Required Practical: Temperature Changes

This lesson covers the AQA GCSE Chemistry Required Practical for investigating the variables that affect temperature changes in reacting solutions. You must know the method, apparatus, variables, how to record and process results, and how to evaluate this experiment. This practical directly examines your understanding of exothermic and endothermic reactions from AQA GCSE Chemistry Section 5.

---

Aim of the Practical

To investigate the variables that affect the temperature change in a chemical reaction, such as a neutralisation reaction between an acid and an alkali.

A common version of this practical involves adding increasing volumes of an acid to a fixed volume of alkali (or vice versa) and measuring the temperature change at each addition.

---

Apparatus and Materials

Apparatus	Purpose
Polystyrene cup (calorimeter)	Insulates the reaction mixture to reduce heat loss
Lid for the cup	Further reduces heat loss to the air
Thermometer (or temperature probe)	Measures the temperature of the solution accurately
Measuring cylinder (various sizes)	Measures volumes of acid and alkali accurately
Dilute hydrochloric acid (e.g., 1.0 mol/dm3)	One of the reacting solutions
Dilute sodium hydroxide (e.g., 1.0 mol/dm3)	The other reacting solution
Stirring rod	Ensures thorough mixing and even heat distribution
Safety goggles	Eye protection — acids and alkalis are corrosive or irritant

---

Method — Step by Step

1. Measure 25 cm3 of sodium hydroxide solution using a measuring cylinder and pour it into the polystyrene cup.
2. Record the initial temperature of the sodium hydroxide solution using the thermometer.
3. Measure 5 cm3 of hydrochloric acid using a separate measuring cylinder.
4. Add the acid to the alkali in the polystyrene cup.
5. Stir the mixture gently with the stirring rod.
6. Record the highest temperature reached (for an exothermic reaction).
7. Repeat by adding further 5 cm3 portions of acid, recording the temperature after each addition.
8. Continue until you have added 40 cm3 of acid in total (8 additions).
9. Record all results in a suitable table.

graph TD
    A["1. Measure 25 cm3 NaOH into polystyrene cup"] --> B["2. Record initial temperature"]
    B --> C["3. Measure 5 cm3 HCl"]
    C --> D["4. Add HCl to NaOH in cup"]
    D --> E["5. Stir and record highest temperature"]
    E --> F{"Added 40 cm3 total?"}
    F -->|No| C
    F -->|Yes| G["6. Plot results on a graph"]

Exam Tip: In the exam, you may be asked to describe the method for this practical. Always mention: (1) the polystyrene cup as an insulator, (2) measuring initial temperature BEFORE adding, (3) stirring to ensure even mixing, and (4) recording the MAXIMUM or MINIMUM temperature reached.

---

Variables

Variable Type	Variable	Details
Independent variable	Volume of acid added	Changed in 5 cm3 increments from 5 cm3 to 40 cm3
Dependent variable	Temperature change	Measured using a thermometer after each addition
Control variables	Concentration of acid and alkali	Keep at 1.0 mol/dm3 throughout
	Volume of alkali	Keep at 25 cm3
	Starting temperature	Should be the same (room temperature)
	Type of acid and alkali	Use the same chemicals throughout
	Insulation	Use the same polystyrene cup with a lid

---

Recording Results

A suitable results table would look like:

Volume of HCl added (cm3)	Temperature before addition (degrees C)	Temperature after addition (degrees C)	Temperature change (degrees C)
5	20.0	23.5	+3.5
10	23.5	27.0	+3.5
15	27.0	30.2	+3.2
20	30.2	33.0	+2.8
25	33.0	33.5	+0.5
30	33.5	33.0	-0.5
35	33.0	32.5	-0.5
40	32.5	32.0	-0.5

Exam Tip: Notice that the temperature rises initially (exothermic neutralisation), reaches a maximum, and then starts to decrease. The decrease after the maximum is because the excess acid is simply cooling the mixture (dilution effect). The point of maximum temperature corresponds to the exact neutralisation point.

---

Processing Results — Drawing a Graph

Plot a graph with:

• x-axis: Volume of acid added (cm3)
• y-axis: Temperature of the mixture (degrees C)

Key Features of the Graph

• Temperature rises steeply at first as neutralisation occurs (exothermic).
• Temperature reaches a maximum at the neutralisation point.
• Temperature decreases after the neutralisation point as excess acid dilutes and cools the mixture.
• Draw two best-fit straight lines — one for the rising section and one for the falling section.
• Extrapolate both lines so they intersect. The point of intersection gives the true maximum temperature and the exact volume of acid needed for complete neutralisation.

---

Why Use a Polystyrene Cup?

Reason	Explanation
Good insulator	Polystyrene has a very low thermal conductivity, so less heat is lost to the surroundings
Lightweight	Does not absorb much heat itself, so more of the energy goes into heating the solution
Cheap and disposable	Practical for school laboratories
Low heat capacity	Unlike glass or metal, it does not absorb significant energy from the reaction

A lid is also used to prevent heat loss by evaporation and convection.

---

Sources of Error and Improvements