6.5 - Why Yield Is Less Than Expected

Why a reaction gives less product than expected

• The mass of product a reaction should make can be calculated from the balanced equation and the masses of the reactants used.
• The mass actually obtained is almost always less than this theoretical amount.

1. The reaction doesn't finish

• A reaction stopped before it's complete leaves unreacted starting material.
• A reversible reaction can't fully complete because products turn back into reactants (e.g. the Haber process making ammonia).

2. Side reactions occur

Reactants can react with gases in the air or with impurities, forming unintended products instead of the wanted one.

3. Product is lost during separation and purification

• Solid product is left behind stuck to filter paper or container walls.
• Liquid product is lost wetting the filter paper or remaining on glassware.
• A gas product can escape into the air before it's collected.