27 Nov 2015

Chemical Reactions (Ch 1)

Important Terms
Important Questions solved (CBSE)

Important Terms

  • Chemical reaction
Reactants reacts to form products

  • Chemical equation
A chemical equation represents a chemical reaction with their respective chemical formulas.

  • Skeletal Chemical Equation
A simple chemical equation which represents symbols of compounds or elements of reactants and products taking part in a chemical reaction

  • Balanced Chemical Equation
A Balanced Chemical Equation is the one in which the total number of atoms of each elements of the reactants is equal to the total number of atoms of reacted products

  • Representation of symbols of various physical states
Solids  (s)
Liquid  (l)
Gases  (g)
Aqueous (aq) {NOTE: Aqueous is not same as liquid}

Writing Chemical Equations 

1. In a chemical reaction, the reactants are written on the left hand side and the products on the right hand side of the equation.

2. An arrow (→) pointing towards the products is inserted between the reactants and the products. It also represents the direction of the reaction.

3. A single arrow (→) indicates the direction in which the reaction proceeds.

4. A double arrow ( ) indicates a reversible reaction, i.e. the products recombine to form the reactants.

5. A plus sign (+) is inserted between two or more reactants or products formed.

6. If reactions are carried out under specific conditions of temperature, pressure, catalyst etc., then these conditions are mentioned on the arrow.

7. The chemical equation can be made more informative by mentioning the physical states of the reactants and products.

8. If gas is liberated as a product then it is represented by an arrow pointing upwards (↑). If the product formed is in the form of a precipitate, it is represented by an arrow pointing downwards (↓). Balancing the Chemical Equations

9. In a balanced chemical equation, the total number of atoms of each element of the reactants on the left hand side of the equation is equal to the number of atoms of the products formed on the right hand side of the equation.

10. The total mass of the reactants is equal to the total mass of the products or the number of atoms of each element before the reaction and after the reaction is equal.

Types Of  Reaction

Combination Reaction
When two or more substances combine to form a single product, the reaction is known as a combination
For example:
In the laboratory, iron sulphide is prepared by mixing iron and sulphur.
Fe(s) + S(s)  FeS(s)
⇒ Endothermic Reaction: The reactions accompanied by the absorption of heat are called
endothermic reactions.
⇒ Exothermic Reaction:
The reactions accompanied by the evolution of heat are called endothermic

Decomposition Reaction
A chemical reaction in which a single compound splits into two or more simple substances is called a
decomposition reaction.
For example:
When mercuric oxide is heated in a crucible, the orange-red powder begins to darken and a silver mirror
begins to deposit on the cooler parts of the crucible.
2HgO(s)   →    2Hg(s) + O2 ↑
Mercuric oxide Mercury Oxygen

Displacement Reaction
Reactions in which the more reactive element displaces the less reactive element from its compound are called displacement reactions.
For example:
Zinc displaces copper in copper sulphate to form zinc sulphate.
Zn(s) + CuSO4 (aq) → ZnSO4 (aq) + Cu(s)
Zinc Copper sulphate  Zinc sulphate Copper

Double Displacement Reaction
Reactions in which ions of the reactants exchange places to form two new compounds, are called double displacement reactions.
For example:
Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Redox Reaction
The chemical reaction in which oxidation and reduction takes place simultaneously is known as a redox reaction.

Oxidation Reactions
Reactions which involve the addition of oxygen or the removal of hydrogen are called oxidation reactions.
For example:
C(s) + 2H2 (g)  CH4 (g)

Reduction Reactions
Chemical reactions in which the reactants gain hydrogen are reduction reactions.
For example:
 Fe2O3 + 3CO 2Fe + 3CO2↑
Ferric oxide Carbon monoxide Iron Carbon dioxide

The slow process of decay and destruction of metals due to the action of air, moisture or acids is called corrosion.
For example:
Iron combines with oxygen present in the air, in the presence of water, to form a red-brown flaky
substance called rust. This process is commonly called the rusting of iron.
The chemical formula of rust is Fe2O3. x H2O.

Prevention of Corrosion

Corrosion damages buildings, bridges, ships, automobiles and other articles made of iron. Hence,
prevention of corrosion is necessary. This will not only save money but can also prevent the
occurrence of accidents.

It can be prevented by processes like galvanising and electroplating with other metals.

Oils and fats react with oxygen and get oxidised or turn rancid. This process is called rancidity.
Rancidity can be prevented by keeping food in air tight containers or by using antioxidants.
Antioxidants are used to prevent oxidation of food containing fats and oils.
Storage of food in air tight containers also decelerates oxidation.

Important Questions (CBSE)

Important questions will be published here soon.


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