ii. Expressing the heat changes: Heat is liberated in exothermic reactions and heat is absorbed in endothermic reactions. See the following examples.

1. C(s) + O2 (g) –––––> CO2 (g) + Q (exothermic reaction)

2. N2 (g) + O2 (g) –––––> 2NO (g) - Q (endothermic reaction)

‘Q’ is heat energy which is shown with plus (+) sign on product side for exothermic reactions and minus (–) sign on product side for endothermic reactions.

iii. Expressing the gas evolved:If a gas is evolved in a reaction, it is denoted by an upward arrow or (g)

Eg: Zn (s) + H2SO4 (aq) -–––––> ZnSO4 (aq) + H2 (g)

iv. Expressing precipitate formed: If a precipitate is formed in the reactions it is denoted by a downward arrow.

Eg: AgNO3 (aq) + NaCl (aq)-> AgCl(s)down arrow + NaNO3 (aq)

Sometimes the reaction conditions such as temperature, pressure, catalyst etc., are indicated above and/or below the arrow in the equation.

For example,

2AgCl(s) sunlight–––––> 2 Ag (s) + Cl2(g)

6CO2 (g) + 6 H2O (l) sunlight –––––>Chlorophyll C6H12O6 (s) + 6O2 (g)

Interpreting a balanced chemical equation

i. A chemical equation gives information about the reactants and products through their symbols and formulae.

ii. It gives the ratio of molecules of reactants and products.

iii. As molecular masses are expressed in ‘Unified Masses’ (U), the relative masses of reactants and products are known from the equation.

iv. If the masses are expressed in grams then the equation also gives the molar ratios of reactants and products.

v. If gases are involved, we can equate their masses to their volumes and calculate the volumes or those gases liberated at given condition of temperature and pressure using molar mass and molar volume relationship.

vi. Using molar mass and Avagadro’s number we can calculate the number of molecules and atoms of different substances from the equation.

It gives information about relative masses of reactants and products. from the equation we get,

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