Chemical equations, their significances and limitations

Chemical Equation:

A chemical equation is a shorthand way of describing the events that occur in a chemical change or reaction.

The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.

Example:

The substances that combine or react are known as reactants.

The new substances produced in a reaction are known as products.

Certain conventions and symbols in addition to those for elements are used in equation writing. The following are common:

• Reactants are written on the left side separated by + signs.

• Products are written on the right, also separated by + signs.

• 'D' (the greek capital letter delta) placed over or below the arrow means that “heat was added to the reactants”.

• (g) means that the substance is gaseous.

• (l) means that the substance is liquid.

• (s) means that the substance is solid.

• (aq) indicates an aqueous solution i.e. the substance is dissolved in water. The designations for the physical state appear as subscripts.

• A number in front of the formula or symbol is a coefficient.

• Conditions required for the reaction are specified above or below the arrow.

Eg. Catalyst, temperature, pressure, etc.

The following example illustrates these aspects of chemical equations:

01. Limestone when heated, yields solid calcium oxide and gaseous carbon dioxide. (In words)

(In symbols)

02. The electrolysis of water in its liquid form yields hydrogen and oxygen gases.

(In symbols)

Significances of chemical equation:

- coefficients of a chemical reaction indicate relative, not absolute, amounts of reactants and products

o usually shows the smallest numbers of atoms, molecules or ions that will satisfy the law of conservation of mass

- relative masses of the reactants and products of a chemical reaction can be determined from the reaction’s coefficients

o can convert moles to mass in grams by multiplying by the molar mass

- reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction

- chemical equations give useful quantitative information but it does NOT give an indication of whether or not the reaction will ever take place

Limitations of chemical equation:

• It does not mention the state of the substances. So (s) for solid, (l) for liquid, (g) for gas and (vap) for vapor may be added.

• The reaction may or may not be complete. Equation does not reveal it.

• It does not give any information regarding the speed of the reaction.

• It does not give the concentration of the substances. In some cases, terms like diluted and concentrated may be added.

• It does not give the conditions of temperature, pressure, catalyst, etc. This is overcome by mentioning these above or below the arrow e.g.,

• It does not give any idea about color changes, which has to be mentioned separately.

• It does not give any indication regarding the production or absorption of heat. This is mentioned separately.

• Some reactions are reversible. They are represented by

  

  or

  

  .