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STOICHIOMETRIC CALCULATIONS, LIMITING REACTANT AND YIELD

STOICHIOMETRY 
The branch of chemistry which deals with the quantitative relationship of the reactants and the products as given in the balanced chemical equation.
  • A statement that describes a chemical reaction in terms of symbols and chemical formulas is called a chemical equation.
Limitations of a chemical equation:
The limitations of a chemical equation are as follows:
(i) Chemical equation does not show the rate of the reaction.
(ii) It does not tell about the conditions necessary for the reaction.
(iii) The chemical equation can even be written to describe a chemical change that doesn’t occur.
Stoichiometric amounts:
The amounts of the reactants or the products as given by the balanced chemical equation are called stoichiometric amounts.
2H2            +           O2             2H2O   
(i) 2moles        1mole                 2moles      Stoichiometric amounts     
(ii) 4g               32g                       36g
Assumptions to perform stoichiometric calculations:
Following are the assumptions to perform stoichiometric calculation.
  • All the reactants (in stoichiometric amounts) are completely converted into the products (Law of conservation of mass is obeyed).
  • No side reaction occurs. (Law of definite proportions is obeyed).
  • The reaction must be irreversible.
Stoichiometric relations:
The following types of relationships can be studied with the help of a balanced chemical equation.
  • Mass-mass relationship.
  • Mass-mole relationship.
  • Mass-volume relationship.
  • Mole-mole relationship.
LIMITING REACTANT 
A reactant which is completely consumed earlier leaving behind the other reactant in excess is called limiting reactant.
A limiting reactant:
(i) Is consumed earlier
(ii) Is in lesser than stoichiometric amount
(iii) Limitize (controls) the chemical reaction.
(iv) Limitize (controls) the amount of product.
(v)  Gives least amount of product.
Example 1:
Formation of water:
If two moles of hydrogen (4g) and two moles of oxygen (64g) are made to react, 2moles of water will be formed.
Actually, 2moles of O2 requires 4moles of H2 for complete reaction. To react with 2moles of O2, H2 is in (2moles) lesser than stoichiometric (required) amount (4moles). Hence, will be consumed earlier and will be the limiting reactant.

Significance of limiting reactant:
The significance of limiting reactant is as follows.
(i)     Expensive reactant is taken as limiting to ensure that it is completely used up in a chemical reaction.
(ii)   Sometimes one of the reactant is taken as limiting reactant to make the reaction faster e.g. Taking oxygen in excess to burn a substance makes the burning faster.
(iii) To determine the effect of concentration on the rate of reaction.
Identification of limiting reactant:
To identify a limiting reactant, the following three steps are performed:
(i) Calculate the number of moles from the given amount of reactant.
(ii) Find out the number of moles of product with the help of a balanced chemical equation.
(iii) Identify the reactant which produces the least amount of product as limiting reactant.

YIELD 
The amount of the product calculated from balanced chemical equation or produced as a result of chemical reaction is known as yield.
Types of yield:
The types of yield are as follows.
(i) Theoretical yield
(ii) Actual yield
(i) Theoretical yield:
  • The amount of product calculated from the balanced chemical equation is called theoretical yield.
  • It is the maximum amount of product that should be produced according to the balanced chemical equation.
(ii) Actual yield:
The amount of the product obtained experimentally as a result of chemical reaction is called actual yield.
It is also called practical or experimental yield.
Actual yield is mostly less than theoretical yield:
In most of the reactions, actual yield is less than the theoretical yield due to:
  1. Side reaction/ competing reaction.
  2. Reversible reaction
  3. Incomplete reaction
  4. Inexperience worker
  5. Actual yield may be lesser due to inefficiency during the washing and drying of crystals.
  6. Mechanical Loss of products during separation by
  • Solvent extraction
  • Filtration
  • Distillation
  • Crystallization
PERCENTAGE YIELD 
The ratio of actual yield to theoretical yield multiplied by 100 gives us percentage yield. A chemist is usually interested in the efficiency of a reaction. The efficiency of the reaction is expressed in the form of percentage yield.
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