Enthalpy Change (Oxford AQA International A Level Chemistry)

Enthalpy Change

• The total chemical energy inside a substance is called the enthalpy (or heat content)

• When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes

• Enthalpy change is defined as:

  'The heat change measured under conditions of constant pressure'

• It is represented by the symbol ΔH

  • Δ = change; H = enthalpy

• Enthalpy change can be a positive or negative value depending on whether the reaction is exothermic or endothermic

• These changes can be shown on enthalpy level diagrams

Exothermic reactions

• In an exothermic reaction:

  • Heat energy is given out to the surroundings

    • The temperature of the surroundings increases

    • The products will have less energy than the reactants

    • The enthalpy change will be negative

Endothermic reactions

• In an endothermic reaction:

  • Heat energy is taken in from the surroundings

    • The temperature of the surroundings decreases

    • The products will have more energy than the reactants

    • The enthalpy change will be positive

Standard Enthalpy Changes

• To fairly compare the enthalpy changes between reactions, all reactions should be carried out under standard conditions

• These standard conditions are:

  • A pressure of 100 kPa (atmospheric pressure)

  • A temperature of 298 K (25 ^oC / room temperature)

  • Each substance involved in the reaction is in its normal physical state (solid, gas or liquid)

• To show that a reaction has been carried out under standard conditions, the symbol Ꝋ is used

  • ΔH^Ꝋ = the standard enthalpy change

• There are two common enthalpy changes you need to know about:

  • The standard enthalpy of combustion

  • The standard enthalpy of formation

• The standard enthalpy of combustion is:

  • The enthalpy change when one mole of substance is completely burned in oxygen, under standard conditions with all reactants and products in their standard states

  • The symbol is ΔH_c^Ꝋ

  • Combustion reactions are exothermic reactions so the enthalpy change of combustion will always be negative

• The standard enthalpy of formation is:

  • The enthalpy change when one mole of substance is formed from its constituent elements, under standard conditions with all reactants and products in their standard states

  • The symbol is ΔH_f^Ꝋ

  • The reactions to form substances can be exothermic or endothermic so the enthalpy change could be positive or negative

  • The standard enthalpy of formation for elements is 0

• The enthalpy change for a reaction can be measured using calorimetry