7.7 Required Practicals: Energy Changes

A free GCSE Chemistry lesson for AQA (8462) from OBA School: required practical 4: investigating the variables that affect temperature changes in reacting solutions. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how to measure the temperature change of a reaction in solution
• Investigate how the volume of alkali (or mass of a solid) affects the temperature change
• Name the independent, dependent and control variables and write a risk assessment
• Plot results, draw two lines of best fit and use their intersection
• Make and test quantitative predictions for displacement, dissolving and neutralisation
• Evaluate the method, including energy losses, and suggest improvements

Key words

• Temperature change: Highest (or lowest) temperature − starting temperature, in °C.
• Insulation: A material that reduces energy transfer to the surroundings, e.g. a polystyrene cup and lid.
• Resolution: The smallest change a measuring instrument can show, e.g. 0.1 °C for a digital thermometer.
• Random error: Errors that make readings vary unpredictably. Repeat and take a mean to reduce their effect.
• Systematic error: Errors that make all readings too high or too low, e.g. energy lost to the surroundings.

Aim, equipment and variables

Aim: to find out how the volume of sodium hydroxide solution added to hydrochloric acid affects the temperature change. The same method works for displacement reactions (metal powder + copper(II) sulfate), metal + acid, and carbonate + acid.

You need: polystyrene cup and lid, 250 cm³ beaker, thermometer (0.1 °C resolution if possible), two 25 cm³ measuring cylinders (or a burette), 2.0 mol/dm³ hydrochloric acid, 2.0 mol/dm³ sodium hydroxide solution, eye protection.

• Independent; In this practical volume of sodium hydroxide solution added
• Dependent; In this practical highest temperature reached (or temperature change)
• Control; In this practical volume and concentration of acid; concentration of alkali; starting temperature; same cup and lid; stirring

Method

• 1. Measure 25 cm³ of hydrochloric acid into the polystyrene cup. Stand the cup in the beaker.
• 2. Measure and record the starting temperature of the acid.
• 3. Add 5 cm³ of sodium hydroxide solution. Put on the lid, stir and record the highest temperature reached.
• 4. Add a further 5 cm³, stir and record the highest temperature. Repeat until 40 cm³ has been added.
• 5. Repeat the whole experiment and calculate the mean highest temperature for each volume.

• 2.0 mol/dm³ sodium hydroxide; Risk corrosive / irritant to eyes and skin; Control measure eye protection; wash splashes off with plenty of water
• 2.0 mol/dm³ hydrochloric acid; Risk irritant to eyes and skin; Control measure eye protection; mop up spills
• glass thermometer; Risk breaks and cuts; Control measure do not use it to stir; use a stirring rod or digital probe

Other reactions you can investigate

The same insulated cup method works for any change involving at least one solution. Choose the independent variable, keep everything else the same, and record the highest (or lowest) temperature.

• Displacement: metal powder + copper(II) sulfate solution; Independent variable type of metal (same moles, in excess), or mass of one metal; What you see bigger rise for more reactive metals; copper gives no reaction
• Dissolving a solid in water; Independent variable type or mass of solid; What you see some solids cool the water (endothermic), some warm it (exothermic)
• Metal + acid; carbonate + acid; Independent variable concentration of acid, or mass of solid; What you see rise (metal + acid); citric acid + hydrogencarbonate falls

Predict: Making a quantitative prediction. The temperature change depends on the energy released and the mass of solution it heats. Doubling the concentrations (same volumes) doubles the moles reacting, so the rise roughly doubles. Doubling both volumes (same concentrations) doubles the energy but also the mass heated, so the rise stays about the same.

Processing the results

Results (made-up): starting temperature 18.5 °C. Highest temperatures after adding 0, 5, 10 … 40 cm³: 18.5, 21.0, 23.2, 25.8, 28.0, 30.4, 29.7, 28.8, 28.1 °C.

Worked example: using the graph

Draw one straight line of best fit through the rising points and another through the falling points.
Where they cross: volume ≈ 25 cm³ and highest temperature ≈ 30.5 °C.
Maximum temperature rise = 30.5 − 18.5 = 12.0 °C.
Why it rises: more acid and alkali react, so more energy is released. Why it falls after 25 cm³: all the acid has reacted, so the extra (cooler) alkali only absorbs energy and dilutes the mixture.

Answer: about 25 cm³ of alkali neutralises the acid; maximum rise about 12 °C

Evaluating the method

• Energy is lost to the surroundings, so the temperature rise is too small (systematic error); Improvement use a lid and extra insulation (e.g. cotton wool, a second cup); take readings quickly
• Thermometer only reads to 1 °C; Improvement use a digital thermometer or probe with 0.1 °C resolution
• Measuring cylinder volumes not accurate; Improvement use a burette or pipette
• Highest temperature hard to catch; Improvement use a data logger; stir continuously
• Readings vary between repeats (random error); Improvement repeat, ignore anomalies and calculate a mean

Maths skill: Uncertainty in a mean ≈ ± half the range of the repeat readings. For 6.8, 7.1 and 6.9 °C: range = 0.3 °C, so mean = 6.9 ± 0.15 °C.

Common mistakes

• Calling the highest temperature the temperature change. Subtract the starting temperature.
• Joining the points dot to dot. Draw two straight lines of best fit and find where they cross.
• Saying “use a more accurate thermometer” without explaining. Say what is improved, e.g. a resolution of 0.1 °C.
• Thinking energy losses make the measured rise too big. They make it smaller than the true value.

Revision questions

Try these, then check your answers below each one.

Question 1. Why is a polystyrene cup with a lid used for this practical? (Choose from: It reduces energy transfer to the surroundings, It speeds up the reaction, It reacts with the acid, It makes the temperature easier to read.)

Answer: It reduces energy transfer to the surroundings

Working: Less energy is lost, so the measured temperature change is closer to the true value.

Question 2. Sodium hydroxide solution is added in 5 cm³ portions to hydrochloric acid. What is the dependent variable? (Choose from: The highest temperature reached, The volume of sodium hydroxide added, The volume of acid, The type of cup.)

Answer: The highest temperature reached

Working: You measure the temperature. The volume of alkali is the independent variable; acid volume and cup are controls.

Question 3. A reaction mixture starts at 19.5 °C and reaches a highest temperature of 31.0 °C. Calculate the temperature rise.

Answer: 11.5 °C

Working: 31.0 − 19.5 = 11.5 °C.

Related lessons

• Previous lesson: 7.6 Fuel Cells
• Next lesson: 8.1 Rate of Reaction
• In this chapter: 7.1 Exothermic and Endothermic Reactions
• In this chapter: 7.2 Using Energy Transfers from Reactions
• In this chapter: 7.3 Reaction Profiles

Continue in the full course

This lesson is section 7.7 (ARPCH0707) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.