Stoichiometry is the calculation of the relative quantities of the reactants and the products in chemical reactions. Stoichiometry is based on the law of conservation of mass where the sum total of all the masses of the reactants is equal to the total mass of the products. This leads to an insight in the relations among the quantity of reactants and the products, forming a ratio of the positive integers.
This in short means that the amount of products can be easily calculated if the amount of separate reactants is known. Also, if one reactant is known and empirically the quantity of the product can be determined, then the amount for the other reactants can be easily calculated.
TYPES OF STOICHIOMETRY
Stoichiometry measures the quantitative relationships and determines the amount of products or reactants that are required or produced in a given chemical reaction.
- Reaction stoichiometry is describing the quantitative relationships that exist between the substances while participating in chemical reactions.
- Composition stoichiometry- owing to the well known relationship of the moles to the atomic weights, the ratios derived by stoichiometry can also be used in determining the quantities by weight in a reaction, also described by the balanced equation.
- Gas stoichiometry is dealing with the reactions that involve gases in the cases where the gases are ideal gases i.e. they are at a known temperature, pressure and volume. The volume ratio of the gases is ideally as per the ideal gas law, but the mass ratio of single reaction needs to be calculated using the molecular masses of the reactants and of the products. Molar masses are now used for calculating mass ratio owing to the existence of the isotopes.
STOICHIOMETRY IN DETAIL
The calculations from stoichiometry are based on the fact that the atoms are conserved. Atoms cannot be created or destroyed. Number of atoms before and after the reactions remains same.
Stoichiometry at the atomic level becomes complicated when the quantities of atoms are measured in g, kg, l or ml. Ratios are easier if the quantities are in moles.
Eg. One mole of oxygen reacts with two moles of hydrogen giving-
2 H2 + O2 -> 2 H2O
Here, 4 hydrogen atoms and 2 oxygen atoms are on each side of the equation. One needs twice the number of hydrogen atoms to the oxygen atoms. This number suggests and tells us the amount of atoms we need for the reaction to happen.
In a chemical reaction, not all reactants can be necessarily consumed. Some reactants might be in excess while some might fall less. The completely consumed reactant is known as the limiting reactant while the unreacted reactants are known as the excess reactants. Yields are the produced quantities or the products.
We use stoichiometric coefficients i.e. the numbers used to make the equation balanced. Ratios are made using the stoichiometric coefficients, and the ratios are helpful in calculating the relative proportions of chemicals in our reactions.
Through this equation, we know that 1 mole of Fe?2??O?3 would react with 2 moles of Al for yielding 2 moles of Fe and 1 mole of Al?2??O?3.
Now, the mole ratio between Al and Fe?2??O?3 = 2 mole Al / 1 mole of Fe?2??O?3