Stoichiometry is a section of chemistry that involves using relationships between reactants and/or products in a chemical reaction to determine desired quantitative data. In Greek, stoikhein means element and metron means to measure, so stoichiometry literally translated means the measure of elements.

Outline

## Stoichiometric and non-stoichiometric reaction

Chemical Reaction occurs in a perfect stoichiometry among reactants and products. However, the reactants of a reaction are not necessarily in an ideal stoichiometric mixture.

The reactant that is totally consumed or totally converted into the product is called the limiting reactant.

If the mole ratios of the reactants with respect to each other are the same, then the reactants are stoichiometric.

If the mole ratios of the reactants are different then these reactants are not stoichiometric.

The one with the lower mole ratio is called the limiting and it determines the amount of product formed. The one with the higher mole ratio is called the excess reactant.

## Excess reactant

An excess reactant is a substance that is not wholly consumed or entirely reacted in a chemical reaction. The amount of product formed is independent of the quantity of excess reactant.

A + 2B → C

The above equation indicates that 1 mole of A will react with 2 moles of B. If we have equivalent moles of A and B.

then B will be a limiting reactant, while A will be an excess reactant. How to find out excess reactants?

In a reaction chamber, 8 mol of Na reacts with 5 mol of O2 to give Na2O3, finding the excess reagent.

Na + O2  →   Na2O

To calculate the excess reactant; follow the below steps.

1) Balance the chemical equation:

4 Na + O2  →  2 Na2O

2) Find the stoichiometric ratios of the reactants (mole ratio).

nNa/4 = 8/4  = 2

nO2/1 = 5/1  = 5

3)  Compare the ratios, the one of higher value is the excess.

5 > 2, thus O2 is the excess reactant.

4) To find the remained quantity in the excess:

The number of moles remained in excess = initial number of moles – number of moles reacted

(number of moles reacted is calculated according to stoichiometry using limiting reactant)

The initial number of mole of O2  = 5 moles

nNa/4 = n O2/1

8/4 = n O2/1

number of moles of O2 reacted = 2 mole

number of moles remained in O2 = 5-2 = 3 moles.

## Concepts Berg

Give an example of an excess reactant.

A combustion reaction occurs when a candle is burned in the air, i.e. oxygen. The amount of product formed is independent of oxygen. Thus oxygen is an excess reactant here.

How do you find limiting and excess reactants?

To calculate the excess reactant:

1) Balance the chemical equation.

2) Find the stoichiometric ratios of the reactants (mole ratio).

3)  Compare the ratios, the one of higher value is the excess.

What is meant by an excess reagent?

The excess reactant is the reactant in a chemical reaction with a greater amount than necessary to react completely with the limiting reactant. It is the reactant(s) that remain after a chemical reaction has reached equilibrium.

What are the calculations needed to find the amount of excess reactant?

To find the remained quantity in the excess reactant use this formula:

The number of moles remained in excess = initial number of moles (initially given) – number of moles reacted

(number of moles reacted is calculated according to stoichiometric using limiting reactant)