3.3 - Yield & Atom Economy of Chemical Reactions

Why isn’t it always possible to obtain the theoretical amount of product in a chemical reaction?

• The reaction may not go to completion because it’s reversible.

• Some of the product may be lost when it’s separated from the reaction mixture.

• Some of the reactants may react in ways different to the expected reaction (side reactions may occur).

What is the equation to find percentage yield?

% yield = (actual mass of a product ÷ max theoretical mass of product) × 100

Calculate the percentage yield from the following data.

45, 24, 20 (in this order)

What is the % yield of NH₃ if 40.5 g NH is produced from 20.0 mol H₂ and excess N₂?

• N₂ + 3H₂ → 2NH₃

• 13.3 × 17 (M of NH) = 227

• (40.5 ÷ 227) × 100 = 17.8%

What is atom economy?

• A measure of the amount of starting materials that end up as useful products.

• It’s a ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants.

Look at the equations for the two reactions that produce CuCl₂

• Reaction 1: CuCO_{3 (s)} + 2HCl_(aq) → CuCl_{2 (aq)} + H₂O_(l) + CO_{2 (g)}

• Reaction 2: CuO_(s) + 2HCl_(aq) → CuCl_{2 (aq)} + H₂O_(l)

• Reactive formula masses: CuO = 79.5 || HCl = 36.5 || CuCl₂ = 134.5 || H₂O = 18

Which reaction has a better atom economy?

• Reaction 2

• Total formula mass of reactants = 152.5

• Formula mass of CuCl₂ = 134.5

• (134.5 ÷ 152.5) × 100 = 88.2%