Help your students to observe and monitor reactions, and recognise their use in everyday life
Bubbles, fizzing, colours, heat! Visible signs of a chemical reaction are often very exciting to younger students in their early science education, conjuring enthusiasm and curiosity for the subject. Ask young students why they think there is a chemical reaction and the answers are bound to include, ‘look, there are bubbles’, ‘it changed colour’ or ‘it’s getting hot!’ These obvious signs pave the way for a more detailed look at what is happening in a chemical reaction, even when signs of a reaction are not so obvious to the naked eye.
Why monitor reactions?
For our students, trying to figure out what is going on in the contents of a reaction vessel may seem very remote from everyday life. However, giving examples of real-world contexts can help them to realise the advantages of checking what is going on. For example, in making jam, the fruity mixture is observed boiling in the pan – but in order to set successfully upon cooling, is it boiling vigorously enough and for how long? In monitoring the reaction, you can put small samples onto a cold plate to see if it looks set, and if not, the jam is kept boiling. An easier method uses a sugar thermometer to monitor temperature, ensuring that the jam reaches 105°C where it will set.
Monitoring the pH of soil is useful in helping to maintain optimal conditions for the uptake of nutrients and growth of different crops. Owners of solar panels (photovoltaics) can monitor the output of UV light and the conversion of sun’s energy to electricity so they can maximise the use of this ‘free’ electricity as it is produced, rather than paying to import it at a later time.
The use of thin layer chromatography (TLC) and other chromatography techniques to monitor the progress of reactions in modern organic synthesis and drugs research remains effective. A tiny sample of the reaction mixture is examined on a TLC plate against the starting material and also the product if it’s known. Is there a new product – a new spot on the TLC – and is there still a spot for the starting material? The chemist can judge if the reaction mixture is ready or should be left going for a longer time. The paracetamol resource on Learn Chemistry, while more advanced, includes a practical example of monitoring by TLC.
The use of thin layer chromatography (TLC) and other chromatography techniques to monitor the progress of reactions in modern organic synthesis and drugs research remains effective. A tiny sample of the reaction mixture is examined on a TLC plate against the starting material and also the product if it’s known. Is there a new product – a new spot on the TLC – and is there still a spot for the starting material? The chemist can judge if the reaction mixture is ready or should be left going for a longer time. The paracetamol resource on Learn Chemistry, while more advanced, includes a practical example of monitoring by TLC (rsc.li/2E3y1ej).
Progression in monitoring reactions
| 7–11 years (Key stage 2) | 11–14 years (Key stage 3) | 14–16 years (Key stage 4) | 16–18 years (Key stage 5) |
|---|---|---|---|
|
Make systematic and careful observations and, where appropriate, take accurate measurements using standard units, using a range of equipment, including thermometers and data loggers |
Make and record observations and measurements using a range of methods for different investigations Interpret observations and data, including identifying patterns and using observations, measurements and data to draw conclusions |
Use of appropriate apparatus and techniques for conducting and monitoring chemical reactions, including appropriate reagents and/or techniques for the measurement of pH in different situations |
Measure pH using indicators, pH meter or pH probes to monitor a titration reaction Measure rates of reaction by at least two different methods, eg (i) an initial rate method such as a clock reaction; (ii) a continuous monitoring method |
Underpinning chemistry and progression
The idea that reactions are the result of a rearrangement of particles into new substances and that mass is conserved during a reaction is an important concept for students to understand. And just as mass is conserved: energy is conserved too. While chemical reactions involve energy transfers, the overall energy of the system doesn’t change. The concept of bonds breaking and new bonds forming leading to the overall net energy change of a reaction should be fully understood, and it is associated with a change in temperature which can be measured.
Basic chemical concepts underpinning monitoring reactions
Pre-11
- Macroscopic (things we can see) to sub-microscopic (things that are too small to see)
- Conservation of mass: no matter destroyed or created, simply the rearrangement of particles
11–14
- Signs of a chemical reaction – change of temperature, colour change, bubbles
- Mixtures and compounds
- Grouping substances as acids and alkalis (use of indicator or indicator paper)
- Word equations as a simple way to describe chemical reactions and formulae equations
14–16
- Classes of reaction, eg reactions of acids, neutralisation, decomposition
- Use of chemical indicators in reactions of acid and bases/alkalis
- Energy changes in reactions – exothermic and endothermic (as sum of bonds broken/formed)
- Modelling reactions
- Writing a balanced equation (for titration and energy calculations)
- The mole and ratios from equations (for titration and energy calculations)
Post-16
- TLC in organic synthesis, eg preparation of paracetamol (understanding chemical purity and mixtures)
- Choosing the correct indicator for various strong/weak acid/alkali titrations
It is worth checking your students’ understanding of the basic concepts of chemical reactions before tackling more advanced practical activities involving monitoring chemical reactions.
| Resource | Comment |
|---|---|
|
What happens to particles when new materials are made? (rsc.li/2BTcp3b) |
Assessment for Learning is an effective way of actively involving students in their learning. Students’ understanding of particles and chemical reactions can be explored with activities, lesson plans, student worksheets and suggestions of how to organise activities |
|
Words and formulas equations (rsc.li/2KW7XUb) |
These activities allow students to practise word and symbol equations and use patterns in reactions to make predictions about other reactions |
|
Acids and bases (rsc.li/2PmGBHg) |
Concept map of acids and alkalis to reinforce and revise what students have learned about acids and alkalis. |
|
Sorting heat from temperature (rsc.li/2E3hQxS) |
This article will encourage you to think! It discusses concepts underpinning energy changes in chemical reactions, particularly temperature and heat. |
Practical problems and suggested solutions
Monitoring reactions by temperature changes
Practical activities that are engaging, robust and allow clear observations and measurements to interpret, are important for developing understanding. An example of a reliable practical activity monitoring temperature change (often required across GCSE specifications), is the neutralisation reaction of a strong acid with a strong alkali. Small and measured aliquots of a strong acid, eg hydrochloric acid from a burette can be added to a strong alkali in a polystyrene cup with a thermometer to measure the increase in temperature as the neutralisation reaction proceeds. The reaction gives out heat energy as the net result of bonds breaking and new bonds forming. This activity can be used to determine the point of neutralisation by identifying the maximum temperature as the point of neutralisation.
Practical activities that are engaging, robust and allow clear observations and measurements to interpret, are important for developing understanding. An example of a reliable practical activity monitoring temperature change (often required across GCSE specifications), is the neutralisation reaction of a strong acid with a strong alkali (rsc.li/2zLW4vo). Small and measured aliquots of a strong acid, eg hydrochloric acid from a burette can be added to a strong alkali in a polystyrene cup with a thermometer to measure the increase in temperature as the neutralisation reaction proceeds. The reaction gives out heat energy as the net result of bonds breaking and new bonds forming. This activity can be used to determine the point of neutralisation by identifying the maximum temperature as the point of neutralisation (rsc.li/2RvCmes).
One problem with this experiment is students not knowing when to stop adding the acid. Even after the temperature falls, if they stop too soon it will not be possible to draw the downward ‘line of best fit’ effectively that is needed to extrapolate the end point. A possible solution is for students working in pairs to be encouraged to plot the graph as they go along, see it develop, and think about what exactly is happening in the flask at various points. Using data loggers would assist plotting it as the experiment proceeds.
Representing experimental data in a graph and drawing lines of best fit for extrapolating a value is worthwhile practice for students’ general practical skills and data handling.
Questions to encourage your students’ understanding
- Why does the temperature rise?
- What is the significance of the extrapolated temperature maximum? Why is this not exactly the same as the highest measured temperature?
- Why does the temperature fall?
- What could you add to the set-up to show explicitly that neutralisation occurred?
- How could the set-up be modified to minimise heat loss to surroundings even more?
- If sulfuric acid was used instead, what would the graph look like?
- Do you really need a burette for this activity? What else could you use?
The reaction of acid and alkali in this activity gives a clear change in temperature. These practical techniques can be applied in different contexts, giving students plenty of practice. For example, what would happen if the strong acid was reacted with magnesium metal instead of alkali? That is, finely divided metals or metal filings can be added to a strong acid while monitoring the change in temperature (see box).
The reaction of acid and alkali in this activity gives a clear change in temperature. These practical techniques can be applied in different contexts, giving students plenty of practice. For example, what would happen if the strong acid was reacted with magnesium metal instead of alkali? That is, finely divided metals or metal filings can be added to a strong acid while monitoring the change in temperature (rsc.li/2SvcE9M, see box).
An exploration of temperature change in the context of heat energy from different alcohols as fuels can investigate the effect of increasing carbon chain length of the alcohol on the energy given out. Students could develop their own hypothesis and test it.
An exploration of temperature change in the context of heat energy from different alcohols as fuels can investigate the effect of increasing carbon chain length of the alcohol on the energy given out (rsc.li/2rmFzkR). Students could develop their own hypothesis and test it.
Synoptic approach
Example: measuring temperature change from metals reacting with hydrochloric acid
Some possible questions to get your students started:
- Which powdered metals will you investigate?
- Where are the metals positioned in the reactivity series?
- What equipment will you need?
- How could you minimise heat loss?
- What gas is given off?
Planning the activity
Explain how you would carry out each step in your investigation. Give details of all the equipment and chemicals needed and experimental procedures.
Monitoring reactions by the production of gases
The evolution of gas from reactions can often be seen, yet counting numerous bubbles would be tricky, and methods of collecting gases are needed. For a younger age group, the catalytic decomposition of hydrogen peroxide by testing several catalysts, in parallel, provides a captivating practical activity. Washing up liquid already added to each peroxide sample before dropping in a catalyst provides the means of visualising the volume of evolving gas as foam – giving rise to the ‘elephant’s toothpaste’ familiar name – and allowing a comparison of which catalyst provides the most foam immediately.
The evolution of gas from reactions can often be seen, yet counting numerous bubbles would be tricky, and methods of collecting gases are needed. For a younger age group, the catalytic decomposition of hydrogen peroxide by testing several catalysts, in parallel, provides a captivating practical activity (rsc.li/2rkzghM). Washing up liquid already added to each peroxide sample before dropping in a catalyst provides the means of visualising the volume of evolving gas as foam – giving rise to the ‘elephant’s toothpaste’ familiar name – and allowing a comparison of which catalyst provides the most foam immediately.
When a more quantitative approach is required for collecting gases, there are several options. These include gas syringe (find practical tips in How to teach rate experiments) upturned measuring cylinder and upturned burette over water (see figure). These all might be interchangeable as gas collection vessels, but which method would your students choose for a particular activity? It depends upon the experiment, the measuring accuracy required, availability of equipment and upon the solubility of the gas evolving. If it’s insoluble it can be collected over water. Giving your students practice with lots of example contexts will help them cope with an unfamiliar context.
When a more quantitative approach is required for collecting gases, there are several options. These include gas syringe (find practical tips in How to teach rate experiments) upturned measuring cylinder and upturned burette over water (see figure). These all might be interchangeable as gas collection vessels, but which method would your students choose for a particular activity? It depends upon the experiment, the measuring accuracy required, availability of equipment and upon the solubility of the gas evolving. If it’s insoluble it can be collected over water. Giving your students practice with lots of example contexts will help them cope with an unfamiliar context.
| Reaction mixture | Gas evolved | Suggested collection method |
|---|---|---|
|
Magnesium + hydrochloric acid |
Hydrogen |
A, B, C (over water possible). C is the most accurate if this reaction is used to determine the volume of one mole of gas. |
|
Zinc carbonate decomposition (heating) |
Carbon dioxide |
A is more accurate for measuring (as carbon dioxide is soluble in water). |
|
Zinc + sulfuric acid |
Hydrogen |
A, B, C (over water possible) – a less familiar example of a metal + acid reaction. |
|
Photosynthesis |
Oxygen |
B or could use any method, although slow. Normally inverted test tube over water to test gas, but could be measured. |
In filling the measuring cylinder with water and inverting it, students can spill the contents easily. Making sure their hand is firmly over the brimming full measuring cylinder until it is fully inverted and submerged in the trough will require practice.
Colour changes
Significant practical work associated with colour changes at school level involves acids, alkalis and indicators, and requires titrations as a key practical technique. A popular activity (often required) is the titration of a strong acid and strong alkali using a pH indicator. While the chemistry is quite clear, the practical process can be enormously challenging for some students. Moles and titrations: Scary stuff? Looks at possible problems and solutions, and you can find more supporting resources on Learn Chemistry, including a possible homework task of a screen titration. This interactive simulation, with example contexts, allows students to gain familiarity and confidence with practical procedures in order to free up class time for developing practical skills. For example, they can learn the order of the steps they are taking and why: they can take readings, practise calculations and they can appreciate time savers such as not refilling the burette to the zero mark each time.
Significant practical work associated with colour changes at school level involves acids, alkalis and indicators, and requires titrations as a key practical technique. A popular activity (often required) is the titration of a strong acid and strong alkali using a pH indicator (rsc.li/2KWhfiO). While the chemistry is quite clear, the practical process can be enormously challenging for some students. Moles and titrations: Scary stuff? looks at possible problems and solutions (rsc.li/2BT8fbz), and you can find more supporting resources on Learn Chemistry (www.rsc.org/learn-chemistry), including a possible homework task of a screen titration (rsc.li/2rnvIv2). This interactive simulation, with example contexts, allows students to gain familiarity and confidence with practical procedures in order to free up class time for developing practical skills. For example, they can learn the order of the steps they are taking and why: they can take readings, practise calculations and they can appreciate time savers such as not refilling the burette to the zero mark each time.
Colour changes of pH indicators for reactions of acids and bases, or clear solutions becoming opaque in rate experiments, can bring up the practical issue of judging the end point. You can find helpful tips in How to Teach Rate Experiments and How to Teach Observation Skills. Electrolysis experiments also monitor reactions by colour changes or gases produced, and Practical Electrolysis will help with practical problems and solutions.
Colour changes of pH indicators for reactions of acids and bases, or clear solutions becoming opaque in rate experiments, can bring up the practical issue of judging the end point. You can find helpful tips in How to Teach Rate Experiments (rsc.li/2Qxu6gI) and How to Teach Observation Skills (rsc.li/2Uiw6Iz). Electrolysis experiments also monitor reactions by colour changes or gases produced, and Practical Electrolysis will help with practical problems and solutions (rsc.li/2rkBkX4).
Morag Easson is a Royal Society of Chemistry trainer and independent science education consultant
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