What is meant by a chemical reaction? Give any 2 examples.

A chemical reaction expressed by writing the full names of reactants and products is called a word equation. For example:

Hydrogen + oxygen -----------> Water

A chemical equation is an equation that represents the chemical reaction with the symbols and molecular formulae of the reactants and products.

For example:    2H₂ + 0₂ --------> 2H₂O

The elements or compounds that take part in a chemical reaction are reactants and the elements or compounds that are formed as a result of a chemical reaction are called products.

for example

H₂SO₄ + Zn ----------> ZnSO₄ + H₂

Reactants                    Products

A chemical eqn. which contains an equal number of atoms of each element in both reactant and product side is called balanced chemical eqn.

Example: 2H₂ + O₂ --------> 2H₂O

Any three pieces of info that can be obtained from a balanced chemical eqn are:

1. The type of chemical reaction.
2. The name and symbol of reactants and products.
3. The total number of atoms of reactants and products.

Four factors that bring about a chemical change are:

> Heat                               > Catalyst
> Light                               > Solution
> Pressure ...

A catalyst is a chemical substance that increases or decreases the rate of a chemical reaction without taking part in a chemical reaction.

The +ve catalyst is the chemical substance that increases the rate of a chemical reaction. For example MnO₂. The catalyst is the chemical substance that decreases the rate of a chemical reaction. For example glycerine.

A chemical reaction in which two or more reactants are combined together to give a single product is called a combination reaction. Examples are:

C + O₂ --------> CO₂
C + 2H₂ ---------> CH₄

A chemical reaction in which a single reactant is broken down into two or more products called a decomposition reaction. For example:

CaCO₃ -------> CaO + CO₂
CuCO₃ -------> CuO + CO₂

A chemical reaction in which an atom of a compound is displaced by another element is called a displacement reaction. Its types are:

1. Single displacement rxn.
2. Double displacement rxn.

A chemical reaction in which aid and base react together to gee salt and water is called an acid-base reaction.

It is called a neutralization reaction because the acidic and basic nature of compounds is neutralized during the reaction.

The atomic weight of an element expressed in grams is called gram atomic weight.

The molecular weight of a substance expressed in grams is called molecular weight.

The amount of a substance that contains 6.023 X 10²³ atoms or ions is called moles.

Avogadro's number is the number of particles in one mole of a substance.

N_A = 6.023 X 10²³

The burning of magnesium in the air conducts that reaction where magnesium and oxygen combine to form magnesium oxide. As two elements combine each other, it is called a combination reaction.

2Mg + O₂ ------> 2MgO

We know, a chemical reaction in which an acid and base react together to give salt and water is called an acid-base reaction. Here acidic and basic nature et compounds are neutralized. So, it called a neutralization reaction.

We know, water breaks down into hydrogen and oxygen during its electrolysis. As two elements decompose themselves in this chemical reaction, it is called a decomposition reaction.

2H₂0 --------> 2H₂ + O₂

The positive change in the concentration of a reactant or a product per unit time is called the rate of a chemical reaction. Any four factors that affect the rate of a chemical reaction are:

1. The concentration of reactants
2. Temperature
3. Physical nature of reactants
4. Chemical nature of reactants

An increase in temperature increases the rate e chemical reaction whereas a decrease in temperature decreases the rate of a chemical reaction.

Positive catalyst increases and negative catalyst decreases the rate of a chemical reaction.

When the concentration of Sodium thiosulphate was increased the rate of reaction increased and the time taken to reach equilibrium decreased, so therefore the rate of reaction is directly proportional to the concentration. So rate of reaction increases.

a. Reactants and products.

b. Word equation and formulae equation.