Required Practical 2 • CH02 / CH05

RP2: Measurement of an Enthalpy Change

Determine molar enthalpy changes for reactions in aqueous solution using an insulated polystyrene cup calorimeter, cooling curve extrapolation to mixing time, q = mc delta T calculations, and heat loss reduction.

1. Practical Aim & Reaction Context

The aim of Required Practical 2 is to determine the molar enthalpy change (ΔH) for a chemical reaction in solution. Typical reactions tested in OxfordAQA examinations include:

  • Metal Displacement Reaction: e.g. Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) (strongly exothermic displacement).
  • Acid-Base Neutralisation: e.g. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) (ΔH_neut ~ -57 kJ mol^-1).
  • Dissolution of an Ionic Solid: e.g. NH4NO3(s) → NH4+(aq) + NO3-(aq) (endothermic enthalpy of solution).

2. Polystyrene Cup Calorimeter Construction

To minimize heat exchange with the surrounding laboratory atmosphere, a simple polystyrene cup calorimeter is constructed:

Apparatus Components & Functions

  • Expanded Polystyrene Cup: Polystyrene has very low thermal conductivity (excellent insulator) and a negligible heat capacity, ensuring heat is retained in the water.
  • Outer Glass Beaker: Provides mechanical stability and creates an insulating dead air jacket around the cup.
  • Plastic / Cardboard Lid: Prevents heat loss via evaporation and convection. Features a small hole for the thermometer.
  • High-Resolution Thermometer: Graduated in 0.1 deg C or 0.2 deg C divisions, or a digital temperature probe (+/- 0.1 deg C).

Sources of Heat Loss & Prevention

  • Convective Loss: Hot air escaping from surface → prevented by placing lid securely on cup.
  • Conductive Loss: Heat transferring through walls → minimized by nested polystyrene cups and air gap.
  • Cooling During Slow Reaction: Even with insulation, heat escapes while the reaction progresses → compensated by temperature-time extrapolation.
Polystyrene Cup Calorimeter Setup Polystyrene Cup Calorimeter Glass beaker (insulation & stability) Polystyrene cup (low heat capacity) Reaction solution (50.0 cm3) Plastic lid (prevents evaporation) Thermometer bulb immersed in liquid; not touching base

3. Sequential Temperature Logging Protocol

Simply recording the initial temperature and the maximum observed temperature introduces significant systematic error because heat begins dissipating to the surroundings before the maximum temperature is reached. To compensate for this, a strict temperature-time logging procedure is used:

  1. Using a volumetric pipette, measure 50.0 cm3 of copper(II) sulfate solution into the polystyrene cup.
  2. Start the stopwatch. Record the temperature every minute for 3 minutes (minutes 1, 2, and 3) with gentle, continuous stirring to establish a stable pre-reaction baseline.
  3. At minute 4, add the second reactant (e.g. an excess of powdered zinc metal, approximately 3.0 g). Do NOT record the temperature at minute 4. Immediately place the lid on the cup and stir vigorously.
  4. Resume temperature logging at minute 5. Record the temperature every minute from minute 5 to minute 15.
  5. Plot a graph of temperature (vertical axis) against time (horizontal axis).

4. Temperature-Time Cooling Curve Extrapolation

Plotting the recorded data yields two distinct regions: an initial flat baseline (minutes 1 to 3), and a cooling curve as the reaction finishes and heat dissipates (minutes 5 to 15).

Extrapolation Method for delta T 1. Draw a straight line through the initial baseline points (minutes 1 to 3) and extrapolate forward to minute 4.
2. Draw a line of best fit through the cooling points (minutes 5 to 15) and extrapolate backward to minute 4.
3. The vertical distance between these two extrapolated lines at exactly minute 4 represents the theoretical maximum temperature change (delta T_theoretical).
Cooling Curve Extrapolation to Mixing Time Time / minutes Temperature / deg C Min 4 (Mixing) Theoretical delta T Cooling curve extrapolated back to mixing time (min 4)

5. Quantitative Enthalpy Calculations (q = mcΔT)

To convert the measured temperature change into the standard molar enthalpy change of reaction (ΔH), follow this two-stage quantitative sequence:

Stage 1: Calculate Heat Energy Transferred (q) q = m * c * delta T
where:
- q = heat energy transferred (in Joules, J)
- m = mass of solution (in g). Assume density = 1.00 g cm^-3 (so 50.0 cm3 = 50.0 g)
- c = specific heat capacity of solution = 4.18 J g^-1 K^-1
- delta T = theoretical temperature change from extrapolation (in K or deg C)
Stage 2: Calculate Molar Enthalpy Change (Delta H) Delta H = - [ q / (n_limiting * 1000) ]
where:
- n_limiting = moles of the limiting reactant that fully reacted (mol)
- Division by 1000 converts Joules (J) to kilojoules (kJ)
- The Negative Sign: Must be inserted manually for exothermic reactions (where temperature rises). Omitting the sign forfeits the final mark.

6. Key Calorimetric Assumptions & Examiner Pitfalls

Paper 5 questions frequently challenge candidates to state the approximations made in calorimetry calculations:

Calorimetric Assumption Physical Reality Impact on Final Delta H Value
No heat is lost to the surroundings Some heat escapes through polystyrene walls and air gap during reaction. Measured delta T is slightly too small; calculated ΔH is less negative (underestimated).
Specific heat capacity c = 4.18 J g^-1 K^-1 The solution contains dissolved ions (e.g. Cu2+, SO4^2-) whose heat capacity is slightly lower than pure water. Assumes 4.18; slight deviation in calculated q.
Density of solution = 1.00 g cm^-3 A 1.0 mol dm^-3 metal sulfate solution is denser than pure water (~1.05 g cm^-3). Assuming 1.00 g cm^-3 slightly underestimates mass m.
Heat capacity of polystyrene cup is zero The cup and thermometer bulb absorb a tiny quantity of heat. Calculated q is slightly lower than true energy released.

7. Worked Calorimetry Problem: Zinc Displacement of Copper Sulfate

Worked Example: Calculating Molar Enthalpy of Displacement

Problem: In an RP2 experiment, 50.0 cm3 of 0.500 mol dm^-3 CuSO4 solution is placed in a polystyrene cup. Powdered zinc (3.00 g, an excess) is added at minute 4. The initial temperature baseline is 20.2 deg C. Extrapolation of the cooling curve back to minute 4 gives a theoretical maximum temperature of 41.6 deg C.

Assuming the solution has a density of 1.00 g cm^-3 and a specific heat capacity of 4.18 J g^-1 K^-1, calculate the molar enthalpy change for this displacement reaction in kJ mol^-1 to 3 significant figures.

Step 1: Calculate theoretical temperature rise (delta T)

delta T = 41.6 - 20.2 = 21.4 deg C (or 21.4 K)

Step 2: Calculate heat energy transferred (q)

m = 50.0 cm3 * 1.00 g cm^-3 = 50.0 g
q = m * c * delta T = 50.0 * 4.18 * 21.4 = 4472.6 J = 4.4726 kJ

Step 3: Calculate moles of limiting reactant (CuSO4)

Zinc is in excess.
Moles of CuSO4 = c * V = 0.500 * (50.0 / 1000) = 0.0250 mol

Step 4: Calculate molar enthalpy change (Delta H)

Delta H = - (q / n) = - (4.4726 kJ / 0.0250 mol) = -178.9 kJ mol^-1

Final Answer: Delta H = -179 kJ mol-1 (3 significant figures)

8. Practice Exam Questions

Question 1: Why is the temperature of the reaction mixture NOT recorded at exactly the fourth minute during RP2?

Show Answer & Explanation

Correct Answer: B

At minute 4, the solid reactant is tipped in, the lid replaced, and stirring begun. The temperature is changing rapidly and is not homogeneous throughout the liquid. Recording is resumed at minute 5 once heat has distributed evenly.

Question 2: In calculating the heat change q for the reaction of 25.0 cm^3 of 1.0 mol dm^-3 HCl with 25.0 cm^3 of 1.0 mol dm^-3 NaOH, what value of mass m should be substituted into q = mc delta T?

Show Answer & Explanation

Correct Answer: B

The total volume of liquid heated by the reaction is 25.0 + 25.0 = 50.0 cm^3. Assuming a solution density of 1.00 g cm^-3, the total mass m = 50.0 g.

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