Edexcel IGCSE Chemistry 复习笔记 1.5.5 Calculate Percentage 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 & 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:

Answer

• • Actual yield of copper(II) sulfate = 1.6 g
  • Percentage yield of copper(II) sulfate = (1.6 / 2.0) x 100
  • Percentage yield = 80%