CHAPTER – 1 ChemicalReactionsandEquations
❑
Chemical Reaction : – Whenever
a chemical change occurs we can say that a chemical reaction has taken place
eg – Food gets
digested in our body – Rusting of iron.
❑
Chemical Equation :–Achemical
reaction can be expressed symbolically by using chemical equation
eg magnesium is burnt into air to form magnesium oxide can
be represented as
Mg + O2 → MgO
–
We can observe or recognise a chemical reaction
by observing change in state, colour, by evolution of gas or by change in
temperature.
❑
Physical state of the reactant and products are mentioned to make
chemical reaction more informative. eg we use (g) for gas, (l) for liquid, (s)
for solid and (aq) for aqueous.
❑
Balancing Equation :– We balance
the chemical equation so that no. of atoms of each element involved in the
reaction remain same at the reactant and product side.
eg Fe + H2O →
Fe2O3 + H2
can be written as
3 Fe(s) + 4H2O(g) → Fe2O3(s)
+4H2(g)
❑ Combination Reaction :– The reaction in which two or more
substances combine to form a new single substance
eg CaO(s) + H2O(l) → Ca(OH)2 (aq)
Calcium Water Calcium hydroxide
oxide (slaked lime)
Quick lime
–
Ca(OH)2 slaked lime is used for white
washing walls. It reacts will CO2 to form CaCO3 and gives
a shiny finish to the walls.
Ca(OH)2(aq)+ CO2(g) →
CaCO3(s)+ H2O (l)
|
Calcium |
Calcium |
|
Hydroxide |
Carbonate |
–
Burning of Coal
C(s) + O2(g) → CO2(g) +
heat + light
–
Formation of water
2H2(g) +
O2(g) → 2H2O(l)
❑ Exothermic Reactions :– Reaction in which heat is released along
with the formation of products. eg. CH4(g) + 2O2(g) →CO2(g) +
2H2O(g) –
Respiration is also exothermic reaction.
–
De composition of vegetable matter into compost.
❑
De compositon Reactions :– The
reaction in which a single substance decomposes to give two or more substances.
De composition reactions can be of three types
Thermal Decompositon :– When a decompositon reaction is carried out
by heating
–
Silver bromide behaves similarly
2Ag Br →Sunlight 2Ag(s) + Br2(g)
–
The above two reactions are used in black and
white photography.
Endothermic
Reactions – The reactions which require energy in the form of heat, light
or electricty are called Endothermic Reactions.
2Ba(OH)2 + NH4Cl → 2BaCl2 + NH4OH
–
Displacement
Reaction : The chemical Reaction in which an element displaces another
element from its solution
Fe(s) + CuSO4(aq) → FeSO4 +
Cu(s)
Copper (aq)
Sulphate Iron Sulphate
–
The nail becomes brownish in colour and the blue
colour of Copper Sulphate solution fade.
–
Other examples Zn(s)
+ CuSO4 → ZnSO4
+ Cu(s)
(aq) (aq)
Copper Zinc
Sulphate Sulphate
Pb(s) +
CuCl2 → PbCl2 + Cu(s)
(aq) (aq)
Copper Lead
Chloride Chloride
–
Zinc and lead are more reactive elements than
copper. They displace copper from its compounds.
Double Displacement
Reaction : The reaction in which two different atoms or group of atoms are
mutually exchanged
eg. Na2 SO4 + BaCl2 → BaSO4(s) + 2NaCl
(aq) (aq) (aq)
Sodium Barium Barium Sodium
Sulphate Chloride Sulphate Chloride
A white substance is formed due to above reaction. The
insoluble substance is called precipitate.
Precipitation Reaction –Any reaction that produces a
precipitate is called a precipitation reaction.
eg. Pb(NO3)2 + 2KI
→ PbI2 +2KNO3
(aq) (aq) (aq)
Lead
Nitrate Potassium Lead Potassium
Iodide Iodide Nitrate
–
Oxidation : Oxidation is the gain of oxygen or
loss of hydrogen eg. 2Cu + O2 →Heat 2CuO
When Copper is heated a black colour appears. If this CuO
is reacted with hydrogen gas then again Cu becomes brown as reverse reaction
takes place
CuO + H2 →Heat Cu + H2O
–
Reduction
: Reduction is the loss of oxygen or gain of hydrogen.
–
Redox
Reaction : The reaction in which one reactant gets oxidised while other
gets reduced
eg. ZnO + C → Zn +
CO
MnO2 + 4HCl → MnCl2 + 2H2O
+ Cl2
Corrosion
: When a metal is attacked by substances around it such as moisture, acids
etc. eg. Reddish brown coating on iron.
(ii) Black coating on Silver.
–
Rancidity
: When fats and oils are oxidised they become rancid and their smell and
taste change.
–
Antioxidants are added to foods containing fats
and oil.
EXERCISE
(Question Bank)
(1 Mark)
Answer the following questions very breifly
1.
What happens when the milk is left at room
temperature during summer?
2.
Write a chemical equation when magnesium is
burnt in air to give magnesium oxide.
3.
A substance under goes chemical reactions to
produce simpler products, what type of reaction is this?
4.
Why do copper vessels lose their shine when
exposed to air?
5.
Which gas is produced by the action of dilute
hydrochloric acid on zinc granules?
6.
Fe2O3 + 2Al →
Al2O3 + 2Fe
The above reaction is an example
of which type of reaction.
7.
Name the type of reaction in which energy is
absorbed.
8.
Why does the colour of copper sulphate solution
change when an iron nail is dipped into it?
9.
Give an example of decomposition reaction which
proceeds by absorbing electric energy.
10.
Why do we balance the chemical equation?
Answer
the following questions briefly (2 mark)
1.
Write down the observations which indicate the
occurence of a chemical reaction.
2.
Why is respiration considered as an exothermic
reaction? Explain.
3.
Transfer the following statements into Chemical
equations and then balance them.
a)
Hydrogen gas combines with nitrogen to form
ammonia.
b)
Potassium metal reacts with water to give
potassium hydroxide and hydrogen gas.
4.
Identify the substances that are oxidised and
the substances that are reduced to the folllowing reactions.
1. Na + O2 → Na2O (s) (g) (s)
CuO + H2 → Cu + H2 O(l)
(s) (g) (s)
5.
What happens when silver chloride is exposed to
sunlight? Give one practical application of this reaction. Write the equation
also.
6.
Why is the bag used for potato chips flushed
with nitrogen gas?
Answer the following questions
in detail (3 marks)
1.
Write down the balanced chemical equations for
the following reactions.
a) Zinc
Carbonate(s) → Zinc
Oxide + Carbon Dioxide(g)
b) Aluminium(s)
+ Chlorine(g) → Aluminium
Chloride(s)
c) Magnesium(s)+Water(l) →Heat MagnesiumHydroxide(l)+Hydrogen(g)
2.
Choose combination, displacement and double
displacement reactions out of the given reactions.
i) MnO2(s) + 4HCl(l) →MnCl2(s) +
Cl2(g)
+ 2H2O(l) ii) CaO(s) +
CO2(g)
→ CaCO3(s) iii) 2AgCl(s) → 2Ag(s) + Cl2(g)
3.
What happens when CO2(g) is passed
through slaked lime? Write the balanced chemical equation. Write the type of
reaction that has occured.
Explain the following questions
detail (5 marks)
1.
Balance the following chemical equation and
identifythe type of reaction they represent
KClO3 → KCl + O2
NH3 + O2 → NO + H2O
Na2O + H2O → NaOH
Na + H2O →
NaOH + H2
FeCl3 + NaOH → Fe (OH)3 +
NaCl.
2.
Define various types of chemical reactions.
Write one chemical equation for each type.
