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3.3 Yield and atom economy of chemical reactions

3.3.1 Percentage yield

_{(stolen from spec)}
- Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because:

• the reaction may not go to completion because it is reversible
• some of the product may be lost when it is separated from the reaction mixture
• some of the reactants may react in ways different to the expected reaction.

Calculating Yield

The percentage yield can be calculated using the formula:
Percentage Yield = (Actual Yield / Theoretical Yield) × 100%

• You can calculate the percentage yield of a product using the actual yield (the amount you actually get from the experiment) and the theoretical yield (the maximum possible amount predicted by the balanced equation).
• Example: If the actual yield of a product is 8g and the theoretical yield is 10g:
  Percentage Yield = (8g / 10g) × 100% = 80%
• To calculate the theoretical mass of a product:
  1. Write the balanced chemical equation.
  2. Calculate the number of moles of reactant using its mass and molar mass.
  3. Use the mole ratio from the equation to find the moles of product.
  4. Calculate the mass of product using its molar mass.
  Example: If 5g of reactant A (molar mass = 50 g/mol) reacts to produce product B (molar mass = 20 g/mol) in a 1:2 ratio:
  Moles of A = 5g / 50g/mol = 0.1 mol
  Moles of B = 0.1 mol × 2 = 0.2 mol
  Mass of B = 0.2 mol × 20g/mol = 4g
• You can then compare the theoretical yield with the actual yield to find the percentage yield.
  For example, if you actually obtained 3g of product B:
  Percentage Yield = (3g / 4g) × 100% = 75%

3.3.2 Atom economy

- The atom economy (atom utilisation) is a measure of the amount of starting materials that end up as useful products.
- It is similar to percentage yield, just operates on different variables.
- It is important for sustainable development and for economic reasons to use reactions with high atom economy.
- The formula for atom economy is:
Atom Economy = (Relative formula mass of desired product / Sum of relative formula masses of all reactants) × 100%

Calculating Atom Economy

To calculate the atom economy of a reaction:

1. Write the balanced chemical equation.
2. Determine the relative formula mass of the desired product.
3. Determine the sum of the relative formula masses of all reactants.
4. Use the formula to calculate the atom economy.