8.1 Rate of Reaction

A free GCSE Chemistry lesson for AQA (8462) from OBA School: what rate of reaction means, the main ways to measure it, and how to read and use rate graphs. 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

• Calculate the mean rate of a reaction from the amount of reactant used or product formed and the time taken
• Describe how to measure rate by mass loss, gas volume, a disappearing cross and a light sensor
• Interpret graphs of amount of product or reactant against time
• Draw a tangent to a curve and use its gradient to find the rate at a particular time (Higher tier)
• Calculate a rate in mol/s (Higher tier)

Key words

• Rate of reaction: How fast reactants are turned into products: the amount of reactant used up, or product formed, per unit time.
• Mean rate: The average rate over a time interval: quantity used or formed ÷ time taken.
• Gradient: The steepness of a graph line: change in y ÷ change in x. On a rate graph, gradient = rate.
• Tangent: A straight line that just touches a curve at one point and has the same gradient as the curve there.
• Precipitate: An insoluble solid that forms when two solutions react. It makes the mixture cloudy.
• Data logger: A device that records readings from a sensor or balance automatically at set time intervals.

What is the rate of a reaction?

Some reactions, such as a firework exploding, are over in a fraction of a second. Others, such as iron rusting, take weeks. The rate of reaction tells us how quickly reactants are used up or products are made.

mean rate of reaction = quantity of reactant used ÷ time taken

mean rate of reaction = quantity of product formed ÷ time taken

Units: g/s (mass), cm³/s (gas volume), mol/s (amount in moles, Higher tier).

Worked example: mean rate from gas volume

A student collects 60 cm³ of carbon dioxide in the first 40 s of a reaction.
mean rate = volume of product ÷ time = 60 ÷ 40

Answer: 1.5 cm³/s

Worked example: mean rate from mass

The mass of a flask falls by 1.20 g in 80 s as gas escapes.
mean rate = 1.20 ÷ 80 = 0.015

Answer: 0.015 g/s

Ways to measure the rate

• Loss in mass; What you measure Mass of the flask and contents on a balance at regular times. Mass falls as gas escapes.; Good for Reactions that give off a gas that is not too light (e.g. CO₂). Not great for H₂: the mass change is tiny.
• Volume of gas; What you measure Volume of gas collected in a gas syringe (or an upturned measuring cylinder over water) at regular times.; Good for Any reaction that makes a gas, e.g. magnesium + acid.
• Precipitate (disappearing cross); What you measure Time for a cross under the flask to vanish as a solid forms. Shorter time = faster rate.; Good for Reactions that make a precipitate, e.g. sodium thiosulfate + acid. Gives one result per run, not a graph.
• Light sensor; What you measure A light sensor and data logger record how much light passes through the mixture. The reading falls as the precipitate forms.; Good for Precipitate reactions. Gives a full graph, and nobody has to judge by eye.

A balance or a light sensor can be linked to a data logger, which records readings automatically many times a second. You get a continuous record and avoid timing and reading errors.

Exam tip: For the disappearing cross method you get a single time. Use rate = 1 ÷ time (units s⁻¹) to turn it into a rate: a shorter time means a bigger rate.

Getting good gas readings: sources of error

• Gas escapes while the bung is being put in. Fix: hang the solid in a small tube inside the flask and tip it in once the bung is on.

• The syringe plunger sticks, then jumps. Fix: check it moves freely before you start.

• Reading the scale or the stop clock late. Fix: use a data logger, or one person times while another reads.

• The cotton wool in the mass method lets gas out but stops acid spray. Without it, droplets escape and the mass loss is too big.

Reading rate graphs

If you plot volume of gas (or mass lost) against time you get a curve. The gradient of the curve is the rate. The curve is steepest at the start, when there are most reactant particles. It gets less steep as the reactants are used up. When the line goes flat, one reactant (the limiting reactant) has run out and the reaction has stopped.

A graph of the mass of reactant left (or the mass of the flask) against time slopes downwards and levels off. The rate is still the steepness of the line. The shape tells you what was measured:

Exam tip: If you plot mass lost instead of the mass of the flask, the line rises and levels off, just like the gas volume graph.

Rate at a particular time (Higher tier)

The mean rate is an average. To find the rate at one instant, draw a tangent to the curve at that time and work out its gradient. Make the tangent long, then use a big triangle so your readings are accurate.

Worked example: gradient of a tangent

The tangent passes through (0 s, 21 cm³) and (60 s, 80 cm³).
gradient = change in volume ÷ change in time = (80 − 21) ÷ (60 − 0) = 59 ÷ 60 = 0.983…

Answer: rate at 30 s ≈ 0.98 cm³/s

Worked example: rate in mol/s

0.48 g of magnesium (Ar = 24) reacts completely with acid in 120 s.
moles of Mg = mass ÷ Ar = 0.48 ÷ 24 = 0.020 mol
mean rate = 0.020 ÷ 120 = 0.000 166 7 mol/s

Answer: 1.7 × 10⁻⁴ mol/s

Key fact: To change a gas volume into moles, use 1 mol of any gas = 24 dm³ = 24 000 cm³ at room temperature and pressure.

Common mistakes

• Using the total time when a question asks for the mean rate over the first part of a reaction. Read the time interval from the question.
• Forgetting units, or writing cm³ instead of cm³/s. A rate is always “per second” (or per minute).
• Thinking a flat line means the reaction is very slow. It means the reaction has stopped.
• Drawing a tangent that crosses the curve, or using a tiny triangle. The tangent should only touch the curve at the chosen point.
• Saying a longer time for the cross to disappear means a faster reaction. A longer time means a slower rate.

Revision questions

Try these, then check your answers below each one.

Question 1. A student measures the volume of gas given off by a reaction every 10 seconds. Which unit should she use for the rate of reaction? (Choose from: cm³, cm³/s, s/cm³, g/s.)

Answer: cm³/s

Working: Volume (cm³) ÷ time (s) gives a rate in cm³/s.

Question 2. A reaction gives off 90 cm³ of carbon dioxide in the first 60 s. Calculate the mean rate of reaction over this time.

Answer: 1.5 cm³/s

Working: 90 ÷ 60 = 1.5 cm³/s.

Question 3. A flask of marble chips and acid on a balance loses 0.84 g in 120 s. Calculate the mean rate of reaction in g/s.

Answer: 0.007 g/s

Working: 0.84 ÷ 120 = 0.007 g/s.

Related lessons

• Previous lesson: 7.7 Required Practicals: Energy Changes
• Next lesson: 8.2 Collision Theory and Surface Area
• In this chapter: 8.3 The Effect of Temperature
• In this chapter: 8.4–8.5 The Effect of Concentration, Pressure and Catalysts
• In this chapter: 8.6–8.7 Reversible Reactions and Energy Changes

Continue in the full course

This lesson is section 8.1 (AGCCH0801) 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.