Limiting reactant

What is a Limiting Reactant?

Basically A reaction is said to be complete when its reactants are consumed. Limiting reagent is all about this.

In a chemical reaction, the limiting reagent is the first to be completely used up and prevents any further reaction from occurring. This is because the reaction cannot proceed further without it; on the other hand other reactants are present in excess of the quantities required to react with it.

We can understand it like this:

A + B = C

In this reaction if A is in excess quantity and B is in limited one than B is known as limiting reactant. Because B will be first to consumed and in the absence of B, A cannot make C so reaction will stop.

Since this reactant limits the amount of product that can be formed so it is known as limiting reactant. The reaction will stop when all of the limiting reactant is consumed.

Determine the limiting reactant:

In a reaction where two reactants are present, we can identify the limiting reactant by going through these steps.

1)   Balance the chemical equation that describes the reaction

2)   Compare the mole ratios of the amounts of reactants used

3)   Comparing the amount of products that can be formed from each reactant

Balanced the chemical equation that describes the reaction:

A balanced equation helps us to know the proportion of each reactant. It reflects the law of conservation of mass, which states that mass of substances produced (products) by a chemical reaction, is always equal to the mass of the reacting substances (reactants).

For example:

2H₂ + O₂ --> 2H₂O

Now the above chemical reaction states that to make one water atom we need two hydrogen atoms and one oxygen atom.  

Compare the mole ratios of the amounts of reactants used:

By comparing the mole ratio of the reactants, we can find out the exact amount of the reactants used in the reaction. . A mole is equal to 6.023 x 10²³ units of the substance and weighs the same as the molecular weight of that substance. Means

1 mole = molecular weight of substance

 Number of molecule present in 1 mole = 6.023 X 10²³ units

Now Let us observe the following reaction:

2 H₂ + O₂ → 2 H₂O

 This chemical equation implies that 2 mol of dihydrogen (H₂) and 1 mol of dioxygen (O₂) react to form 2 mol of water (H₂O). 

Since the molecular weight of hydrogen is approximately equal to 2 grams, So a mole of hydrogen molecules would also weigh approximately 2 grams and be roughly equal to 6.023 x 10²³ molecules of hydrogen

In the same way, the molecular weight of oxygen - approximately 32 grams –is roughly equal to one mole of oxygen molecules

Comparing the amount of products that can be formed from each reactant:

Once the equation is balanced properly and a clear knowledge of the proportion of each reactant is known is obtained, then it is simple to determine which reactant is the limiting reactant.

 For example: In the balanced equation for making water, we can see that it takes twice as many moles of hydrogen atoms than oxygen atoms to make water. In other words, each oxygen atom requires two hydrogen atoms in order to make water. The hydrogen would run out before the oxygen does, and once that happens, the reaction would come to an end.

 Thus it is clear that hydrogen is limiting reactant of the following reaction.

Let us take some more examples to understand it better:

1)    NH₃ (g) + 5O₂ (g) --> 4NO (g) +  H₂O (g) 

In the following reaction, ammonia reacts with oxygen to give nitrous oxide and water.

To find out limiting reactant in the following reaction, first balance the equation. So it will be:

4NH₃ (g) + 5O₂ (g) --> 4NO (g) + 6 H₂O (g)

To complete the reaction, we need 4 mole of ammonia and 5 mole of oxygen. It is clear that ammonia will be the first reactant to consume completely. Thus ammonia is limiting reactant of the above reaction.

2)    2C₆H₆ + 15O₂ --> 12CO₂ + 6H₂O

In the following reaction, benzene reacts with oxygen to give carbon dioxide and water.

The above reaction is balanced. In this reaction 2 mole of benzene is used to complete the reaction while oxygen is present in excess. Thus benzene will be will be the first reactant to consume completely. Thus benzene is limiting reactant of the above reaction.