Percentage Yield

• % yield tells you about the overall success of an experiment. It compared what you think you should get (theoretical yield) with what you get in practice (actual yield)
• % yield compares actual and theoretical yield
• The amount of product you get is known as the yield
• The more reactants you start with, the higher the actual yield will be. But the percentage yield doesn't depend on the amount of reactants you started with - it's a percentage
• (Mass of product actually made (g) / maximum theoretical mass of product (g)) x 100 = % yield
• The maximum yield can be calculated from the balanced reaction equation
• % yield is always somewhere between 0 and 100%
• 100% yield means that you got all the product you expected to get
• 0% yield means that no reactants were converted into product, i.e. no product was made
• Industrial processes should have as high percentage yield as possible to reduce waste and reduce cost
• Yields are always less than 100%
• In real life, you never get 100% yield. Some product or reactant always gets lost along the way - and that goes for big industrial processes as well as school lab experiments. How this happens depends on what sort of reaction it it and what apparatus is being used
• Lots of things can go wrong, but some common problems are:

  Not all reactants react to make a product - In reversable reactions, the products can turn back into reactants, so the yield will never be 100%. E.g. in the Haber process, at the same time as the reaction N2 + 3H2 -> 2NH3 is taking place, the reverse reaction 2NH3 -> N2 + 3H2 is also happening. The reactants will never get completely used up
• Lots of things can go wrong, but some common problems are:

  There might be side reactions - The reactants sometimes react differently to how you expect. They might react with gases in the air, or impurities in the reaction mixture, so they end up forming extra products other than the ones you want
• Lots of things can go wrong, but some common problems are:

  You lose some product when you separate it from the reaction mixture -When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or a bit of solid

  • If you want to keep the liquid, you'll lose the bit that remains with the solid and filter paper
  • If you want to keep the solid, some of it'll get left behind when you scrape it off the filter paper

  You'll also lose a bit of material when you transfer it from one container to another - even if you manage not to spill it