Heat energy transferred at constant pressure, in kJ mol⁻¹.
Exothermic
Releases energy to the surroundings: ΔH negative, temperature of surroundings rises.
Endothermic
Absorbs energy from the surroundings: ΔH positive, temperature of surroundings falls.
Activation energy (Ea)
Minimum energy colliding particles need to react.
Standard conditions
100 kPa and a stated temperature, usually 298 K; solutions at 1 mol dm⁻³.
In an exothermic reaction the products are at a lower enthalpy than the reactants. Energy profiles show reactants, products, the activation energy hump and ΔH.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Three classic slips: using the reactant mass for m, forgetting to convert J to kJ, and leaving off the negative sign.
R1.2Energy cycles in reactions
Key terms
Bond enthalpy
Energy needed to break one mole of a bond in gaseous molecules (an average for most bonds).
Hess's law
The enthalpy change of a reaction is independent of the route taken.
Standard enthalpy of formation (ΔHf°)
Enthalpy change when one mole of a compound forms from its elements in their standard states.
Standard enthalpy of combustion (ΔHc°)
Enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
Breaking bonds is endothermic; making bonds is exothermic. A reaction is exothermic overall if more energy is released forming bonds than is absorbed breaking them.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Draw every bond in full before counting: a CO₂ molecule has two C=O bonds, not one.
R1.3Energy from fuels
Key terms
Complete combustion
Burning in excess oxygen, giving CO₂ and H₂O.
Incomplete combustion
Burning in limited oxygen, giving CO and/or C (soot) as well as H₂O.
Specific energy
Energy released per unit mass of fuel, in kJ g⁻¹ (or MJ kg⁻¹).
Biofuel
A fuel made from recently living material, such as bioethanol or biodiesel.
Fuel cell
A cell that converts the chemical energy of a continuous fuel supply directly into electricity.
Fossil fuels (coal, oil, natural gas) are non-renewable; burning them releases CO₂, a greenhouse gas. Carbon monoxide is toxic (binds to haemoglobin); soot causes respiratory problems.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Write combustion equations by balancing C first, then H, then O last; halves are fine (and allowed for one mole of fuel).
R1.4Entropy and spontaneity HL
Key terms
Entropy (S)
A measure of the dispersal of energy and matter (disorder) in a system, in J K⁻¹ mol⁻¹.
Gibbs energy (G)
Determines spontaneity: ΔG = ΔH − TΔS.
Spontaneous reaction
A reaction that proceeds without continuous outside input once started: ΔG < 0.
Entropy increases from solid to liquid to gas, with more moles of gas, with dissolving and with higher temperature.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Mismatched units (kJ with J) is the most common lost mark in ΔG questions.