Chem 2 - Bonding and Calculations - % Yield and Reversible Reactions

The percentage yield compares actual yield and the predicted (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, its a percentage

To calculate percentage yield, use this equation:

Percentage yield = actual yield (grams)/predicted yield (grams) x 100

The percentage yield is always somewhere between 0 and 100%. A 100% yield means you get all the product you expected, but percentage yields are always less than 100%

Even though no atoms are gained or lost in reactions, in real life you never get a 100% percentage yield. Some product or reactant always gets lost aling the way.

There are several reasons for this:

- The reaction is reversible. This means that the reactants will never be completely converted to the products, as the reaction goes both ways. Some of the products are always reacting together to change back to the orginal reactants. This will mean a lower yield.

- When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or solid. So, some of the product may be lost when its seperated from the reaction mixture, meaning a lower yield.

- Things don't always go exactly to plan. Sometimes there are unexpected reactions happening that use up the reactants. This means there isn't as much reactant to make the product you want, meaning a lower yield.