Required practical: rates of reaction (including use of colorimeter and gas syringe)

What you'll learn

This required practical investigates how changing variables affects the rate of reaction. You'll measure how quickly products form or reactants are used up using different techniques including gas collection, precipitation reactions, and colour change. Understanding this practical is essential because exam questions regularly test your ability to describe methods, analyse results, and evaluate experimental techniques.

Key terms and definitions

Rate of reaction — the speed at which reactants are converted into products, measured as the amount of reactant used or product formed per unit time (e.g. cm³/s or g/s)

Independent variable — the factor you deliberately change in an experiment (e.g. concentration, temperature, surface area)

Dependent variable — the factor you measure in response to changes in the independent variable (e.g. volume of gas produced, time taken for cross to disappear)

Control variables — factors kept constant throughout the experiment to ensure a fair test (e.g. temperature, volume of solution, mass of reactant)

Colorimeter — an instrument that measures the absorbance or transmission of light through a coloured solution, providing quantitative data on colour intensity

Tangent — a straight line that touches a curve at one point only, used to calculate the rate of reaction at a specific time from a graph

Mean rate of reaction — the average rate over a time period, calculated by dividing the total change in quantity by the total time taken

Gas syringe — a graduated cylinder with a movable plunger used to accurately measure volumes of gas produced in a reaction

Core concepts

Methods for measuring rate of reaction

There are three main methods you must know for this practical:

Gas production method

Used when a gas is produced during the reaction (e.g. marble chips with hydrochloric acid producing CO₂, or magnesium with acid producing H₂).

Equipment options:

• Gas syringe — most accurate method, typically measures up to 100 cm³
• Measuring cylinder over water (downward displacement) — suitable for insoluble gases
• Measuring cylinder filled with water, inverted in trough

Procedure:

1. Set up apparatus with reactants separated initially
2. Start stopwatch when reactants are mixed
3. Record volume of gas at regular time intervals (e.g. every 10 seconds)
4. Continue until no more gas is produced
5. Plot volume of gas (y-axis) against time (x-axis)

Precipitation/turbidity method

Used when a precipitate forms that makes the solution cloudy (e.g. sodium thiosulfate reacting with hydrochloric acid to produce sulfur).

Procedure:

1. Place conical flask on paper marked with a black cross
2. Add reactants and start stopwatch
3. Look down through the solution from above
4. Stop timing when the cross can no longer be seen
5. Record time taken
6. Repeat with different concentrations/temperatures
7. Calculate rate as 1/time (s⁻¹)

Colour change method

Used when reactions involve coloured reactants or products.

Visual method:

• Observe colour change at regular intervals
• Subjective and less reliable

Colorimeter method:

1. Calibrate colorimeter with distilled water (0% absorbance)
2. Select appropriate wavelength filter (complementary to solution colour)
3. Measure absorbance or % transmission at regular time intervals
4. Plot absorbance against time
5. Higher absorbance = more concentrated colour

The colorimeter provides quantitative data, making it more reliable than visual observation.

Variables to investigate

The AQA specification requires you to investigate how different factors affect reaction rate:

Concentration (of solutions)

Method: Vary the concentration of one reactant whilst keeping volume constant

• Use different concentrations (e.g. 2.0, 1.5, 1.0, 0.5 mol/dm³)
• Or dilute with water (e.g. 50 cm³ acid + 0 cm³ water, 40 cm³ acid + 10 cm³ water)
• Keep total volume constant for fair test

Temperature

Method: Carry out the same reaction at different temperatures

• Use water bath to heat reactants to desired temperature before mixing
• Common temperatures: 20°C, 30°C, 40°C, 50°C, 60°C
• Ensure both reactants reach the same temperature
• Control: concentration, volume, surface area

Surface area (of solid reactants)

Method: Use different sized particles of the same solid

• Large marble chips, medium chips, small chips, powdered calcium carbonate
• Keep mass of solid constant
• Control: concentration and volume of solution, temperature

Catalyst

Method: Compare reaction with and without catalyst

• Add same mass of catalyst each time
• Common example: manganese(IV) oxide with hydrogen peroxide decomposition
• Control: all other variables

Analyzing results and calculating rates

From graphs

For reactions producing gas or changing colour continuously:

1. Plot quantity (volume of gas/absorbance) on y-axis
2. Plot time on x-axis
3. Draw a smooth curve of best fit

Calculating rate at a specific time:

• Draw a tangent to the curve at that time point
• Calculate gradient: rate = change in y ÷ change in x
• Units depend on quantities measured (e.g. cm³/s)

Calculating mean rate:

• Mean rate = total volume produced ÷ total time taken
• Or: mean rate = change in mass ÷ time taken
• Less accurate than instantaneous rate from tangent

From precipitation experiments:

• Faster reaction = shorter time for cross to disappear
• Rate is proportional to 1/time
• Calculate rate as 1/t (units: s⁻¹)
• Allows comparison: higher value = faster rate

Identifying patterns:

• Steeper initial gradient = faster initial rate
• Curve levels off when reaction complete
• Same final volume/mass = same amount of product formed
• All reactants give same maximum product when in excess

Safety and accuracy considerations

Safety precautions:

• Wear safety goggles throughout (acids and alkalis are irritants/corrosive)
• Work in well-ventilated area when producing gases
• Take care with hot water baths to prevent scalding
• Avoid inhaling gases produced

Improving accuracy:

• Use measuring cylinder/pipette for accurate volumes (not beakers)
• Use digital thermometer (±0.1°C) rather than alcohol thermometer
• Ensure gas syringe moves freely without sticking
• Start stopwatch at precise moment reactants mix
• Use colorimeter instead of visual observation for colour changes
• Repeat measurements and calculate means to identify anomalies

Common errors:

• Gas escaping before bung inserted properly
• Not swirling flask during precipitation method (uneven mixing)
• Temperature not kept constant (if not investigating temperature)
• Solid reactant not completely submerged
• Reading gas syringe at eye level inconsistently

Sources of uncertainty:

• Reaction time (human reaction time ±0.2 s)
• Gas syringe readings (±0.5 cm³ typically)
• Judging when cross disappears (subjective)
• Maintaining constant temperature

Expected results and patterns

Effect of concentration:

• Higher concentration → faster rate
• Graph shows steeper initial gradient at higher concentrations
• Relationship often proportional (double concentration = double rate)
• Particles more crowded → more frequent collisions

Effect of temperature:

• Higher temperature → faster rate
• 10°C rise approximately doubles rate for many reactions
• Graph shows steeper gradient at higher temperatures
• Particles have more kinetic energy → more frequent successful collisions

Effect of surface area:

• Larger surface area (smaller pieces) → faster rate
• Powder reacts fastest, large lumps slowest
• Same total mass gives same total gas produced
• More surface exposed → more collision sites

Effect of catalyst:

• Catalyst increases rate significantly
• Same total product formed with or without catalyst
• Steeper gradient with catalyst present
• Catalyst provides alternative pathway with lower activation energy

Worked examples

Example 1: Gas syringe investigation

A student investigates the effect of concentration on rate of reaction between hydrochloric acid and marble chips. They measure the volume of carbon dioxide produced.

Here is some data:

(a) Calculate the mean rate of reaction for the 2.0 mol/dm³ acid over the first 60 seconds. (2 marks)

(b) Explain why the final volume of gas would be the same regardless of concentration if excess acid was used. (2 marks)

Mark scheme answers:

(a)

• Mean rate = 68 ÷ 60 (1 mark)
• = 1.13 cm³/s or 1.1 cm³/s (1 mark)

(b)

• The marble chips (calcium carbonate) are the limiting reactant (1 mark)
• Same mass of marble produces same amount of CO₂/same number of moles of product (1 mark)

Example 2: Sodium thiosulfate investigation

A student investigates how temperature affects the rate of reaction between sodium thiosulfate and hydrochloric acid. They time how long it takes for a cross to disappear.

Results:

(a) Explain why the student calculated 1/time rather than just using time. (2 marks)

(b) The student used the same person to judge when the cross disappeared each time. Suggest why. (1 mark)

Mark scheme answers:

(a)

• Shorter time means faster reaction (1 mark)
• Calculating 1/time gives a value that increases as rate increases / makes rate proportional to temperature increase (1 mark)

(b)

• Different people might judge the disappearance differently / to keep it a fair test / to reduce subjectivity (1 mark)

Example 3: Calculating rate from a graph

A student plots a graph of volume of hydrogen gas against time for the reaction between magnesium and sulfuric acid.

At 20 seconds, they draw a tangent that passes through coordinates (10, 15) and (40, 60).

Calculate the rate of reaction at 20 seconds. Include units. (3 marks)

Mark scheme answer:

• Change in volume = 60 - 15 = 45 cm³ (1 mark)
• Change in time = 40 - 10 = 30 s (1 mark)
• Rate = 45 ÷ 30 = 1.5 cm³/s (1 mark for value and unit)

Common mistakes and how to avoid them

• Not keeping total volume constant when varying concentration — if you dilute 50 cm³ of acid to 25 cm³, add water to bring it back to 50 cm³, otherwise you're changing two variables

• Confusing rate with time — a shorter time means a faster rate; rate = 1/time for the precipitation method, not just time itself

• Reading the gas syringe while gas is still being produced — wait for the reading to stabilize before recording; read at eye level to avoid parallax error

• Not stating units in calculations — rate needs units like cm³/s, g/s, or s⁻¹; always include them in your final answer

• Drawing tangents incorrectly — use a ruler, make the tangent touch the curve at only one point, and draw it long enough to calculate an accurate gradient

• Forgetting to identify the limiting reactant — when one reactant is in excess, the other limits the total product; this explains why changing excess reactant concentration doesn't change total volume

Exam technique for "Required practical: rates of reaction (including use of colorimeter and gas syringe)"

• Method questions (3-6 marks): Write a numbered list of steps; include specific volumes/masses, equipment names (gas syringe not just "syringe"), what you measure and when, and at least one control variable. Command words like "describe" or "plan" require a complete method.

• Graph analysis (2-4 marks): To find rate, you must show your working for the tangent gradient. State two coordinates clearly, show subtraction for change in y and x, then divide. Don't just draw on the graph without calculations.

• Evaluation questions (3-6 marks): Expected to identify errors (random vs systematic), suggest improvements with justification (not just "repeat" — explain why), and consider which variables had the biggest impact on uncertainty. Higher-tier students should calculate percentage uncertainty.

• Explaining patterns (2-3 marks): Link to collision theory — mention frequency of collisions and/or energy of collisions. Don't just say "particles move faster" — explain that this leads to more frequent successful collisions exceeding activation energy.

Quick revision summary

Rates of reaction can be measured by collecting gas in a syringe or measuring cylinder, timing a precipitation until a cross disappears, or using a colorimeter for colour changes. Key variables to investigate include concentration, temperature, surface area, and catalysts. Calculate rate from graphs using tangents (gradient = rate) or from precipitation as 1/time. Higher concentration, temperature, and surface area increase rate due to more frequent successful collisions. Always control variables for fair testing, use accurate equipment, and take safety precautions with acids and gases.