1.6.4Physical Chemistry
Calorimetry — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 36 of 102
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.
Part 1 of 5The Method
Principle
- Principle The energy transferred to or from a reaction mixture is found from the temperature change of a known mass of water or solution.
Equation
- Equation The heat energy is q = m × c × ΔT, where m is mass in grams, c is specific heat capacity and ΔT is the temperature change.
Water value
- Water value For water and dilute aqueous solutions, c is taken as 4.18 J g⁻¹ K⁻¹, and the density is taken as 1.00 g cm⁻³.
Enthalpy change
- Enthalpy change Divide q by the moles of the limiting reactant, convert to kJ,
- and add a negative sign for a temperature rise: ΔH = −q ÷ n.What the symbols mean: ΔH = enthalpy change (kJ mol−1) · q = heat energy measured · n = amount of the limiting reactant (mol); negative sign = exothermic
- (J, divide by 1000 for kJ)
- and add a negative sign for a temperature rise: ΔH = −q ÷ n.
Types
- Types Solution reactions are done in an insulated polystyrene cup, while combustion uses a spirit burner heating water in a metal can.
Part 2 of 5Errors and Improvements
Errors and Improvements
Heat loss
- Heat loss Energy escapes to the surroundings, so the temperature rise is lower than it should be, especially in combustion experiments.
Incomplete combustion
- Incomplete combustion Soot formation in a fuel experiment means less energy is released than for complete combustion.
Evaporation
- Evaporation Fuel evaporating from the wick between weighings alters the mass used and affects the result.
Improvements
- Improvements Use a lid, insulation, a draught shield, and plot a cooling curve to extrapolate the maximum temperature change.
Part 3 of 5Calculation Steps
1
Record the mass of solution or water heated and the temperature change ΔT in kelvin or °C.
2
Calculate q = m × c × ΔT in joules and convert to kilojoules by dividing by 1000.
3
Work out the moles of the substance defined in the enthalpy change, using concentration × volume or mass ÷ Mr.
4
Divide the energy in kJ by moles to get ΔH in kJ mol⁻¹.
5
Give the sign: negative if the temperature rose (exothermic) and positive if it fell.
Part 4 of 5Worked 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.
- The temperature rises by 6.8 K.
- Calculate the enthalpy of neutralisation.
- (c = 4.18 J g⁻¹ K⁻¹, density 1.00 g cm⁻³)
- Mass of solution = 50.0 cm³ × 1.00 g cm⁻³ = 50.0 g.
- q = 50.0 × 4.18 × 6.8 = 1421 J = 1.421 kJ.
- Moles of water formed = 0.0250 dm³ × 1.00 = 0.0250 mol.
- ΔH = −1.421 ÷ 0.0250 = −56.8 kJ mol⁻¹.
Answer Enthalpy of neutralisation = −56.8 kJ mol⁻¹, which is about −57 kJ mol⁻¹.
Part 5 of 5Exam Checklist & Tips
1.6.4 – Key Points for the Exam
- Calorimeter An insulated container used to measure the energy transferred in a reaction.
- Specific heat capacity The energy needed to raise the temperature of 1 g of a substance by 1 K.
- q = mcΔT The equation used to calculate heat energy from mass, specific heat capacity and temperature change.
- Limiting reagent The reactant that is not in excess, which decides the moles used for ΔH.
Common Mistakes
- Using the mass of the reagent rather than the mass of the whole solution in q = mcΔT.
- Forgetting to convert joules to kilojoules before dividing by moles.
- Leaving ΔH positive for a reaction in which the temperature increased.
- Using total moles of reactants when only the limiting reagent should be used.
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