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Standard Enthalpy Change (CIE AS Chemistry)

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Defining Enthalpy Changes

  • To fairly compare the changes in enthalpy between reactions, all reactions should be carried out under standard conditions
  • These standard conditions are:
    • A pressure of 101 kPa
    • A temperature of 298 K (25 oC)
    • 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 θ is used
    • ΔHθ = the standard enthalpy change
  • These are a number of key definitions for common language relating to enthalpy change that all chemists need to know

Enthalpy definitions table

Standard Enthalpy Change of... Definition Symbol Exothermic /
Endothermic
Reaction The enthalpy change when the reactants in the stoichiometric equation react to form the products under standard conditions ΔHθr  Both
Formation The enthalpy change when one mole of a compound is formed from its elements under standard conditions ΔHθf  Both
Combustion The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions ΔHθc  Exothermic
Neutralisation The enthalpy change when one mole of water is formed by reacting an acid and an alkali under standard conditions ΔHθneut  Exothermic

Worked example

Calculating the enthalpy change of reaction of water

One mole of water is formed from hydrogen and oxygen, releasing 286 kJ of energy.

H2 (g) + ½O2 (g) → H2O (l)   ΔHθr = –286 kJ mol-1 

Calculate ΔHr for the reaction below:

2H2 (g) + O2 (g) → 2H2O (l)

Answer

  • The ΔHθr value of –286 kJ mol-1 is for one mole of water being formed
  • Two moles of water molecules are formed for the equation in question
  • So, the energy released is simply:
    • ΔHr  = 2 mol x (–286 kJ mol-1)
    • ΔHr  = –572 kJ mol-1 

Worked example

Calculating the enthalpy change of formation

Calculate ΔHθf of the reaction below:

4Fe (s) + 3O2 (g) → 2Fe2O3 (s)   ΔHθf [Fe2O3 (s)] = –824.2 kJ mol-1 

Answer

  • The ΔHθf value of –824.2 kJ mol-1 is for one mole of Fe2O3 (s)being formed
  • Two moles of Fe2O3 (s) are formed for the equation in question
  • So, the energy released is simply:
    • ΔHθf  = 2 mol x (–824.2kJ mol-1)
    • ΔHθf  = –1648.4 kJ mol-1 

Worked example

Calculating enthalpy changes

Identify each of the following as ΔHθr, ΔHθf, ΔHθc or ΔHθneut:

  1. MgCO3 (s) → MgO (s) + CO2 (g)
  2. C (graphite) + O2 (g) → CO2 (g)
  3. HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

Answers:

Answer 1

  • It cannot be ΔHθf as there is more than one compound being formed
  • It cannot be ΔHθc as there is no reaction with oxygen
  • It cannot be ΔHθneut as there is no acid or alkali involved 
  • This is just a chemical reaction, therefore, it is ΔHθr

Answer 2

  • ΔHθf as one mole of CO2 is formed from its elements in their standard states
    AND
    ΔHθc as one mole of carbon is burnt completely in oxygen

Answer 3

  • ΔHneut as one mole of water is formed from the reaction of an acid and alkali

Examiner Tip

The ΔHθf of an element in its standard state is zero. For example, ΔHθf of O2 (g) is 0 kJ mol-1

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Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.