Does a chemist need to know about percentage yield while balancing a chemica–l Equation?

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The ratio between actual yield and theoretical yield is called percentage yield. To be precise, actual yield is the total amount of conversion obtained as a result of chemical reaction while theoretical yield is the pre calculated amount of products produced after conversion to limiting reactant under described set of reaction parameters. Percentage yield is a unit less term often written in %.

Percentage Yield and Balanced Chemical Equation:

It is often seen in chemistry and chemical engineering that a variety of chemical reactions occur at different temperatures and pressures. As a result, many new products are formed that find a range of applications in industrial and domestic lifes. It is worth mentioning here, that each chemical reaction independent of its type can be written in a chemical equation. A balanced chemical equation, shows the formula and state of reagents and products. Moreover, reaction temperature, enthalpies and pressures can also be found indicated on a chemical equation. The percentage yield depends on the theoretical and actual yield. Hence, it is very important to balance a chemical equation otherwise the limiting reactant will not be identified and hence, the correct percentage yield will not be calculated. 

Let us take a chemical equation written in a balanced and unbalanced form and calculate percentage yield & theoretical yield. 45 gram of iron (Fe) combine with 50 grams of oxygen (O2) to give iron oxide (Fe2O4). The actual yield of the process is 0.2 moles. Calculate the percentage Yield? 

Percentage Yield with Balanced Chemical Equation

The balanced chemical equation of the above reaction is 

3 Fe + 2 O2 → Fe3O4

To find out the percentage yield we need to calculate the theoretical yield first and for that we need to identify the limiting reactant. 

Mass of Fe = 45 g

Molar Mass of Fe = 55.8 g/mole

Mass of O2 = 50 g

Molar Mass of O2 = 32 g 

Actual Yield = 0.2 moles

Amount of iron oxide produced by

iron = 45g Fe ×1 mol Fe 55.8 g Fe ×1 mol Fe3O43 mol Fe: = 0.269 mole Fe3O4

Amount of iron oxide produced by

oxygen = 50g O2 × 1 mol O232 g of O2 ×1 mol Fe3O42 mol O2 = 0.781 mole Fe3O4

The amount of iron oxide produced by iron is less. Hence, it can be concluded that Fe is a limiting reactant. Hence, the theoretical yield of 0.269 moles. 

Percentage Yield=Actual YieldTheoretical Yield×100

Percentage Yield=0.20.269×100

Percentage Yield=74%

So, the percentage yield via the balanced chemical equation is 74%. 

Percentage Yield with Balanced Chemical Equation

The percentage yield of an unbalanced equation can be calculated by two ways. 

Unbalanced Fe:

 Fe + 2 O2 → Fe3O4

Mass of Fe= 45 g

Molar Mass of Fe= 55.8 g/mole

Mass of O2= 50 g 

Molar Mass of O2= 32 g 

Actual Yield= 0.2 moles

Amount of iron oxide produced by iron= 45 g Fe ×1 mol Fe 55.8 g Fe×1 mol Fe3O41 mol Fe: = 0.86 mole Fe3O4

Amount of iron oxide produced by oxygen= 50 g O2 ×1 mol O232 g of O2×1 mol Fe3O42 mol O2 = 0.781 mole Fe3O4

Hence, for the unbalanced Fe in the chemical equation, the limiting reactant has been changed from iron to oxygen. Now the theoretical yield is 0.781 moles. 

Percentage Yield=Actual YieldTheoretical Yield×100

Percentage Yield=0.20.781×100

Percentage Yield=25%

So, the percentage yield via the unbalanced chemical equation is 25%. 

Unbalanced O2

3 Fe + O2 → Fe3O4

Mass of Fe= 45 g

Molar Mass of Fe= 55.8 g/mole

Mass of O2= 50 g 

Molar Mass of O2= 32 g 

Actual Yield= 0.2 moles

Amount of iron oxide produced by iron= 45 g Fe ×1 mol Fe 55.8 g Fe×1 mol Fe3O43 mol Fe: = 0.269 mole Fe3O4

Amount of iron oxide produced by oxygen= 50 g O2 ×1 mol O232 g of O2×1 mol Fe3O41 mol O2 = 1.5625 mole Fe3O4

Hence, in the current scenario, the limiting reactant is the same and that is Fe and the theoretical yield is 0.269 moles. If we calculate the percentage yield then it will come out to be 74% as in case 1. Hence, the yield remains the same as long as the limiting reactant is not affected. 

Conclusion: 

It can be concluded from the above discussion that a chemist should be very well aware of percentage yield when he is balancing the chemical equation. The percentage yield is the ratio in between actual yield and theoretical yield. When the chemical equation is not balanced correctly then the identification of the limiting reactant gets disturbed. For instance, in the example stated below when the equation was not balanced for oxygen the limiting reactant got changed from iron to oxygen and hence, the percentage yield changed from 75% to 25%. Hence, the relationship between percentage yield and balancing chemical equation is very vital in chemistry.

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