4 Ways To Represent Enthalpy Changes
** remember that –ve is exothermic and +ve is endothermic **
1) Including the energy value as a term in the thermochemical equation
Endothermic: in electrolysis of water, energy is absorbed
H2O(l) + 285.8 kJ  H2(g) + ½ O2 (g)
Exothermic: energy is released when magnesium burns in oxygen
Mg(s) + ½ O2 (g)  MgO(s) + 601.6 kJ
Example:
Write a thermochemical equation to represent the exothermic
reaction that occurs when 2 mol of butane burn in excess oxygen gas.
The molar enthalpy of combustion is -2871 kJ/mol.
2)
a) Writing a balanced chemical equation and stating its enthalpy
change beside it
*make sure that ΔH has the correct symbol
CO(g) + H2(g)  CH3OH (l)
ΔH = -128.6 kJ
Notice that these are written in kJ not kJ/mol … this is because the
moles have already been taken into account with the given equation.
Example:
Sulfur dioxide and oxygen react to form sulfur trioxide. The molar
enthalpy for the combustion of sulfur dioxide, ΔHcomb, in this reaction is 98.9kJ/mol SO2. What is the enthalpy change for this reaction?
b) Stating the molar enthalpy of a specific reaction
-Where one mole of a particular reactant or product is specified.
CO(g) + H2(g)  CH3OH (l)
ΔHf = -128.6 kJ/mol CH3OH
Example: Write an equation whose enthalpy change is the molar enthalpy
of reaction of calcium with hydrochloric acid to produce hydrogen gas and
calcium chloride solution.
3) Drawing a chemical potential energy diagram to represent the energy
transferred in a reaction
Potential energy: stored in the relative positions of particles and in
the bonds between them.
Examples: