Page 27 - Chemistry ICSE Class IX

The Language of Chemistry 15

How is a chemical equation written
Chemical equation for a chemical reaction is written as follows:
Step 1. Identify the reactants and the products of the chemical reaction.
Step 2. Write down the formulae or symbols of the reactants on the left-hand
side with a sign of plus (+) between them.
The formulae or symbols of the products formed in the reaction are
written on the right-hand side with a sign of plus (+) between them. The
reactants and products are separated by o or =.
Such a chemical equation is called a skeleton equation.
Step 3. Count the number of atoms of each element on both the sides. If the
number of atoms of each element on both the sides are equal, then the
equation is called a balanced chemical equation.
If the number of atoms of any one or more of the elements on both the
sides is not equal, then it is made equal by ad usting the coef cients before
the symbols and formulae of the reactants and products. The process by
which the number of atoms of each element on both sides is made equal,
is called balancing of chemical equation.

Step 4. In the end, the chemical equation is made molecular, if required.

How can a chemical equation be made more informative
A chemical equation can be made more informative by adding additional
information to the chemical equation. This is done as follows:
ƒ Reaction conditions. The information regarding temperature,
pressure and catalyst, etc., is provided above the arrow (o or =)
separating the reactant and products. A reaction taking place at t°C
and R atm pressure, and in the presence of a catalyst can be described
as follows.
t°C, R atm
Reactants o Products
catalyst
For example, nitrogen and hydrogen react to form ammonia under
the conditions; temperature = 450°C, pressure = 200 – 900 atm, and in
the presence of a catalyst (a mixture of iron and molybdenum). The
chemical equation for this reaction is written as follows.

450°C, 200 – 900 atm
N + 3H o 2NH 3
2
2
Fe + Mo
ƒ Physical states of reactants and products. Information regarding ŐĂƐĞŽƵƐ ƐƵďƐƚĂŶĐĞ ǁŚŝĐŚ ŝƐ
the physical states of the reactants and products can be provided by ĞǀŽůǀĞĚ ;ŐŝǀĞŶ ŽƵƚͿ ĚƵƌŝŶŐ ƚŚĞ
using the letters (U), (N), (I) and (CS) for solid, liquid, gas and a solution ƌĞĂĐƟŽŶ ŵĂLJ ĂůƐŽ ďĞ ŝŶĚŝĐĂƚĞĚ ďLJ ĂŶ
in water, respectively, at the end of the formula of the substance ĂƌƌŽǁ ƉŽŝŶƟŶŐ ƵƉǁĂƌĚƐ ;nͿ͘
involved. For example, solid sodium metal reacts with water at room ͻ
temperature to produce hydrogen gas, and a solution of sodium ƐŽůŝĚ ƐƵďƐƚĂŶĐĞ ǁŚŝĐŚ ƉƌĞĐŝƉŝƚĂƚĞƐ
hydroxide in water. The complete chemical equation is, ŽƵƚ ĨƌŽŵ ƚŚĞ ƌĞĂĐƟŽŶ ŵŝdžƚƵƌĞ
ŵĂLJ ĂůƐŽ ďĞ ŝŶĚŝĐĂƚĞĚ ďLJ ĂŶ ĂƌƌŽǁ
2Na(U) + 2H O(N) o 2NaOH(CS) + H (I) ƉŽŝŶƟŶŐ ĚŽǁŶǁĂƌĚƐ ;pͿ Žƌ ďLJ
2
2
ĂĚĚŝŶŐ ƚŚĞ ǁŽƌĚ ƉƉƚ ďĞůŽǁ ƚŚĞ
ƒ Heat absorbed or evolved. Chemical reactions proceed with the ĨŽƌŵƵůĂ ŽĨ ƚŚĞ ƐƵďƐƚĂŶĐĞ͘
evolution or absorption of heat. The reactions in which heat is ͻ
absorbed are called endothermic reactions. The reactions in which dŚĞ ĐŚĞŵŝĐĂů ĞƋƵĂƟŽŶƐ ŝŶ ǁŚŝĐŚ
heat is given out are called exothermic reactions. This information is ƚŚĞ ŚĞĂƚ ƚĞƌŵ ŝƐ ŝŶĐůƵĚĞĚ ĂƌĞ ĐĂůůĞĚ
provided by adding a heat term on the product-side (right-hand side) ƚŚĞƌŵŽĐŚĞŵŝĐĂů ĞƋƵĂƟŽŶƐ.
of the chemical equation. For example,