Stoichiometry of Chemical Reactions

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A chemical reaction can be represented by a chemical equation, which may be defined as a chemical change in terms of symbols and formulae of the substances involved in the reaction. The substances that react among themselves to bring about the chemical changes are known as reactants, where as the substances that are produced as a result of the chemical change, are known as products.

Essentials of a Chemical Equation

A chemical equation should satisfy the following conditions:

• It should represent a true chemical reaction.

• It should be molecular in nature. The ionic reactions must be represented by ionic equations.

• It should be a balanced equation i.e. the number of atoms of each elements on both sides should be equal.

Writing of a Chemical Equation

Steps followed to write a chemical equation are:

• The reactants and products of the reaction are identified and written down.

• The formulae or symbols of the reactants are written on the left hand side with a '+' sign in between them.

• The formulae or symbols of the products are written on the right hand side with a '+' sign in between them.

• The two sides (reactants and products) are separated either by a sign of equality (=) or that of an arrow () pointing towards the products.

• For a reversible reaction a sign () is used in place of an arrow.

• The number of atoms of each element are counted on both the sides. If they are equal, then it is termed as balanced chemical equation. If they are not equal, the balancing of the equation is done by adjusting the coefficients before the symbols and formulae of the reactants and products.

In qualitative terms, a chemical equation conveys:

• The names of various reactants and the products.

In quantitative terms it represents:

• The relative number of atoms and molecules (reactant and product species) taking part in the reaction.

• The relative number of moles of the reactants and products.

• The relative masses of the reactants and products.

• The relative volumes of gaseous reactants and products.

Thus the chemical equation gives us the quantitative relationship between the reactants and products or the stoichiometry of the reaction.

Making a Chemical Equation more Informative

• The physical states of the reactants and products are specified. We use 's' for solids, 'l' for liquids and 'g' for gases. Sometimes 'aq' is used for aqueous - to represent that the given substance has been dissolved in excess of water.

• The strength of acid or base used in the reaction is prefixed by the words 'conc'. for concentrated and 'dil'. for diluted.

• The conditions of the reaction such as catalyst, temperature, pressure etc. are written on the arrow between the reactants and products.

The above equation indicates that the reaction has been carried out in the presence of a catalyst Fe/Mo at 723 K at 456000 mm of Hg pressure.

• Heat changes (absorption and evolution) in the reaction may be expressed in the equation as

 

 

• The distinction between slow and fast reactions can be made by writing the words slow and fast on the arrow head.

• The evolution of a gas in the reaction can be indicated by an arrow pointing upward (

  

  ) while the formation of a precipitate can be indicated by an arrow pointing downwards (

  

  ) or by writing the word 'ppt'.

• The reversible nature of the reaction is indicated by a double headed arrow, indicating that the reaction occurs in the forward as well as backward direction.