Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 6.7 CORE PRACTICAL 2: Hess’s Law-Study Notes - New Syllabus

6.7 CORE PRACTICAL 2: Determination of Enthalpy Change using Hess’s Law

This core practical determines the enthalpy change of a reaction that cannot be measured directly, by using Hess’s Law and experimental data from related reactions.

Aim

To determine the enthalpy change of a reaction using Hess’s Law by measuring enthalpy changes of two related reactions.

Typical Reactions Used

Example system (commonly used):

Reaction 1: \( \mathrm{NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)} \)

Reaction 2: \( \mathrm{Na_2CO_3(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)} \)

From these, ΔH for another reaction (e.g. involving \( \mathrm{NaHCO_3} \)) can be calculated using Hess’s Law.

Apparatus

• Polystyrene cup (insulated calorimeter)
• Thermometer
• Measuring cylinder / pipette
• Stirrer

Procedure (General)

• Measure known volumes of reactants.
• Record initial temperature.
• Mix reactants in insulated cup.
• Stir and record maximum/minimum temperature.
• Repeat for second reaction.

Calculations

Step 1: Calculate energy change

\( q = mc\Delta T \)

Step 2: Calculate ΔH for each reaction

\( \Delta H = \frac{q}{n} \) (convert to kJ mol\(^{-1}\))

Step 3: Apply Hess’s Law

• Add/subtract equations appropriately.
• Combine ΔH values.

Key Assumptions

• No heat loss to surroundings.
• Density = \( \mathrm{1.00\,g\,cm^{-3}} \).
• Specific heat capacity = \( \mathrm{4.18\,J\,g^{-1}\,°C^{-1}} \).

Sources of Error

• Heat loss to surroundings.
• Incomplete mixing.
• Inaccurate temperature readings.

Improvements

• Use lid on calorimeter.
• Stir continuously.
• Use more precise thermometer or data logger.

Conclusion

• ΔH for reactions can be determined indirectly.
• Hess’s Law ensures validity of calculation.

Therefore, this practical demonstrates how experimental data and theory combine to determine enthalpy changes.