Required practical: temperature changes (energy changes in reactions)

What you'll learn

This required practical focuses on investigating the temperature changes that occur during chemical reactions. You will learn how to safely measure and record temperature changes when different substances react, and use your results to classify reactions as exothermic or endothermic. Understanding this practical is essential for both paper 1 and paper 2 examinations, where you may be asked to describe the method, analyse results, or evaluate the experimental procedure.

Key terms and definitions

Exothermic reaction — a reaction that transfers energy to the surroundings, causing the temperature of the surroundings to increase

Endothermic reaction — a reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease

Temperature change (ΔT) — the difference between the final temperature and the initial temperature, measured in degrees Celsius (°C)

Control variable — a factor that must be kept constant throughout an experiment to ensure a fair test

Polystyrene cup — an insulated container used in this practical to reduce heat loss to the surroundings

Neutralisation — a reaction between an acid and an alkali (or base) that produces a salt and water

Limiting reactant — the reactant that is completely used up in a reaction, limiting the amount of product formed

Systematic error — an error that affects all readings by the same amount in the same direction, such as a thermometer reading 2°C too high

Core concepts

The purpose of the practical

This investigation allows you to measure and compare temperature changes in different reactions. The main aims are to:

• Determine whether reactions are exothermic or endothermic
• Measure the temperature change accurately using appropriate apparatus
• Identify patterns in how temperature changes relate to the quantities of reactants used
• Develop practical skills in fair testing and hazard assessment

Common reactions investigated include:

• Neutralisation reactions (acid + alkali)
• Displacement reactions (reactive metal + metal salt solution)
• Dissolving substances in water
• Adding metals to acid solutions

Method for neutralisation reactions

The standard method for investigating temperature changes in neutralisation follows these steps:

1. Use a measuring cylinder to measure 25 cm³ of dilute hydrochloric acid (typically 1.0 mol/dm³) into a polystyrene cup
2. Place the polystyrene cup inside a beaker for stability
3. Use a thermometer to measure and record the initial temperature of the acid
4. Measure 25 cm³ of sodium hydroxide solution (same concentration as the acid) in a clean measuring cylinder
5. Add the sodium hydroxide solution to the acid and stir continuously with the thermometer
6. Record the maximum temperature reached by the mixture
7. Calculate the temperature change (ΔT = final temperature - initial temperature)

Key equipment choices:

• Polystyrene cup: Provides insulation to minimise heat loss to surroundings, making temperature measurements more accurate
• Thermometer: Digital thermometers offer precision to 0.1°C; traditional thermometers typically measure to 1°C
• Measuring cylinder: Ensures accurate volumes of reactants are used (control variable)
• Stirring: Ensures even heat distribution throughout the solution for accurate temperature readings

Method for displacement reactions

A typical displacement investigation uses copper(II) sulfate solution and zinc powder:

1. Measure 25 cm³ of copper(II) sulfate solution (e.g., 0.5 mol/dm³) into a polystyrene cup
2. Measure and record the initial temperature
3. Add an excess of zinc powder (approximately 1-2 g)
4. Stir the mixture and record the maximum temperature reached
5. Calculate the temperature change

The reaction is: zinc + copper(II) sulfate → zinc sulfate + copper

This is an exothermic displacement reaction because zinc is more reactive than copper.

Variables and fair testing

For valid results, you must control variables effectively:

Independent variable (what you change):

• Volume or concentration of one reactant
• Type of acid or alkali used
• Type of metal in displacement reactions

Dependent variable (what you measure):

• Temperature change (ΔT)

Control variables (what you keep constant):

• Volume of solution(s) used
• Concentration of solutions
• Starting temperature of reactants
• Same apparatus (polystyrene cup, same thermometer)
• Time allowed for reaction to complete

When investigating how concentration affects temperature change, you must:

• Keep the volume of both reactants constant
• Use the same starting temperature
• Change only the concentration of one reactant
• Repeat readings and calculate means to improve reliability

Classifying reactions and analysing results

Exothermic reactions show:

• Temperature increase (positive ΔT)
• Heat energy released to surroundings
• The reaction mixture feels hot

Examples include:

• Neutralisation: hydrochloric acid + sodium hydroxide
• Displacement: zinc + copper(II) sulfate
• Combustion reactions
• Many oxidation reactions

Endothermic reactions show:

• Temperature decrease (negative ΔT)
• Heat energy absorbed from surroundings
• The reaction mixture feels cold

Examples include:

• Thermal decomposition reactions
• Dissolving some salts (ammonium nitrate in water)
• Citric acid + sodium hydrogencarbonate

When analysing results tables, look for:

• Patterns in temperature changes as concentration or volume changes
• Anomalous results that don't fit the pattern
• The maximum temperature change (usually when reactants are in stoichiometric proportions)

Sources of error and improvements

Understanding experimental limitations is crucial for evaluation questions:

Heat loss to surroundings:

• Energy escapes through the polystyrene cup
• Effect: Temperature change measured is lower than actual
• Improvement: Use a lid on the cup, use a more insulated container, or perform the reaction in a vacuum flask

Incomplete mixing:

• Reactants not fully mixed before temperature measured
• Effect: Lower maximum temperature recorded
• Improvement: Stir continuously and consistently

Thermometer not calibrated correctly:

• Systematic error affecting all readings
• Effect: All temperatures shifted by same amount (doesn't affect ΔT calculation)
• Improvement: Use a calibrated digital thermometer

Heat from stirring:

• Vigorous stirring adds mechanical energy
• Effect: Slightly increased temperature readings
• Improvement: Stir gently but consistently

Reaction time delays:

• Taking too long to mix reactants and measure temperature
• Effect: More heat lost before maximum temperature recorded
• Improvement: Work quickly and systematically

Resolution of measuring equipment:

• Thermometer only measures to nearest 1°C
• Improvement: Use a digital thermometer with 0.1°C resolution for more precise measurements

Worked examples

Example 1: Neutralisation investigation

Question: A student investigates the temperature change when sodium hydroxide solution reacts with hydrochloric acid. The student measures 25 cm³ of hydrochloric acid into a polystyrene cup. The initial temperature is 19°C. After adding 25 cm³ of sodium hydroxide solution and stirring, the maximum temperature reached is 26°C.

(a) Calculate the temperature change. [1 mark] (b) State whether this reaction is exothermic or endothermic. [1 mark] (c) Explain why a polystyrene cup is used instead of a glass beaker. [2 marks]

Mark scheme answers:

(a) Temperature change = 26°C - 19°C = 7°C [1 mark]

(b) Exothermic [1 mark]

(c) Polystyrene is an insulator [1 mark] which reduces/minimises heat loss to the surroundings / makes the experiment more accurate [1 mark]

(Note: Both marks needed for full answer. Simply stating "polystyrene insulates" would only earn 1 mark)

Example 2: Displacement reaction analysis

Question: A student investigates the temperature change when different masses of zinc powder are added to 30 cm³ of copper(II) sulfate solution. The results are shown in the table below.

(a) State the control variables in this investigation. [2 marks] (b) Describe the pattern shown in the results. [2 marks] (c) Suggest why the temperature change is the same for 1.5 g and 2.0 g of zinc. [2 marks]

Mark scheme answers:

(a) Any two from:

• Volume of copper(II) sulfate solution [1 mark]
• Concentration of copper(II) sulfate solution [1 mark]
• Initial/starting temperature [1 mark]
• Same apparatus/polystyrene cup [1 mark]

(b) As the mass of zinc increases, the temperature change increases [1 mark] until it reaches a maximum of 18°C / until the mass reaches 1.5 g then it stays constant [1 mark]

(c) All the copper(II) sulfate has reacted / the copper(II) sulfate is the limiting reactant [1 mark], so adding more zinc produces no further reaction / no more heat released [1 mark]

Example 3: Evaluating an experiment

Question: A student obtains the following results when investigating the temperature change of neutralisation:

Test 1: 6.5°C, Test 2: 7.0°C, Test 3: 11.5°C

The student calculates a mean temperature change of 8.3°C.

(a) Identify the anomalous result. [1 mark] (b) Calculate the correct mean, excluding the anomalous result. [2 marks] (c) The actual temperature change for this reaction should be 7.5°C. Suggest one reason why the student's results are different. [1 mark]

Mark scheme answers:

(a) 11.5°C [1 mark]

(b) (6.5 + 7.0) ÷ 2 [1 mark] = 6.75°C or 6.8°C [1 mark]

(Note: Accept 6.75°C or 6.8°C for final mark)

(c) Any one from:

• Heat lost to surroundings [1 mark]
• Incomplete mixing / not stirred properly [1 mark]
• Delay in measuring maximum temperature [1 mark]
• Measuring cylinder/thermometer not accurate [1 mark]

Common mistakes and how to avoid them

• Confusing exothermic and endothermic definitions: Remember that exothermic reactions give OUT heat (temperature goes UP). Use the phrase "EXothermic = EXit of heat" to help recall. Endothermic reactions take IN heat (temperature goes DOWN).

• Forgetting to subtract initial temperature from final temperature: The temperature change is not just the final temperature. Always calculate ΔT = final temperature - initial temperature. Show your working clearly.

• Not identifying control variables correctly: When asked what must be kept constant, think about all the factors that could affect temperature change: volumes, concentrations, starting temperature, and apparatus. Don't just list one.

• Poor evaluation answers: When suggesting improvements, be specific. Don't just say "use better equipment" — explain what equipment and why (e.g., "use a digital thermometer with 0.1°C resolution to measure temperature more precisely").

• Ignoring anomalous results when calculating means: Always check data for outliers. If one result is significantly different from the others, identify it as anomalous and exclude it from your mean calculation. State clearly which result you've excluded.

• Not explaining why polystyrene cups are used: Simply stating "polystyrene is an insulator" earns only one mark. You must also explain the consequence: it "reduces heat loss to surroundings" or "makes the results more accurate" for the second mark.

Exam technique for "Required practical: temperature changes (energy changes in reactions)"

• Command word awareness: "Describe" requires you to state what happens (e.g., "temperature increases from 20°C to 28°C"). "Explain" requires you to give reasons why (e.g., "temperature increases because the reaction is exothermic and releases energy to the surroundings"). "Suggest" means the answer may not be directly in the specification but can be worked out from your knowledge.

• Method questions structure: When asked to describe a method, use numbered steps in a logical order. Include specific measurements (volumes, concentrations), safety precautions if relevant, and how you would ensure a fair test. Typically worth 4-6 marks, so aim for at least 5 clear points.

• Calculation accuracy: Always show your working for temperature change calculations. Even if your final answer is wrong, you can gain method marks. Include units (°C) in your final answer. Check your arithmetic carefully.

• Evaluation and improvement: Higher-tier questions often ask you to evaluate the method or suggest improvements. Structure answers as: identify the problem, explain its effect on results, suggest a specific improvement. Each improvement is usually worth 2 marks (1 for the improvement, 1 for the explanation).

Quick revision summary

Temperature change practicals measure how reaction temperature changes to classify reactions as exothermic (temperature increases) or endothermic (temperature decreases). Use a polystyrene cup to reduce heat loss, measure reactant volumes precisely, and record initial and final temperatures accurately. Calculate ΔT = final - initial temperature. Control variables include volumes, concentrations, and starting temperature. Common sources of error include heat loss to surroundings and incomplete mixing. Always identify anomalous results before calculating means and suggest specific improvements when evaluating the method.