Enthalpy of Reaction
- The standard enthalpy change for a chemical reaction, ΔH°, 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 M 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 25 oC (298 K) 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° = 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 | ΔH°r |
Both |
| Formation | The enthalpy change when one mole of a compound is formed from its elements under standard conditions | ΔH°f |
Both |
| Combustion | The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions | ΔH°c |
Exothermic |
| Neutralisation | The enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions |
ΔH°neut |
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) ΔH°r= -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)
- ΔHr = - 572 kJ
Worked example
Calculate ΔHr for the reaction below
4Fe (s) +O2 (g) → 2Fe2O3 (s)
given that ΔH°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:
- ΔHf = 2 mol x ( -824 kJ mol-1)
- ΔHf = - 1648 kJ
Worked example
Identify each of the following as ΔH°r, ΔH°f, ΔH°c or ΔH°neut
- MgCO3 (s) → MgO (s) + CO2 (g)
- C (graphite) + O2 (g) → CO2 (g)
- HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
Answers:
- Answer 1: ΔH°r
- Answer 2: ΔH°f as one mole of CO2 is formed from its elements in standard state and ΔH°c as one mole of carbon is burnt in oxygen
- Answer 3: ΔH°neut as one mole of water is formed from the reaction of an acid and alkali
