What is the sign of ΔH for an exothermic reaction?

Negative: heat is given out and the surroundings get hotter.

What are standard conditions?

A pressure of 100 kPa and a stated temperature, usually 298 K.

Where are products on the enthalpy level diagram of an exothermic reaction?

Below the reactants, with the ΔH arrow pointing down.

Define standard enthalpy change of formation.

The enthalpy change when 1 mol of a compound is formed from its elements in their standard states under standard conditions.

Define standard enthalpy change of combustion.

The enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions.

Define standard enthalpy change of neutralisation.

The enthalpy change when an acid and an alkali react to form 1 mol of water under standard conditions.

Define standard enthalpy change of atomisation.

The enthalpy change when 1 mol of gaseous atoms is formed from an element in its standard state under standard conditions.

What is the standard state of water at 298 K?

Liquid, so combustion equations show H₂O(l).

Ionic equation for neutralisation of a strong acid by a strong alkali?

H⁺(aq) + OH⁻(aq) → H₂O(l)

Why are state symbols needed in enthalpy equations?

The enthalpy change depends on the physical states of the substances.

Enthalpy changes and enthalpy level diagramsEdexcel International A Level Chemistry: Flashcards

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What is enthalpy change, ΔH?

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What is enthalpy change, ΔH?: The heat energy change at constant pressure.
What is the sign of ΔH for an exothermic reaction?: Negative: heat is given out and the surroundings get hotter.
What are standard conditions?: A pressure of 100 kPa and a stated temperature, usually 298 K.
Where are products on the enthalpy level diagram of an exothermic reaction?: Below the reactants, with the ΔH arrow pointing down.
Define standard enthalpy change of formation.: The enthalpy change when 1 mol of a compound is formed from its elements in their standard states under standard conditions.
Define standard enthalpy change of combustion.: The enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions.
Define standard enthalpy change of neutralisation.: The enthalpy change when an acid and an alkali react to form 1 mol of water under standard conditions.
Define standard enthalpy change of atomisation.: The enthalpy change when 1 mol of gaseous atoms is formed from an element in its standard state under standard conditions.
Write the equation for ΔatH of chlorine.: ½Cl₂(g) → Cl(g)
What is the standard state of water at 298 K?: Liquid, so combustion equations show H₂O(l).
What is ΔfH of an element in its standard state?: Zero.
Ionic equation for neutralisation of a strong acid by a strong alkali?: H⁺(aq) + OH⁻(aq) → H₂O(l)
Why are state symbols needed in enthalpy equations?: The enthalpy change depends on the physical states of the substances.

Exam questions on Enthalpy changes and enthalpy level diagrams

A food company sells self-heating drink cans. The base of each can holds solid calcium oxide separated from water by a seal. Pulling a tab mixes them: CaO(s) + H₂O(l) → Ca(OH)₂(s), for which ΔH = –65 kJ mol⁻¹ at constant pressure.
Describe the enthalpy level diagram for this reaction, stating what is on the vertical axis and what the labels and arrow show.2 marks
A teacher demonstrates two energy changes. First, ethanol is burned in a spirit burner; the standard enthalpy change of combustion of ethanol is –1367 kJ mol⁻¹. Second, dilute hydrochloric acid is mixed with aqueous sodium hydroxide in an insulated cup; the standard enthalpy change of neutralisation of a strong acid by a strong alkali is –57 kJ mol⁻¹.
Write the ionic equation, including state symbols, for the neutralisation reaction, and use it to explain why the enthalpy change of neutralisation is almost the same for any strong acid and strong alkali.2 marks
A chemist is studying the enthalpy changes of chlorine and its compounds. The standard enthalpy change of atomisation of chlorine is +122 kJ mol⁻¹ and the standard enthalpy change of formation of hydrogen chloride gas is –92 kJ mol⁻¹.
Define the term standard enthalpy change of atomisation and write the equation for the standard enthalpy change of atomisation of chlorine.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).