Chemical Equation more Informative!

In three ways chemical equations can be made more informative.

• By indicating 'physical states' of the reactants and products.

• By indicating 'heat changes' taking place in the reaction.

• By indicating the 'conditions' under which the reaction takes place.

To indicate physical states of reactants and products in an equation

There can be four physical states of the components of a chemical reaction i.e., solid, liquid, aqueous solution or gas.

Here Zn is solid so we write Zn_(s).

Dilute sulphuric acid is aqueous state, so we write H₂SO_4(aq) and Zinc sulphate is also an aqueous solution, so we write ZnSO_4(aq) (and hydrogen which is a gas is written as H_2(g)). An arrow pointing upwards is used to indicate a gaseous product in an equation. Hence, H₂ is written instead of H_2(g).

An arrow pointing downwards is used to indicate an insoluble product or precipitate in an equation.

Precipitate is an insoluble product formed.

To indicate heat changes in an equation

There are two types of reactions based on heat changes involved: exothermic and endothermic.

Exothermic - Most of reactions we call “burning” are exothermic. They give out heat. This is indicated by writing '+ Heat' on the products side of an equation.

For Example:

Combustion of Coal

Burning of Magnesium

Cellular Respiration

Cellular Respiration helps in maintaining the body temperature at 37⁰C irrespective of the external environment. Some of the energy is used for other activities such as movement.

Endothermic - Those reactions in which heat is absorbed are known as endothermic reactions. The decomposition of NiO_(s) is endothermic.

An endothermic reaction is usually indicated by writing '+ heat' on the reactants' side of an equation.

In chemical reactions, bonds are either formed or broken. Making or breaking of bonds results in the rearrangement of atoms.

BOND MAKING IS EXOTHERMIC

BOND BREAKING IS ENDOTHERMIC

The heat of reaction generally gives the change in energy in a chemical reaction. This change in energy is the difference between heat content of the reactants (H_reactants) and the heat content of the products (H_products). It is also called enthalpy change. It is represented as H.

H = H_products - H_reactants.

The equations can be illustrated as follows:

EXOTHERMIC

(or)

ENDOTHERMIC

or

To indicate the conditions under which the reaction takes place

• If heat is required for a reaction to take place, then the sign (delta) is placed over the arrow of the equation.

• If the reaction takes place in the presence of a catalyst then the symbol or formula of the catalyst is written above or below the arrow.

• If the reaction requires special conditions of temperature and pressure that is also mentioned on the arrow above or below by mentioning their values.

• A reversible reaction is represented by