IB Chemistry — Reactivity 1.1: Measuring Enthalpy Changes

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What is the sign of ΔH for an exothermic reaction?

This sub-topic is examined identically at SL and HL. Every reaction transfers energy between the system (the chemicals) and the surroundings, and total energy is always conserved. Whether it feels hot or cold depends on the direction of that transfer.

👆 Toggle exothermic/endothermic · type into the calorimetry calculator

1. Exothermic vs endothermic R1.1.1–2

An exothermic reaction releases heat to the surroundings, so the surroundings warm up and the enthalpy change ΔH is negative (the products store less energy than the reactants). An endothermic reaction absorbs heat, so the surroundings cool down and ΔH is positive. (Don't confuse heat — a transfer of energy — with temperature, a measure of average kinetic energy.)

2. Reaction (energy) profiles R1.1.3

An energy profile plots potential energy (y) against the reaction coordinate (x). The peak is the transition state; the climb up to it is the activation energy (Eₐ). Whether products sit lower or higher than reactants decides the sign of ΔH. Toggle:

3. Measuring ΔH — calorimetry R1.1.4

Run the reaction in a known mass of water (or solution) and measure the temperature change. The heat is q = m c ΔT (m = mass in g, c = specific heat capacity, ΔT = temperature change). Then the molar enthalpy change is ΔH = −q ÷ n — the minus sign makes an exothermic reaction (temperature rise) come out negative. Try it:

Calorimetry calculator · c = 4.18 J g⁻¹ K⁻¹ for water

Mass of water m (g)

Specific heat c (J g⁻¹ K⁻¹)

Temp change ΔT (°C, + for rise)

Moles reacting n (mol)

q = 5016 JΔH = −q/n = −502 kJ mol⁻¹ (exothermic)

Coming up in Reactivity 1.2 & 1.3Once you can measure ΔH, the next sub-topics use Hess's law and bond enthalpies to calculate it without an experiment (R1.2), and apply it to fuels (R1.3). At HL, R1.4 adds entropy and spontaneity (ΔG = ΔH − TΔS).

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Common mistakes examiners see

What is the sign of ΔH for an exothermic reaction?✗ Positive. ✓ Negative — energy leaves the system, so the products have less enthalpy than the reactants.

In q = mcΔT, what mass do you use for a reaction in solution?✗ The mass of the reacting chemical. ✓ The mass of the solution/water being heated (usually taken as its volume in cm³ ≈ g), not the mass of solute.

Why is there a minus sign in ΔH = −q/n?✗ It is a typo. ✓ Heat gained by the water (q positive, temperature rise) was released by the reaction, so the reaction's ΔH is negative — the sign flips.

Are heat and temperature the same thing?✗ Yes. ✓ No — heat is energy transferred; temperature measures the average kinetic energy of the particles.

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Measuring Enthalpy Changes