Enthalpy changes and standard conditionsAQA A-Level Chemistry: Revision notes
Section 1
Exothermic and endothermic reactions
The enthalpy change, ΔH, is the heat energy change measured at constant pressure. Its units are kJ mol⁻¹.
- Exothermic: heat is released to the surroundings, the temperature of the surroundings rises and ΔH is negative (the products have less enthalpy than the reactants). Examples: combustion, neutralisation, oxidation.
- Endothermic: heat is taken in from the surroundings, the temperature falls and ΔH is positive (the products have more enthalpy). Examples: thermal decomposition, dissolving ammonium nitrate, photosynthesis.
A reaction in which the surroundings get colder is endothermic, not exothermic. The sign of ΔH refers to the system, not the surroundings.
Section 2
Standard conditions
Enthalpy changes depend on pressure, temperature and the physical states, so values are compared under standard conditions: a pressure of 100 kPa, a stated temperature (usually 298 K, written ΔH(298 K)), and every substance in its standard state, the physical state it has under those conditions (e.g. H₂O(l), O₂(g), C(s) as graphite).
Solutions are at a concentration of 1 mol dm⁻³.
Always state symbols in enthalpy equations. The enthalpy change differs for H₂O(l) and H₂O(g).
Section 3
Standard enthalpy of formation, ΔfH
The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
Equation rules: one mole of the compound on the right, only elements in their standard states on the left, with fractions allowed.
Example for ethanol: 2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l).
The ΔfH of an element in its standard state is zero.
Section 4
Standard enthalpy of combustion, ΔcH
The standard enthalpy of combustion is the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions, with all reactants and products in their standard states.
Equation rules: one mole of the fuel on the left, oxygen in excess, products CO₂(g) and H₂O(l).
Example: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l), ΔcH = −1367 kJ mol⁻¹. Combustion is always exothermic.
Writing H₂O(g) in a combustion equation, or doubling the equation to avoid a fraction, does not give the standard enthalpy of combustion for one mole.
Section 5
Telling the definitions apart
Ask two questions of any equation:
- Does it make one mole of a compound from its elements? Then it is a formation equation.
- Does one mole of a substance burn completely in oxygen? Then it is a combustion equation.
Some equations are both: C(s) + O₂(g) → CO₂(g) is the formation of CO₂ and the combustion of carbon. H₂(g) + ½O₂(g) → H₂O(l) is the formation of water and the combustion of hydrogen. Neither applies to partial combustion such as C(s) + ½O₂(g) → CO(g), which is only a formation.
Section 6
Using enthalpy values
Values are given per mole, so scale for other amounts: energy = moles × ΔH. Moles = mass ÷ M.
Example: energy released when 5.0 g of methane (M 16.0) burns, ΔcH = −890 kJ mol⁻¹: moles = 5.0 ÷ 16.0 = 0.3125 mol, energy = 0.3125 × 890 = 278 kJ released. State the answer as released (or ΔH negative); ΔH for the reaction is −278 kJ for that amount.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Enthalpy changes and standard conditions
- An instant cold pack used for sports injuries contains solid ammonium nitrate and a separate bag of water. When the bag is broken and the solid dissolves, the temperature of the mixture falls. An instant hand warmer works in the same way using calcium chloride, and the temperature rises. Both are measured at constant pressure.Explain why the temperature of the cold pack falls when the ammonium nitrate dissolves.2 marks
- Ethanol, C₂H₅OH, is used as a biofuel. Its standard enthalpy of combustion is −1367 kJ mol⁻¹. Standard enthalpy changes refer to 100 kPa and 298 K.Calculate the enthalpy change when 2.00 mol of ethanol burn completely under standard conditions, and state what the sign of the value shows.2 marks
- Hydrogen is a possible clean fuel for vehicles. Under standard conditions, H₂(g) + ½O₂(g) → H₂O(l) has ΔH = −286 kJ mol⁻¹, and the standard enthalpy of combustion of methane, the main component of natural gas, is −890 kJ mol⁻¹. The relative molecular masses are 2.0 for hydrogen and 16.0 for methane.Define standard enthalpy of combustion, and explain why the value for hydrogen is also the standard enthalpy of formation of liquid water.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).