Enthalpy changes and standard conditionsEdexcel A-Level Chemistry: Revision notes
Section 1
Enthalpy change and standard conditions
Enthalpy change, ΔH, is the heat energy change measured at constant pressure. It is measured in kJ mol⁻¹.
Standard conditions let enthalpy changes be compared fairly: a pressure of 100 kPa and a specified temperature, usually 298 K. Substances must be in their standard states, the physical states they are in under those conditions (water is a liquid, oxygen is a gas). The standard enthalpy change is written ΔH⦵.
Section 2
Exothermic and endothermic changes
- Exothermic: heat energy is given out to the surroundings, so the temperature of the surroundings rises. ΔH is negative.
- Endothermic: heat energy is taken in from the surroundings, so the temperature falls. ΔH is positive.
Examples: combustion and neutralisation are exothermic; dissolving ammonium nitrate is endothermic.
The sign of ΔH is from the point of view of the reacting system. A hot beaker feels warm because the reaction is exothermic, with ΔH negative.
Section 3
Enthalpy level diagrams
An enthalpy level diagram has enthalpy on the vertical axis, the reactants on one horizontal level and the products on another.
- Exothermic: products below reactants, with a downward arrow labelled ΔH (negative)
- Endothermic: products above reactants, with an upward arrow labelled ΔH (positive)
Activation energy is shown on a reaction profile, not on an enthalpy level diagram, because the level diagram shows only the overall change between reactants and products.
Label the axis (enthalpy, H), both levels with formulae and state symbols, and the arrow with the sign and value of ΔH.
Section 4
Defining standard enthalpy changes
All definitions are under standard conditions, with all substances in their standard states:
- Reaction, ΔrH⦵: the enthalpy change when the amounts in the equation react
- Formation, ΔfH⦵: one mole of a compound is formed from its elements, e.g. H₂(g) + ½O₂(g) → H₂O(l)
- Combustion, ΔcH⦵: one mole of a substance is completely burned in oxygen, e.g. C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
- Neutralisation, ΔneutH⦵: the enthalpy change when an acid and an alkali react to form one mole of water, H⁺(aq) + OH⁻(aq) → H₂O(l), about −57 kJ mol⁻¹ for strong acid and strong base
A formation equation must make exactly one mole of the compound, so fractions such as ½O₂ are allowed. 2H₂ + O₂ → 2H₂O is not a formation equation.
Section 5
State symbols and why they matter
State symbols change the value. Forming liquid water from its elements gives −286 kJ mol⁻¹ but forming steam gives −242 kJ mol⁻¹, because 44 kJ mol⁻¹ is needed to turn liquid water into gas.
The standard enthalpy change of formation of an element in its standard state is zero.
Always give state symbols and state the conditions when quoting or using enthalpy data.
That's the notes covered.
Carry on to the next subtopic.
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
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).