Scientific Principles & Theory
Scientific Background: Chemical reactions are accompanied by energy changes. Exothermic reactions release thermal energy, increasing the temperature of the surroundings. Endothermic reactions absorb thermal energy, decreasing the temperature.
By measuring the temperature change (ΔT) of a known mass of water or solution (m), the heat energy change (Q) can be calculated:
Where c is the specific heat capacity of water (4.18 J/g/°C). The molar enthalpy change (ΔH) is then determined by dividing Q by the moles of the limiting reactant, adding a negative sign for exothermic reactions:
Experimental Variables
Independent Variable
The identity or amount of reactant added (e.g. mass of zinc added to copper sulfate).
Dependent Variable
The temperature change (ΔT) of the mixture.
Control Variables
Volume of solution (50 cm³), concentration of solution, insulation (polystyrene cup with lid).
⚠️ Lab Risk Assessment
| Hazard | Associated Risk | Control Measure |
|---|---|---|
| Copper(II) sulfate solution | Harmful and toxic to aquatic life | Wear safety goggles; do not pour down the sink if local regulations prohibit it (dispose in heavy metal waste). |
| Corrosive acids/alkalis | Chemical burns / skin irritation | Wear safety goggles and gloves; clean spills immediately with water. |
Apparatus & Procedure
Required Apparatus
- Polystyrene cup with a lid
- Glass beaker (100 cm³ - to support the cup)
- Thermometer (0 to 50 °C, 0.1 °C divisions)
- Measuring cylinder (50 cm³)
- Electronic balance (0.01 g resolution)
- Glass stirring rod
- Zinc powder (excess)
- 1.0 mol/dm³ copper(II) sulfate solution
Step-by-Step Procedure
- Use a measuring cylinder to place 25.0 cm³ of 1.0 mol/dm³ copper(II) sulfate solution into a polystyrene cup.
- Place the polystyrene cup inside a glass beaker to support it and prevent it from tipping over.
- Insert the thermometer through the hole in the cup's lid into the solution. Record the temperature every minute for 3 minutes to establish a stable baseline.
- Weigh approximately 2.0 g of zinc powder (an excess) on a digital balance.
- At the 4th minute, add the zinc powder to the cup. Immediately replace the lid and stir the mixture with the thermometer.
- Record the temperature of the mixture every 30 seconds for 6 minutes.
- The temperature will rise as the exothermic displacement reaction occurs:
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). - Plot a graph of temperature against time.
- Draw lines of best fit for the points before and after the zinc was added, and extrapolate them to the 4th minute to determine the theoretical maximum temperature rise, compensating for any heat loss.
Fig 1. Laboratory experimental setup for Core Practical 3.8.
Sample Data & Calculations
This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:
| Parameter Measured | Value |
|---|---|
| Volume of 1.0 mol/dm³ CuSO₄ solution used | 25.0 cm³ |
| Mass of zinc powder added (excess) | 2.00 g |
| Initial temperature (at minute 3) | 21.0 °C |
| Maximum extrapolated temperature (at minute 4) | 38.2 °C |
| Temperature change (ΔT) | 17.2 °C |
Data Processing & Analysis
- 1. Heat energy change: Q = m * c * ΔT. (Assume solution density = 1.0 g/cm³, so m = 25.0 g)
- Q = 25.0 g * 4.18 J/g/°C * 17.2 °C = 1797.4 J = 1.797 kJ
- 2. Moles of CuSO₄ reacted = Concentration * Volume = 1.0 mol/dm³ * 0.025 dm³ = 0.025 mol
- 3. Molar enthalpy change: ΔH = -Q / n = -1.797 kJ / 0.025 mol = -71.9 kJ/mol
Conclusion & Evaluation
Chemical Explanation: Saturated solutions are heavily dependent on temperature. Heating shifts solubility limits, allowing more solute to form coordinate bonds or ion-dipole interactions with solvent molecules. When cooled, the reverse process happens and solute precipitates out.
Experimental Error Analysis
| Error Type & Source | Effect on Final Result | Mitigation Strategy |
|---|---|---|
| Systematic Error Heat loss to the surroundings |
The maximum temperature recorded is lower than the theoretical maximum, making the calculated Q and ΔH too low. | Use a polystyrene cup (an insulator), a lid to prevent convection, and extrapolate the cooling curve on a graph back to the time of mixing. |
| Systematic Error Heat capacity of the thermometer and polystyrene cup ignored |
Some heat is absorbed by the equipment rather than the water, underestimating Q. | Use a double-walled calorimeter or perform calibration runs to determine the calorimeter constant. |
Exam Practice
A student wants to determine the molar enthalpy change for the reaction between zinc and copper(II) sulfate. Describe how the student should carry out the calorimetry experiment to obtain the data, and how to use a temperature-time graph to compensate for heat loss.
View Model Answer & Mark Scheme
Model Answer (6/6 Marks):
- Setup: Measure 25.0 cm³ of copper(II) sulfate solution into a polystyrene cup supported in a beaker.
- Baseline: Record the temperature of the solution every minute for 3 minutes using a thermometer to establish the initial temperature.
- Addition: At the 4th minute, add an excess of zinc powder, quickly place the lid on the cup, and stir.
- Monitoring: Record the temperature every 30 seconds for 6 minutes.
- Graphing: Plot temperature against time. Draw a line of best fit through the cooling points and extrapolate it back to the 4th minute (time of mixing).
- Compensation: The difference between the extrapolated maximum temperature at the 4th minute and the baseline temperature gives the corrected temperature change, compensating for heat lost during mixing.
Examiner Tip:
Always explain the extrapolation method on the temperature-time graph: it is the standard way to compensate for heat loss to the surroundings during the reaction.