Topic 15.1: Standard enthalpy changes of reaction

15.1.1 Define and apply the terms standard state, standard enthalpy change of formation and standard enthalpy change of combustion.

The standard state of a substance is the pure form of the substance under standard conditions of 298 kelvins and 1.00 10^5 Pa.

Standard enthalpy change of formation is the energy change on the formation of one mole of substance from its constituent elements in their standard states.

• It gives a measure of the stability of a substance relative to its elements
• It can also be used to calculate the enthalpy changes of all reactions, either hypothetical or real

Standard enthalpy change of combustion is the heat evolved on the complete combustion of one mole of a substance.

• This data can be used to calculate the enthalpy change of all reaction
• They can also be used more directly to compare the heat output of different fuels.

15.1.2 Determine the enthalpy change of a reaction using standard enthalpy changes of formation and combustion.

Standard enthalpy changes of combustion

Using the Hess's law, we could find Hf by using the following equation. 

Hf = Total combustion of reactants - Total combustion of products

Standard enthalpy change of formation

Using the Hess's law, we could find Hf by using the following equation.