Enthalpy Changes

Full Notes

Enthalpy change (ΔH) is the heat change during a chemical reaction at constant pressure.

Exothermic reactions release heat (ΔH is negative).
For Example: Combustion of fuels.

Endothermic reactions absorb heat (ΔH is positive).
For Example: Thermal decomposition reactions.

Enthalpy Level Diagrams

Enthalpy level diagrams shows the enthalpies of reactants and products for a given reaction:

Exothermic: products lie below reactants.

Endothermic: products lie above reactants.

The vertical difference = ΔH.

Note: These diagrams do not show activation energy, only the net enthalpy change. Reaction profile diagrams show activation energy.

Activation Energy (E_a)

Activation energy (E_a) is the minimum amount of energy required to initiate a reaction.

On an reaction profile diagram, it’s the energy ‘hump’ between reactants and the highest energy reached in the reaction.

Standard Enthalpy Terms

Standard conditions: 100 kPa, 298 K, and standard states.
All ΔH values refer to 1 mole.

• Enthalpy of Reaction (ΔH_r):
  Enthalpy change when a reaction occurs in molar quantities as shown in a balanced equation.
  Example: H₂ + ½ O₂ → H₂O, ΔH_r = −286 kJ mol⁻¹


• Enthalpy of Formation (ΔH_f):
  Enthalpy change when 1 mole of a compound is formed from its elements in standard states.
  Example: C(s) + 2H₂(g) → CH₄(g), ΔH_f = −75 kJ mol⁻¹


• Enthalpy of Combustion (ΔH_c):
  Enthalpy change when 1 mole of a substance is completely burned in oxygen.
  Example: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH_c = −890 kJ mol⁻¹


• Enthalpy of Neutralisation (ΔH_neut):
  Enthalpy change when acid and alkali form 1 mole of water.
  Example: HCl + NaOH → NaCl + H₂O, ΔH_neut = −57 kJ mol⁻¹

Calculating Enthalpy Changes from Experiments

Calorimetry is an experimental technique used to measure enthalpy changes.

The key equation is:

q = mcΔT

where:
q = heat energy change (J)
m = mass of substance heated (g)
c = specific heat capacity (J g⁻¹ K⁻¹) (for water, c = 4.18 J g⁻¹ K⁻¹)
ΔT = temperature change (K)

The enthalpy change per mole of reactant can now be calculated using:

ΔH = -q / n

where n = moles of the limiting reactant.

Matt’s exam tip

Remember the m in q = mcΔT refers to the mass of surroundings, not the mass of reactants used. You need the mass of reactants only to find moles (n) for ΔH.

Summary

• ΔH is the heat change at constant pressure.
• Exothermic = ΔH negative, Endothermic = ΔH positive.
• Activation energy is the minimum energy required for reaction.
• Standard enthalpy definitions apply under 298 K and 100 kPa.
• Calorimetry and q = mcΔT are used for experimental ΔH calculations.