Chemical reaction
Chemical reaction are the processes in which new substance with new properties are formed.
During a chemical reaction, atom of one element do not change into those of another element. Only a rearrangement of atom takes place in a chemical reaction.
1. The substance which take part in a chemical reaction are called reactants.
2. The new substance produced as a result of chemical reaction are called products.
Magnesium +Oxygen-->Magnesium oxide.
The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.
Characteristics of Chemical Reaction:
The important characteristics of chemical reaction are:
i. Evolution of a gas,
ii. Formation of a precipitate,
iii. Change in colour,
iv. Change in temperature, and
v. Change in state.
The method of representing a chemical reaction with the help of symbols and formulae of the substance involved in it is known as chemical equation.
Zinc + sulphuric acid -->zinc
sulphate + hydrogen.
Now, symbol of zinc is Zn
Formula of sulphuric acid is H2SO4
Formula of zinc sulphate is ZnSO4
And, formula of hydrogen is H2
So, putting the symbols and formula of all the substances in the above word-equation, we get the following chemical equation.
The substance which combine or react are known as reactants.
The new substance produced in a reaction are known as products.
5. Types of Chemical Equations:
1. Combination Reactions
2- Decomposition Reactions:
Those reaction in which a compound splits up into two or more simpler substances are known as decomposition reactions.
A decomposition reaction is just the opposite of a combination reaction.
1. When Calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide.
CaCO3 (s) CaO(s) + CO2 (g)
3. Displacement Reactions:
Those reaction in which one element takes the place of another element in a compound, are known as displacement reaction.
1. When of zinc metal is placed in copper sulphate solution, then zinc sulphate and copper are obtained.
CuSO4 (aq) + Zn --->ZnSO4 (aq) + Cu (s)
4. Double Displacement Reactions
Those Reactions in which two compounds react by an exchange of iron to form two new compounds are called double displacement reactions.
A double displacement reaction usually occurs in solution and one of the products, being insoluble, and precipitation out (separates as a solid). Some of the examples of double displacement reaction are given below.
1. When silver nitrate solution is added to sodium chloride solution, then a white precipitate of silver chloride is formed along with sodium nitrate solution.
AgNO3 (aq0) + NaCl (aq)--> Agcl(s)+ NaNO3 (aq)
5. Oxidation and Reduction Reactions
Oxidation:
i.) The addition of oxygen to a substance is called oxidation.
ii.) The removal of hydrogen from a substance is also called oxidation.
Reduction:
i.) The addition of hydrogen to a substance is called reduction.
ii.) The removal of oxygen from a substance is also called reduction.
Oxidising Agent:
a.) The substance which gives oxygen for oxidation is called an oxidising agent.
b.) The substance which removes hydrogen is also called an oxidising agent.
Reducing Agent:
a.) The substance which gives hydrogen for reduction is called a reducing agent.
b.) The substance which removes oxygen is also called a reducing agent.
The oxidation and reduction reactions are also called redox reaction (In the name ‘redox’, the term “red” stands for ‘reduction’ and “ox” stands for ‘oxidation’).
Ex: when copper oxide is heated with hydrogen, then copper metal and water are formed.
CuO + H2 Cu + H2O
Copper oxide Hydrogen copper Water
In this reaction, CuO is changing into Cu. That is, oxygen is being removed from copper oxide. Now, by definition, removal of oxygen from a substance is called reduction, so we can say that copper oxide is being reduced to copper.
5. Oxidation and Reduction Reactions
Oxidation:
i.) The addition of oxygen to a substance is called oxidation.
ii.) The removal of hydrogen from a substance is also called oxidation.
Reduction:
i.) The addition of hydrogen to a substance is called reduction.
ii.) The removal of oxygen from a substance is also called reduction.
Oxidising Agent:
a.) The substance which gives oxygen for oxidation is called an oxidising agent.
b.) The substance which removes hydrogen is also called an oxidising agent.
Reducing Agent:
a.) The substance which gives hydrogen for reduction is called a reducing agent.
b.) The substance which removes oxygen is also called a reducing agent.
The oxidation and reduction reactions are also called redox reaction (In the name ‘redox’, the term “red” stands for ‘reduction’ and “ox” stands for ‘oxidation’).
Ex: when copper oxide is heated with hydrogen, then copper metal and water are formed.
CuO + H2 Cu + H2O
Copper oxide Hydrogen copper Water
In this reaction, CuO is changing into Cu. That is, oxygen is being removed from copper oxide. Now, by definition, removal of oxygen from a substance is called reduction, so we can say that copper oxide is being reduced to copper.
6. Corrosion:
Corrosion is the process in which metal are eaten up gradually by the action of air, moisture or a chemical (such as acid) on their surface. Corrosion is caused mainly by the oxidation of metals by the oxygen of air. Rusting of iron metal is the most common form of corrosion. When an iron object is left in damp air for a considerable time, it gets covered with a red-brown flaky substance called “Rust”. This is called rusting of iron.
During the corrosion of iron (or rusting of iron), iron metal is oxidised by the oxygen of air in the presence of water (moisture) to form hydrated iron(III) oxide called rust.
4Fe + 3O2 + 2x H2O à 2Fe2O.xH2O
Iron Oxygen water hydrated iron (III) oxide (Rust)
Corrosion weakens the iron and steel objects and structures such as railings, car bodies, bridges and ships, etc., and cuts short their life.
7. Rancidity:
The condition produced by aerial oxidation of fats and oils in food marked by unpleasant and taste is called rancidity.
Rancidity spoils the food materials prepared in fats and oils which have been kept for considerable time and makes them unfit for eating. The characteristics of a rancid food are that it gives out unpleasant smell and also has an unpleasant taste. Rancidity is called “vikritgandhita” in Hindi.
The development of rancidity of food can be prevented or retarded (slowed down) in the following ways:
1. Rancidity can be prevented by adding anti-oxidants to food containing fats and oils.
2. Rancidity can be prevented by packaging fat and oil containing food in nitrogen gas.
3. Rancidity can be retarded by keeping in a refrigerator
Rancidity can be retarded by storing food in air-tight containers.
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