• Chemical reaction— Chemical changes or chemical reactions are the changes in which one or more new substances are formed.

• Chemical Equations – Representation of a chemical reaction in terms of symbols and formulae of the reactants and products is known as chemical equation.

• Balanced Chemical equations– The chemical equation in which the no. of atoms of different elements is same on both sides of the arrow is called balanced chemical equation.

The chemical reactions can be classified into different types such as—

a) Combination reaction– The reactions in which two or more substances combine to form a new substance are called combination reaction.

For example, 2Mg(s)+O2(g)→2MgO(s)

b) Decomposition reaction- The reaction in which a single compound breaks up into two or more simpler substances are called decomposition reactions.

For example, 2Pb (NO3)2 (s)→2PbO (s) + 4NO2 (g) +O2 (g) •The decomposition of a substance by passing electric current through it is known as electrolysis.

•The decomposition of a substance on heating is known as thermal decomposition.

•The decomposition of a substance by absorbing light energy is called photochemical decomposition.

c) Displacement reactions-The chemical reactions in which a more reactive element displaces a less reactive element from a compound are known as displacement reactions.

For example, i) Zn(s) + CuSO4(aq)→ZnSO4(aq) + Cu(s).

ii) Cu(s) + 2AgNO3(aq)→Cu (NO3)2(aq) +2Ag(s).

d) Double Displacement Reactions - The chemical reactions in which compounds react to form two different compounds by mutual exchange of ions are called double displacement reactions.

These reactions take place in solution two common types of this reaction are precipitation reactions and neutralization reactions

i) Precipitation reaction: In this reactions, aqueous solution of two salts are mixed whereby Some salts precipitate due to mutual exchange of ions between the two salts.

For example: AgNO3(aq) + NaCI(aq) → AgCl(s) + NaNO3.

ii)Neutralization reaction: In this type of reaction an acid reacts with a base to form salt and water by exchange of ions.

NaOH(aq) + HCl(aq)→NaCl(aq) +H20.

The process in which new substances with new properties are formed from one or more substances is called Chemical Reaction.

• The substances which take part in chemical reaction are called Reactants.

• The substances which are formed in a chemical reaction are called Products. Examples :

  (i) Digestion of food

  (ii) Respiration

  (iii) Rusting of iron

  (iv) Burning of Magnesium ribbon.

  (v) Formation of curd

• Chemical reaction involves :

  • Change in state

  • Change in colour

  • Change in temperature

  • Evolution of gas

• Ways of Representing a Chemical Reaction

A) Word Equation

Zinc + Sulphuric Acid → Zinc sulphate + Hydrogen LHS(Reactant) RHS (Product)

B) Chemical Equation

Zn + H2 SO4 → ZnSO4 + H2

LHS (Reactant) RHS (Product)

Chemical Equation

• A chemical reaction can be represented by chemical equation.

• It involves uses of symbol of elements or chemical formula of reactant and product with mention of physical state.

  • The necessary conditions such as temperature, pressure or any catalyst should be written on arrow between reactant and products. e.g., Magnesium is burnt in air to form magnesium oxide. Mg + O2 → MgO Balancing Chemical Equation

  • Law of conservation of Mass: Matter can neither be created nor destroyed in a chemical reaction.

  • So number of elements involved in chemical reaction should remain same at reactant and product side. STEPWISE BALANCING (Hit and Trial)

• Step 1. Write a chemical equation and draw boxes around each formula. Fe + H2O → Fe3O4 + H2. Donot change anything inside the box.

• Step 2. Count the number of atoms of each element on both the sides of chemical equation.

• Step 3. Equalise the number of atoms of element which has maximum number by putting in front of it. Fe + 4H2O → Fe3O4 + H2

• Step 4. Try to equalize all the atoms of elements on reactant and product side by adding coefficient in front of it. 3Fe + 4H2O → Fe3O4 + 4H2. Now all the atoms of elements are equal on both sides.

• Step 5. Write the physical states of reactants and products. 3Fe (s) + 4H2O (g) → Fe3O4 ( s) + 4H2(g)

• Solid state = (s)

  Liquid state = (l)

  Gaseous state = (g)

  Aqueous state = (aq)

• Step 6. Write necessary conditions of temperature, pressure or catalyst on arrow above or below.

  TYPES OF CHEMICAL REACTIONS

  I. COMBINATION REACTION :

  The reaction in which two or more reactant combine

  to form a single product.

• e.g. (i) Burning of coal - C(s) + O2 (g) → CO2(g)

  (ii) Formation of water -2H2(g) + O2 (g) → 2H2O (l)

  (iii) CaO (s) + H2 O (l) → Ca(OH)2 (aq)

  CaO- Quick Lime

  Ca(OH)2- Slaked lime

• Exothermic Reactions : Reaction in which heat is released along with formation of products.

  e.g., (i) Burning of natural gas-

  CH4 (g) + O2 (g) → CO2 (g) + 2H2 O (g) + Heat

  (ii) Respiration is also an exothermic reaction-

  C6H12O6 (aq) + 6O2 (g) → 6CO2 (aq) + 6H2O (l) + energy .

II. DECOMPOSITION REACTION :

• The reaction in which a compound splits into two or more simple substances is called decomposition reaction. A → B + C

  • Thermal decomposition: When decomposition is carried out by heating.

  e.g., (i) 2FeSO4(s)→{Heat} Fe2O3(s) + SO2(g) + SO3(g) (Ferrous sulphate) (Ferric oxide)

  Green colour Red-brown colour

• (ii) CaCO3(s)→{Heat} CaO(s) + CO2(g)

  CaCO3- Lime stone

  CaO- Quick lime

  •Electrolytic Decomposition : When decomposition is carried out by passing electricity.

  e.g., 2H2O→{Electric current}2H2+O2

  •Photolytic Decomposition : When decomposition is carried out in presence of sunlight.

  e.g., 1)2AgCl (s)→Sunlight 2Ag (s) + Cl2(g)

  2. 2AgBr (s) →Sunlight 2Ag (s) + Br2 (g)

  reaction of AgCl and AgBr is used in black & white photography.

  • Endothermic Reactions : The reactions which require energy in the form of heat, light or electricity to break reactants are called endothermic reactions.

III. DISPLACEMENT REACTION :

The chemical reaction in which more reactive element displaces less reactive element from its salt solution.

Eg.Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)

The iron nail becomes brownish in colour by deposition of Cu and blue colour of CuSO4 changes dirty green colour due to formation of FeSO4

Zn + CuSO4 → ZnSO4 + Cu: Zn is more reactive than copper.

IV. DOUBLE DISPLACEMENT REACTION :

A reaction in which new compounds are formed by mutual exchange of ions between two compounds.

Eg.Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)

Sodium Barium Barium Sodium

sulphate chloride sulphate chloride White precipitate of BaSO4 is formed, so it is also called precipitation reaction.

V. OXIDATION AND REDUCTION :

Oxidation:

(i) The addition of oxygen to substance.

(ii) The removal of hydrogen from a substance.

C + O2 → CO2

2Cu + O2→{Heat}2CuO

CuO + H2→{Heat}Cu + H2O

Reduction :

(i) The addition of hydrogen to substance.

(ii) The removal of oxygen from a substance.

          |----/> oxidation<\\---|

CuO + H2→ {Heat} Cu + H2O

|---/>Reduction<\\---|

In the reaction CuO is reduced to Cu and H2 is oxidized to H2O. So, oxidation and reduction taking place together is redox reaction.

Effects of Oxidation in Daily Life

1) Corrosion :

• When a metal is exposed to substances such as moisture, acid for some time a layer of hydrated oxide is formed which weakens the metal and hence metal is said to be corrode.

• Rusting of iron, black coating on silver and green coating on copper are examples of corrosion.

• Corrosion can be prevented by galvanization, electroplating or painting.

2) Rancidity :

The oxidation of fats and oils when exposed to air is known as

rancidity. It leads to bad smell and bad taste of food.

•Methods to Prevent Rancidity :

(i) By adding antioxidants

(ii) Keeping food in air tight containers

(iii) Replacing air by nitrogen

(iv) Refrigeration