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1.8.3Physical Chemistry

Calorimetry — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 35 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6The Heat Equation

Formula
  • Formula q = mcΔT, where q is heat energy in J, m is mass in g, c is specific heat capacity and ΔT is the temperature change.
Specific heat capacity
  • Specific heat capacity Water has c = 4.18 J g⁻¹ K⁻¹, and dilute aqueous solutions are assumed to have the same value.
Mass of solution
  • Mass of solution Assume that 1.00 cm³ of dilute solution has a mass of 1.00 g.
Enthalpy change
  • Enthalpy change ΔH = −q ÷ n for an exothermic reaction, converted to kJ mol⁻¹.

Part 2 of 6Calorimetry Procedure

1
Measure the volume of each solution and record the starting temperatures.
2
Mix the solutions in an insulated polystyrene cup with a lid and stir.
3
Record the highest or lowest temperature reached, using a graph if heat loss is significant.
4
Calculate q using the total mass of solution and the temperature change.
5
Divide by the moles of the limiting reactant to find the enthalpy change per mole and attach the correct sign.

Part 3 of 6Combustion Calorimetry

Combustion Calorimetry
Setup
  • Setup A spirit burner containing the fuel heats a known mass of water in a metal can.
Measurements
  • Measurements Record the water temperature rise and the mass of burner before and after burning.
Calculation
  • Calculation Find q from the water, then divide by the moles of fuel burned to get ΔcH.
Typical error
  • Typical error Much of the heat is lost to the surroundings and the fuel burns incompletely, so the result is less exothermic than the data book value.
Source of errorEffect on measured ΔH
Heat lost to surroundingsSmaller magnitude
Incomplete combustionSmaller magnitude
Heat absorbed by apparatusSmaller magnitude

Part 4 of 6Reducing Errors

Insulation
  • Insulation Use a lid and an insulated cup to reduce heat loss.
Extrapolation
  • Extrapolation Plot temperature against time and extrapolate to the moment of mixing for a better ΔT.
Windshield
  • Windshield Shield the burner from draughts in combustion experiments.
Repeat
  • Repeat Take repeat readings and calculate a mean to reduce random error.

Part 5 of 6Worked Example

Question

  • 25.0 cm³ of 1.00 mol dm⁻³ HCl is mixed with 25.0 cm³ of 1.00 mol dm⁻³ NaOH and the temperature rises by 6.8 K.
  • Calculate the enthalpy of neutralisation.
  • (c = 4.18 J g⁻¹ K⁻¹)

  1. Mass of solution = 50.0 g.
  2. q = 50.0 × 4.18 × 6.8 = 1421 J = 1.42 kJ.
  3. Moles of water formed = 0.0250 dm³ × 1.00 = 0.0250 mol.
  4. ΔH = −1.42 ÷ 0.0250 = −56.8 kJ mol⁻¹.

Answer ΔH ≈ −57 kJ mol⁻¹

Part 6 of 6Exam Checklist & Tips

1.8.3 – Key Points for the Exam

  • Specific heat capacity Heat needed to raise 1 g of a substance by 1 K.
  • Calorimeter Apparatus used to measure heat changes.
  • Neutralisation Reaction of an acid with an alkali to form a salt and water.
  • Extrapolation Extending a graph line to estimate a value outside the data.

Common Mistakes

  • Using the mass of the reactant instead of the mass of the solution in q = mcΔT.
  • Leaving q in joules while giving ΔH in kJ mol⁻¹.
  • Forgetting the negative sign for an exothermic reaction.
  • Dividing by moles of the excess reagent instead of the limiting reagent.

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