10th Notes Chemical reactions and equations

     SCIENCE NOTES    

    CHAPTER-1    

   CHEMICAL REACTIONS AND EQUATIONS    

→ Chemical reaction 

Chemical reaction is the process by which two or more substance react with each other to form new substance with different properties.

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

➤ The substances which are formed in a chemical reaction are called Products. 

These are the following changes to determine that the chemical reaction has taken place:

(i) Change in state : The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas. 

(ii) Change in colour : Shiny brown colored Copper on heating in air becomes a black colored compound. 

2Cu + O2 → 2CuO 

(iii) Evolution of gas : The chemical reaction between zinc and dilute sulphuric acid is characterised by hydrogen gas. 

Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) 

(iv) Change in temperature : The reaction between quicklime and water to form slaked lime is characterised by an increase in temperature. 

Ca(OH)2 + CO2 → CaCO3 + H2O 

(v) Formation of a precipitate : When an aqueous solution of sodium sulphate is mixed with the aqueous solution of barium chloride, barium sulphate comes in the form of white precipitate 

Na2SO4 (aq) + BaCl2(aq) → BaSO4(↓) + 2NaCl(aq) 

→ Chemical equations 

A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae. It is a way to represent the chemical reaction in a concise and informative way. 

For example:

Magnesium + Oxygen  → Magnesium oxide 

         (Reactants)                          (Product)

This equation is called word equation. The word equation can be written into chemical equation by writing symbols and formulae of the substance in place of their name. 

2Mg + O2 → 2MgO

Writing a chemical equation:

(i) The symbols of elements and the formulae of reacting substances (reactants) are written on the left hand side of the equation, with a plus (+) sign between them. 

(ii) The symbols and formulae of the substances formed (products) are written on the right hand side of the equation, with a plus sign (+) between them. 

(iii) An arrow sign (→) is put between the reactants and the products. (iv) The physical states of the reactants and products are also mentioned in a chemical equation. 

(v) The upward arrow sign (↑) denotes the evolution of gas and the downward arrow sign (↓) denotes the formation of precipitate   

Balancing the chemical Equation: 

➤ The equation in which atoms of various elements on both sides of a chemical equation are equal in accordance with the law of conservation of mass. 

➤ Law of conservation of mass: It states that, "The matter can neither be created nor be destroyed in a chemical reaction. The total mass of reactants = total mass of products. 

➤ The process of equalizing the atoms of various elements both on either sides of an equation is called the balancing of chemical equation.  

Example 1 : 

Skeletal equation : Mg + O2 → MgO

Balanced equation : 2Mg + O2 → 2MgO 

Example 2 : 

Skeletal equation : Fe + H2O → Fe3O4 + H2 

Balanced equation: 3Fe+ 4H2O → Fe3O4 + 4H2

Example 3 : 

Skeletal equation : AgCl → Ag + Cl2 

Balanced equation : 2AgCl → 2Ag + Cl2 

Example 4 : 

Skeletal equation : Al + O2 → Al2O3

Balanced equation : 4Al + 3O2 → 2Al2O3

→ Types of Chemical Reactions 

I. Combination Reaction: The reaction in which two or more reactants combine to form a single product. 

e.g., 

(i) Burning of coal : C + O2 → CO2 

(ii) Formation of water: 2H2 + O2 → 2H2O 

 (iii)   CaO     +      H2O →   Ca(OH)2    +   Heat 

      (Quick lime)              (Slaked lime) 

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

e.g.,

(i) Burning of natural gas. 

CH4 + 2O2 → CO2 + 2H2O + Heat 

(ii) Respiration is also an exothermic reaction. 

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy 

II. Decomposition Reaction: The reaction in which a compound splits into two or more simpler substances is called decomposition reaction. 

A → B + C 

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

e.g., 

                          Heat

(i)    2FeSO4       →           Fe2O3   +   SO2 + SO3 

 (Ferrous sulphate)        (Ferric oxide) 

      Green colour            Red-brown colour 

                    Heat

(ii) CaCO3     →     CaO         +    CO2 

   (Lime stone)    (Quick lime) 

                        Heat

(iii) 2Pb(NO3)2 → 2PbO   +   4NO2      +   O2 

     (Lead nitrate)(Lead oxide) (Nitrogen dioxide) 

                                                       (Brown fumes) 

(b) Electrolytic Decomposition: When decomposition is carried out by passing electricity. 

 e.g., Electrolysis of water 

           Electric current

 2H2O          →           2H2   +   O2