C2aL14  Percentage Yield

Key Words

Atom economy - proportion of atoms that react that end up in useful products.

Percentage yield - how much product is formed as a fraction of what should be formed in theory (multiplied by 100 to give a percentage).

Product - new substance made in a reaction

Reaction - chemical interaction in which new substances are formed.

Sustainable development - development that doesn't (literally) cost the Earth.

Theoretical Yield - amount of product that should be formed, based on the reaction equation.

Yield - amount of product that is actually formed.

Test Yourself

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Grade E

The yield is the amount of product that is formed in a reaction.  Think about the reaction we did in the last lesson, the reaction of magnesium and oxygen:

 

2Mg + O2 ® 2MgO

When we reacted 6 g Mg with 4 g O2, we got 10 g MgO.  That is the theoretical yield, the yield we get if every atom reacts, and we don't lose anything at all.

Suppose we weighed the product, and found that we had 8.5 g.  We can work out the percentage yield:

Percentage Yield =       actual yield     × 100 = 8.5 = 85 %

                         theoretical yield             10

Why the loss?  In this case, we have to open the crucible lid to let in oxygen, otherwise the reaction is not completed.  Some of magnesium oxide might have escaped as the fine white powder you see when magnesium is burned.  Or the reaction might not have gone to completion.

This reaction has a high atom economy.  Most of the magnesium (and oxygen) end up as the product we want.

Grade C

We may end up with a yield less than 100 % because:

  • the reaction is reversible;
  • the reaction didn't go to completion;
  • some product may get lost;
  • some reactants may react in other ways, to produce substances we don't want.

Reactions need to have a high atom economy, otherwise we need to use a lot of reactants.  High atom economy reactions form part of sustainable development, because they don't waste natural resources.

We can't get more than 100 % yield from the reaction, since that would involve mass being created from nothing.  If you do get more than 100 %, you may have made a mistake in:

  • your reaction equation;
  • your measurements;
  • your calculation.

Check it out.

Grade A

The percentage yield is important for industrial chemists, as wasted materials mean wasted money.  They need to ensure that their expensive resources are used as efficiently as possible.