Chemical reactions and equations

➣  Physical Change: Change in physical properties.
– Melting
– Boiling
– Condensation
– [Note- No change occurs in the identity of the substance].

➣  Chemical Change:
– Atoms in the reactants are rearranged to form one or more different substances.
– Old bonds are broken, new bonds are formed.
– Reactants lose their properties to form product of different properties.

4 Fe(s) + 3O₂ →2Fe2O₃ (rust). Iron Oxygen Ferric oxide

➣   Chemical equation:
The symbolic representation of a chemical reaction is called a
chemical equation.
Features of a chemical equation:

•    The reactants are written on the left hand side with a plus sign between them.
•    The products are written on the right hand side with a plus sign between them.
•    An arrow separates the reactants from the products. The arrow head points towards the products and indicates the direction of the reaction.

Skeletal chemical equation: A chemical equation which simply represents the symbols and formulas of reactants and products taking part in the reaction is known as skeletal chemical equation for a reaction.

For example: For the burning of Magnesium in the air, Mg + O₂ → MgO is the skeletal equation.

•   Balanced chemical equation: A balanced equation is a chemical equation in which number of atoms of each element is equal on both sides of the equation i.e number ofatoms of an element on reactant side = number of atoms of that element on the product side.

➣    Identification:

Combustion :	AB + O2 → Oxide of A & B.
Combination :	A + B → C
Decomposition :	AB → A + B
Displacement :	A + BC (aq) → AC (aq) + B
Double Displacement :	AB (aq) + CD (aq) → AD (aq) + CB

Definitions with examples :

1. Combination Reaction :Two or more reactant combine to form a single product.
2 Mg (s) + O₂ (g) → 2 MgO Magnesium Oxygen Magnesium oxide (White ash)
(basic) turns Red litmus blue

2. Decomposition Reaction:A single compound decomposes or break down to give two or moresimpler substances.
2 FeSO₄ (s). --> Fe2 O₃ (s) + SO₂ (g) + SO₃(g) Ferrous Sulphate Ferric oxide Sulphur Sulphur (green) (brown) dioxide trioxide

3. Displacement Reaction: A more reactive element [metal] displaces less reactive elemen [metal] from its aqueous salt soln

Fe (s) + ZnSO₄ (aq)      → FeSO₄ (aq) + Zn (s) (Colourless) (green)

4. Double Displacement Reaction: Aqueous soln of two ionic compounds react by exchange of their ions is called double displacement Reaction

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

Pb(NO₃)₂ + 2 KI (aq) → PbI₂ ( ↓ ) + 2 KNO₃ (aq)

5. Oxidation Reaction: In oxidation reaction, addition of oxygen or removal of hydrogen or loss of electron takes place.

2 Mg(s) + O₂ (g)  →  2MgO (s)  2 Cu + O₂  →  2 CuO (Black ) (Copper II Oxide) 

6. Reduction Reaction:In reduction Reaction addition of hydrogen or removal of oxygen or gain of electron takes place. CuO(s) + H₂ (g)  →Cu(s) + H₂ O (l) 

7. Redox Reaction: Reaction involving both oxidation and reduction simultaneously

CuO (s)+ H₂   →  Cu(s) + H₂ O (l). 

8. Exothermic Reaction: Reaction in which heat is evolved.

C (s) + O₂ (g)   → CO₂ (g) + Heat

10. Neutralisation Reaction : When an acid and a base react together to form salt and water.  

HCl (aq)                      +  NaOH (aq)  →  H2O (l)  +  NaCl (aq)  

(acid)                                              (base)                        (Water)                (Salt)  

Hydrochloric acid     Sodium hydroxide                                                     Sodium Chloride

Law of Conservation of Mass

In a chemical reaction matter is conserved. Total no. of atoms = Total no. of atoms Total mass = Total mass. [While Balancing a Chemical Equation Formula of reactants and products should not be changed]. Balancing:

      

➣   Corrosion: Process of slowly reacting up of metals due to attack of atmospheric gases like O₂ ,CO_2M etc.

Rust(hydrated Iron (III)oxide)

Prevention:Painting, Galvanization, oiling greasing.
Corrosion of Aluminium has advantage , since Al2O3 formed as a result
of corrosion act as protective layer.

➣   Rancidity:

Oxidation of oils or fats in a flood, resulting into a bad smell and taste.
Prevention: Adding anti-oxidants.
Vacuum Packing
Replacing air by Nitrogen
Refrigeration of food stuf