Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 6.5 Calorimetry calculations-Study Notes - New Syllabus

6.5 (i) Energy Transferred in a Reaction

In calorimetry experiments, the energy transferred during a reaction is calculated using temperature change of a substance (usually water or solution).

Key Formula

energy transferred (\( q \)) = mass × specific heat capacity × temperature change

\( q = mc\Delta T \)

Where:

• \( q \) = energy transferred (J)
• \( m \) = mass of solution (g)
• \( c \) = specific heat capacity (J g\(^{-1}\) °C\(^{-1}\))
• \( \Delta T \) = temperature change (°C or K)

Important Notes

• For aqueous solutions, \( c = 4.18 \,\mathrm{J\,g^{-1}\,°C^{-1}} \).
• \( \Delta T = \text{final temperature} – \text{initial temperature} \).
• Assume density of solution ≈ \( 1.00 \,\mathrm{g\,cm^{-3}} \).

Sign of Energy Change

• Temperature increase → exothermic → heat released
• Temperature decrease → endothermic → heat absorbed

Experimental Context

• Reactions carried out in insulated containers (e.g. polystyrene cups).
• Minimises heat loss to surroundings.
• Assumes all heat transfer affects the solution.

6.5 (ii) Enthalpy Change of Reaction (ΔH) in kJ mol\(^{-1}\)

After calculating the energy transferred (\( q \)) using calorimetry, the enthalpy change of the reaction is determined per mole of reactant in kJ mol\(^{-1}\).

Key Steps

• Step 1: Calculate energy transferred using \( q = mc\Delta T \) (in J).
• Step 2: Convert \( q \) to kJ → divide by 1000.
• Step 3: Calculate number of moles of limiting reagent.
• Step 4: Divide energy by moles.

Formula

\( \Delta H = \frac{q}{n} \)

(convert \( q \) to kJ first)

Sign Convention

• Temperature increase → exothermic → \( \Delta H < 0 \)
• Temperature decrease → endothermic → \( \Delta H > 0 \)

Important Notes

• Use limiting reagent for moles.
• Units must be kJ mol\(^{-1}\).
• Assume no heat loss (ideal condition).