KEY CONCEPTS- Chemical Reaction, Reactants, Products, Notations or
symbols of physical states of reactants and products, Types of chemical
reactions, Combination Reaction, Decomposition Reaction, Displacement Reaction,
Decomposition Reaction, Reactivity
series of metals, Double Displacement
Reaction, Exothermic reactions, Endothermic reactions, Balanced chemical
equations, Precipitation reaction, Oxidation Reaction, Reduction Reaction,
Oxidizing and reducing agents, Effects of oxidation reactions in daily life, Redox
reactions, Corrosion, Rusting, Rancidity,
CHEMICAL
REACTION- The process
in which two or more substances react to give new substances having different
properties is known as a chemical reaction. The chemical reaction is written in
form of a word chemical equation. The word chemical equation does not give true
picture of a chemical reaction. The chemical reaction is written in form of
symbol equation.
e.g.- Carbon + Oxygen Carbon dioxide [word chemical equation]
C + O2 CO2 [symbol
equation]
On the left hand side
of the arrow, reactants are written and on the right hand side of the arrow
products are written. The direction of arrow decides the position of reactants
and products. Some chemical reactions are balanced by writing equal number of
atoms of reactants and products. All chemical reactions take place at N.T.P.
(Normal Temperature and Pressure) conditions i.e. at 273 K temperature and 1
atmospheric pressure.
REACTANTS-The substance which react in a chemical reaction are
called as reactants. e.g.- Carbon and oxygen are reactants in the above
reaction.
PRODUCTS-The substance which is formed as a result of a chemical
reaction is known as a product. The product may be one or more. The properties
of products are quite different from their reactants.
e. g.- The carbon
dioxide formed in the above reaction has quite different properties than carbon
and oxygen.
NOTATIONS
OR SYMBOLS FOR PHYSICAL STATES OF REACTANTS AND PRODUCTS-The chemical reactions are written
in form of symbols of their reactants and products. It helps in balancing of
atoms of reactants and products of chemical reactions. The physical states are
also written in a chemical reaction to make it more informative. The symbols of
physical states are changed into italics to differentiate them from reactants
and products symbols and
the notation or symbol used for solids is (s), for liquids is
(l), for gases is (g) and for aqueous solutions is (aq.). The gas evolved in a
chemical reaction can be shown by an upward arrow (↑). The solid precipitate formed in a
chemical reaction can be written by a downward arrow (↓) or as (ppt.). Dilute solvents are written
as (dil.) and concentrated as (conc.).
TYPES OF CHEMICAL REACTIONS- On the basis of ways of exchange of
partners to form product, the chemical reactions are broadly categorized into
following four types.
(a) COMBINATION
REACTION-The reactions in which two or more reactants combine to form a single
substance under suitable conditions are called combination reaction.
e. g.- C (s) + O2 (g) --- Sunlight------>
CO2 (g)
e. g.- H2 (g)
+ Cl2 (g) ----Heat-----> 2 HCl (g)
(b) DECOMPOSITION
REACTION- The chemical reactions in which a single compound splits into two or
more simple substances under suitable conditions are called decomposition
reactions.
e. g.- 2 H2O
(l) ---Electrolysis----> 2 H2
(g) + O2 (g)
e.g.- 2AgCl (s)
-----Sunlight----> 2 Ag (s) +
Cl2 (g)
e.g.- ZnCO3 (s)
----Heat------> ZnO (s) + CO2 (g)
(c) DISPLACEMENT REACTION-The chemical reactions in which one
element takes the position of other element in a compound by displacing it, are
called displacement reaction reactions. The reactivity of one metal over the
other metal decides the displacement reaction.
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e. g.-Ag NO3
(aq.) + Cu (s) -----------> Cu(NO3)2 (aq.)
+ 2Ag (s)
e.g.- H2SO4 (aq.) + Zn (s)
-----------> ZnSO4 (aq.) + H2 (g)
(d) DOUBLE DISPLACEMENT REACTION- The chemical reactions in
which one component each of both the reacting molecules get exchanged to form
the products are called as double displacement reactions.
e.g. BaCl2 (aq.)
+ Na2 SO4 (aq.) -----------> BaSO4 (s)
+ NaCl (aq.)
Barium sulphate
(White Precipitate)
e.g.- NaCl (aq.)
+ AgNO3 (aq.) -------------> NaNO3 (aq.)
+ AgCl (s)
EXOTHERMIC
AND ENDOTHERMIC REACTIONS-Those chemical reactions, in which heat is liberated, are called as
exothermic reactions. The container in which exothermic reaction occur gets
heated up. e. g.- 3 H2 (g) + N2 (g)
------->2 NH3 (g) +92 kJ
NaOH (aq.) +
HCl (aq.) ---------> NaCl (aq.) + H2O (aq.)
+ 57.5 kJ
Combustion and
respiration are also exothermic reactions.
Those chemical
reactions, in which heat is absorbed, are called as endothermic reactions. The
container in which endothermic reaction occur gets cold. e. g.-
C(s) + H2O
(g) ----------> CO (g) + H2 (g) -130kJ
N2 (g)
+ O2 (g) -----------> 2 NO (g) -180kJ
BALANCED
CHEMICAL EQUATIONS-A
chemical equation in which the number of atoms of each type of element is equal
on both sides of the arrow is called a balanced chemical equation. In a
balanced chemical reaction the total mass of reactants is equal to the total mass
of the products. Balancing of a chemical reaction is done according to the law
of conservation of mass. According to this law the mass can neither can be
created nor can be destroyed in a chemical reaction. There is no change in mass
of reactants and the products.
The balancing of a
chemical reaction or the mass balancing can be done in following steps-
1. Write the chemical
equation in form of a word equation. The reactants are kept on left and
products on right hand side. There is + (plus sign) between two or among more
reactants are there. Similarly, there is + (plus sign) between two or among
more products. Reactants and products are separated by an arrow pointing
towards products.
2. The word equation is changed into symbol equation, by
writing symbols and formulae of all the reactants and products.
3. The atoms of reactants and products are balanced. The
formulae of substances will remain unchanged. By keen observation, the
coefficients are adjusted on both sides to maintain law of conservation of
mass. Count the number of atoms of both side and adjust them.
e.g.- Methane burns to produce carbon dioxide gas and water.
or, Methane + Oxygen -------> Carbon dioxide + Water [Word chemical equation]
or, CH4 + O2 ------> CO2 + H2O [Symbol Equation]
or, CH4 + 2 O2 ------> CO2 +2 H2O [Balanced Chemical
Equation]
PRECIPITATION REACTION-The chemical reaction in which a
precipitate (an insoluble salt) is formed is called as precipitation reaction.
The precipitate is represented by a symbol (↓) or as (ppt.).
e.g.- Ca(OH)2(s) + CO2 (g) -------->
CaCO3 ( ↓ ) + H2O (l)
e.g.- AgNO3(aq.)
+NaCl (aq.) ----> AgCl (s) + NaNO3(aq.)
(White ppt.)
e.g.- BaCl2
(aq.) + Na2SO4 (aq.) ------> BaSO4 (s) +
2NaCl (aq.)
(White ppt.)
OXIDATION
REACTION- The
chemical reaction in which there is loss of hydrogen or gain of oxygen by any
reactant is called as oxidation reaction.
{Old concept}
It is also defined as the chemical reaction in
which an atom, molecule or ion looses one or more electrons. {Electronic
Concept}
e. g.- 2Cu (s)
+ O2 (g) --------> 2 CuO (s) {Cu is oxidized to CuO}
e.g.- N2H4 + O2 ---------> N2 + 2 H2
O { O2 is reduced to H2O}
Hydrazine
REDUCTION
REACTION- The
chemical reaction in which there is gain of hydrogen or loss of oxygen by any
reactant is called as oxidation reaction. {Old concept}
It is also defined as the chemical reaction in
which an atom, molecule or ion gains one or more electrons. {Electronic
Concept}
e.g.- ZnO (s) +H2 (g) ---------> Zn (s) + H2O (l)
e.g.- Cl2 (g) + H2
(g) -----Sunlight---> 2HCl (g)
OXIDISING
AND REDUCING AGENTS-
The reactants that brings about the oxidation is called as oxidizing agents.
Oxidizing agents are themselves get reduced in a chemical reaction. The
reactants that bring about the reduction are called as reducing agents.
Reducing agents are themselves get oxidized in a chemical reaction.
e.g.- Oxygen, ZnO,
Chlorine etc are oxidizing agents.
e.g.- Hydrogen,
Copper, Hydrazine etc are reducing agent.
The electron loosing
species (atom or molecule or ion) is called as reducing agent while electron
gaining species is called as oxidizing agent. ‘X’ is a reducing agent while ‘Y’
is an oxidizing agent.
e. g.- Iron is losing
electrons to oxygen, therefore iron like metal is an example of reducing agent.
Oxygen is accepting electrons from iron, therefore, oxygen like non-metal is an
example of oxidizing agent.
EFFECTS
OF OXIDATION REACTIONS IN DAILY LIFE-
Following are the
common effects of oxidation reaction in our daily life.
(i) The cellular respiration inside our body
cells ours due to oxidation.
(ii) The burning of
fuels for cooking, running of vehicles of fossil fuels are due to oxidation.
(iii) Corrosion is
slow oxidation of some metals. It is an uneconomical chemical process causing
loss of metals.
(iv) Rancidity of
edible food items is an oxidation process that make them unfit for eating.
REDOX
REACTIONS- The chemical reaction in
which both oxidation and reduction occurs simultaneously is called as redox
reaction. In a redox reaction, one reactant loses certain electrons (i.e.
undergoes oxidation) and other reactant gains electrons (i.e. undergoes
reduction)
e. g. -4 Fe + 3 O2 --------> 2
Fe2O
e.g.- CuO + H2 ------> Cu + H2O
CuO-------->
Cu {Reduction}
H2
----------> H2O {Oxidation}
In this reaction, iron
is oxidized and oxygen is reduced. The oxidation number of iron has increased
and oxidation state of oxygen has reduced.
CORROSION-The slow oxidation
of some metals in the presence of air and moisture is called as corrosion
reaction. Corrosion is an oxidation reaction. Corrosion is usually seen in
iron, copper, silver, brass, steel etc. In case of iron, it is called as rusting.
Rusting occurs in presence of oxygen and moisture i.e. water vapours. Near sea
shores the rate of rusting increases. Rusting causes great economic loss as the
bridges, iron objects, vehicles etc get rusted gradually. Alloy formation,
galvanization, chromium plating, applying paint, oil and grease are common
methods to prevent rusting.
Iron + Oxygen + Moisture -------> Rust
Fe + O2 + H2O -------> Fe2O3.xH2O
In rust, iron forms hydrated iron (III)
oxide.
The
copper utensils often get coated with a dull green coating. This coating is a
mixture of copper hydroxide and copper carbonate.
Copper
+ Oxygen + Water vapour + Carbon dioxide --> copper hydroxide + copper
carbonate
Or,
Cu + O2 + H2O + CO2 -------> Cu (OH) 2
+ CuCO3
The
silver develops a black coating in moist and polluted areas. The polluted air
contains Sulpher which combines with silver to form silver sulfide (Ag2S).
Hydrogen sulfide (H2S) gas contains Sulpher.
Silver
+ Hydrogen Sulfide + Oxygen + Water Vapour --> Silver sulfide + Hydrogen
Or,
2 Ag + H2S + O2 + H2O ---> Silver sulfide
(Ag2S) + H2
All
the above are example of corrosion in iron, copper and silver metals. So, the
corrosion tarnishes the metals. Applying of oil, paint, grease, galvanization,
alloy formation, chromium plating etc. are some of the methods of preventing
rusting.
RANCIDITY-The chemical reaction in which food
items are oxidized resulting into production of foul smell and taste is called
rancidity. The oil and fats found in the oily foods are more prone to
rancidity. e.g.- Milk and curds become rancid if left overnight in summer
season.
Prevention of
rancidity-
(a) Adding of
antioxidants to oily food substances.
(b) Keeping foods in
air tight containers.
(c) Keeping the food
in nitrogen filled packets or in liquid nitrogen.
(d) Keeping food item
at low temperature in refrigerators.
IN-TEXT
(WITHIN THE CHAPTER)
Q.1 Why should magnesium ribbon be
cleaned before burning?
Ans-Magnesium is a reactive metal. When
magnesium is kept in air for a long time, a layer of magnesium oxide (MgO) is
formed on its surface. The magnesium metal reacts with oxygen of air, to form
magnesium oxide. When flame is brought near the magnesium ribbon having coating
of magnesium oxide, it does not catch fire easily. Therefore, the surface of
magnesium ribbon should be properly cleaned with sand paper before burning of
it in air.
Q.2 Write the balanced equations for the
following chemical reactions:
(i) Hydrogen + Chlorine
Hydrogen chloride
(ii) Sodium + Water
Sodium hydroxide + Hydrogen
(iii) Barium chloride + Aluminium sulphate Barium sulphate + Aluminium chloride
Ans-
(i) H2 + Cl2 2 HCl
(ii) 2Na + 2H2O
2NaOH + H2
(iii) 3BaCl2 + Al2(SO4)3 3BaSO4 + 2AlCl3
Q.3 Write the balanced equations with state symbols for
the following reactions:
(i) Solutions of barium chloride and
sodium sulphate in water react to give insoluble barium sulphate and the
solution of sodium chloride.
(ii) Sodium hydroxide solution (in
water) reacts with hydrochloric acid solution (in water) to produce sodium
chloride (in water) and water.
Ans-
(i) BaCl2 (aq.)
+ Na2SO4 (aq.) ------> BaSO4 (s)
+ 2NaCl (aq.)
(White ppt.)
(ii) NaOH (aq.) + HCl (aq.)
---------> NaCl (aq.) + H2O (l)
Q.4 A solution of the substance ‘X’ is used for white
washing.
(i) Name the substance ‘X’ and write
its formula.
(ii) Write the reaction of the
substance ` X` with water.
Ans.-
(i) The substance `X` is calcium
oxide (also called as quick lime). It’s formula is CaO.
(ii) CaO(s) + H2O (l) → Ca(OH)2 (s)
‘X’
Calcium
Hydroxide
Q.5 With the help of an experiment, show that in the
electrolysis of acidulated water, the volume of one gas is twice the volume of
the other gas. Name the gas.
Ans-The gas released in twice volume by the electrolysis
of acidulated water is hydrogen.
2H2O (aq.)
---Electrolysis----> 2H2 (g) + O2 (g)
Q.6 Why does the colour of copper sulphate change when an
iron nail is dipped in it?
Ans- When an iron nail is dipped into the copper sulphate
solution, a reddish brown coating of copper is acquired by it due to
displacement reaction.
CuSO4 (aq.) + Fe
(s) --------> FeSO4
(aq.) +
Cu(s)
Iron nail
Copper deposited on iron nail
Q.7 Give an example of double displacement reaction.
Ans- BaCl2 (aq.) + Na2
SO4 (aq.) -----------> BaSO4 (s) + NaCl
(aq.)
Barium sulphate
(White Precipitate)
Q.8 Identify the substances that are
oxidized and the substances that are reduced in the following reactions:
(i) 4 Na (s) +
O2 (g) ------> 2 Na2O (s)
(ii) CuO (s) + H2 (g) ------> Cu (s)
+ H2O (l)
Ans- (i) In this reaction, Sodium (Na) is oxidized to
Sodium oxide (Na2O). So,
Oxygen (O2) is reduced.
(ii) In this reaction, Copper (II)
oxide (CuO) is reduced to copper (Cu). So, Hydrogen is oxidized to water (H2O).
EXERCISE QUESTIONS
Q. 1 Which of the following statements
about the reaction given below are incorrect?
2 PbO (s) + C (s) ------------> 2 Pb (s) + CO2
(g)
(a)Lead is getting reduced. (b) Carbon dioxide is getting
oxidized.
(c) Carbon is getting oxidized. (d) Lead oxide is getting reduced.
(i) a and b both are incorrect
(ii) a and c
(iii) a, b and c
(iv) all are incorrect.
Ans- (i) a and b both are
incorrect
It is because lead is getting
oxidized to PbO in backward direction. CO2 is getting reduced to C
in backward direction.
Q.2 Fe2 O3 +
2Al ---------> Al2 O3 + 2 Fe
The above reaction is an example of:
(a) Combination reaction (b) Double displacement reaction
(c) Decomposition reaction (d) Displacement reaction
Ans-(d) Displacement reaction
Aluminium lies above Iron in the
reactivity series of metals. Since, Al is replacing Fe from Fe2 O3.
Therefore it is an example of displacement reaction.
Q.3 What happens when dilute hydrochloric acid is added
to iron filings? Tick the correct answer:
(a) Hydrogen gas and iron chloride
are produced.
(b) Chloride gas and iron hydroxide
are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Ans- (a) Hydrogen gas and iron chloride
are produced.
Fe (s) + 2HCl (dil.) ---------> FeCl2 (aq.)
+ H2 (g)
This reaction is an example of
displacement reaction.
Q.4 What is a balanced
chemical equation? Why should chemical equations be balanced?
Ans- A chemical
equation is said to be balanced if- (a) atoms of different elements on both
sides of equation are equal and (b) the equation is molecular i.e., molecules
of reactants and products are written.
Balancing of a chemical reaction is done according to the law
of conservation of mass. According to this law the mass can neither can be
created nor can be destroyed in a chemical reaction. There is no change in mass
of reactants and the products.
Q.5 Transfer the following into chemical
equations and balance them.
(a) Hydrogen gas
combines with nitrogen to form ammonia.
(b) Hydrogen sulphide
gas burns in air to give water and Sulpher dioxide.
(c) Barium chloride
reacts with aluminium sulphate to give aluminium chloride and precipitate of
barium sulphate.
(d) Potassium metal
reacts with water to give potassium hydroxide and hydrogen gas.
Ans-
(a) H2 + N2 -------> NH3
or, 3 H2 + N2 ------->2 NH3
(b) H2S + O2
---------> H2O + SO2
or, 2 H2S + 3 O2
---------> 2 H2O + 2 SO2
(c) BaCl2 (aq.) + Al2(SO4)3
(aq.) -----------> BaSO4 (s) +
AlCl3 (aq.)
Barium sulphate
or, 3 BaCl2
(aq.) + Al2(SO4)3 (aq.)
-----------> BaSO4(s) + NaCl (aq.)
Barium sulphate (ppt.)
(d) K + H2O ------>
KOH + H2
or, 2 K + 2 H2O ------> 2 KOH + H2
Q.6 Balance the
following chemical equations:
(a) HNO3 + Ca(OH)2 ----------> Ca(NO3)2
+ H2O
(b) NaOH + H2SO4------------->
Na2SO4 + H2O
(c) NaCl + AgNO3
--------------> AgCl + NaNO3
(d) BaCl2 + H2SO4 ---------> BaSO4
+ HCl
Ans-
(a) HNO3 + Ca(OH)2 ----------> Ca(NO3)2
+ H2O
|
Element
|
No of atoms in reactants(L.H.S.)
|
No of atoms in products(R.H.S.)
|
|
H
|
3
|
2
|
|
N
|
1
|
2
|
|
O
|
5
|
7
|
|
Ca
|
1
|
1
|
So, 2HNO3 + Ca(OH)2 ----------> Ca(NO3)2
+ 2 H2O
(b) NaOH + H2SO4------------->
Na2SO4 + H2O
|
Element
|
No of atoms in reactants(L.H.S.)
|
No of atoms in products(R.H.S.)
|
|
H
|
3
|
2
|
|
S
|
1
|
1
|
|
O
|
5
|
5
|
|
Na
|
1
|
2
|
So, 2 NaOH + H2SO4------------->
Na2SO4 + 2 H2O
(c) NaCl + AgNO3
--------------> AgCl + NaNO3
|
Element
|
No of atoms in reactants(L.H.S.)
|
No of atoms in products(R.H.S.)
|
|
Na
|
1
|
1
|
|
Cl
|
1
|
1
|
|
Ag
|
1
|
1
|
|
N
|
1
|
1
|
|
O
|
3
|
3
|
So, NaCl + AgNO3
--------------> AgCl + NaNO3
It is already a
balanced chemical reaction.
(d) BaCl2 + H2SO4 --------->
BaSO4 + HCl
|
Element
|
No of atoms in reactants(L.H.S.)
|
No of atoms in products(R.H.S.)
|
|
Ba
|
1
|
1
|
|
Cl
|
2
|
1
|
|
H
|
2
|
1
|
|
S
|
1
|
1
|
|
O
|
4
|
4
|
So, BaCl2 + H2SO4
---------> BaSO4 + 2 HCl
Q.7 Write the balanced equations for the
following reactions.
(a) Calcium hydroxide
+ Carbon dioxide ------------> Calcium carbonate + water
(b) Aluminium + Copper
Chloride ----------> Aluminum chloride + Copper
(c) Barium chloride +
Potassium sulphate -----------> Barium sulphate + Potassium chloride
(d) Zinc + Silver nitrate --------> Zinc
nitrate + Silver
Ans-
(a) Ca(OH)2
+ CO2 --------> CaCO3 + H2O
(b) 2 Al + 3 CuCl2
----------> 2 AlCl3
+ 3Cu
(c) BaCl2 +
K2SO4 -----------> BaSO4 + 2KCl
(d) Zn + 2 AgNO3
-----------> Zn(NO3)2 + 2Ag
Q.8 Write the balanced chemical
equations for the following reactions and identify the type of reaction:
(a) Potassium bromide (aq.)
+ Barium iodide (aq.) ----> Potassium iodide (aq.) + Barium
bromide (aq.)
(b) Zinc carbonate (s)
-------> Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g)
+ Chlorine (g) ---------> Hydrogen chloride (g)
(d) Magnesium (s) +
Hydrochloric acid (aq.) -----------> Magnesium chloride (aq.)
+ Hydrogen (g)
Ans-
(a) 2 KBr (aq.)
+ BaI2 (aq.) ---------> 2KI (aq.) + BaBr2 (aq.)
– double displacement reaction.
(b) ZnCO3 (s)
----------> ZnO (s) + CO2 (g) –decomposition
reaction
(c) H2 (g)
+ Cl2 (g) --------> 2HCl (g)-combination reaction
(d) Mg(s) + + 2HCl (dil.)
---------> MgCl2 (aq.) + H2 (g)-displacement
reaction
Q.9 What does one mean by exothermic and
endothermic reactions? Give examples.
Ans-Those chemical reactions, in which
heat is liberated, are called as exothermic reactions. The container in which
exothermic reaction occur gets heated up. e. g.-
3 H2 (g) + N2 (g)
------->2 NH3 (g) +92 kJ
NaOH (aq.) +
HCl (aq.) ---------> NaCl (aq.) + H2O (aq.)
+ 57.5 kJ
Those chemical
reactions, in which heat is absorbed, are called as endothermic reactions. The
container in which endothermic reaction occur gets cold.
e. g.-
C(s) + H2O
(g) ----------> CO (g) + H2 (g) -130kJ
N2 (g)
+ O2 (g) -----------> 2 NO (g) -180kJ
Q.10 Why respiration is considered as an
exothermic reaction?
Ans- Respiration releases energy in form
of A.T.Ps in the cell. The oxygen breaks down the glucose
with the help of
enzymes as a result carbon dioxide gas, water vapours and energy is released.
C6H12O6
(s) +6 O2 (g) --------> 6CO2 (g)
+ 6 H2O (l) +2820kJ
Q.11 Why are decomposition reactions
called opposite to combination reactions? Write equations for these reactions.
Ans- The chemical reactions in which a
single compound splits into two or more simple substances under suitable
conditions are called decomposition reactions.
e.g.- 2AgCl (s)
-----Sunlight----> 2 Ag (s) +
Cl2 (g)
e.g.- ZnCO3 (s)
----Heat------> ZnO (s) + CO2 (g)
The reactions in which
two or more reactants combine to form a single substance under suitable
conditions are called combination reaction.
e. g.- C (s) + O2 (g) ---
Sunlight------> CO2 (g)
e. g.- H2 (g)
+ Cl2 (g) ----Heat-----> 2 HCl (g)
Since, there is
breaking of substance in decomposition reaction whereas there is formation of
any substance in combination reaction. Therefore, both reactions are called
opposite to each other.
Q.12 Write equations for each
decomposition reaction, where energy is supplied in the form of heat, light and
electricity.
Ans-
(i) 2AgCl (s)
-----Sunlight----> 2 Ag (s) +
Cl2 (g)
(ii) ZnCO3
(s) ----Heat------> ZnO (s) + CO2 (g)
(iii)2 NaCl
------Electricity---> 2Na + Cl2
Q.13 What is the difference between
displacement and double displacement reactions?
Ans-The chemical reactions in which one
element takes the position of other element in a compound by displacing it, are
called displacement reaction reactions. The reactivity of one metal over the
other metal decides the displacement reaction.
e.g.- H2SO4
(aq.) + Zn (s) -----------> ZnSO4 (aq.) + H2
(g)
The chemical reactions
in which one component each of both the reacting molecules get exchanged to
form the products are called as double displacement reactions.
e.g. BaCl2 (aq.)
+ Na2 SO4 (aq.) -----------> BaSO4 (s) +
NaCl (aq.)
Q.14 In the refining of silver, the
recovery of silver from silver nitrate solution involved displacement of copper
metal. Write chemical equation involved.
Ans-The chemical reaction involved in
refining of silver is-
Cu(s) +2AgNO3
(aq.) --------Cu (NO3) (aq.) + 2Ag(s)
Q.15 What do you mean by precipitation
reaction? Explain giving examples.
Ans-
The chemical reaction in which a precipitate (an insoluble salt) is formed is
called as precipitation reaction. The precipitate is represented by a symbol
( ) or as (ppt.).
e.g.- Ca(OH)2(s) + CO2 (g) -------->
CaCO3 ( ↓ ) + H2O (l)
e.g.- AgNO3(aq.)
+NaCl (aq.) ----> AgCl (s) + NaNO3(aq.)
(White ppt.)
Q.16 Explain the following in terms of gain
or loss of oxygen with two examples of each-
(a) oxidation (b) reduction
Ans-
(a) oxidation- The
chemical reaction in which there is gain of oxygen by any reactant is called as
oxidation reaction.
e. g.- 2 Zn (s)
+ O2 (g) -------> 2 ZnO(s)
e. g.- 2Cu (s)
+ O2 (g) --------> 2 CuO (s)
(b) reduction- The
chemical reaction in which there is loss of oxygen by any reactant is called as
reduction reaction.
e. g.- ZnO (s) +H2 (g) ---------> Zn (s) + H2O (l)
e. g.- CuO (s)
+ H2 (g) -----------> Cu (s) + H2O (l)
Q.17 A shining brown coloured element ‘X’
on heating in air becomes black in colour. Name the element
‘X’ and the black
coloured compound formed.
Ans- The element ‘X’ is copper and the
black coloured compound is copper (II) oxide.
2 Cu
+ O2
------Heat-----------> 2 CuO
‘X’
Copper oxide (Black)
Q.18 Why do you apply paint on iron
articles?
Ans- Paints applied on the surface of
iron articles cut off the direct contact of oxygen and moisture. Due to this
protective coating the iron objects remain protected from the rusting.
Q.19 Oil and fat containing food items
are flushed with nitrogen. Why?
Ans- The chemical reaction in which oil
and fats found in the oily food items are oxidized resulting into production of
foul smell and taste of such food is called rancidity. The oil and fat
containing food items when filled with nitrogen, stops rancidity i.e. oxidation
of such food items as nitrogen is a non-reactive gas.
Q.20 Explain the following terms with one
example of each- (a) Corrosion (b) Rancidity
Ans-
(a) Corrosion-
The slow oxidation of some metals in the presence of air and moisture is called
as corrosion reaction. Corrosion is an oxidation reaction. Corrosion is usually
seen in iron, copper, silver, brass, steel etc. In case of iron, it is called
as rusting. Rusting occurs in presence of oxygen and moisture i.e. water
vapours. Near sea shores the rate of rusting increases. Rusting causes great
economic loss as the bridges, iron objects, vehicles etc get rusted gradually.
Alloy formation, galvanization, chromium plating, applying paint, oil and
grease are common methods to prevent rusting.
Iron +
Oxygen + Moisture -------> Rust
Fe + O2 + H2O -------> Fe2O3.xH2O
In rust, iron forms hydrated iron
(III) oxide.
(b) Rancidity-The
chemical reaction in which food items are oxidized resulting into production of
foul smell and taste is called rancidity. The oil and fats found in the oily
foods are more prone to rancidity.
e. g.- Milk and curds
become rancid if left overnight in summer season.
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