The chemical substances, whenever exposed in a certain condition, make themselves open for the transformation in another substance of different compositions.
These certain conditions can be required temperature, or for many chemical reactions, it can be the presence of a catalyst.
The substance or substances that are primarily involved in the formation of a chemical product are called reactants or reagents.
These reacting agents are usually combined voluntarily with each other and produce new products in the air, and they can sometimes be pushed to complete a specific reaction under the roof of a laboratory by putting them in the glass beaker and flask tubes.
These chemical reactants can either be converted entirely into a product or partially, depending on circumstances.
If the reactants completely transform into the product, it refers to limiting reagent; otherwise, it will be called excess reagent.
From the amount of limiting reactants in a chemical reaction, the total amount of resultant products can be calculated, which is called a Theoretical Yield.
At the same time, it is not always necessary that you will end up getting an absolute theoretical yield after a chemical reaction.
If the amount of actual resultant product does not meet the theoretical yield, then it will be known as Actual Yield.
In order to determine the percent yield, we will calculate the ratio of the percentage of actual yield and the theoretical yield.
Percent yield = Actual Yield * 100
Theoretical Yield
You can also use a percent yield online calculator if you just wanna keep your focus on the theoretical aspect of it.
Example
Let clarify the discussed concept with the example of the production of Methyl Alcohol.
Today, in the industry, the Methanol or methyl alcohol has been produced by the hydrogenation of carbon monoxide.
It requires a high temperature and amount of pressure.
CO(g)+2H2 (g)→CH3OH(l)
Assume that if we produce 6.1 metric tons of methanol from the 1.2 metric tons of hydrogen reacting with the excess amount of CO.
How can we calculate the theoretical and the percentage yield of this chemical reaction?
Relationship between Percent Yield and Theoretical Yield
Before diving deep inside the calculations, we need to understand a few important things midway.
The equations of chemical reaction state the typical stoichiometric relationship between the products and reactants involved in a chemical reaction.
Therefore, we can estimate the theoretical yield directly from the computations of reaction stoichiometry.
It has been experimentally proved that for many chemical reactions, the amount of actual yield is usually less as compared to the theoretical yield.
This might have many reasons; including the loss of energy in the completion of the process, or it might be the result of using inappropriate lab apparatus.
Now, we need to frame the above equation of methanol stoichiometrically, i.e., keeping their masses intact.
As the molar mass of carbon monoxide is 28.01 g/mol, and the molecular weight of 2H2 will be 4.0318 g/mol.
This subsequently produces CH3OH with a molar mass of 32.4 g/mol. There comes the concept of limiting reactant in the frame, as it is a reactant that limits the amount of product that can be formed in a chemical reaction.
In our case, we have hydrogen as a limiting reactant, and we know the amount of hydrogen reactant, i.e., 1.2 metric tons.
Theoretical Yield = 32.0 CH3OH * 1.2 tons H2
4.0 H2
= 9.6 tons CH3OH
Again, if you wish to understand the concept and not wanna go through all the calculation yourself, you can use a theoretical yield measuring tool to get the job done.
Hence, we will get a theoretical yield of 9.6 tons of methanol from 1.2 tons of H2 and an excess amount of CO.
Now, in a bid to find out the percent yield will simply put the values in the ratio of the percentage of actual yield to theoretical yield.
Percent yield = 6.1 tons × 100
9.6 tons
= 64%
In a nutshell, we can say that actual yield belongs to the experimental evidence, while the theoretical yield deals with the absolute paperwork calculation, and last but not least, the percent yield reports the ratio of these both kinds of yield.