Enthalpy Change, ΔH

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Understand that chemical reactions are accompanied by enthalpy changes and these changes can be exothermic (ΔH is negative) or endothermic (ΔH is positive)
  2. Construct and interpret a reaction pathway diagram, in terms of the enthalpy change of the reaction and of the activation energy
  3. Define and use the terms:
    • (a) Standard conditions (298K and 101kPa) shown by ⦵
    • (b) Enthalpy change with particular reference to: reaction (ΔH_r), formation (ΔH_f), combustion (ΔH_c), neutralisation (ΔH_neut)
  4. Understand that energy transfers occur during chemical reactions because of the breaking and making of chemical bonds
  5. Use bond energies (ΔH positive, i.e. bond breaking) to calculate enthalpy change of reaction, ΔH_r
  6. Understand that some bond energies are exact and some bond energies are averages
  7. Calculate enthalpy changes from appropriate experimental results, including the use of the relationships q = mcΔT and ΔH = –mcΔT/n

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1. What is Enthalpy and Enthalpy Change?

Enthalpy (H) is the total heat energy (or heat content) stored inside a substance at constant pressure. Think of it like the total amount of energy "trapped" in the chemical bonds and forces within a substance.

We cannot directly measure the total enthalpy of a substance. However, we can measure how much the enthalpy changes during a chemical reaction. This change is called the enthalpy change.

Enthalpy change (ΔH) is the difference between the enthalpy of the products and the enthalpy of the reactants.

Formula:

ΔH=Hproducts−Hreactants\Delta H = H_{\text{products}} - H_{\text{reactants}}

Where:

  • ΔH = enthalpy change (measured in kJ mol⁻¹)
  • H = enthalpy
  • Δ (delta) = "change in"

Understanding the System and Surroundings:

When a chemical reaction happens, energy is exchanged between two parts:

  • The system = the chemicals that are reacting (the reactants and products themselves)
  • The surroundings = everything else around the reaction (the container, the air, the thermometer, your hand if you're holding the test tube)

Energy can move from the system to the surroundings, or from the surroundings into the system.

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