CLASS :- X
Chapter 1
Subject science
Chemical reactions and equations
Chemical reactions are the
processes in which new substances with new properties are formed.
- Only a rearrangement of atoms
takes place in a chemical reaction.
- The substances which take part in a chemical reaction are called reactants.
- The new substances produced as
a result of chemical reaction are called products.
- In a chemical reaction,
reactants are transformed into products.
- The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.
- Take about 2cm long and clean it by rubbing its surface with sand paper.
- Hold it with a pair of tongs.
Burn it using a burner.
- The magnesium ribbon starts
burning with a dazzling whit flame.
- Hold the burning magnesium ribbon over a watch glass so that the magnesium oxide powder being formed collects in the watch glass.
Note: Before burning in air, the magnesium
ribbon is cleaned by rubbing with a sand paper. This is done to remove the
protective layer of basic magnesium carbonate from the surface of magnesium
ribbon so that it may readily combine with the oxygen of air.
Characteristics of Chemical reactions
- The conversion of reactants
into products in a chemical reaction is often accompanied by some features
which can be easily observed easily.
- The important characteristics
of chemical reactions are:
- Evolution
of a gas
- Formation
of a precipitate
- Change in
colour
- Change in
temperature
- Change in
state
Evolution of a Gas:
Some
chemical reactions are characterized by the evolution of a gas.
The chemical reaction
between zinc and dilute sulphuric acid is characterized by the evolution of
hydrogen gas.
- Take some zinc granules in a
conical flask.
- Add dilute sulphuric acid over
zinc granules.
- We will see the bubbles of
hydrogen gas being formed around zinc granules.
- If we touch the conical flask
with our hand, we will find that it is somewhat hot. So, a change in
temperature also occurs in this chemical reaction.
Formation of a precipitate:
Some
chemical reactions are characterized by the formation of precipitate. A
precipitate is a ‘solid product’ which separates out from the solution during a
chemical reaction.
The chemical between
potassium iodide and lead nitrate is characterized by the formation of a yellow
precipitate of lead iodide.
- Take some lead nitrate solution
in test tube.
- Add potassium iodide solution
to it.
- A yellow precipitate of lead
iodide is formed at once.
- A change in colour also takes place in this chemical reaction.
- Change in colour:
Some
chemical reactions are characterized by a change in color.
The chemical reaction between citric acid and purple colored potassium permanganate solution is characterized by a change in color from purple to colorless.
- Take some dilute potassium
permanganate solution in a test tube. It has purple colour.
- Add some lemon juice (it
contains citric acid) to it with the help of a dropper and shake the test
tube.
- The purple color of potassium
permanganate solution goes on fading and ultimately it becomes colorless.
Some
chemical reactions are characterized by a change in temperature.
The chemical reaction between quicklime and water to form slaked lime is
characterized by a change in temperature.
- Take a little of quicklime in a
hard-glass beaker.
- Add water to it slowly.
- Touch the beaker.
- The beaker feels to be quite
hot.
Change in state:
- Some chemical reactions are
characterized by a change in state.
- When wax is burned (in the form
of wax candle), then water and carbon dioxide are formed.
- Now, wax is a liquid whereas carbon dioxide is a gas. This means that during the combustion reaction of wax, the physical state changes from solid to liquid and gas.
Chemical equations:
· The method of representing a chemical reaction with the help of symbols and formula of the substances involved in it is known as a chemical equation.
· Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
· This is known as word equation. Putting the symbols and formula of all the substances in the above word equation, we get the following chemical equation:
·
The substances which combine or react are known as reactants.
·
The new substances produced in a reaction are known as products.
·
A chemical equation is a short-hand method of representing a
chemical reaction.
Balanced and unbalanced chemical equations:
A balanced chemical
equation has an equal number of atoms of different elements in the reactants
and products.
Zinc
metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas
.Count the number of atoms of all the elements in the reactants and products separately.
In reactants |
In products |
|
No. of Zn atoms |
1 |
1 |
No. of H atoms |
2 |
2 |
No. of S atoms |
1 |
1 |
No. of O atoms |
4 |
4 |
There are an equal number of atoms of different elements in the reactants and products, so the above chemical equation is a balanced equation.- An unbalanced
chemical equation has an unequal number of atoms of one or more elements
in the reactants and products.
- Hydrogen reacts with oxygen to form water
Count the number of atoms of all the elements in the reactants and products separately.
In reactants |
In products |
|
No. of H atoms |
2 |
2 |
No. of O atoms |
2 |
1 |
The
above chemical equation contains an unequal number of oxygen atoms in reactants
and products, so it is an unbalanced equation.
Types of chemical reactions
- Combination
reactions
- Decomposition
reaction
- Displacement
reactions
- Double
displacement reactions
- Oxidation
and Reduction reactions
Types of chemical reactions
Some
of the important types of chemical reactions are:
- Combination reactions
- Decomposition reaction
- Displacement reactions
- Double displacement reactions
- Oxidation and Reduction
reactions
Combination reactions:
Those reactions, in
which two or more substances combine to form a single substance, are called
combination reactions.
- In a combination reaction, two
or more elements can combine to form a compound; two or more compounds can
combine to form a new compound; or an element and a compound can combine
to form a new compound. Examples:
Hydrogen burns in oxygen to form water:
In this reaction, two elements, hydrogen and oxygen are combining to form single compound water, so this is an example of a combination.- Ammonia reacts with hydrogen chloride to form ammonium chloride.
- In this reaction, two compounds, ammonia and hydrogen chloride, combine together to produce a new compound ammonium chloride. So, this is a combination reaction.
- Carbon monoxide reacts with oxygen to form carbon dioxide.
- In
this reaction, carbon dioxide compound reacts with oxygen element to form
a new compound, carbon dioxide. So, this is a combination reaction.
Decomposition reactions:
- Those
reactions in which a compound splits up into two or more simpler substances
are known as decomposition reactions.
The decomposition reactions are carried out by applying heat, light or electricity.
Example: When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide:
In this reaction, one substance, calcium carbonate is breaking up into two simpler substances, calcium oxide and carbon dioxide, so this is a decomposition reaction.
- When a
decomposition reaction is carried out by heating, it is called ‘thermal
decomposition’.
Example: When lead nitrate is heated strongly, it breaks down to form simpler substances like lead monoxide, nitrogen oxide and oxygen. - Some
decomposition reactions are carried out by using electricity.
Example: When electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas.
This decomposition reaction takes place by the action of electricity. It is called electrolysis of water.
- Some
decomposition reactions are carried out by light energy.
Example: When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.
Uses of Decomposition reactions:
- The decomposition reactions
carried out by electricity are used to extract several metals from their
naturally occurring compounds like chlorides and oxides.
- For example, sodium metal is
extracted by the electrolysis of molten aluminum oxide.
Decomposition reactions in our body:
- The digestion of food in the
body is an example of decomposition reaction.
- When we eat foods like wheat,
rice or potatoes, then the starch present in them decomposes to give
simple sugar like glucose in the body; and protein decompose to form amino
acids.
Displacement reactions:
- Those
reactions, in which one element takes the place of another element in a
compound, are known as displacement reactions.
- In general, a more reactive
element displaces a less reactive element from its compound.
Examples:
- When as strip of zinc metal is placed in copper sulphate solution, then zinc sulphate solution and copper are obtained.
- In
this reaction, zinc displaces copper from copper sulphate compound so
that copper is free. This displacement reaction takes place because zinc
is more reactive than copper.
- When a piece of iron metal (or iron nail) is placed in copper sulphate solution, then iron sulphate solution and copper metal are formed.
In this reaction, iron displaces copper from copper sulphate solution. The deep blue color of copper sulphate solution fades due to the formation of green solution of iron sulphate. A red-brown coating (or layer) of copper metal is formed on the surface of iron metal (or iron nail).- This displacement reaction
takes place because iron is more reactive than copper.
Double Displacement reactions:
- Those
reactions, in which two compounds react by an exchange of ions to form two
new compounds, are called double displacement reactions.
Example:
When barium chloride solution is added to sodium sulphate solution, then a white precipitate of barium sulphate is formed along with sodium chloride solution.
In this reaction, two compounds barium chloride and sodium sulphate react to form two new compounds, barium sulphate and sodium chloride. An exchange of ions takes place in this reaction.
In this reaction, barium sulphate is formed as a white, insoluble solid called precipitate which separates out suddenly from the solution.
Note: any reaction in which an insoluble solid called precipitate is formed that separates from the solution is called a precipitation reaction.
Oxidation and Reduction reactions:
Oxidation:
The addition of oxygen to a substance is
called oxidation.
The removal of hydrogen from a substance is
called oxidation.
Reduction:
The addition of hydrogen to a substance is
called reduction.
The removal of oxygen from a substance is
called reduction.
- The oxidation and reduction
reactions are also called redox reactions.
- Example:
When copper oxide is heated with hydrogen, then copper metal and water are formed.
- In the above
reaction, copper oxide (CuO) is changing into copper (Cu), so
copper oxide is being reduced to copper.
Hydrogen is changing into water (H2O), so hydrogen is being oxidized to water.
Copper oxide is giving oxygen required for the oxidation of hydrogen, therefore, copper oxide is oxidizing agent and hydrogen is reducing agent. - Effect of oxidation reactions in everyday
life:
Oxidation has damaging effect on metals as well as on food. There are two common effects of oxidation reactions which we observe in daily. These are:
- Corrosion of metals:
Corrosion is the process in which metals are eaten up gradually by the action of air, moisture or a chemical (such as an acid) on their surface.
- Corrosion is caused mainly by the oxidation of metals by
oxygen of air. Rusting
of iron metal is the most common form of corrosion.
- During the corrosion of iron (rusting of iron), iron metal is oxidized by the oxygen of air in the presence of water (moisture) to form hydrated iron (III) oxide called rust.
Corrosion weakens the iron and steel objects and structures such as railings, car bodies, bridges and ships, etc., and cuts short their life.- Rancidity
- When the
fats and oils present in food materials get oxidized by the oxygen (of
air), their oxidation products have unpleasant smell and taste.
- The condition
produced by aerial oxidation of fats and oils in foods marked by
unpleasant smell and taste is called rancidity.
- Rancidity spoils the food materials prepared in fats
and oils which have been kept for a considerable time and make them
unfit for eating.
- The
development of rancidity of food can be prevented or retarded (slowed
down) in the following ways:
a.
Rancidity can be prevented by adding anti-oxidants to foods
containing fats and oils:
Anti-oxidant is a substance (or chemical) which prevents oxidation. Anti-oxidants are actually reducing agent.
The two common
anti-oxidants used in foods to prevent the development of rancidity are BHA
(Butylated Hydroxy – Anisole) and BHT (Butylated Hydroxy-Toluene).
b.
Rancidity can be prevented by packaging fats and oils containing
foods in nitrogen gas:
When the packed is
surrounded by unreactive gas nitrogen, there is no oxygen to cause its
oxidation and make it rancid.
The manufacturers of potato chips fill the plastic bags containing chips with
nitrogen gas.
c.
Rancidity can be retarded by keeping food in a refrigerator.
d.
Rancidity can be retarded by storing food in air-tight
containers.
e.
Rancidity can be retarded by storing food away from light.