1.4.1

Enthalpy Changes

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Enthalpy Changes

For a given process or reaction, we can measure the enthalpy change.

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Definition of enthalpy change

  • An enthalpy change is a measure of the heat given out or taken in during a process.
  • When objects are heated, they use energy to expand.
  • Enthalpy takes into account the energy used in the expansion.
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Enthalpy vs energy changes

  • We use enthalpy instead of energy because we cannot easily measure energy changes. This is because objects expand when heated.
  • To measure an energy change, we would have to fix the volume of the object.
  • Enthalpy is much easier to use as it allows for expansion.
    • Enthalpy changes are instead measured under constant pressure.
    • The atmosphere is at a constant pressure, so we require no extra equipment.
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Endothermic vs exothermic reactions

  • An exothermic reaction is one which gives out heat energy.
  • An exothermic reaction has a negative enthalpy change.
  • An endothermic reaction is one which takes in heat energy.
  • An endothermic reaction has a positive enthalpy change.
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Combustion is exothermic

  • Combustion is an exothermic process as it gives out heat!
    • E.g. Burning methane:
      • CH4 + 2O2 → CO2 + 2H2O
      • ΔH = −882.00kJmol-1
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Thermal decomposition is endothermic

  • Thermal decomposition is an endothermic process.
    • E.g. The thermal decomposition of calcium carbonate:
      • CaCO3 → CaO + CO2
      • ΔH = +178.30kJmol-1

Jump to other topics

1Physical Chemistry

2Physical Chemistry 2 (A2 Only)

3Inorganic Chemistry

4Inorganic Chemistry 2 (A2 Only)

5Organic Chemistry 1

6Organic Chemistry 2 (A2 Only)

6.1Optical Isomerism (A2 Only)

6.2Aldehydes & Ketones (A2 Only)

6.3Carboxylic Acids & Esters (A2 Only)

6.4Aromatic Chemistry (A2 Only)

6.5Amines (A2 Only)

6.6Polymers (A2 Only)

6.7Biological Organic (A2 Only)

6.8Organic Synthesis (A2 Only)

6.9NMR Spectroscopy (A2 Only)

6.10Chromatography (A2 Only)

6.11A-A* (AO3/4) - Organic 2

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