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Chemistry · 5.1 Exothermic & endothermic · Paper 5/6 practical

Energy Changes. Feel the heat.

Mix reactants in an insulated polystyrene cup and follow the temperature. A rise means the reaction is exothermic; a fall means it is endothermic. Measure Δθ, then calculate the energy released or absorbed with q = mcΔθ and the molar enthalpy change.

0620 Topic 5.1 — Exothermic & endothermic q = mcΔθ Paper 5/6 — Practical
Setup — choose a reaction, then mix and read the thermometer every 30 s.
00:00
θ 20.0 °C

Shortcuts Space mix · Enter record · R reset.

Variables

exo
1.00
70%

Live readouts

Temperature θ
20.0 °C
Max change Δθ
0.0 °C
Energy q = mcΔθ
0 J
ΔH (experimental)
— kJ/mol
ΔH (data book)
— kJ/mol
Type
—
m = mass of solution ≈ 50 g, c = 4.18 J/g·°C. Heat loss makes |ΔH| a little smaller than the true value.

θ vs time — read every 30 s

Mix the reactants and record θ every 30 s.

θ vs t — extrapolate to find max Δθ

📋 Method (Cambridge practical procedure)
  1. Measure a fixed volume of the first solution into a polystyrene cup standing in a beaker for support; record its starting temperature.
  2. Add the second reactant, stir with the thermometer and start the stopwatch.
  3. Record the temperature every 30 s for several minutes, through and beyond the peak (or trough).
  4. Plot temperature against time and extrapolate the cooling line back to the moment of mixing to find the true maximum Δθ.
  5. Calculate the heat change q = mcΔθ (m = mass of solution, c = 4.18 J/g·°C).
  6. Divide by the moles of the limiting reactant to find ΔH per mole; quote the sign (− exo, + endo).
⚠ Sources of error & precautions
  • Heat loss to the surroundings — use a lid and a polystyrene cup; extrapolate the graph to correct for it. This is why measured |ΔH| < true value for exothermic reactions.
  • Heat absorbed by the cup and thermometer is ignored — a small systematic error.
  • Stir well for an even temperature before reading.
  • Assume the solution has the density and specific heat capacity of water.
  • Use excess of one reactant so the other is fully used up (the limiting reactant fixes the moles).
🎯 Syllabus reference (0620)
  • 5.1 Exothermic and endothermic reactions — describe reactions as exothermic or endothermic from temperature change; interpret reaction pathway diagrams; state that bond breaking is endothermic and bond making is exothermic.
  • 12.1 — measurement of temperature and volume; q = mcΔθ.

Ask the lab assistant