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.
Observations:
(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