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C7 RP Required Practical: Temperature Changes

GCSE Combined Science · AQA · Chapter C7: Energy Changes

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: required practical 10: 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 using an insulated cup
  • Name the independent, dependent and control variables and write a risk assessment
  • Plot results, draw two lines of best fit and use where they cross
  • Investigate other reactions: displacement, dissolving, metal + acid and carbonate + acid
  • Evaluate the method, including energy losses, resolution and repeatability

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 repeat readings vary. 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 solution), metal + acid, and carbonate + acid.

You need: polystyrene cup and lid, 250 cm³ beaker, thermometer (0.1 °C resolution if possible), measuring cylinders (or a burette), dilute hydrochloric acid, dilute 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 so it cannot fall over.
  • 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.
  • sodium hydroxide solution; Risk irritant / corrosive to eyes and skin; Control measure eye protection; wash splashes off with plenty of water
  • 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 stir with it; use a stirring rod or a digital probe

Processing the results

Results (made-up): starting temperature 19.5 °C. Highest temperatures after adding 0, 5, 10 … 40 cm³: 19.5, 22.1, 24.4, 27.1, 29.4, 29.0, 28.4, 28.1, 27.4 °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 ≈ 20 cm³ and highest temperature ≈ 29.5 °C.
Maximum temperature rise = 29.5 − 19.5 = 10.0 °C.
Why it rises: more acid and alkali react, so more energy is released. Why it falls after about 20 cm³: all the acid has reacted, so the extra (cooler) alkali just absorbs energy and the mixture cools.

Answer: about 20 cm³ of alkali neutralises the acid; maximum rise about 10 °C

Other reactions you can investigate

  • Displacement: metal powder + copper(II) sulfate solution; Independent variable type of metal (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
  • Metal + acid; carbonate + acid; Independent variable concentration of acid, or mass of solid; What you see rise for metal + acid; citric acid + sodium hydrogencarbonate gives a fall

Predict: Doubling both volumes (same concentrations) doubles the energy released but also doubles the mass of solution heated, so the temperature rise stays about the same. Doubling the concentrations (same volumes) roughly doubles the rise.

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. a second cup); take readings quickly
  • Thermometer only reads to 1 °C; Improvement use a digital thermometer 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 all the time
  • 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 5.2, 5.6 and 5.3 °C: range = 0.4 °C, so mean = 5.4 ± 0.2 °C.

Is the result valid? A valid result measures what it is meant to measure. Here that means the temperature change is caused only by the reaction, so the starting temperatures of the acid and alkali should be the same, the mixture must be stirred so the reading is the temperature of the whole solution, and only one variable is changed at a time. Results are repeatable if you get similar values when you repeat the experiment, and reproducible if another student with different equipment gets similar values.

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 used instead of a glass beaker? (Choose from: It is a good insulator, so less energy is lost to the surroundings, It does not react with acids, It is easier to read the volume, It speeds up the reaction.)

Answer: It is a good insulator, so less energy is lost to the surroundings

Working: Polystyrene is a good thermal insulator, so less energy is transferred to the surroundings and the temperature change is more accurate.

Question 2. A student adds different volumes of sodium hydroxide solution to 25 cm³ of hydrochloric acid and measures the highest temperature. What is the independent variable? (Choose from: The volume of sodium hydroxide solution, The highest temperature, The volume of hydrochloric acid, The type of cup.)

Answer: The volume of sodium hydroxide solution

Working: The student changes the volume of alkali, so it is the independent variable. The highest temperature is the dependent variable.

Question 3. Three repeats gave temperature rises of 6.4 °C, 6.8 °C and 6.6 °C. Calculate the mean temperature rise.

Answer: 6.6 °C

Working: (6.4 + 6.8 + 6.6) ÷ 3 = 19.8 ÷ 3 = 6.6 °C.

Related lessons

Continue in the full course

This lesson is section C7 RP (AGCSC2405) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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