Chapter : 1 Chemical Reactions And Equations

 Class 10 SCIENCE

Chapter : 1
Chemical Reactions And Equations

NOTES

• 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 chemical reaction are called Products.

Examples :

1. Digestion of food

2. Respiration

3. Rusting of iron

4. Burning Magnesium ribbon

5. Formation of curd

Chemical Reaction Involves :

• Change in state

• Change in colour

• Change in temperature

• Evolution of gas

Ways of Representing a Chemical Reaction

• Word Equation

Zinc + Sulphuric  ➝  Acid Zinc Sulphate + Hydrogen

        LHS (Reactants)     ➝    RHS (Products)

• Chemical Equation

Zn + H2SO4 ➝ ZnSO4 + H2

       LHS (Reactants)   ➝     RHS (Products)

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.

2Mg + O2 ➝  2MgO

Balancing Chemical Equation

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

• So number of elements involved in chemical reaction should remain same at reactant and product side.

Types of Chemical Reactions

• Combination Reaction

• Decomposition Reaction

• Displacement Reaction

• Double Displacement Reaction

• Endothermic & Exothermic Reaction

• Oxidation And Reduction

1. 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) + H2O (l) ➝ Ca(OH)2 (aq) 

Quick lime + water  ➝  Slaked lime

2. DECOMPOSITION REACTION : The reaction in which a compound splits into two or more simple substances is called decomposition reaction.

A ➝ B + C

Types of Decomposition Reaction

• Thermal Decomposition

• Photolytic Decomposition

• Electrolytic Decomposition

(i) Thermal decomposition: When decomposition is carried out by heating.

(ii) Photolytic Decomposition: When decomposition is carried out in the presence of sunlight.

• Silver chloride turns grey on exposure to sunlight.
• This reaction is used in black & white photography.

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

3. Displacement Reaction: The chemical reaction in which more reactive element displaces less reactive element from its salt solution.

e.g.- 

(i)     Zn + CuSO4   ➝  ZnSO4 + Cu

•    Zn is more reactive than Cu.

(ii) 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 to dirty green colour due to formation of FeSO4 .

4. Double Displacement Reaction : A reaction in which new compounds are formed by mutual exchange of ions between two compounds.

(i) Na2SO4 (aq) + BaCl2 (aq) ➝  BaSO4 (s) + 2NaCl (aq)

• White precipitate of BaSO4 is formed, so it is also called Precipitation reaction.

(ii)          2KCI + Pb(NO3)2 ➝ PbCI2 + KNO3

Potassium Iodine + Lead Nitrate (Yellow ppt.) ➝ Lead Iodide (Yellow ppt.) + Potassium Nitrate

(iii)              2KBr + BaCI2 ➝  2KCI + BaBr2

Potassium Bromide + Barium Iodide ➝ Potassium Iodide + Barium Bromide

5. Endothermic & Exothermic Reaction

• Endothermic Reaction :

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

•  Exothermic Reaction :

The reactions in which heat is released along with the formation of products.

 (i) Burning of natural gas

CH4 (g) + O2 (g) ➝ CO2 (g) + 2H2O (g) + Heat

(ii) Respiration is also an exothermic reaction.

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

6. Oxidation and Reduction :

• Oxidation: (i) The addition of oxygen to reactant.

 (ii) The removal of hydrogen from a reactant.

e.g. - 

C + O2 ➝ CO2

• Reduction: (i) The addition of hydrogen to a reactant.

(ii) The removal of oxygen from a reactant.

• In this reaction, CuO is reduced to Cu and H2 is oxidized to H2O. So, oxidation and reduction taking together is Redox Reaction.

 

Effects of Oxidation in Daily Life

1. Corrosion

• When a metal is exposed to moisture, air, acid etc. for some time, a layer hydrated oxide is formed which weakens the metal and hence metal is said to be corroded.
• Rusting of iron, black coating on silver and green coating on copper are examples of corrosion.
• Corrosion can be prevented by galvanization, electroplating or by putting paints.

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

• By adding antioxidants.

• Keeping food in air tight containers.

• Replacing air by nitrogen.

• Refrigeration.