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Course: Oxford AQA International A Level Chemistry
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Oxford AQA International A Level Chemistry

1. Physical Chemistry

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2. Inorganic Chemistry

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3. Organic Chemistry

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Text lesson

1.4.1 Enthalpy Change

1.4.1 Enthalpy Change: Define enthalpy change, distinguish exothermic and endothermic reactions using enthalpy level diagrams, and recall the definitions of standard enthalpy changes.

1.4.1 Enthalpy Change
Enthalpy change, ΔH
  • The heat energy change measured at constant pressure, usually in kJ mol⁻¹.
Exothermic
  • Heat is given out to the surroundings, so the temperature of the surroundings rises and ΔH is negative.
Endothermic
  • Heat is taken in from the surroundings, so their temperature falls and ΔH is positive.
Bond breaking and making
  • Breaking bonds absorbs energy and making bonds releases energy, and the balance decides the sign of ΔH.
Enthalpy Level Diagrams
Exothermic
  • Products sit lower than reactants on the energy axis, and an arrow downwards marks a negative ΔH.
Endothermic
  • Products sit higher than reactants, and an arrow upwards marks a positive ΔH.
Activation energy
  • The energy barrier between reactants and the top of the curve is the activation energy, Ea.
Labels
  • Label the axes as enthalpy and progress of reaction, and include state symbols with the species.
Pressure
  • 100 kPa, close to atmospheric pressure.
Temperature
  • 298 K, usually written as 25 °C, with the temperature stated for the data.
States
  • Each substance is in its standard state, the physical state it normally has under these conditions.
Symbol
  • The superscript ° shows standard conditions, as in ΔH°, with 298 as a subscript if wanted.
Standard enthalpy of formation, ΔfH°
  • The enthalpy change when one mole of a compound forms from its elements in their standard states under standard conditions.
Standard enthalpy of combustion, ΔcH°
  • The enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
Standard enthalpy of neutralisation
  • The enthalpy change when an acid and a base react to form one mole of water under standard conditions.
Elements
  • The standard enthalpy of formation of an element in its standard state is zero.
Reaction
  • Standard enthalpy of reaction is the change for the molar quantities shown in the balanced equation.

1.4.1 – Key Points for the Exam

  • Enthalpy change: The heat energy change of a reaction at constant pressure.
  • Exothermic: A reaction that releases heat energy to the surroundings.
  • Endothermic: A reaction that absorbs heat energy from the surroundings.
  • Standard conditions: A pressure of 100 kPa and a stated temperature, usually 298 K.

Common Mistakes

  • Writing a positive ΔH for an exothermic reaction.
  • Forgetting that the formation of one mole of the compound is required, so fractions in the equation may be needed.
  • Saying 'heat is made' in a reaction; energy is transferred, not created.
  • Leaving out the physical states when writing equations for ΔfH° or ΔcH°.
YOUR STUDY PATH

1.4.1 Enthalpy Change

Oxford AQA International A Level Chemistry / 1. Physical Chemistry

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