Measuring Enthalpy Changes

Enthalpy changes can be measured experimentally using a technique called calorimetry. At A-Level, this typically involves measuring the temperature change of water (or a solution) when a reaction takes place, and then using that temperature change to calculate the energy transferred.

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The Principle of Calorimetry

The core idea is straightforward: if a reaction occurs in aqueous solution (or heats a known mass of water), the energy released or absorbed by the reaction is transferred to or from the water. By measuring how much the water temperature changes, we can calculate how much energy was transferred.

The key equation is:

q = mcΔT

where:

• q = energy transferred (in joules, J)
• m = mass of the water or solution being heated (in grams, g)
• c = specific heat capacity of water = 4.18 J g⁻¹ K⁻¹
• ΔT = temperature change (in K or °C — the magnitude is the same for both)

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Worked Example: Combustion of Ethanol

A student burns 0.46 g of ethanol (C₂H₅OH, Mᵣ = 46.0) in a spirit burner beneath a copper calorimeter containing 150 g of water. The water temperature rises from 21.0 °C to 33.5 °C.