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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

3.2.1 Enthalpy

Bond enthalpiesEnthalpy changesHess’ law and enthalpy cycles

Hess’ Law, Enthalpy Cycles and Calorimetry

Specification Reference 3.2.1 (a)–(h)

Quick Notes

  • Hess’s Law: Total enthalpy change is the same no matter the path taken.
  • Enthalpy change (ΔH): Heat energy change under constant pressure.
  • Standard conditions: 100 kPa and 298 K (25 °C).
  • Exothermic reactions release heat (ΔH negative), endothermic reactions absorb heat (ΔH positive).
  • Types of standard enthalpy change:
    • Reaction, ΔHr° – for any reaction in molar quantities given by balanced reaction equation, under standard conditions.
    • Formation, ΔHf° – from elements to 1 mole of compound.
    • Combustion, ΔHc° – 1 mole of substance completely burned in O2.
    • Neutralisation, ΔHn° – 1 mole of water formed in neutralisation.
  • Q = mcΔT used in calorimetry (specific heat capacity of water = 4.18 J g⁻¹ °C⁻¹).
  • ΔH = –Q / n converts energy change to kJ mol⁻¹.

Full Notes

Hess’s Law

Hess’s Law is based on the Law of Conservation of Energy, which states that energy cannot be created or destroyed, only transferred.

Definition of Hess’s Law:
The total enthalpy change for a reaction is the same, regardless of the route taken, provided the initial and final conditions are the same.

Hess’s Law and Enthalpy Cycles

Hess’s Law is useful when it is difficult to directly measure an enthalpy change. Instead, we use enthalpy cycles to calculate it indirectly.

Edexcel A-Level Chemistry generic Hess cycle diagram linking alternative routes between reactants and products.

You need to know how we can use Hess cycles for:

Hess’s Law for Enthalpy of Formation

The enthalpy of formation (ΔHf) is the enthalpy change when one mole of a compound is formed from its elements in their standard states.

Elements in their standard states (for example, O2(g)) have an enthalpy of formation, ΔHf, of zero — this is really important for calculations involving standard enthalpies of formation.

Formula for Hess’s Cycle using enthalpy of formation:
ΔHr = ΣΔHf (products) − ΣΔHf (reactants)

Worked Example

Calculation for the formation of CO2
Calculate the enthalpy change for the combustion of carbon, based on the following.

  1. Reaction
    C (s) + O2 (g) → CO2 (g)
  2. Given Data
    ΔHf° (CO2) = −393 kJ mol−1
    ΔHf° (O2) = 0 (since elements in their standard states have ΔHf° = 0)
  3. Using Hess’s Law
    ΔHr = ΣΔHf° (products) − ΣΔHf° (reactants)
    ΔHr = ΔHf° (CO2) − (ΔHf° (C) + ΔHf° (O2))
    ΔHr = −393 − (0 + 0) = −393 kJ mol−1

Hess’s Law for Enthalpy of Combustion

The enthalpy of combustion (ΔHc) is the enthalpy change when one mole of a substance is completely burned in oxygen.

Worked Example

WorkedExample: Find the enthalpy of formation (ΔHf) of propane (C3H8) using the following data.

  1. Reaction:
    Edexcel A-Level Chemistry Hess cycle setup for propane using combustion data.
    ΔH꜀ for C(s) = −394 kJ mol⁻¹
    ΔH꜀ for H2(g) = −286 kJ mol⁻¹
    ΔH꜀ for C3H8(g) = −2220 kJ mol⁻¹
  2. Draw a Hess Cycle:
    Showing two possible routes - we can combust the 3C(s) and 4H2(g) directly to form 3CO2(g) and 4H2O(g) or we can can combust the C3H8(g) to also form 3CO2(g) and 4H2O(g).
    Edexcel A-Level Chemistry Hess cycle diagram comparing combustion route of elements vs combustion of propane to determine ΔHf of propane.
  3. Apply Hess’s Law:
    Using Hess’s Law we know route 1 = route 2
    Meaning
    ΔH1 = ΔH? + ΔH2
    ΔH? = ΔH1 − ΔH2
    ΔH? = (−2326) − (−2220) = −106 kJ mol⁻¹

Photo of Matt
Matt’s exam tip

When drawing or using enthalpy cycles, you must ensure equations are balanced and include molar ratios when calculating enthalpy changes for each route. You may also be asked to find enthalpy changes for unfamiliar enthalpy cycles (such as with neutralisation reactions), plenty of past paper practice (especially year 1 AS papers) can help build experience with this.

Constructing and Using Hess’s Cycles

  1. Identify known enthalpy values (formation or combustion).
  2. Draw an enthalpy cycle showing the different reaction pathways.
  3. Apply Hess’s Law equation to calculate the unknown enthalpy change.

Measuring Enthalpy Changes – Calorimetry

Calorimetry is an experimental technique used to measure enthalpy changes.

The key equation used is:

q = mcΔT

where:

The enthalpy change per mole of reactant is found using:
ΔH = -q / n
where n = moles of the limiting reactant.

Assumptions made:

Measuring Enthalpy Change of Combustion (ΔHc)

Edexcel A-Level Chemistry combustion calorimetry setup with burner heating water in a metal can.

Method:

  1. Measure a known volume of water in a calorimeter (beaker or copper can).
  2. Record the starting temperature of the water.
  3. Weigh the spirit burner containing the fuel.
  4. Light the burner and allow it to heat the water.
  5. Stir and measure the final temperature of the water.
  6. Reweigh the burner to determine mass of fuel burned.
  7. Calculate q using q = mcΔT, then use ΔH = q / n.

Sources of Error:

Measuring Enthalpy Change of Neutralisation (ΔHneut)

Edexcel A-Level Chemistry polystyrene cup calorimetry for neutralisation with thermometer and stirring.

The enthalpy of neutralisation is the energy change when one mole of water is formed from an acid-alkali reaction.

Method:

  1. Use a polystyrene cup (to reduce heat loss).
  2. Add a known volume of acid and record the starting temperature.
  3. Add a known volume of alkali, stir, and record the maximum temperature.
  4. Use q = mcΔT to calculate heat energy change, then use ΔH = q / n.

Sources of Error:

Measuring Enthalpy Change of Solution (ΔHsol)

Edexcel A-Level Chemistry calorimetry for enthalpy of solution using a polystyrene cup.

Method:

  1. Add a known mass of solute to a known volume of water in a polystyrene cup.
  2. Stir and record the temperature change.
  3. Use q = mcΔT to calculate the heat energy change, then use ΔH = q / n.

Sources of Error:

Summary