EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.5.5 Calculate Percentage Yield

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.5.5 Calculate Percentage Yield

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:3.3.1-Percentage-Yield-Formula-1

 

Worked Example

Copper(II) sulfate may be prepared by the reaction of dilute sulfuric acid on copper(II) oxide. A student prepared 1.6 g of dry copper(II) sulfate crystals. Calculate the percentage yield if the theoretical yield is 2.0 g.

 

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%

     

 

Exam Tip

The actual yield can be determined by experiment only, while the theoretical yield can be calculated assuming there is 100% conversion of reactants to products.

转载自savemyexams

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