Chemical Reaction

Many changes take place in our surroundings everyday.

Cooking of food, burning of fuel are the reactions in which substances are totally transformed. Such transformations are called chemical changes.

The event in which substance gets transformed permanently ( Chemical changes ) is called chemical reaction.

The things we use daily are formed because of chemical reactions  e.g : The painted walls, things made from plastics

To represent chemical reaction they are written in the form of equation e.g When Calcium Oxide reacts with water, it produces a single compound, Calcium hydroxide and energy is released  This reaction is represented in equation form as under  CaO + H2O  Ca(OH)2 + energy Calcium Oxide Water Calcium Hydroxide

CaO + H 2 O  Ca(OH) 2 In a chemical reaction, reactants are written on left hand side of the arrow whereas products are written on right hand side of arrow. The substances taking part in a chemical reaction are called reactants and the substances formed during the reaction are called products.

Types of Chemical reactions Combination reactions Reactants two or more than two Product only one e.g  C + O 2  CO 2

Decomposition reaction Reactant only one  Products more than one e.g  2H2 0  2H2 + O2

Displacement Reaction Two or more reactants and products One constituent from the reactant displaces another constituent and separates it to form the product  e.g :  Zn + 2HCl  ZnCl 2 + H 2

Oxidation and reduction Oxidation- Oxygen combines with an element or a compound   e.g  C + O 2  CO 2 Reduction - A reaction in which hydrogen combines with an element or compound or a reaction in which oxygen is lost from a compound Cl 2 + H 2  2HCl

The speed of chemical reactions Some chemical reactions are fast whereas some reactions takes place slowly. e.g : Ripening of fruits, rusting of iron(Slow), Burning of Paper, Lightening of fire crackers(Fast)

Various factors such as the size of the reactant particles, their temperature and concentration and the presence of catalysts affect the speed of reaction.

Catalyst doesn’t take part in chemical reaction, it’s mere presence speeds up a chemical reaction. e.g: Manganese dioxide, Raney nickel

In some chemical reactions, heat is given out. ( Exothermic Reaction ) e.g : Burning of Magnesium in air.

In some chemical reactions heat is absorbed. (Endothermic reactions) e.g: dissolving of urea in water ADD EXPERIMENT

Following changes are observed during a chemical reaction -Change in the state of the substance - Change in temperature - Change in colour - Generation of gas