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8.10 Required Practicals: Rates and Equilibrium

GCSE Chemistry · AQA · Chapter 8: Rates and Equilibrium

A free GCSE Chemistry lesson for AQA (8462) from OBA School: required practical: how concentration affects the rate of reaction, using a gas-volume method and a disappearing-cross (turbidity) method. 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 investigate the effect of concentration on rate by measuring the volume of gas produced
  • Describe how to investigate the effect of concentration on rate by timing a colour change / cloudiness (disappearing cross)
  • Identify the independent, dependent and control variables, and write a risk assessment
  • Process results: means, anomalies, rate = 1 ÷ time (or 1000 ÷ time), mean rate from gas volumes, and graphs
  • Evaluate the methods and suggest improvements

Key words

  • Independent variable: The variable you change on purpose: here, the concentration of a reactant.
  • Dependent variable: The variable you measure: volume of gas, or time for the cross to disappear.
  • Control variable: A variable kept the same so that the test is fair.
  • Turbidity: Cloudiness of a liquid caused by tiny solid particles (here, a sulfur precipitate).
  • Anomalous result: A result that does not fit the pattern of the others. Leave it out when calculating a mean.

Method 1: volume of gas (magnesium and hydrochloric acid)

Aim: to find how the concentration of hydrochloric acid affects the rate of its reaction with magnesium. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) Equipment: conical flask, bung with delivery tube, gas syringe (or trough of water and upturned 100 cm³ measuring cylinder), measuring cylinder for the acid, magnesium ribbon cut to equal lengths, stop clock, hydrochloric acid at several concentrations (e.g. 0.4, 0.8, 1.2, 1.6, 2.0 mol/dm³).

  • Step 1: Measure 50 cm³ of 0.4 mol/dm³ hydrochloric acid into the conical flask.
  • Step 2: Set up the gas syringe (or fill the measuring cylinder with water and turn it upside down in the trough).
  • Step 3: Add a 3 cm strip of magnesium ribbon, put the bung in straight away and start the stop clock.
  • Step 4: Record the volume of gas every 10 s until no more gas is made.
  • Step 5: Repeat with each acid concentration. Repeat each concentration at least twice more.

Plan it: Hypothesis: “As the concentration of the acid increases, the rate of reaction increases, because there are more acid particles in the same volume, so collisions with the magnesium are more frequent.” A good hypothesis gives a prediction and a scientific reason. Alternative method: time how long it takes to collect a fixed volume of gas (e.g. 30 cm³) for each concentration; mean rate = 30 ÷ time.

Method 2: disappearing cross (sodium thiosulfate and acid)

Aim: to find how the concentration of sodium thiosulfate affects the rate of its reaction with hydrochloric acid. The sulfur precipitate makes the mixture cloudy (turbid). Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l)

  • Step 1: Draw a black cross on a piece of paper and put a conical flask on it.
  • Step 2: Measure 10 cm³ of sodium thiosulfate solution and 40 cm³ of water into the flask (see the table (drawn in the printable lesson notes) for the other mixtures).
  • Step 3: Add 10 cm³ of dilute hydrochloric acid, swirl, and start the stop clock.
  • Step 4: Look down through the flask. Stop the clock when the cross can no longer be seen. Record the time.
  • Step 5: Repeat with the other concentrations, and repeat each one. Calculate mean times.
  • 50; Water (cm³) 0; Concentration (g/dm³) 40
  • 40; Water (cm³) 10; Concentration (g/dm³) 32
  • 30; Water (cm³) 20; Concentration (g/dm³) 24
  • 20; Water (cm³) 30; Concentration (g/dm³) 16
  • 10; Water (cm³) 40; Concentration (g/dm³) 8

The total volume is always 50 cm³, so the depth of liquid (and how cloudy it must get to hide the cross) is the same each time. Concentration = 40 g/dm³ × volume of thiosulfate ÷ 50.

Variables and risk assessment

  • Independent; Gas method concentration of hydrochloric acid; Disappearing cross concentration of sodium thiosulfate
  • Dependent; Gas method volume of hydrogen produced over time; Disappearing cross time for the cross to disappear
  • Control; Gas method volume of acid; length (mass) of magnesium; temperature; same apparatus; Disappearing cross total volume; volume and concentration of acid; temperature; same flask and cross; same person judging
  • Hydrochloric acid (irritant); Risk splashes in the eyes or on skin; Control measure wear eye protection; wash splashes off with water
  • Sulfur dioxide gas (toxic); Risk breathing it in; can trigger asthma attacks; Control measure well-ventilated room; small volumes; pour mixture away in a fume cupboard; asthmatic students take extra care
  • Hydrogen (flammable); Risk catches fire if ignited; Control measure no flames nearby

Processing the results

Gas method: plot volume of gas (y-axis) against time (x-axis) for each concentration, on the same axes. Compare the initial gradients (Higher tier: draw tangents at t = 0), or calculate a mean rate, e.g. volume collected in the first 20 s ÷ 20.

Worked example: mean rate from gas volumes

With 1.0 mol/dm³ acid, 25 cm³ of hydrogen was collected in the first 20 s. With 0.5 mol/dm³ acid, 13 cm³.
mean rate = 25 ÷ 20 = 1.25 cm³/s and 13 ÷ 20 = 0.65 cm³/s.

Answer: doubling the concentration roughly doubles the rate

Disappearing cross: rate is proportional to 1 ÷ time. Using 1000 ÷ time gives easier numbers (units s⁻¹). First check repeats for anomalies, then find a mean.

Worked example: mean time and rate

At 24 g/dm³ the three times were 30 s, 32 s and 31 s (no anomaly).
mean = (30 + 32 + 31) ÷ 3 = 31 s; rate = 1000 ÷ 31 = 32.3 s⁻¹

Answer: mean 31 s; rate 32 s⁻¹ (2 s.f.)

Errors and improvements

  • Judging when the cross “disappears” is subjective; Effect times vary between people (random error); Improvement same person judges every time; or use a light sensor and data logger
  • Gas escapes before the bung is put in; Effect volumes too low; Improvement put the magnesium in a small tube inside the flask and tip it over after the bung is in
  • Temperature changes during the day; Effect rates change for another reason; Improvement use a water bath; measure the temperature

Common mistakes

  • Calling the time for the cross to disappear “the rate”. A longer time means a slower rate; use 1 ÷ time.
  • Changing the total volume in the disappearing cross method. Add water so the total volume stays the same.
  • Including an anomalous result in a mean. Identify it first and leave it out.
  • Forgetting to link the conclusion to collision theory: more particles in the same volume → more frequent collisions.
  • Naming “the amount of chemicals” as a control variable. Be specific: volume of acid, length of magnesium, temperature.

Revision questions

Try these, then check your answers below each one.

Question 1. A student investigates how the concentration of sodium thiosulfate affects the time for a cross to disappear. What is the independent variable? (Choose from: The time for the cross to disappear, The concentration of sodium thiosulfate, The temperature, The volume of acid.)

Answer: The concentration of sodium thiosulfate

Working: She changes the concentration (independent) and measures the time (dependent). Temperature and volume of acid are control variables.

Question 2. Sodium thiosulfate solution has a concentration of 40 g/dm³. 20 cm³ is mixed with 30 cm³ of water. Calculate the new concentration in g/dm³.

Answer: 16 g/dm³

Working: 40 × 20 ÷ 50 = 16 g/dm³.

Question 3. True or false? In the disappearing cross method, water is added so that the total volume of the mixture is the same for every concentration.

Answer: True

Working: True. Keeping the total volume constant keeps the depth of liquid the same, so the test is fair.

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