol⁻¹. Burning 4.0 g of methane (Mr = 16.0): n = 4.0 ÷ 16.0 = 0.25 mol, so energy released = 0.25 × 890 = 222.5 kJ.

If an equation is multiplied by 2, ΔH is multiplied by 2; if it is reversed, the sign of ΔH changes.

Must Know

  • ΔH is heat change at constant pressure; exothermic negative, endothermic positive
  • Standard conditions: 100 kPa, 298 K (usually), standard states
  • Level diagram: exothermic arrow down, endothermic arrow up
  • ΔfH: 1 mol compound from elements; ΔcH: 1 mol burned completely in oxygen
  • ΔneutH: 1 mol water formed; ΔatH: 1 mol gaseous atoms formed
  • Include state symbols in every enthalpy equation

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Enthalpy changes and enthalpy level diagrams

  1. A food company sells self-heating drink cans. The base of each can holds solid calcium oxide separated from water by a seal. Pulling a tab mixes them: CaO(s) + H₂O(l) → Ca(OH)₂(s), for which ΔH = –65 kJ mol⁻¹ at constant pressure.
    Describe the enthalpy level diagram for this reaction, stating what is on the vertical axis and what the labels and arrow show.2 marks
  2. A teacher demonstrates two energy changes. First, ethanol is burned in a spirit burner; the standard enthalpy change of combustion of ethanol is –1367 kJ mol⁻¹. Second, dilute hydrochloric acid is mixed with aqueous sodium hydroxide in an insulated cup; the standard enthalpy change of neutralisation of a strong acid by a strong alkali is –57 kJ mol⁻¹.
    Write the ionic equation, including state symbols, for the neutralisation reaction, and use it to explain why the enthalpy change of neutralisation is almost the same for any strong acid and strong alkali.2 marks
  3. A chemist is studying the enthalpy changes of chlorine and its compounds. The standard enthalpy change of atomisation of chlorine is +122 kJ mol⁻¹ and the standard enthalpy change of formation of hydrogen chloride gas is –92 kJ mol⁻¹.
    Define the term standard enthalpy change of atomisation and write the equation for the standard enthalpy change of atomisation of chlorine.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).