Yields (OCR GCSE Chemistry A (Gateway))

Reaction Yield

• Yield is the term used to describe the amount of product you get from a reaction

• In practice, you never get 100% yield in a chemical process for several reasons

• These include:

  • Some reactants may be left behind in the equipment

  • The reaction may be reversible and in these reactions a high yield is never possible as the products are continually turning back into the reactants

  • Some products may also be lost during separation and purification stages such as filtration or distillation

  • There may be side reactions occurring where a substance reacts with a gas in the air or an impurity in one of the reactants

  • Products can also be lost during transfer from one container to another

Actual Yield & Theoretical Yield

• The actual yield is the recorded amount of product obtained

• The theoretical yield is the amount of product that would be obtained under perfect practical and chemical conditions

• It is calculated from the balanced equation and the reacting masses

• The percentage yield compares the actual yield to the theoretical yield

• For economic reasons, the objective of every chemical producing company is to have as high a percentage yield as possible to increase profits and reduce costs and waste

Percentage Yield

• The percentage yield is a good way of measuring how successful a chemical process is

• There are often several methods of creating a compound and each method is called a reaction pathway

• Reaction pathways consist of a sequence of reactions which must occur to produce the required product

• Companies often investigate and try out different reaction pathways and these are then compared and evaluated so that a manufacturing process can be chosen

• The percentage yield of each pathway is a significant factor in this decision making process

• The equation to calculate the percentage yield is: