Enthalpy

• a measure of the heat content of a substance at constant pressure
• you cannot measure the actual enthalpy of a substance
• you can measure an enthalpy change at constant pressure
• written as the symbol  ∆Η , “delta H ”

Standard Enthalpy change (∆Η) = Enthalpy of products - Enthalpy of reactants

• examples of exothermic reactions: resipiration, combustion of fuel
• examples of endothermic reactions: photosynthesis, thermal decomposition (e.g. of calcium carbonate)

In A-Level Chemistry, there are different types of enthalpy:

Standard enthalpy of formation ΔH_f

• Enthalpy change when one mole of a substance is formed from its elements
• With all reactants and products in their standard states
• Under standard conditions, temperature of 298K and pressure of 100kPa
• Usually, but not exclusively, exothermic

Example: 2C(graphite)+ ½O₂(g) + 3H₂(g) ——> C₂H₅OH(l)

(One mole of C₂H₅OH is formed.)

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• Elements In their standard states have zero enthalpy of formation.
• Carbon is usually taken as the graphite allotrope

Standard enthalpy of combustion ΔH_c

• The enthalpy change when one mole of a substance undergoes complete combustion under standard conditions.
• Always exothermic

Examples:

C(graphite)+ O₂(g)——> CO₂(g)

C₂H₅OH(l) + 3O₂(g)——> 2CO₂(g) + 3H₂O(l)

Standard enthalpy of neutralisation ΔH_n

• Enthalpy change when 1 mole of water is formed from its ions in dilute solution.
• H⁺(aq)  + OH ̄ (aq) ——> H2O(l)
• Exothermic

Enthalpy of atomisation ΔH_at /Bond dissociation enthalpy

• Energy required to break one mole of gaseous bonds of gaseous elements to form gaseous atoms
• Endothermic (Energy must be put in to break any chemical bond)

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• the strength of a bond depends on its environment so mean values are quoted
• for diatomic gases, the bond enthalpy is twice the enthalpy of atomisation
• the smaller the bond enthalpy, the weaker the bond and the easier it is to break

First ionisation enthalpy ΔH_ie1 and second ionisation enthalpy ΔH_ie2

• First ionisation enthalpy is the enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions
• Second ionisation enthalpy is the enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

First electron affinity ΔH_ea1 and second electron affinity ΔH_ea2

• First electron affinity is the enthalpy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous 1-ions.
• Second electron affinity is the enthalpy change when one mole of electrons is added to one mole of gaseous 1-ions to form one mole of gaseous 2-ions.

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Drafted by Eunice Wong (Chemistry)