Stoichiometry - JEE Important Topic

Stoichiometry is a very important concept in Chemistry which helps us understand various reactions, the proportion in which it can occur, and calculates the amount of reactants or products in a chemical reaction. Stoichiometry is a combination of two Greek words. These are ‘stoicheion’ which means element and ‘metron’ which means measure. 

Stoichiometry and stoichiometric calculation help in finding masses of the reactants and the products involved and the energy requirement in a chemical reaction. Stoichiometry means a measurement of the amount and energy of elements involved in chemical reactions. Stoichiometry is a branch of Chemistry where we apply the laws of definite proportions and laws of the conservation of mass to the chemical reactions in order to find the quantity of chemicals required in a chemical reaction.

Mole Concept

“One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12g of the Carbon-12 isotope”.  The number of chemical entities in 1 mole of a substance is always the same and is equal to the Avogadro number denoted by N_A =6.023$\times$ 10²³ chemical entities (atoms, molecules etc.). The mass of one mole of a substance in grams is known as its molar mass. The molar mass expressed in grams is equal to atomic/molecular/ formula mass in u. Thus, 1 mole of oxygen molecule has 6.023$\times$ 10²³ oxygen molecules in it and the Molar mass of oxygen is 32 g mol^-1.

Stoichiometric Coefficient 

Consider a balanced chemical reaction combustion of propane as given below: 

$\mathrm{C}_{3} \mathrm{H}_{8}(\mathrm{~g})+5 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 3 \mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{H}_{2} \mathrm{O}(\mathrm{g})$

In this reaction, propane and oxygen are the reactants and carbon dioxide and water are the products. We can see there are some coefficients associated with the chemicals. These coefficients represent the number of chemical entities or number of moles of chemical entities like atoms /molecules/ ions of reactants taking part or the number of  chemical entities or number of moles of chemical entities formed as products during the reaction. They are called stoichiometric coefficients. The stoichiometric coefficient for propane is 1, oxygen is 5, carbon dioxide is 3, and water is  4.

The balanced reaction stated above gives us the following information:

• One mole or molecule of propane reacts with 5  moles or molecules of oxygen and produces 3 moles or molecules of carbon dioxide and 4 moles or molecules of water.

• In terms of volume, 22.7 L (At NTP, 1 mole of ideal gas has a molar volume of 22.7 L. NTP means Normal temperature and pressure P =100 kPa and T= 293.15 K) of propane reacts with 113.5 L of oxygen and produces 68.1 L of carbon dioxide and 90.8 L of water.

• In terms of weight, 44 g of propane reacts with 160 g (5 × 32) of oxygen and produces 132 g (3 × 44) of carbon dioxide and 72 g (4× 18) of water.

Mole Ratio

Mole ratio is the ratio of moles of one component in reaction to moles of another component. The mole ratio of reactants in the above reaction is 1:5 (propane:Oxygen). In the mole ratio, if the moles of components are the same as the coefficients of the corresponding components in the balanced chemical reactions, then it is called a stoichiometric ratio. 

Stoichiometric Proportion

When the ratio of moles of reactants entering a reactor is in the same ratio as in a balanced chemical reaction, then the reactants are said to be in stoichiometric proportion. When we take 2 moles of methane and 4 moles of oxygen and allow it to react, then we can say that reactants are in stoichiometric proportion (ratio of reactants is 1:2).

Stoichiometric Compounds

A stoichiometric compound is a compound in which atomic components are present in exact and fixed composition with a small integer ratio. For example, compounds like NaCl and MgO have a 1: 1 ratio of composition of their respective atomic components. 

Limiting Reagent 

In most chemical reactions, the reactants may not be in stoichiometric proportion due to technical, economical, or safety factors. This means some reactants may be more and some reactants may be less when compared to the stoichiometric proportion. The reactant which is in the least quantity gets consumed faster when compared to the reactant which is in excess. Thus, the reactants which are less in quantity decide the conversion of reactant as it limits the amount of product formed. The reactant which is less compared to the stoichiometric requirement is thus called the limiting reagent. The reactant which is present in excess compared to the stoichiometric requirement is called excess reactant. Consider the stoichiometric equation for a chemical reaction,

$\mathrm{CO}_{2}+2 \mathrm{NH}_{3} \rightarrow \mathrm{NH}_{2} \mathrm{CONH}_{2}+\mathrm{H}_{2} \mathrm{O}$

Here, Urea is produced by the reaction of carbon dioxide and ammonia. The reactants are supplied in the mole ratio (CO₂ : NH₃) 1 : 4.5. Which means ammonia is the excess reactant and CO₂ is the limiting reactant.

Composition of Liquid Mixtures

Different methods are available to express the composition of solids, liquids, and gases in mixtures. The composition of solid and liquid mixtures is generally expressed in mass per cent or mole fraction. The composition of gases is expressed in volume or mole fraction.

Many reactions in the laboratories are carried out in solutions. So, we need to know the methods to express the composition of components present in solution. The following are the methods to express the composition of liquid mixture.

1. Mass or weight percent 

2. Mole fraction

3. Molarity

4. Molality

Mass percent: It is the mass of a solute divided by the total mass of a solution multiplied by 100.

\[Mass\text{ p}er\text{ }cent\text{ =}\dfrac{Mass\text{ }of\text{ solute}}{Mass\text{ }of\text{ solution}}\times 100\]

Mole Fraction: It is the ratio of moles of one component to the total moles of solution or mixture. If solute A is dissolved in solvent B, and if n_A is the number of moles of solute and n_B is the number of moles of solvent, then the mole fraction of solute is 

$\text { Mole fraction }=\dfrac{n_{A}}{n_{A}+n_{B}}$

Molarity: It is defined as the number of moles of solute dissolved in 1 litre of solution. It is denoted by M.

\[Molarity(M)\text{ =}\dfrac{Moles\text{ of solute}}{Volume\text{ of solution in litres}}\]

Molality (m): It is the number of moles of solute which is present in 1 kg of solvent.

\[Molality\text{ }(m)\text{ =}\dfrac{Moles\text{ of solute}}{\text{Mass of solvent in kg}}\]

In the laboratory, sometimes we encounter situations where we have a solution of one particular concentration, but we want to prepare the same solution in a different concentration. For example, if we have a solution of higher concentration, we can make a lower concentration solution from this. Assume, we have a 1M solution of oxalic acid, to prepare a 0.2M solution from a 1M solution, we can use the following mixing formula.

M₁ $\times$ V₁ = M₂ $\times$ V₂ 

Here, M₁ and M₂ = initial and final molarity, respectively, and V₁ and V₂ = Initial and final volume, respectively.

If M₁=1 M, M₂ = 0.2M, and V₂= 1000mL.

Substituting in above equation, we get V₁ = 200 mL

This means that we need to take 200 mL of 1M oxalic acid solution and add water to make it to 1 litre.

Conclusion

Stoichiometry and Stoichiometric calculations help to find the quantity of reactants required or the quantity of product formed in a chemical reaction. The coefficients of chemicals in a balanced chemical equation also known as stoichiometric coefficients indicate the molar ratios and the respective number of particles taking part in a particular reaction. We can see stoichiometry examples in our daily life or in the chemical manufacturing industry. The reactors or chemical systems cannot be designed without proper stoichiometric calculations.

The limiting reactant in a reaction gives the extent to which the reaction can be carried out as it is the reactant present in the least amount when compared to stoichiometric proportion. Hence, limiting reactants get consumed first in a chemical reaction. The composition of a liquid mixture or solution can be represented by mass per cent, mole fraction, molarity, and molality.