Types of chemical reactions

Types of chemical reactions.Some of the important classes of chemical reactions depending on the type of chemical change taking place in a chemical reaction are:
Those reactions in which a single compound is formed by the combination of two or more elements or compounds are called combination reactions.
Activity: Take a small amount of calcium oxide or quick lime in a beaker. Slowly add water to the quick lime taken in the beaker as they combine vigorously. Pour enough water so that the quick lime remains submerged in water. Touch the beaker and note the temperature.

Calcium oxide and water react vigorously to form slaked lime i.e. calcium hydroxide releasing a large amount of heat. The reaction is thus highly exothermic. The reaction is represented as;
In this reaction a single compound, calcium hydroxide, is formed by the combination of two compounds- calcium oxide and water. Thus the above reaction is an example of combination reaction.
Some more examples of combination reactions:
1) Combination between iron and sulphur to form iron sulphide.
2) Combination between ammonia and hydrogen chloride gas to form ammonium chloride.
3) Combination between sulphur dioxide and oxygen to form sulphur trioxide.

Decomposition Reactions

The reactions in which a single reactant breaks down into two or more simpler products is called a decomposition reaction. These reactions are thus reverse of combination reactions. Decomposition reactions are of three types- Thermal, electrolytic and photolytic.
Thermal decomposition reactions: Those decomposition reactions which take place by absorption of heat. For example, thermal decomposition of ferrous sulphate.
Activity: Take small amount of ferrous sulphate crystals in a dry boiling tube. Note the colour of the crystals. Hold the test tube in a test tube holder and heat the test tube strongly over the flame of spirit lamp. Observe the colour of the crystals after heating.

Observations: The green coloured ferrous sulphate crystals first lose water on heating and form anhydrous ferrous sulphate with a change in colour. This on further heating decomposes to give reddish brown ferric oxide, sulphur dioxide and sulphur trioxide. A characteristic smell of burning sulphur is also observed. Following reaction takes place;
Electrolytic decomposition reactions (Electrolysis): Those decomposition reactions which take place when electric current is passed through the compound. For example, electrolytic decomposition of water.
Activity: Take a plastic mug and drill two holes at its base. Fit rubber stoppers into these holes. These rubber stoppers are fitted with carbon electrodes. Connect these electrodes to a 6 volt battery. Fill the mug with water to nearly half of its capacity such that the electrodes are immersed. Add a few drops of dilute sulphuric acid to the water to make it a good conductor of electricity. Take two test tubes filled with water and invert them over the two carbon electrodes as shown in the figure below. Switch on the current and leave the apparatus undisturbed for some time. Observe the formation of bubbles at both the electrodes and the levels of water in the two test tubes start falling. Once the test tubes are filled with the respective gases, remove them carefully. Switch off the battery now.

(i) The amount of gas collected in the test tube covering the cathode is double than that of the gas in the test tube covering the anode.
(ii) Hydrogen gas burns with a light blue flame with a pop sound and is collected at cathode. Oxygen gas (supporter of combustion-tested by bringing a lighted candle) is collected in the test tube which covers the anode.
Thus on supplying electricity water decomposes into hydrogen and oxygen according to the equation;

Photolytic decomposition reactions (Photolysis): Those decomposition reactions which take place on absorption of light.
Activity: Take some silver chloride in a china dish. Note its colour. Place the china dish in sunlight for some time. Observe the colour of silver chloride after some time.
Observations: The silver chloride crystals slowly turn greyish. On analysis it has been found that silver chloride is decomposed by sunlight into silver and chlorine.
Silver bromide also decomposes in a similar manner.
These reactions are used in black and white photography.   
The decomposition reactions are mostly endothermic in nature because these reactions require energy either in the form of heat, light or electricity.
These are the chemical reactions in which a more reactive element displaces a less reactive element. These reactions are found to occur in solutions. Both metals and non-metals take part in displacement reactions.
Activity: Take a small amount of silver nitrate in a test tube and dissolve it in water. Place a clean copper wire in the test tube. Keep it undisturbed for some time.
Observations: Copper wire becomes shiny after some time due to the deposition of silver on it. The colour of the solution becomes bluish as some copper nitrate is formed.
Copper being more reactive displaces silver from silver nitrate solution. If silver plate is dipped in copper nitrate, no reaction will take place because silver is less reactive than copper.
Some more examples of displacement reactions:
1) The more active metal, iron displaces the less active metal copper from copper sulphate solution.
2) Zinc displaces hydrogen from sulphuric acid or hydrochloric acid. Thus zinc is more reactive than hydrogen.
3) Lead is more reactive than copper and displaces copper from copper chloride solution.

Double Displacement Reactions

The reactions in which there is an exchange of ions between the reactants are called double displacement reactions. These reactions usually occur between two ionic compounds in the solution. The ionic compounds taken as reactants are water soluble. One of the products formed is either insoluble and separates out as a solid, called precipitate or it is a gas. The cations and anions of two different compounds switch their places forming entirely different compounds. The general equation for a double displacement reaction is;
Activity: Take a small quantity of sodium sulphate solution in a test tube. Take small volume of barium chloride solution in another test tube. Mix the two solutions with gentle shaking. Observe the change.
Observations: A white substance which is insoluble in water is formed. This insoluble substance formed is known as precipitate and the reactions which produce a precipitate are known as precipitation reactions. The white precipitate of barium sulphate which has been formed is due to the following reaction;

Neutralisation Reactions

The reactions in which acids and bases react together to from salts and water along with the evolution of heat are called neutralisation reactions.
These reactions are also double displacement reactions.
Examples of neutralisation reactions;

Oxidation-Reduction Reactions

Oxidation is the chemical reaction which involves either gain of oxygen or removal of hydrogen and Reduction is defined as a process involving either gain of hydrogen or loss of oxygen. If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidised. If a substance loses oxygen or gains hydrogen during a reaction, it is said to be reduced. Thus, reduction is just the opposite of oxidation.
It is worthwhile to mention here that oxidation and reduction reactions do not occur alone. If one substance is oxidised in a reaction, the other must be reduced. Thus, oxidation and reduction reactions go side by side. Oxidation-reduction reactions are also known as redox reactions.
Oxidising agent is a substance which loses oxygen or gains hydrogen or in simple terms we can say a substance which undergoes reduction.
Reducing agent is a substance which loses hydrogen or gains oxygen or in simple terms we can say a substance which undergoes oxidation. For example,

In the above reaction, CuO loses oxygen and is thus oxidising agent. H2 has gained oxygen and thus acts as a reducing agent.
Activity: Take some copper powder in a china dish. Heat the china dish with the help of burner or spirit lamp. Observe the changes.
Observations: The surface of reddish brown copper powder turns black which is found to be copper oxide. The reaction taking place is:

Copper has been oxidised to copper oxide.
Some other examples of redox reactions:

Effects of oxidation reactions in everyday life

A number of oxidation reactions take place around us which affect our daily lives. Some of these are discussed as follows:
1) Corrosion: It is the slow oxidation of metals caused by moist air containing oxygen, carbon dioxide and hydrogen sulphide, water vapour etc. Due to corrosion, layers of undesired compounds such as metallic oxides, metallic hydroxides, metallic carbonates are formed on the surface of metals.
The most common example of corrosion is rusting. The slow conversion of iron into hydrated ferric oxide in presence of moist air is called rusting.

Copper is attacked by moist water containing carbon dioxide to form green coloured hydrated copper carbonate CuCO3.Cu(OH)2.
Silver metal gets tarnished i.e. loses lustre and becomes dull on exposure to air. This is due to the formation of a coating of black silver sulphide (Ag2S) on its surface by the action of H2S gas present in the air. Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, especially those of iron. Corrosion of iron is a serious problem. Every year an enormous amount of money is spent to replace damaged iron. Corrosion can be prevented by:
(i) Painting
(ii) Greasing and oiling
(iii) Galvanisation (coating the surface of iron objects with a thin layer of zinc).
2) Rancidity: The slow oxidation of oils and fats present in the food materials resulting in the bad odour and taste of food is called Rancidity. The products formed as a result of this oxidation are volatile and have a bad smell. Rancidity can be prevented by:
(i) Adding anti-oxidants to the food substances.
(ii) Vacuum packing
(iii) Replacing air by nitrogen

(iv) Refrigeration of the food stuff
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