2.1.3Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Using Calorimetry — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 12 of 53 Unit quiz
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.
Section 1 of 6The Basic Calculation
Calorimetry turns a measured temperature change into an enthalpy change per mole.
Heat transferred
- Heat transferred q = mcΔT, where m is the mass of water or solution in g, c is 4.18 J g⁻¹ K⁻¹ and ΔT is the temperature change.
Moles
- Moles Calculate n from concentration and volume for solutions, or from mass divided by molar mass for a fuel.
Enthalpy change
- Enthalpy change ΔH = −q ÷ n, converted to kJ mol⁻¹, with a negative sign if the temperature rose.
Assumptions
- Assumptions The solution is treated as having the density and specific heat capacity of water, and the heat capacity of the container is ignored.
Section 2 of 6Neutralisation and Solution Experiments
Neutralisation and Solution Experiments
Apparatus
- Apparatus A polystyrene cup with a lid, a thermometer and a measuring cylinder reduce heat losses from the reaction mixture.
Method
- Method Measure the initial temperature of each solution, mix them quickly, stir and record the highest or lowest temperature reached.
Temperature change
- Temperature change If the starting temperatures differ slightly, use their average as the initial temperature.
Mass used
- Mass used The volume of the final mixture in cm³ is taken as its mass in grams, because the density is about 1 g cm⁻³.
Section 3 of 6Combustion Calorimetry
1
Place a known volume of water in a metal can and record its starting temperature.
2
Weigh a spirit burner containing the liquid fuel, light it and heat the water by a measured rise such as 20 K.
3
Extinguish the flame, then reweigh the burner to find the mass of fuel burned.
4
Calculate q for the water, then the moles of fuel burned, and divide to get ΔcH.
5
Compare the result with the data book value; the experimental value is smaller in magnitude because of heat losses.
Section 4 of 6Errors and Improvements
Errors and Improvements
Heat loss
- Heat loss Energy escapes to the surroundings, so the temperature rise is too small;
- use lids, lagging or a draught shield.
Incomplete combustion
- Incomplete combustion Soot on the can shows the fuel did not burn fully, giving less energy than complete combustion.
Evaporation
- Evaporation Fuel may evaporate from the wick, so the mass loss is larger than the mass burned.
Cooling correction
- Cooling correction For solutions, plot temperature against time and extrapolate the cooling line back to the time of mixing to find the true ΔT.
Section 5 of 6Worked Example
Question
- In a neutralisation, 50.0 cm³ of 1.00 mol dm⁻³ HCl is mixed with 50.0 cm³ of 1.00 mol dm⁻³ NaOH.
- The temperature rises by 6.8 K.
- Calculate ΔH, using c = 4.18 J g⁻¹ K⁻¹.
- Mass of solution = 100 g, so q = 100 × 4.18 × 6.8 = 2842 J.
- Moles of water formed = 0.0500 dm³ × 1.00 mol dm⁻³ = 0.0500 mol.
- ΔH = −2842 ÷ 0.0500 = −56 848 J mol⁻¹, which is −56.8 kJ mol⁻¹ to 3 significant figures.
Answer ΔH = −56.8 kJ mol⁻¹
Section 6 of 6Exam Checklist & Tips
2.1.3 – Key Points for the Exam
- Calorimeter A container used to measure heat transferred in a reaction.
- Specific heat capacity The energy required to raise the temperature of 1 g of a substance by 1 K.
- q = mcΔT The equation for the heat energy transferred to or from a mass of water or solution.
- Extrapolation Extending a graph line beyond measured points to estimate a value.
- Polystyrene cup An insulating container used to limit heat loss.
Common Mistakes
- Using the mass of the reactants instead of the mass of water in q = mcΔT.
- Forgetting to convert J to kJ before quoting ΔH.
- Giving ΔH a positive sign when the temperature rises.
- Explaining the low experimental value of a combustion enthalpy by the fuel being impure only.
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