Enthalpy changes

• Chemistry
• Secondary Education
• Age 12,13

Enthalpy Change

07 December 2023

Enthalpy

• total chemical energy inside a substance/heat content

Enthalpy change

• happens when chemical reactions take place, changes in chemical energy take place 
• ΔH (Δ= change; H = enthalpy)
• An enthalpy change can be positive or negative

Exo vs Endothermic Reactions

Exo vs Endothermic Reactions

Reaction pathway diagrams

• A diagram that shows the energies of the reactants, the transition state(s) and the products of the reaction with time
• Transition state = stage during the reaction when chemical bonds are partially broken and formed.

Reaction pathway diagrams

• The transition state is very unstable – it cannot be isolated and is higher in energy than the reactants and products.
• The activation energy (Ea) is needed to reach the transition state.

Reaction pathway diagrams

• Activation energy = ‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction.

Exothermic Reactions

Endothermic Reactions

Sample

• CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)

Sample

Standard Enthalpy Change

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

Standard Enthalpy Change

Sample

• Since two moles of water molecules are formed in the question above, the energy released is:

• ΔHrꝊ = 2 mol x (-286 kJ mol -1)
• =   - 572 kJ mol -1

Sample

• Since two moles of Fe2O3 (s) are formed the total change in enthalpy for the reaction above is:

• ΔHfꝊ = 2 x (-824.2 kJ mol-1)
• = - 1648 kJ mol -1

Sample

• Answer 1: ΔHr Ꝋ
• Answer 2: ΔHf Ꝋ as one mole of CO2 is formed from its elements in standard state and ΔHcꝊ as one mole of carbon is burnt in oxygen
• Answer 3: ΔHneutꝊ as one mole of water is formed from the reaction of an acid and alkali

Type two things you learned today.