Sunday, March 13, 2011

Chemicals Equations Extra score Science Notes

CBSE ADDA Chemicals Equations

1. Chemical reactions- Chemical changes or chemical reactions are the changes in which one or more new substances are formed.

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

3. Balanced Chemical equation – 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.

4. The chemical reactions can be classified into different types such as

(a) Combination reactions- The reactions in which two or more substances combine to form a new substance are called combination reactions. For example,

2Mg(s) + O2 (g) → 2 MgO (s)
(b) Decomposition reactions – The reactions 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 on heating is known as thermal decomposition. The decomposition of a substance by passing electric current through it is known as electrolysis.
For example, when electric current is passed through acidified water, it decomposes into hydrogen and oxygen. Electric current
2H2O (l) → 2H2 (g) + O2 (g)
Certain compounds when exposed to light undergo decomposition. For example, Silver chloride when exposed to sunlight turns grey due to its decomposition into silver and chlorine. Sunlight 2AgCl(s) → 2Ag(s) + Cl2 (g)
The decomposition of a substance by absorbing light energy is called photolysis or 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 double displacement reactions are precipitation reactions and neutralization reactions.

(i) Precipitation reaction : In precipitation 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) + NaCl (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) + H2O

(e) Redox reaction : If a substance gains oxygen or looses hydrogen during a reaction it is oxidized. On the other hand if a substance loses oxygen or gains hydrogen during a reaction, it is reduced. Both oxidation and reduction take place simultaneously and hence these are called redox reaction. Oxidation

ZnO               +            C          Zn + CO
Reduction           Oxidation
Cl2                    +        H2S          2HCl + S
(f) Exothermic reaction and endothermic reaction : On the basis of energy changes during chemical reaction, they can be classified as

(i) Exothermic reaction- A chemical reaction in which heat energy is produced is called exothermic reaction .For e.g. (i) C (s) + O2 → CO2 (g) + Heat

(ii) Endothermic reaction- A chemical reaction in which heat energy is absorbed is called endothermic reaction. CaCO3 + Heat → CaO + CO2

5. Effects of some oxidation reaction in everyday life:

(a) Corrosion – The process of slow conversion of metals into their undesirable compounds due to their reaction with moisture and other gases present in the atmosphere is called corrosion of metals. Some common examples of corrosion are rusting of iron, formation of green layer on the surface of copper, tarnishing of silver etc. Formation of brown layer on the surface of iron is called rusting of iron. Formula of rust is Fe2O3.xH2o. Rusting of iron is a serious problem.
 (b) Rancidity- The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is known as rancidity. Rancidity is caused due to oxidation of fat and oil present in food materials. Rancidity can be prevented by using various methods such as by adding antioxidants to the food materials, storing food in air tight container and by flushing out air with some inert gases like nitrogen.

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