How to teach calculations through a green chemistry lens
Thinking back to my time in university labs, calculations seemed a part of the familiar routine: carry out a synthesis, analyse your product and calculate the yield and atom economy of the process. These calculations felt like a judgement on my practical prowess, they weren’t something I considered as meaningful in terms of green chemistry.
My experience was not unique. If I felt this way as a student, it is likely my own students struggle to conceptualise these calculations as a way of evaluating the efficiency of a chemical process. When scientists are prioritising moving towards circular processes, less waste and sustainable production, it’s crucial we highlight to learners how important these chemical calculations are.
My experience was not unique. If I felt this way as a student, it is likely my own students struggle to conceptualise these calculations as a way of evaluating the efficiency of a chemical process. When scientists are prioritising moving towards circular processes, less waste and sustainable production, it’s crucial we highlight to learners how important these chemical calculations are.
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Misconception buster for age range 16–18
Probe and develop understanding with multiple choice questions and differentiated follow-up tasks.
Download the resource from the Education in Chemistry website: rsc.li/4iODG96
What students need to know
Percentage yield and atom economy calculations are quite straightforward, therefore they understandably get lost within the depths of the chemical process – our main focus. Understanding the purpose of these calculations serves as a hook for learners to see the importance of chemical synthesis in the real world.
The calculations form the basis of the 12 principles of green chemistry. Use this as the lens through which you approach teaching them. Place the prominence on yield rather than process. This improves student understanding of the calculations, even when you explore them through abstract concepts. Show students how these simple calculations can help inform greener decision-making about chemical production processes. You will initiate a much more interesting classroom discussion at the end of a practical.
The calculations form the basis of the 12 principles of green chemistry (rsc.li/3DqFNzS). Use this as the lens through which you approach teaching them. Place the prominence on yield rather than process. This improves student understanding of the calculations, even when you explore them through abstract concepts. Show students how these simple calculations can help inform greener decision-making about chemical production processes. You will initiate a much more interesting classroom discussion at the end of a practical.
Common misconceptions
Students generally find percentage yield and atom economy calculations unproblematic but can easily mix these concepts up and find them difficult to evaluate in terms of the key factors which impact them. Exam boards often note these questions to be discriminatory questions. Use simple analogies to conceptualise the calculations; edible examples are a great starting point.
Support student understanding through the simple real-life example of baking cookies. This will build their understanding of losses in all chemical processes. Place the focus of these calculations on the sustainability and efficiency aspect to encourage students to think evaluatively about processes. Exploring the ideas through this context is shown to enrich the curriculum and engage all students. It refocuses them towards the green chemistry, rather than solely on the calculations.
Ideas for your classroom
The scenario: get students to imagine they are baking a batch of cookies for the group to make industrial processes and calculations more familiar.
‘Imagine you are baking a batch of cookies. You gather the flour, sugar, butter, eggs, chocolate chips and baking powder. You follow the recipe precisely, with the expectation of baking a perfect batch of 24 cookies. However, you end up with only 20 perfect, edible cookies.
Neither the percentage yield or atom economy are 100% – but why? Is it a judgement of our baking prowess, an inevitable part of any process, or a combination of both?’
| Percentage yield | Atom economy | |
|---|---|---|
| Explanation | Measures the efficiency by comparing the theoretical yield (the mass of 24 cookies) with the actual yield (the mass of only 20 edible cookies). | A theoretical measure of the extent to which you maximised ingredient use by comparing the desired product (cookies) with any wastage – how much of our ingredients ended up as useful product? 500 g of ingredients only produced 480 g of cookies. |
| Equation | Percentage yield = (actual yield/theoretical yield) × 100 | Atom economy = (mass of atoms in desired product/total mass of atoms within all ingredients) × 100 |
| Calculation |
= (20/24) × 100 = 83% |
= (480/500) × 100 = 96% |
Brainstorm where they may have had losses – dough stuck to the bowl and the spatula, the transfer of ingredients from one bowl to another, spillages or licking the spoon – to help students understand how they contribute to the yield being less than 100%.
Think about non-useful product that contributes to the atom economy being less than 100% – burned pieces, water loss due to evaporation, raising agents producing gases which escape. Ask students to write equations for the process, highlighting desired products and byproducts.
More resources
- Use these chemistry worksheets to prompt students to think critically about how to make everyday processes more environmentally friendly and sustainable.
- Encourage students to explore sustainable development through calculations with this green chemistry worksheet.
- Carry out the preparation of 2-hydroxybenzoic acid (or use the aspirin screen experiment). Then get students to discuss their percentage yields and where losses have occurred.
- Show how Florence, chief technology officer and co-founder of Lixea, turns waste wood into renewable raw materials and fuel to help reduce the world’s reliance on petroleum.
Then move on to industrial applications. Compare two methods of producing ethanol – this resource shows efforts to make the process more green. Introduce examples students may not have come across to stretch them beyond the specification, for example, the greener process for synthesising ibuprofen.
Then move on to industrial applications. Compare two methods of producing ethanol – this resource shows efforts to make the process more green (rsc.li/4iKDZ4K). Introduce examples students may not have come across to stretch them beyond the specification, for example, the greener process for synthesising ibuprofen (bit.ly/49KHggx).
More resources
- Use these chemistry worksheets to prompt students to think critically about how to make everyday processes more environmentally friendly and sustainable: rsc.li/49UTAuM
- Encourage students to explore sustainable development through calculations with this worksheet: rsc.li/4ftL5HW
- Carry out the preparation of 2-hydroxybenzoic acid, then get students to discuss their percentage yields and where losses have occurred: rsc.li/4gKmLCP
- Show how Florence, chief technology officer and co-founder of Lixea, turns waste wood into renewable raw materials and fuel to help reduce the world’s reliance on petroleum: rsc.li/4iIrQgP
Checking for understanding
Integrate the calculations regularly into lessons to help students recall the process of calculating percentage yield and atom economies. Provide opportunities to discuss them in a sustainability context.
Find opportunities to show students alternative synthetic routes that produce the same products and encourage them to evaluate these through yield and atom economy. This works well for aspirin synthesis, where students can analyse their own work.
Find opportunities to show students alternative synthetic routes that produce the same products and encourage them to evaluate these through yield and atom economy. This works well for aspirin synthesis, where students can analyse their own work (rsc.li/426OwBB).
Explore the topic when teaching chemical equilibria. Students already evaluate the optimum conditions for chemical processes such as the Haber process in terms of rate and yield – get them to consider the green aspects as well.
Take-home points
- Maintain a green chemistry approach to teaching percentage yield and atom economy calculations to conceptualise the idea. Build skills in evaluating and thinking critically about what the numbers mean and aspects of industrial chemical processes that can impact them.
- Keep it simple. First, make use of analogies and models, then move on to more complicated chemical processes.
- Revisit these ideas whenever you can find applications to encourage the conversation to continue.
Article and resource written by Louise Glynn, a chemistry teacher at an independent girls’ school in Dulwich
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