Standard Enthalpy Change
- The standard enthalpy change for a chemical reaction, ΔHꝊ, 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-3 for 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. ΔHꝊ = the standard enthalpy change
Standard Enthalpies
- 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 | ΔHꝊr |
Both |
| Formation | The enthalpy change when one mole of a compound is formed from its elements under standard conditions | ΔHꝊf |
Both |
| Combustion | The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions | ΔHꝊc |
Exothermic |
| Neutralisation | The enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions |
ΔHꝊneut |
Exothermic |
- Practice your understanding of enthalpy changes on the following worked examples:
Worked example
One mole of water is formed from hydrogen and oxygen releasing 286 kJ
H2 (g) + ½O2 (g) → H2O (l) ΔHꝊr= -286 kJ mol-1
Calculate ΔHr 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:
ΔHr = 2 mol x (-286 kJ mol-1)
= - 572 kJ
Worked example
Calculate ΔHr for the reaction below
4Fe (s) +O2 (g) → 2Fe2O3 (s)
given that ΔHꝊf [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:
ΔHf = 2 mol x ( -824 kJ mol-1)
= - 1648 kJ
Worked example
Identify each of the following as ΔHꝊr, ΔHꝊf, ΔHꝊc or ΔHꝊneut
- MgCO3 (s) → MgO (s) + CO2 (g)
- C (graphite) + O2 (g) → CO2 (g)
- HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
Answers:
- Answer 1: ΔHꝊr
- Answer 2: ΔHꝊf as one mole of CO2 is formed from its elements in standard state and ΔHꝊc as one mole of carbon is burnt in oxygen
- Answer 3: ΔHꝊneut as one mole of water is formed from the reaction of an acid and alkali
Examiner Tip
You need to learn well the Standard Enthalpy change definitions as they are frequently tested in exam papers
