Rates of reaction

The value of experiencing live practical work cannot be overstated. Numerous studies provide evidence of its value in terms of learner engagement, understanding, results and the likelihood of continuing to study chemistry or work in a related field.

Use this video to complement live practical work, or to help learners understand the methods, equipment and skills when they cannot access the lab.

Chapter titles: 00:09 Introduction to rates of reaction; 01:34 Carrying out the experiment; 05:45 Calculations; 07:40 Evaluating the method.

Teacher notes

Full teacher notes are available in the supporting resources booklet (also available in MS Word), including ideas for how to use this video and the accompanying activites and answers to use as part of your teaching.

Notes on running the practical experiments

Ask learners to work in pairs. Demonstrate how to set up the equipment first (either in-person or via the video) to emphasise health and safety issues. Remind learners that they must wear eye protection and direct them to the relevant student safety sheets (SSS).

It is important that all learners have access to a stop bath to dispose of their waste products as sodium thiosulfate solution (SSS034) reacts with hydrochloric acid (SSS020) to toxic produce sulfur dioxide gas. Explain that the stop bath is a solution of sodium carbonate/hydrogen carbonate (SSS033) with an acid–base indicator such as phenolphthalein (SSS070). It neutralises any remaining acid and the sulfur dioxide reacts with the water to produce sulfuric acid. If the indicator is showing the acidic colour, refresh the stop bath by adding more sodium carbonate solution. Ensure the room is well ventilated. A microscale version of the experiment is available from CLEAPSS.

Health, safety and technical notes

Read our standard health and safety guidance and carry out a risk assessment before running any live practical. Refer to SSERC/CLEAPSS Hazcards, recipe books and student safety sheets. Hazard classification may vary depending on supplier. Download the technician notes for the full equipment list, safety notes and disposal information.

Procedure

1. Measure 10 cm³ of sodium thiosulfate and pour it into the conical flask.

2. Measure 40 cm³ of distilled water and add it to the conical flask.

3. Place the flask on the black cross.

4. Using a clean measuring cylinder, measure 10 cm³ of hydrochloric acid.

5. Add the acid to the flask, start the stop clock and swirl.

6. Place the watch glass on top of the flask to limit breathing in sulfur dioxide gas.

7. Time how long it takes until you can no longer see the black cross. Look at the cross from a distance of at least 20 cm above the top of the flask.

8. Repeat the method using 20, 30, 40 and 50 cm³ of sodium thiosulfate solution with 30, 20, 10 and 0 cm³ of distilled water.

Find the integrated instructions for this experiment in the PowerPoint slides. 

Key terms

Learners will need to have a clear understanding of the following scientific terminology:

• Rate of reaction – the change in the concentration of the reactant or product per unit time.
• Chemical change – a change in which one or more new substances are formed.
• Reactant – a starting substance in a chemical reaction.
• Product – a substance made during a chemical reaction.
• Mass – a measure of the amount of matter in a substance.
• Precipitate – an insoluble solid that forms when two solutions react.
• Collision theory – for a chemical reaction to occur, two or more particles must collide in the correct orientation (the right way around) and with enough energy to break the bonds.
• Activation energy – the minimum amount of energy required for a reaction to occur.
• Temperature – a measure of how hot or cold an object is.
• Concentration – the mass of solute dissolved in a fixed volume of solvent.
• Pressure – the force per unit area.
• Catalyst – a substance that changes the rate of reaction without ‘taking part’ in the reaction.
• Surface area – the area of a solid chemical substance used in a chemical reaction.
• Volume – a measure of the amount of liquid.

You will find a template, example Frayer model and suggested answers for the term ‘collision theory’ in the PowerPoint slides. Find more examples and tips on how to use Frayer models in your teaching. 

Common misconceptions

Difficulties related to chemical change

• Misconception: if you can’t see it, it has disappeared or was never there. Avoid using language surrounding chemical change that confuses learners, eg in this practical try to avoid the phrase ‘the cross has disappeared’ instead use the phrase ‘you can no longer see the cross’.
• Misconception: all reactions are driven by two reactants. Demonstrate reactions with more and less, eg decomposition, than two reactants.
• Misconception: all reactants equally influence how fast a reaction proceeds. Introduce limiting reactants.

Application of collision theory

Learners can struggle to form a full answer including:

• How the changing variable affects the reactants’ behaviours/properties.
• How this relates to collision theory.
• The concept of rate (per unit of time).

There is a lot going on here, so it is important to provide a clear scaffold for learners. Initially, the use of a structure strip (see the example and suggested answer in the supporting resources) will help, followed by plenty of practice providing learners with similar but slightly different questions.

Catalysts

There are several misconceptions around the understanding of catalysts including:

• Catalysts ‘are not involved’ in chemical reactions.
• Catalysts break chemical bonds.
• Catalysts increase the yield of the product.
• Catalysts only affect the rate of the forward reaction.

Be careful with your language. The definition in some pre-16 specifications eg ‘catalysts change the rate of reaction but are not used up during the reaction’ can lead to misconceptions if the definition is not carefully unpacked and linked to the reaction profile, showing an alternative reaction pathway with a lower activating energy. This 5-minute demonstration clearly shows the catalyst is involved in the reactions as the colour changes from pink to green and back to pink.

Graphs

If asked to draw another line on a graph (eg, volume of gas produced (y-axis) versus time (x-axis)) learners can struggle to know how to draw it. Should the start go up more steeply? Where does it plateau? Encourage learners to tell the story of their graph. Linking what is happening in the reaction vessel and their observations to what the graph is showing. Using a Johnstone’s triangle approach can also help here. Download the PowerPoint slides for an example and suggested answers.

Drawing a tangent to a graph

Many learners find this challenging and depending on when you teach rates, they may be meeting the concept for the first time. Talk to your maths colleagues to find out when they teach tangents.