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Enthalpy of Reaction

Chemistry: IB Diploma Higher Level, topic 4.2.1

Enthalpy of Reaction

The enthalpy change of a reaction tells us the amount of heat energy released into the surroundings in an exothermic reaction or absorbed from the surroundings in an endothermic reaction.

Enthalpy change

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.
  • The enthalpy change of a reaction is represented using ΔH.
Enthalpy vs energy

Enthalpy vs energy

  • 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.
Endothermic vs exothermic

Endothermic vs exothermic

  • A negative value of an enthalpy change, ΔH < 0, indicates an exothermic reaction.
  • A positive value, ΔH > 0, indicates an endothermic reaction.
  • If the direction of a chemical equation is reversed, the arithmetic sign of its ΔH is changed.
  • So, a process that is endothermic in one direction is exothermic in the opposite direction.
Molar enthalpy changes

Molar enthalpy changes

  • The ΔH value indicates the amount of heat associated with the reaction involving the number of moles of reactants and products as shown in the chemical equation.
  • For example, consider this equation:
  • H2(g)+½O2(g)⟶H2O(l)                   Δ𝐻=−286kJ
  • This equation indicates that when 1 mole of hydrogen gas and ½ mole of oxygen gas change to 1 mole of liquid water 286 kJ of heat are released to the surroundings.
  • This is assuming constant temperature and pressure throughout the reaction.
Changing coefficients

Changing coefficients

  • If the coefficients of the chemical equation are multiplied by some factor, the enthalpy change must be multiplied by that same factor (ΔH is an extensive property).
  • For example if we double the coefficients of the previous equations, the enthalpy change also doubles:
  • 2H2(g)+ O2(g)⟶2H2O(l)                   Δ𝐻= (2 x −286kJ) = - 572 kJ
State symbols

State symbols

  • The enthalpy change of a reaction depends on the physical states of the reactants and products, so these must be shown.

  • For example, if gaseous rather than liquid water is formed in the reaction mentioned previously, only 242 kJ of heat are released:

  • H2(g)+½O2(g)⟶H2O(g)                   Δ𝐻=−242kJ

Test yourself on Enthalpy of Reaction

  • Which of the following statements about enthalpy changes are true?

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