HOW TO CALCULATE THE YIELD OF A REACTION

HOW TO CALCULATE THE YIELD OF A REACTION

In chemistry, especially in organic chemistry, it is very important to calculate the yield of any reaction. It is reported in percentage. The percent yield of a reaction is an indication of the feasibility of reaction, reactivity and compatibility of the reactants, solvents, and reagents used in the reaction, and validity of the methodology employed in the reaction. This is the reason why scientists talk big about the high yields of products in their synthetic accomplishments. Percent yield is calculated from the following simple equation:

% YIELD = ACTUAL YIELD / THEORETICAL YIELD x 100

Where: Actual yield = Amount of product obtained practically after carrying out the reaction

Theoretical yield = Amount of product expected after calculation from the balanced chem. Eqn.

Following are certain steps needed for correct calculation of the percent yield.

1.      The yield is calculated from the balanced chemical equation, showing the correct molar ratio of reactants used in the reaction.

2.      It is important to pin point the limiting reactant in the reaction. Yield is always calculated from the amount of limiting reactant taken.

3.      If all the reactants are taken in the required molar ratios, as in the balanced chemical equation (no limiting reactant), then the yield may be calculated from any reactant.

FINDING THE LIMITING REACTANT

Well, this is quite simple. For example, if according to the balanced chemical equation, 1 mole of A reacts with 2 moles of B to produce 1 mole of product C. Then reactants A and B must be taken in 1:2 molar ratio. If 0.01 moles of A are taken, then 0.02 moles of B must be taken to produce 0.01 moles of C. In this case none of A or B is a limiting reactant. Please, do not consider A as the limiting reactant, only because it is taken half the moles of B. The amounts taken are exactly according to the chemical equation.

If any of the two reactants, A or B, are taken in less quantity of what is required, then that reactant will be the limiting reactant. There are two conditions:

a.       If 0.005 moles of A are taken along with 0.02 moles of B, then A is the limiting reactant as it is taken in lesser quantity as demanded by the balanced chemical equation.

b.      If 0.01 moles of A are taken along with, say 0.01 moles of B, the B is the limiting reactant because the quantities taken do not fulfil the molar ratio requirements (1:2).

So in case (a), A is the limiting reactant and the yield will be calculated from A. From balanced chemical eqn, if 1 mole of A produces 1 mole of C, then 0.005 moles of A will produce 0.005 moles of C.

In case (b), B is the limiting reactant and the yield will be calculated from B. From balanced chemical eqn, 2 moles of B produces 1 mole of C, then 0.01 moles of B will produce 0.005 moles of C.

Therefore, be careful in pointing out the limiting reactant before calculating yields of reactions.

Let’s see how percent yield is calculated, with the help of an example:

According to balanced chemical equation, 1 mole of benzoic acid reacts with 1 mole of methanol to produce 1 mole of methyl benzoate.

If 0.5 moles of benzoic acid and 0.6 moles of methanol are taken, how much methyl benzoate would be produced?

First we need to know which of the two reactants, is the limiting reactant.

For 1 mole of benzoic acid, 1 mole of methanol is required.

We have taken 0.5 moles of benzoic acid, which would require 0.5 moles of methanol. The amount of methanol taken is in excess (0.6 moles). Therefore, benzoic acid is the limiting reactant. You may look at it from another perspective. For 1 mole of methanol, 1 mole of benzoic acid is required, so for 0.6 moles, we would need 0.6 moles of benzoic acid, but actually we have lesser the amount than what is required (0.5 moles). Therefore, benzoic acid is the limiting reactant, and theoretical yield will be calculated from benzoic acid.

So, 0.5 moles of benzoic acid would yield 0.5 moles of methyl benzoate.

Mass of methyl benzoate to be produced (theoretical yield) = moles x molar mass

                                                                                                = 0.5 x 136 = 68 g

 For instance, Actual yield (amount of methyl benzoate actually produced) = 55 g

Percent yield of methyl benzoate = 55/68 x 100 = 81%

Methyl benzoate is liquid, so you may convert mass into volume and calculate the percent yield.

I hope it works……………