Enthalpy Changes



  • Enthalpy is a measure of the energy content of a system
  • We cannot measure the absolute value of the enthalpy of a real system, but we can measure enthalpy changes that occur when a chemical or physical change takes place
  • Processes can be classified as exothermic (heat given out) or endothermic (heat taken in)

An exothermic reaction produces a lower energy state. The energy that is no longer locked up in the system is released as heat, so the temperature rises

  • e.g. Combustion of fuels and respiration

An endotermic reaction produces a higher energy state. Energy needs to be taken in from the surroundings, so either the reaction must be actively heated or temperature falls

  • e.g. Thermal decomposition of calcium carbonate and photosynthesis
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Standard conditions

  • The enthalpy change in a reaction varies somewhat if changes are made to conditions such as temperature and pressure
  • Standard conditions, under which standard enthalpy changes can be measured and recorded are;

Pressure = 1 atmosphere

Temperature = 25 degrees centigrade

Enthalpy change of reaction

  • Standard enthalpy change of reaction is the enthalpy change when the number of moles shown in the equation react to form the products , under standard conditions

Enthalpy change of combustion

  • Standard enthalpy change of combustion is the enthalpy change when one mole of a substance is burned completely in oxygen, under standard conditions
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Enthalpy change of formation

  • Standard enthalpy change of formation of a substance is the enthalpy change when one mole of a substance is formed from it's elements under standard conditions

Reversible reaction

  • If the enthalpy change for a reaction is known, then the enthalpy change for the reverse reaction has the same magnitude but with the opposite sign

Determining enthalpy changes experimentally

Heat evolved = mC(delta)T

m is the mass of substance

c is the specific heat capacity

(delta)T is the temperature change

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Finding enthalpy change

Summary of steps

  • Find heat change (J) = mC(delta)T
  • Convert units of heat change to kJ: divide by 1000
  • Find heat change per mol: divide by mol reacting
  • Decide on sign of enthalpy change (exothermic and endothermic)
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