Choosing industrial reaction pathways Revision Notes | GCSE Edexcel Higher Chemistry

1. Why Atom Economy Matters

Imagine you're making a cake and half your ingredients end up in the bin. That's wasteful and expensive. The same problem exists in chemistry — many reactions produce unwanted by-products that become waste.

Atom economy tells us how efficient a reaction is at converting reactants into the product we actually want. It's become crucial for sustainable chemistry because waste costs money and harms the environment.

A reaction with 100% atom economy means every atom from the reactants ends up in the desired product — no waste at all. Most real reactions aren't this perfect, but the closer we get, the better.

2. The Atom Economy Formula

The key insight is that we only count the desired product in the numerator, even if the reaction makes other products too. This shows us what fraction of our starting materials actually becomes useful.

**Trap**: Many students include all products in the numerator. Don't do this — only count the product you want. The rest is waste, which is exactly what atom economy measures.

3. Calculating Atom Economy Step by Step

Let's work through a real example to see how this calculation works in practice. The method is always the same: identify the desired product, find the masses, then apply the formula.

Worked example: Calculating Atom Economy for Ammonia Production

Calculate the atom economy for making ammonia: N₂ + 3H₂ → 2NH₃ [4 marks]

1. Desired product = NH₃ (ammonia) — This is what we want to make
2. Mr of NH₃ = 14 + (3 × 1) = 17 But we make 2NH₃, so total = 2 × 17 = 34 — [1 mark] Don't forget the coefficient in the equation
3. Mr of N₂ = 2 × 14 = 28 Mr of 3H₂ = 3 × (2 × 1) = 6 Total reactants = 28 + 6 = 34 — [1 mark] Include all substances on the left side
4. Atom economy = $\frac{34}{34} \times 100\% = 100\%$ — [2 marks] Perfect efficiency — no waste!

Answer: 100% atom economy

Notice how the Haber process has 100% atom economy. Every atom from nitrogen and hydrogen ends up in ammonia — there are no by-products. This is one reason why it's such an important industrial process.

4. Comparing Different Reaction Pathways

Often there are multiple ways to make the same product. Atom economy helps us choose the best route, but it's not the only factor. Let's see how chemists evaluate different pathways.

Worked example: Comparing Routes to Make Ethanol

Compare these two routes to make ethanol (C₂H₅OH): Route A: C₂H₄ + H₂O → C₂H₅OH, Route B: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

1. Mr of C₂H₅OH = (2×12) + (6×1) + 16 = 46 Mr of C2H4 = (2×12) + (4×1) = 28; Mr of H2O = (2×1) + 16 = 18; total Mr of reactants = 46 Atom economy = $\frac{46}{46} \times 100\% = 100\%$ — Perfect efficiency — hydration of ethene
2. Mr of desired product = 2 × 46 = 92 Mr of glucose = (6×12) + (12×1) + (6×16) = 180 Atom economy = $\frac{92}{180} \times 100\% = 51\%$ — Lower efficiency — fermentation produces CO₂ waste
3. Route A: Uses fossil fuels (ethene from oil) Route B: Uses renewable glucose from plants — Atom economy isn't everything!
4. Route A has higher atom economy but Route B is more sustainable long-term — Real decisions consider multiple factors

Answer: Route A: 100% atom economy, Route B: 51% atom economy, but Route B uses renewable resources

This example shows why atom economy alone doesn't determine the best pathway. Route A wastes no atoms but depends on finite oil reserves. Route B wastes nearly half the atoms as CO₂ but uses renewable plant materials.

5. Real Industrial Applications

Let's examine how major chemical companies use atom economy thinking to design better processes. The pharmaceutical industry particularly struggles with low atom economy — many drug syntheses waste 90% of their starting materials.

Consider aspirin production. The traditional route has poor atom economy, but the by-product (ethanoic acid) can be recycled. Modern green chemistry focuses on designing reactions with higher atom economy from the start.

Worked example: Evaluating Aspirin Production

Aspirin is made by: C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH. The aspirin sells for £50/kg, ethanoic acid for £2/kg. Evaluate this process.

1. Mr of aspirin (C₉H₈O₄) = (9×12) + (8×1) + (4×16) = 180 Mr of reactants = 138 + 102 = 240 Atom economy = $\frac{180}{240} \times 100\% = 75\%$ — Reasonably good atom economy
2. By-product: CH₃COOH (ethanoic acid) Mr = 60, so 25% of atoms form this Value ratio: £50/kg ÷ £2/kg = 25:1 — By-product has much lower value
3. Per 240g reactants: Aspirin: 180g × £50/kg = £9.00 Ethanoic acid: 60g × £2/kg = £0.12 Total value = £9.12 — Most value comes from desired product
4. Good atom economy (75%) and by-product has some value, making this an acceptable industrial process — Both efficiency and economics matter

Answer: 75% atom economy with valuable by-product makes this a viable industrial process

6. Exam Technique and Common Mistakes

Atom economy questions are mark-friendly if you know the method, but there are several traps that catch students out. Let's make sure you avoid them and pick up full marks.

**Common Mistake**: Students often forget that atom economy is theoretical — it assumes 100% yield. In reality, you never get 100% yield, so Atom economy and percentage yield are separate measures at GCSE and should be considered separately..

**Exam Language**: When explaining why atom economy matters, use phrases like "reduces waste", "more sustainable", "better use of resources", and "lower environmental impact". Examiners look for these key terms.

Remember that in Higher Tier questions, you might need to evaluate different reaction pathways. Structure your answer by comparing atom economy first, then discussing other factors like yield, rate, raw material costs, and by-product usefulness. Always reach a conclusion about which pathway is best overall.

At GCSE, atom economy tells you how much of the reactant mass ends up in the desired product, while percentage yield tells you how much product was actually obtained compared with the theoretical amount. They are different ideas and should not be multiplied together as if they form one extra formula.