15.1: Standard Enthalpy Changes of Reaction
15.1.1: Define and apply the terms standard state, standard enthalpy change of formation (ΔHfº) and standard enthalpy change of combustion (ΔHcº)
Standard conditions are defined as a temperature of 298K and an atmospheric pressure of 101.3 kPa. The standard state of a substance is its state under standard conditions.
The standard enthalpy of formation is the energy change when 1 mole of a compound is formed from its constituent elements in their standard states. This can be represented by an equation of formation:
The standard enthalpy of formation of sulphuric acid:
H2(g) + S(s) + 2O2(g) H2SO4(l) ΔH = -900 kJ mol-1
The standard enthalpy of combustion is the energy released when 1 mole of a compound burns in excess air or oxygen. This can be represented by an equation:
The standard enthalpy of combustion of ethene:
CH2=CH2(g) + 3O2(g) 2CO2(g) + 2H2O(l) ΔH = -900 kJ mol-1
15.1.2: Determine the enthalpy change of a reaction using standard enthalpy changes of formation and combustion.
We can apply Hess law to reactions using the elements as an alternative route from reactants to products. Changing the reactants to their constituent elements at standard state is the reverse of the enthalpy of formation definition. Consequently the energy change is equal to the reverse of the reactants formation enthalpy changes (i.e. with the opposite sign)
Hence, we can say that the enthalpy of reaction is equal to the enthalpy of formation of the products - the enthalpy of formation of the reactants.
Reaction enthalpy = sum of the products formation enthalpies - the sum of the reactants formation enthalpies.
It is also possible when dealing with flammable substances to go from reactants to products via the combustion products:
In this case step 1 (from the reactants to the combustion products) is equal to the enthalpy of combustion of the reactants. However, step 2 is the reverse of the enthalpy of combustion of the products. Hence, to calculate reaction enthalpy we must use the actual reactant combustion enthalpies plus the reverse of the product reaction enthalpies (i.e. the sign must be changed)
Reaction enthalpy = sum of the reactants combustion enthalpies - the sum of the products combustion enthalpies.