What is enthalpy change?

The heat energy change measured at constant pressure.

On an enthalpy level diagram for an exothermic reaction, where are the products?

Below the reactants, with a downward ΔH arrow.

Why is activation energy not shown on an enthalpy level diagram?

It shows only the overall change between reactants and products; activation energy is shown on a reaction profile.

Define standard enthalpy change of formation.

The enthalpy change when one mole 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 one mole 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 one mole of water under standard conditions.

Write the equation for the standard enthalpy change of formation of water.

H₂(g) + ½O₂(g) → H₂O(l)

Why must state symbols be given in enthalpy equations?

The enthalpy change differs with state, e.g. −286 kJ mol⁻¹ for H₂O(l) and −242 kJ mol⁻¹ for H₂O(g).

Enthalpy changes and standard conditionsEdexcel A-Level Chemistry: Flashcards

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What are standard conditions?

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What are standard conditions?: A pressure of 100 kPa and a specified temperature, usually 298 K.
What is enthalpy change?: The heat energy change measured at constant pressure.
What is the sign of ΔH for an exothermic reaction?: Negative.
What is the sign of ΔH for an endothermic reaction?: Positive.
On an enthalpy level diagram for an exothermic reaction, where are the products?: Below the reactants, with a downward ΔH arrow.
Why is activation energy not shown on an enthalpy level diagram?: It shows only the overall change between reactants and products; activation energy is shown on a reaction profile.
Define standard enthalpy change of formation.: The enthalpy change when one mole 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 one mole 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 one mole of water under standard conditions.
Write the equation for the standard enthalpy change of formation of water.: H₂(g) + ½O₂(g) → H₂O(l)
What is the standard enthalpy change of formation of an element in its standard state?: Zero.
What is the unit of standard enthalpy change?: kJ mol⁻¹
Why must state symbols be given in enthalpy equations?: The enthalpy change differs with state, e.g. −286 kJ mol⁻¹ for H₂O(l) and −242 kJ mol⁻¹ for H₂O(g).

Exam questions on Enthalpy changes and standard conditions

Hydrogen reacts with oxygen to form water, both in fuel cells and in combustion engines. The standard enthalpy change of formation of liquid water is −286 kJ mol⁻¹.
Define the term standard enthalpy change of formation.2 marks
A company makes two types of instant pack. The hot pack contains calcium oxide, which reacts with water in an exothermic reaction. The cold pack contains ammonium nitrate, which dissolves in water in an endothermic process with ΔH = +25.7 kJ per mole of ammonium nitrate.
A cold pack contains 32.0 g of ammonium nitrate, NH₄NO₃ (Mr = 80.0). Calculate the energy absorbed, in kJ, when all of it dissolves.2 marks
The standard enthalpy change of combustion of ethanol is −1367 kJ mol⁻¹, and the standard enthalpy change of neutralisation for a strong acid reacting with a strong alkali is −57 kJ mol⁻¹.
Define the term standard enthalpy change of combustion and write the equation, including state symbols, for the standard enthalpy change of combustion of ethanol.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).