3.1.2Unit 3. Practical Skills in Chemistry I
Determining Enthalpy Change of Reaction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 4 of 10 Unit quiz
Part 1 of 6Calorimetry
Calorimetry measures the energy transferred to or from a solution during a reaction.
Apparatus
- A polystyrene cup with a lid is a cheap, well-insulated calorimeter that keeps heat losses small.
Equation
- The energy transferred is q = mcΔT, where m is the mass of solution, c the specific heat capacity and ΔT the temperature change.
Value of c
- The specific heat capacity of water, 4.18 J g⁻¹ K⁻¹, is used for the solution as an approximation.
Sign of ΔH
- A temperature rise shows an exothermic reaction, so ΔH has a negative sign; a fall gives a positive ΔH.
Part 2 of 6Method and Assumptions
Examplezinc powder displacing copper from copper(II) sulfate solution.
1
Use a measuring cylinder to place 25 cm³ of copper(II) sulfate solution into the polystyrene cup.
2
Record the temperature every 30 seconds for about 2.5 minutes while stirring, to establish the starting trend.
3
At exactly three minutes add an excess of zinc powder, without recording a temperature at that moment.
4
Continue stirring and recording the temperature each half-minute for a further six minutes.
5
Assume the solution has the density and heat capacity of water, there is no heat loss, and the reaction goes to completion.
Part 3 of 6Temperature Correction Graph
Slow reactions lose heat before the highest temperature is reached.
Temperature Correction Graph
Plot
- Plot temperature against time for the whole experiment, including the readings before the reactant was added.
Extrapolate
- Extend the cooling part of the line back to the time when the reactant was added.
Find ΔT
- Measure the vertical distance between the extrapolated cooling line and the line of the starting temperature at that time.
Benefit
- This estimates the temperature rise that would have occurred if the reaction were instantaneous with no cooling.
Part 4 of 6Calculating ΔH
Step 1
- Calculate q = mcΔT in joules, using the mass of the solution and the corrected temperature change.
Step 2
- Convert q from joules to kilojoules by dividing by 1000.
Step 3
- Find the moles of the limiting reagent, which is the substance that is not in excess.
Step 4
- Divide q by the moles and add the sign: ΔH = –q ÷ n for a temperature rise, in kJ mol⁻¹.
Part 5 of 6Worked Example
Question
- 25.0 cm³ of 0.500 mol dm⁻³ copper(II) sulfate solution reacts with excess zinc powder.
- The corrected temperature rise is 26.0 K.
- Calculate the enthalpy change for the reaction per mole of copper(II) sulfate (assume 1 g cm⁻³ and c = 4.18 J g⁻¹ K⁻¹).
- m = 25.0 g, so q = 25.0 × 4.18 × 26.0 = 2717 J = 2.717 kJ.
- n(CuSO₄) = 0.500 × 25.0 ÷ 1000 = 0.0125 mol, which is the limiting reagent because zinc is in excess.
- ΔH = 2.717 ÷ 0.0125 = 217 kJ mol⁻¹, and the temperature rose, so the sign is negative.
Answer ΔH = –217 kJ mol⁻¹ (3 significant figures)
Part 6 of 6Exam Checklist & Tips
3.1.2 – Key Points for the Exam
- Calorimeter An insulated container used to measure heat energy transferred in a reaction.
- Specific heat capacity The energy needed to raise the temperature of 1 g of a substance by 1 K.
- Limiting reagent The reactant that is completely used up and so determines the amount of product.
- Extrapolation Extending a line on a graph beyond the measured data to estimate a value.
Common Mistakes
- Using the mass of the solid added in q = mcΔT instead of the mass of the solution being heated.
- Leaving q in joules while using ΔH in kJ mol⁻¹, which makes the answer 1000 times too large.
- Using the moles of the excess reagent (zinc) rather than those of the limiting reagent.
- Forgetting the negative sign for an exothermic reaction.
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