Measuring rates of reactionIB MYP Chemistry: Revision notes
Section 1
What is the rate of reaction?
The rate of reaction is how quickly reactants are used up or products are made. It is always a change divided by a time:
rate = amount of reactant used or product made ÷ time taken
The amount can be a volume of gas (cm³), a mass (g) or a concentration (mol dm⁻³), so units are such as cm³ s⁻¹ or g s⁻¹.
Example: 30 cm³ of gas in 20 s gives a mean rate of 30 ÷ 20 = 1.5 cm³ s⁻¹. This is a mean rate because the rate changes during the reaction. Reactions are fastest at the start, when the reactants are most concentrated, and slow as they are used up.
Section 2
Method 1: measuring the volume of gas
If a reaction gives off a gas, collect it and measure its volume at regular times, for example every 10 s.
- A gas syringe gives accurate readings and is easy to read.
- An upturned measuring cylinder filled with water over a trough also works, but is less precise.
Example: magnesium + hydrochloric acid gives hydrogen. Keep the apparatus airtight and fit the bung quickly so no gas escapes before timing starts.
Section 3
Method 2: measuring the loss in mass
If a gas escapes, the flask gets lighter. Put the reaction flask on a top-pan balance and record the mass at regular times.
For marble chips and acid, carbon dioxide escapes through a loose cotton wool plug, which lets the gas out but stops spray leaving. The mass decreases and the rate is the mass lost per second.
This works best for a heavy gas such as carbon dioxide. A very light gas such as hydrogen gives only tiny changes in mass.
Do not seal the flask with a tight bung when measuring mass loss; the gas needs to escape or the mass will not change.
Section 4
Method 3: colour change and turbidity
Some reactions change colour or turn cloudy, and the time taken for the change is measured.
In the disappearing cross experiment, sodium thiosulfate reacts with hydrochloric acid to form a yellow precipitate of sulfur. The mixture turns cloudy (turbid), and the time until a cross under the flask can no longer be seen is recorded.
A shorter time means a faster reaction. The relative rate is 1 ÷ time. This method is subjective, because different people judge the end point differently. A light sensor gives a more objective answer.
Section 5
Plotting and reading rate graphs
Plot the dependent variable (volume or mass) on the vertical axis and time on the horizontal axis, and draw a smooth line of best fit.
- A steep line means a fast rate.
- The line becomes less steep as reactants are used up.
- A horizontal line means the reaction has finished.
- The final volume shows how much product was made.
To find the rate at one moment, draw a tangent to the curve and work out its gradient (change in y ÷ change in x). For a mean rate, divide the change in the amount by the time over that period.
Section 6
Planning and evaluating a rate investigation
In a fair test, change only the independent variable (for example the brand of tablet), measure the dependent variable (for example volume of gas) and keep the control variables the same (mass, volume and concentration of acid, temperature).
A good hypothesis is testable and gives a reason, for example: the smaller pieces will react faster because they have a larger surface area.
Evaluate your method by suggesting sources of error (gas lost before the bung, subjective end point) and improvements (repeat and take a mean, use a gas syringe, use a sensor). Wear eye protection when handling acid.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Measuring rates of reaction
- A student in Cape Town reacts magnesium ribbon with excess dilute hydrochloric acid in a conical flask connected to a gas syringe. She records the volume of hydrogen collected at regular intervals and plots volume against time.Between 0 and 20 s she collects 30 cm³ of gas, and between 60 s and 80 s she collects 6 cm³. Calculate the mean rate of reaction, in cm³ s⁻¹, for each period.2 marks
- A technician in Manchester places a conical flask containing marble chips and dilute hydrochloric acid on a top-pan balance. A loose plug of cotton wool is fitted in the neck of the flask, and the balance reading is recorded every 30 seconds as the reaction proceeds.The first reading is 152.40 g and the reading after 60 seconds is 151.92 g. Calculate the mean rate of mass loss, in g s⁻¹, over the first 60 seconds.2 marks
- In a school in Mumbai, a student mixes sodium thiosulfate solution with dilute hydrochloric acid in a flask standing on a cross drawn on paper. A yellow solid forms and makes the mixture cloudy. The student times how long it takes until the cross can no longer be seen when looking down through the mixture.Explain how the time taken for the cross to disappear can be used to compare the rates of reaction in different experiments.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).