Measuring enthalpy changes

Chemistry · Reactivity 1: What drives chemical reactions?. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Chemistry topics

The idea

The reacting chemicals are the system. Everything else, including the water or solution around them, is the surroundings. Energy is never created or destroyed. It only moves between the two.

In an exothermic reaction the system gives out heat, the surroundings get hotter and ΔH is negative. The products have a lower enthalpy than the reactants, so they are more stable. In an endothermic reaction heat is taken in, the surroundings get colder and ΔH is positive.

To measure ΔH, let the reaction heat or cool a known mass of water. Find the heat from Q = mcΔT, then divide by the moles that reacted. Heat lost to the air makes the measured value too small.

Rules to remember

heat = mass of water or solution × specific heat capacity × temperature change (Q = mcΔT), with c = 4.18 J g⁻¹ K⁻¹
enthalpy change = −heat ÷ moles reacted (ΔH = −Q ÷ n), in kJ mol⁻¹, so change Q from J to kJ first
Temperature rises: exothermic, ΔH negative. Temperature falls: endothermic, ΔH positive
On an energy profile: ΔH = enthalpy of products − enthalpy of reactants
activation energy (E_a) = enthalpy of the transition state − enthalpy of the reactants
Lower enthalpy means more stable
Standard conditions for ΔH^⦵: 100 kPa, usually 298 K, solutions at 1 mol dm⁻³

Common mistake

Students put the mass of the fuel or solid into Q = mcΔT. The mass m is the mass of the water or solution that changes temperature. Also remember the sign: a temperature rise means ΔH is negative.

Worked example

0.050 mol of a salt dissolves in 100 g of water and the temperature falls by 3.0 °C. What is the enthalpy change of solution? (c = 4.18 J g⁻¹ K⁻¹)

Q = mcΔT = 100 × 4.18 × 3.0 = 1254 J = 1.254 kJ.
Per mole: 1.254 ÷ 0.050 = 25.1 kJ mol⁻¹.
The temperature fell, so heat was taken in: the process is endothermic and ΔH is positive.

Answer: ΔH = +25 kJ mol⁻¹

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Reactivity 1.1 Measuring Enthalpy Changes

Keep going

← Functional groups: classification of organic compounds Energy cycles in reactions →Take a Chemistry test