CORE PRACTICAL 2

Determination of the Enthalpy Change of a Reaction Using Hess’s Law

Aim

To determine the enthalpy change of a reaction indirectly using Hess’s Law and experimental temperature changes.

Principle (Hess’s Law)

Hess’s Law states that the total enthalpy change of a reaction is independent of the route taken, provided the initial and final conditions are the same.

Method (Overview)

Measure a fixed volume of dilute hydrochloric acid in a polystyrene cup.
Record the initial temperature.
Add a known mass of a solid (e.g. sodium hydroxide or sodium carbonate).
Stir and record the maximum temperature reached.
Repeat with a different solid to obtain a second enthalpy change.
Use Hess’s Law to calculate the required enthalpy change.

Key Equations (Example)

NaOH(s) + HCl(aq) \to NaCl(aq) + H 2 O(l)

Na 2 CO 3 (s) + 2HCl(aq) \to 2NaCl(aq) + H 2 O(l) + CO 2 (g)

Calculations

Calculate heat change using: $q = mcΔT$
Assume density of solution = 1.00 g cm⁻³.
Calculate enthalpy change per mole: $ΔH = -q \div number of moles$
Combine enthalpy changes using Hess’s Law to find the required value.

Sources of Error

Heat loss to surroundings.
Inaccurate temperature readings.
Assumption that solution density equals water.
Incomplete dissolution of solid.

Improvements

Use a lid on the polystyrene cup.
Stir continuously for even temperature distribution.
Repeat the experiment and take an average.

Safety

Wear eye protection.
Handle acids and alkalis carefully.
Clean up spills immediately.

Exam-Style Questions

Q1. Why is a polystyrene cup used in this experiment?

Q2. State the equation used to calculate the heat change of the solution.

Q3. Why is the value of ΔH usually negative for neutralisation reactions?

O-A Note

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CORE PRACTICAL 2 Determination of the enthalpy change of a reaction using Hess’s Law.