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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 ΔHf
- 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 ΔHc
- 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 ΔHn
- 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 ΔHat /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 ΔHie1 and second ionisation enthalpy ΔHie2
- 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 ΔHea1 and second electron affinity ΔHea2
- 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.
Drafted by Eunice Wong (Chemistry)