3.1 EnergeticsEdexcel IGCSE Chemistry: Revision notes
Section 1
What are exothermic and endothermic reactions?
- An exothermic reaction gives out heat energy to the surroundings, so the surrounding temperature increases. Examples: combustion, neutralisation, most oxidation reactions.
- An endothermic reaction takes in heat energy from the surroundings, so the surrounding temperature decreases. Examples: thermal decomposition, dissolving some salts (e.g. ammonium nitrate).
Section 2
How is heat energy change measured experimentally?
Simple calorimetry measures the temperature change of a known mass of a substance (usually water) as a reaction (combustion, displacement, dissolving or neutralisation) occurs, using an insulated container and a thermometer.
Method example (neutralisation): measure a known volume of acid into an insulated cup, record initial temperature, add a known volume of alkali, stir and record the highest (or lowest) temperature reached; calculate the temperature change.
The heat energy change (Q) can then be calculated using:
Q = m × c × ΔT
where m = mass of substance heated/cooled (g), c = specific heat capacity of water (4.2 J/g°C), ΔT = temperature change (°C). Q is measured in joules (J).
Always convert volume of solution (cm3) to mass (g) using density ≈ 1 g/cm3 for dilute aqueous solutions before applying Q = mcT.
Section 3
How do you calculate molar enthalpy change?
Once the heat energy change Q (in J) is found, the molar enthalpy change (ΔH) — the heat energy change per mole of a specified substance reacting — is calculated by dividing Q by the number of moles of that substance:
ΔH = Q ÷ number of moles (usually converted to kJ/mol by dividing by 1000)
ΔH is given a negative sign for exothermic reactions (energy released) and a positive sign for endothermic reactions (energy absorbed).
If 0.05 mol of a substance releases 4200 J of heat, ΔH = -4200 ÷ 0.05 = -84000 J/mol = -84 kJ/mol (negative because exothermic).
Section 4
How are energy level and reaction profile diagrams drawn?
An energy level diagram plots the energy of reactants and products:
- Exothermic: products have LOWER energy than reactants; the diagram shows a downward step, and ΔH is negative (energy released to surroundings).
- Endothermic: products have HIGHER energy than reactants; the diagram shows an upward step, and ΔH is positive (energy absorbed from surroundings).
This links to bond energies: breaking bonds requires energy input (endothermic), while making bonds releases energy (exothermic). The overall energy change of a reaction is found using:
ΔH = (total energy to break bonds in reactants) − (total energy released making bonds in products)
If more energy is released making new bonds than was used breaking old ones, the reaction is exothermic overall (and vice versa).
State clearly: bond breaking is endothermic, bond making is exothermic — examiners want this phrased exactly this way for full marks.
Must Know
- Exothermic reactions release heat (temperature of surroundings rises); endothermic reactions absorb heat (temperature of surroundings falls).
- Calorimetry measures temperature change for combustion, displacement, dissolving and neutralisation reactions.
- Q = mcT calculates heat energy change (m = mass, c = specific heat capacity, T = temperature change).
- Molar enthalpy change ΔH = Q ÷ number of moles; negative for exothermic, positive for endothermic.
- Energy level diagrams: exothermic shows products lower than reactants; endothermic shows products higher than reactants.
- Bond breaking is endothermic; bond making is exothermic; ΔH = energy to break bonds − energy released making bonds.
That's the notes covered.
Carry on to the next subtopic.