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Definitions of enthalpy changes (A Level Chemistry Chemical Energetics)

Define enthalpy changes for A Level Chemistry

Students are expected to know the definition of all the definitions of enthalpy changes tested in A Level Chemistry Chemical Energetics. While questions that involve defining these enthalpy changes are usually 1 mark, it is still essential to know these definitions. You will need to know the definitions in order to proceed with your calculations.

So, in this post, let’s look at all the different enthalpy change definitions in A Level Chemistry Chemical Energetics.

I’ve divided this post into 2 part:

Part 1, we look at the various definitions of enthalpy changes tested in A Level Chemical energetics.

Part 2, we look at what we write for each definitions, so that it’s easier for you to remember them.

Definitions of enthalpy changes

Standard enthalpy change of reaction ( ΔHrθ ) is the energy change in a chemical reaction when the molar quantities of reactants stated in the chemical equation react at 298K and 1 bar.

Standard enthalpy change of combustion (ΔHcθ) is the energy released when 1 mole of the substance is completely burnt in excess oxygen at 298K and 1 bar.

Standard enthalpy change of formation (ΔHfθ) is the energy absorbed or released when 1 mole of the pure substance in a specified state is formed from its constituent elements at 298K and 1 bar.

Standard enthalpy change of neutralisation (ΔHneutθ) is the energy change when an acid and a base react to form one mole of water at 298K and 1 bar.

Standard enthalpy change of hydration (ΔHhydθ) is the energy released when one mole of the gaseous ion is hydrated at 298K and 1 bar.

Standard enthalpy change of solution (ΔHsolθ) is the energy change when one mole of the substance is completely dissolved in a solvent to form an infinitely dilute solution at 298 K and 1 bar.

Lattice energy (LE) of an ionic compound is the energy released when one mole of the solid ionic compound is formed from its constituent gaseous ions at 298 K and 1 bar.

Bond Energy (BE) of a bond is the average energy absorbed when 1 mole of the bonds are broken in the gaseous state.

Standard enthalpy change of solution (ΔHatomθ) of elements is the energy absorbed when one mole of gaseous atoms are formed from the element at 298 K and 1 bar.

Standard enthalpy change of solution (ΔHatomθ) of compounds is the energy absorbed when gaseous atoms are formed from one mole of the compound at 298 K and 1 bar.

First ionisation energy (1st IE) of an element is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of singly charged positive ions.

Second ionisation energy (2nd IE) of an element is the energy required to remove one mole of electrons from one mole of singly charged positive gaseous ions to form one mole of double charged positive gaseous ions.

1st electron affinity of an element is the energy change when one mole of gaseous atoms acquires one mole of electrons to form one mole of singly charge negative gaseous ions.

2nd electron affinity of an element is the energy absorbed when one mole of singly charged negative gaseous ions acquires one mole of electrons to give one mole of doubly charged negative gaseous ions.

Keywords in the definition of enthalpy changes

If you look at all the definitions above, they generally involve writing these keywords or phrases:

Energy change/ absorbed or released

Do you notice the definition always starts with energy change, absorbed or released?

We use energy change if the enthalpy change could be exothermic or endothermic e.g. enthalpy change of reaction and enthalpy change of solution.

The term energy released is used if it’s always exothermic e.g. enthalpy change of formation and enthalpy change of hydration.

We use energy absorbed if it’s always endothermic e.g. bond energy and enthalpy change of atomisation.

1 mole

In all definitions, you see the word 1 mole. For instance, for enthalpy change of combustion, it is one mole of the substance burnt. Make sure you know for each definition, it’s one mole of what.

What happens

Each enthalpy change involves a type of reaction, and the definition describes the type of reaction. For instance, enthalpy change of hydration is to form hydrated ions from gaseous ions.

Standard conditions of 298 K and 1 bar

Take a look at all the definitions above, not all ends with 298 K and 1 bar. Only definitions for standard enthalpy change ends of with 298 K and 1bar. As such, as long as you see the word “standard”, your definition must end off with what standard conditions are – 298 K and 1 bar.

Learn about Chemical Energetics

Now that you have learnt the definitions of the different types of enthalpy change, it’s time to apply them to questions. I If you would like to learn the topic on enthalpy change and chemical energetics in detail, do check out our course on Chemical Energetics here.

Learn how the different types of enthalpy changes in detail, try questions to reinforce your understanding. We go through concepts in detail and questions step-by-step so that you leave the course being prepared for your test or exams on Chemical Energetics.

chemical energetics course for a level chemistry

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