Standard Enthalpy Change

The standard enthalpy change for a chemical reaction, ΔH^Ꝋ_, refers to the heat transferred at constant pressure under standard conditions and states
These standard conditions are:
- A pressure of 100 kPa
- A concentration of 1 mol dm^-3for all solutions
- Each substance involved in the reaction is in its standard state (solid, gas or liquid)
Temperature is not part of the definition of standard state, but a temperature of 298 K (25 ^oC) is usually given as the specified temperature
To show that a reaction has been carried out under standard conditions, the symbol ⦵ is used
- Eg. ΔH^Ꝋ = the standard enthalpy change

There are a few Standard Enthalpy changes which are used commonly in energy calculations and they are summarised below:

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

Practice your understanding of enthalpy changes on the following worked examples:

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

H₂ (g) + ½O₂ (g) → H₂O (l) ΔH^Ꝋ_r= -286 kJ mol^-1

Calculate ΔH_r for the reaction below:

2H₂ (g) + O₂ (g) → 2H₂O (l)

Answer:

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

ΔH_r = 2 mol x (-286 kJ mol^-1)

= - 572 kJ

Calculate ΔH_r for the reaction below

4Fe (s) +O₂ (g) → 2Fe₂O₃ (s)

given that ΔH^Ꝋ_f[Fe₂O₃ (s)] = - 824 kJ mol^-1

Answer:

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

ΔH_f = 2 mol x ( -824 kJ mol^-1)

= - 1648 kJ

Identify each of the following as ΔH^Ꝋ_r, ΔH^Ꝋ_f, ΔH^Ꝋ_c or ΔH^Ꝋneut

Answers:

Answer 1: ΔH^Ꝋ_r
Answer 2: ΔH^Ꝋ_f as one mole of CO₂ is formed from its elements in standard state and ΔH^Ꝋ_c as one mole of carbon is burnt in oxygen
Answer 3: ΔH^Ꝋ_neut as one mole of water is formed from the reaction of an acid and alkali

You need to learn well the Standard Enthalpy change definitions as they are frequently tested in exam papers

Standard Enthalpy Change (HL IB Chemistry)