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4.1.3Physical Chemistry

Measurement of an Enthalpy Change — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 97 of 102

📚 AQA 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 6Principles of Calorimetry

Principles of Calorimetry
Heat transferred
  • Heat transferred The energy transferred to water or solution is q = mcΔT, using the mass of the water or solution.
Specific heat capacity
  • Specific heat capacity Water has c = 4.18 J g⁻¹ K⁻¹, the energy needed to raise 1 g of it by 1 K.
Enthalpy change
  • Enthalpy change ΔH = q ÷ n, with n in moles of the limiting reagent, converted to kJ mol⁻¹ and given a sign.
Assumptions
  • Assumptions The solution has the density and specific heat capacity of water, and the container's heat capacity is ignored.

Part 2 of 6Reactions in Solution

1
Measure a known volume of one solution into an insulated polystyrene cup with a lid and stand it in a beaker for stability.
2
Record the temperature every minute for a few minutes before adding the second reagent.
3
Add the second reagent at a precise time, without recording that reading, and stir continuously.
4
Continue recording the temperature at regular intervals for about ten more minutes.
5
Plot temperature against time and extrapolate both lines back to the time of mixing to find the corrected temperature change.

Part 3 of 6Temperature Correction Graph

Why needed
  • Why needed Some reactions are not instant, so heat is lost while the temperature is still rising.
Before mixing
  • Before mixing The initial readings give a steady line for the starting temperature.
After mixing
  • After mixing The cooling part of the graph is a best-fit line sloping down as heat is lost.
Extrapolation
  • Extrapolation Extend the cooling line back to the time of mixing to estimate the temperature without heat loss.
ΔT
  • ΔT The vertical distance between the extrapolated line and the starting temperature gives the corrected ΔT.

Part 4 of 6Combustion Calorimetry

Combustion Calorimetry
Set-up
  • Set-up A spirit burner heats a known mass of water in a copper calorimeter, and the burner is weighed before and after.
Measurements
  • Measurements Record the temperature rise of the water and the mass of fuel burned.
Errors
  • Errors Heat is lost to the surroundings and absorbed by the calorimeter, and combustion may be incomplete.
Improvements
  • Improvements A lid, a draught shield and placing the flame close to the calorimeter reduce the heat loss.

Part 5 of 6Worked Example

Question

  • 25.0 cm³ of 2.00 mol dm⁻³ HCl was mixed with 25.0 cm³ of 2.00 mol dm⁻³ NaOH in a polystyrene cup.
  • The corrected temperature rise was 13.0 K.
  • Calculate the enthalpy change of neutralisation.
  • (c = 4.18 J g⁻¹ K⁻¹;
    • density = 1.00 g cm⁻³)

  1. Total volume = 50.0 cm³, so mass of solution m = 50.0 g.
  2. q = m c ΔT = 50.0 × 4.18 × 13.0 = 2717 J = 2.717 kJ.
    What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)
  3. Moles of HCl = 2.00 × 0.0250 = 0.0500 mol, which equals the moles of NaOH, so n = 0.0500 mol of water formed.
  4. ΔH = −q ÷ n = −2.717 ÷ 0.0500 = −54.3 kJ mol⁻¹, negative because the temperature rose.
    What the symbols mean: ΔH = enthalpy change (kJ mol−1) · q = heat energy measured (J, divide by 1000 for kJ) · n = amount of the limiting reactant (mol); negative sign = exothermic

Answer ΔH = −54.3 kJ mol⁻¹.

Part 6 of 6Exam Checklist & Tips

4.1.3 – Key Points for the Exam

  • Calorimetry Measuring heat changes in a reaction using the temperature change of a known mass of water.
  • Specific heat capacity The energy needed to raise the temperature of 1 g of a substance by 1 K.
  • Extrapolate Extend a best-fit line beyond the measured points to estimate a value.
  • Enthalpy of combustion The enthalpy change when one mole of a substance burns completely in oxygen.

Common Mistakes

  • Using the mass of the solid or the fuel instead of the mass of water in q = mcΔT.
  • Forgetting the negative sign for an exothermic reaction.
  • Adding 273 to a temperature change; a change of 1 °C equals a change of 1 K.
  • Leaving ΔH in joules when kJ mol⁻¹ was requested.

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