Scientific Principles & Theory
Scientific Background: When chemical reactions occur, energy is transferred to or from the surroundings, resulting in a temperature change. Neutralisation reactions are exothermic (heat energy is released, causing a temperature rise).
To measure energy changes accurately, heat loss to the surroundings must be minimised. This is achieved by using a polystyrene cup as a calorimeter (which provides thermal insulation) and adding a lid. The reaction is neutralisation:
Experimental Variables
Independent Variable
The volume of sodium hydroxide solution added.
Dependent Variable
The maximum temperature reached by the mixture.
Control Variables
The volume of hydrochloric acid (25 cm3), the concentration of hydrochloric acid (2.0 mol/dm3), the concentration of sodium hydroxide (2.0 mol/dm3), and the level of insulation.
⚠️ Lab Risk Assessment
| Hazard | Associated Risk | Control Measure |
|---|---|---|
| Hydrochloric acid and sodium hydroxide | Corrosive and irritating to skin and eyes | Wear safety goggles; wash skin splashes immediately; clean up spills to prevent slipping. |
Apparatus & Procedure
Required Apparatus
- Dilute hydrochloric acid (2.0 mol/dm3)
- Sodium hydroxide solution (2.0 mol/dm3)
- Polystyrene cup
- Beaker (250 cm3) to support the cup
- Thermometer (0 to 50 °C, 0.1 °C divisions)
- Lid with a thermometer hole
- Measuring cylinders (10 cm3 and 25 cm3)
Step-by-Step Procedure
- Measure 25.0 cm3 of hydrochloric acid using a measuring cylinder and pour it into a polystyrene cup.
- Place the polystyrene cup inside a 250 cm3 beaker for stability.
- Measure and record the initial temperature of the acid.
- Measure 5.0 cm3 of sodium hydroxide solution using a smaller measuring cylinder.
- Add the sodium hydroxide to the polystyrene cup, immediately place the lid on, insert the thermometer, and stir the mixture gently.
- Record the highest temperature reached on the thermometer.
- Repeat steps 4 to 6, adding 5.0 cm3 of sodium hydroxide each time, until a total of 40.0 cm3 has been added. Stir and record the maximum temperature after each addition.
- Perform a second trial of the entire experiment and calculate the mean maximum temperature for each volume.
Fig 1. Laboratory experimental setup for Required Practical 4.
Sample Data & Calculations
This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:
| Volume of NaOH added (cm3) | Mean maximum temperature (°C) |
|---|---|
| 0.0 (Acid only) | 20.0 |
| 5.0 | 22.5 |
| 10.0 | 25.0 |
| 15.0 | 27.5 |
| 20.0 | 30.0 |
| 25.0 | 31.0 |
| 30.0 | 29.5 |
| 35.0 | 28.0 |
| 40.0 | 26.5 |
Data Processing & Analysis
- Plot a graph of Volume of NaOH (x-axis) vs. Maximum Temperature (y-axis).
- Draw a line of best fit through the rising points (increasing temperature as neutralisation proceeds).
- Draw a second line of best fit through the falling points (decreasing temperature as excess cold NaOH is added after all acid is neutralised).
- The point where the two best-fit lines intersect gives the exact volume of NaOH required for complete neutralisation (e.g., 23.5 cm3).
- Calculate energy transfer: Q = m * c * Delta T, where m = 25 g acid + 23.5 g alkali = 48.5 g solution. c = 4.2 J/g/°C. Delta T = 31.4 - 20.0 = 11.4 °C. Q = 48.5 * 4.2 * 11.4 = 2326 J = 2.33 kJ.
Conclusion & Evaluation
Chemical Explanation: Evaluating the experimental outcomes against known values ensures validity. Understanding the source of systematic and random deviations allows for better experimental designs in future trials.
Experimental Error Analysis
| Error Type & Source | Effect on Final Result | Mitigation Strategy |
|---|---|---|
| Systematic Error Heat loss to the surroundings during mixing |
The maximum temperature recorded is lower than the true theoretical value, leading to an underestimation of the neutralisation energy. | Use a double-walled polystyrene cup, add an insulated lid, and ensure the thermometer does not touch the bottom or sides of the cup. |
| Random Error Slight variations in starting acid temperature |
Changes the baseline temperature, skewing individual readings. | Allow all reagents to sit in the same room for a few hours before testing to ensure they reach the same ambient starting temperature. |
Exam Practice
Plan an investigation to determine how the volume of sodium hydroxide added affects the temperature rise during neutralisation. Explain how you would analyze your results to find the neutralisation point.
View Model Answer & Mark Scheme
Model Answer (6/6 Marks):
- Setup: Place a polystyrene cup inside a 250 cm3 beaker to stabilize it and provide thermal insulation.
- Acid measurement: Measure 25.0 cm3 of 2.0 mol/dm3 hydrochloric acid using a measuring cylinder and transfer it into the cup. Record the initial temperature of the acid.
- Increments: Measure 5.0 cm3 of 2.0 mol/dm3 sodium hydroxide solution. Add it to the cup, place the lid on immediately, stir with the thermometer, and record the maximum temperature reached.
- Repetition: Repeat the process, adding 5.0 cm3 increments of sodium hydroxide up to a total of 40.0 cm3, recording the maximum temperature after each addition. Repeat the entire trial to obtain average values.
- Graphing: Plot a graph of Volume of NaOH against Mean Maximum Temperature.
- Analysis: Draw two lines of best fit: a rising line for the temperature increase, and a falling line representing the temperature decrease when excess cold NaOH is added. The intersection of these lines indicates the exact volume of NaOH required for complete neutralisation.
Examiner Tip:
Always explain the purpose of the polystyrene cup and lid (insulation to prevent heat loss) and the method of drawing two intersecting lines of best fit to determine the endpoint.