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.
- 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