Lesson no:-3 Chemical Reactions and Equations

                                Chemical reactions and equations

1. Chemical reaction:-A process in which some substance undergo bond breaking and are transformed into new substances by formation of new bonds is called a chemical reaction.
2. Chemical equation:-The representation of a chemical reaction in a considered form using chemical formula is called as the chemical equation.
3. Rules used in writing chemical equation:-a)The reactant are written on the  left hand side(LHS), while the products are written on the right hand side (RHS).                      b)Whenever there are two or more reactants, a plus sign (+) is written between each two of them.                    c)Reactant side and product side are connected with an arrow (---->) pointing from reactants to products. The arrow shows the direction of the reaction.                                                                                                   d)The conditions like temperature, pressure, catalyst, etc., are mentioned above the arrow pointing towards the product side.                                       e)The physical states of the reactants and products are also mentioned in a chemical equation. The notations g,l,sand aq are written in brackets as a subscript along with the symbols/ formula of reactants and products.           f)Special information or names of reactants/products are written below their formula.
4. Balancing a chemical equation:-In a chemical reaction, the number of atoms of the elements in the reactants  is same as the number of atoms of those elements in the product, such an equation is called as balanced equation.                                                                                          Example:AgNO3+NaCl---->AgCl+NaNO3                                                      In the above reaction, the number of atoms of the elements in the reactants is same as the number of atoms of elements in the products.
5. Types of chemical reactions:-a)combination reaction. b)decomposition reaction. c)displacement reaction. d)double displacement reaction.
6. Combination reaction:-when two or more reactants combine in a reactants combine in a reaction to form a single product , it is called a combination reaction.                                           example:NH3+HCl---->NH4Cl
7. Decomposition reaction:- The chemical reaction in which two or more products are formed from a single reactant is called decomposition reaction.                                                                                                       a)Thermal decomposition:-The reaction in which a compound is discomposed by heating it to high temperature is called thermal decomposition.                                                                                             At high temperature, calcium carbonate decomposes into calcium oxide and carbon dioxide.                                                                            b)Electrolytic decomposition :-The reaction in  which compound is discomposed by passing an electric current through its solution or molten mass is called an electrolytic decomposition.
8. Displacement reaction :- The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction.
9. Double displacement reaction:-The reacton in which the ions in the reactants are exchanged to form a  precipitate are called double displacement reaction.
10. Exothermic reaction :- The process in which heat is given out is called an exothermic reaction.
11. Endothermic reaction :- The reaction in which heat is absorbed is called an endothermic process.
12. Rate of chemical reaction :-Some reactions are completed in short time, i.e., occur rapidly while some other require long time for completion, i.e., occur slowly It means that tha rate of different reaction .                                                                                                                          a)more chemical reactions take place during every chemical change .                              b)Strong acid and strong base react instantaneously.                                                                      c)In our body, enzymes increase the rate of physiological reactions.                                            d)If the rate of the chemical reactions is fast, it is profitable for the chemical factories.            e)The rate of chemical reaction is important with respect to environment .                                  f)The ozone layer in the earth's atmosphere protects life on earth from the ultraviolet radiation of the sun. The process of depletion or maintenance of this layer depends upon the rate of production or destruction of ozone molecules.
13. Factors affecting the rate of chemical reaction :-a)Nature of reactants. b)size of the particles. c)concentration of the reactants . d)temperature of the reactions. e)catalyst.
14. Oxidation:-The chemical reaction in which a reactant combines with oxygen or loses hydrogen to form the product is called oxidation.The chemical substances which bring about an oxidation reaction by making oxygen available are called oxidants or oxidizing agents . In the combustion of carbon, oxygen is a oxidant.
15. Reduction :- The chemical reaction in which a reactant gains hydrogen or loses oxygen to form product is called reduction.
16. Redox reaction :-The reaction which involves simultaneous oxidation and reduction is called an oxidation-reaction or redox reaction. In the redox reaction, one reactant gets oxidised while the other gets reduced during a reaction.    Redox reaction=Reduction+Oxidation.
17. Corrosion:-Due to various components of atmosphere, oxidation of metals takes place, consequently resulting in their damage. This is called corrosion. This is also termed as running of iron. 
18. Prevention of corrosion:- Corrosion damages buildings, bridges, automobiles, ships, iron railings, and other articles made of iron. It can be prevented by using an anti-rust solution, coating surface by the paint by process like galvanising and electroplating with other metals.

Rancidity:- Fats and oils in food, is kept for a long time, gets oxidised, it is found to have foul odour called rancidity.

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