Chemical Reactions and Equations- I

Chemical Reactions and Equations- I

1)      A chemical reaction is a change in which one or more substances (reactant) react to form new substances (product) with entirely different properties. Old bond breaks and new bonds form as a result of a chemical reaction.

·        Reactants are the substances that undergo chemical change in the chemical reaction.

·        Products are the new substances that are formed during the reaction.
2)      A chemical reaction can be identified by following ways –

·         Change in state.

·         Change in colour.

·         Evolution of gas.

·         Change in temperature.

·         Formation of a precipitate.

3)      Chemical Equation – A chemical equation is the symbolic representation of a chemical reaction.

    For example: Hydrogen + Oxygen → Water                 (Word equation)

                                (Reactants)            (Product)

                                H₂ (g) + O₂(g)   → H₂O_(l)               (Chemical equation)

4)      Writing the chemical equation

While writing the chemical equation, we have to follow the given steps:

·         Step I: Writing a skeletal chemical equation.

·         Step II: Writing a balanced chemical equation.

Q. Why do we balance a chemical equation? Or Need of balancing a chemical equation. Ans- To satisfy the law of conservation of mass, which states that ‘mass can neither be created nor destroyed during a chemical reaction’.

·         

Step III : Making a chemical equation more informative.

a)      By writing the physical states of substances. (s) for solids, (l) for liquids, (g) for gas and (aq) for aqueous solution. b)      By writing the reaction conditions. c)      Evolution of heat (exothermic) and absorption of heat (endothermic) can be indicated by writing (+ heat) on the right hand side and (- heat) on the left hand side of the equation. d)     (↑) symbol to show evolution of a gas, (↓) symbol to show the formation of precipitate.

5)      Importance of a chemical equation

a)      A chemical equation gives us information about the substance participating in a chemical

        reaction.

b)      It informs us about the symbol and formulae of all the substances involved in a reaction.

c)      It gives us information whether heat is evolved or absorbed during a chemical reaction.

d)     It ensures us that number of atoms present in reactants is equal to the number of atoms present in products.

_{Prepared by Ms. Priti Maan}