2 CHEMICAL-CHANGES AND REACTIONS

0 Types of chemical changes.

SYLLABUS
Direct combination decomposition displacement; double decomposition
(The above to be taught with sultable chemical equations as examples).
() Energy changes in a chemical change.
EXOnermic and endothermic reactions with examples -

evolution/absorption of heat, light and electncny

2.1 CHEMICAL CHANGE
2: Sodium chloride and silver nitrate also react
A chemical change is
permanent change in which
a
in a solution state to form the precipitate or silver
the chemical composifion of a substance is changed chloride and sodium nitrate.
and one or more new substances with different NaCl (aq)+AgNO, (aq) AgCId + NaNO, (2)
chemical compositions and dijferent properties are
white ppt.
formed. The change is not easily reversible. A
(ii) Heat: Some chemical reactions occur only
chemical change always involves a chemical reaction.
on heating.
A chemical reaction is the process of
breaking Examples1:Coppercarbonate decomposes on
the chemical bonds of the reacting substances
heating (A is symbol for heating) into copper oxide
(reactants) and making new bonds to form new
and carbon dioxide.
substances (products).
CuCOs) CuO (s) +CO(8)
A chemical bond is the force that holds the atoms of
2: Lead nitrate decomposes on heating leaving
a molecule together, as in a compound. ro a yellow residue of lead monoxide, brown gas
A chemical change or chemical reaction occurs nitrogen dioxide and colourless gas oxygen.
when particles collide. Collisions occur when 2Pb(NO) 2Pb0+ 4NO, +02
reactants are in close contact or when energy is
supplied. Thus, one or more of the following (iv) Light : Some chemical reactions take place
conditions are necessary for a chemical change by the action of light. These
called photochemical
are
reactions or photolysis. Molecules of the reactants absorb
i) Mixing (close contact) In some cases, a
light energy to get activated, and then react rapidly.
chemical reaction occurs when two substances are
Examples 1: Plants form glucose from carbon
mixed in their solid states.
dioxide and water in the presence of light.
Examples l lodine and phosphorus react
explosively when brought into close contact. 6CO,+ 12H40. Light CH0 +60,+6H,0
(Glucose)
2 Lead nitrate (white) and potassium iodide
2: Hydrogen and chlorine react in the presence
(white) react to make lead iodide (yellow) of sunlight.
PbNOJ()+ 2KI (S) --2KNO, (6) + Pbl,(
(i) Solution: In some cases, a chemical reaction
H+Clh Sunlight 2HCI
3:
Occurs when substances are mixed in either molten If chlorine water is exposed to sunlight,
or aqueous state.
bubbles of a gas are seen, oxygen is evolved.
Clh +H,0 HCI+ HCIO0
Examples 1 : Oxalic acid crystals and sodium
carbonate react in water solution only. 2HCiO Sunlight
2HCI +O
UV radiation
(21)
 . Wnte the chemical mation wthere
the h) Aaythmm is Nat m the hanufa ni
are vhaervad.
flkwing vhanges
) B h e aluthn f y y r lphate chang
(a) Gar is ewlhad when a p i r of inm I8 akkhd o thas subxe
thix
) Coar change is nUNAT
()
Pipitate is fiwmd Cokurless ooncentrated sulyhur acad
(d) hysical state of reactants is (d n
at into
changat nue changes hv baK N ANhng a small

7. Give eason for the following:

(a) Silver nitrate solution pper it.
is kept in cokurad botes
23 TYPES react
OF
CHEMICAL CHANGES
CHEMICAL REACTIONS
OR (ii) nvo or more apounds combine to
form suhst
1. Direct combination (or
single prduct. to ca
synthesis) Exawples Ammonia and hydrogen chhrit give
2. Decomposition both compounds. combine to fom a new compeun
sive
Displacement
4. Double decomposition ammonium chloride.
De
a Direct combination or synthesis NH ()+ HCI (e) NH,CI (s)
(A reaction in which two or more substances ammonia| htngen ammonum
chlonde]
chvnde
combine together to form a single substance is called When calcium axide (quick lime) combins
combination reaction or
a

A+BAB
synthesis with water, a vigorous reacthon takes place with
th
In the
liberation of a large amount ot heat (exothermi
above feact+on, a combination of reaction) and calcium hydroxide. Ca(OH), (slakai en
substances A and B (reactants) takes
molecule of a new substance, AB
place to give a lime) is formed. mo
(Product). CaOXs)+H,O)» Ca(OH)tay)
In combination reactions: Calcium oxide Calcium hydroxide
O elements combine (quick lime) (slaked lime)
to form a
compound.
Examples 1: Carbon burms in oxygen to form Slaked lime is used for white washing. When
a w
a gaseous compound, carbon dioxide. solution of slaked lime is applied to the
wallk
C(s) heat calcium hydroxide reacts slowly with carbon dionide
+
O,g) 0(s) + Heat
carbon) (oxygen] present in the atmosphere to form a thin layer o
carbon dioxide]
calcium carbonate the walls of the
2: The reaction between iron and
sulphur to
on
building
CalOH),(aq)+ CO()Caco;0)+ HOo th
form iron () sulphide.
The layer of calcium carbonate fomed
Fe(s)+S(s)- FeS() gives a
Heat is required to start the reaction but once
shinyfinish.
started, it continues exothermally *. Various
salts be
Thereaction of
photosynthesis is also
can
prepared by this method. considered as
dcombination reaction in which
for example : iron (1I) chloride molecules of carbon
dioxide and water are used by
2Fes)+3Cl(g) > 2FeCl,(s) plants to make glucose.
i/an element and a compound combine to give a Sunlight
new compound. 6CO +6H,0 Chlorophyl GH0,+ 60
Examples Carbon monoxide, a compound.
I: Experiments: To show direct combination reaction
burns in the presence of oxygen, an element, to form Take some black lead sulphide in a test tube an
a single heat it. The black lead sulphide reacts with
product, carbon dioxide.
2C0 () + 0,(8) heat, 2CO, (8) oxygen to form white lead sulphate.
carbon monoxide] (oxygen] carbon dioxide] PbS+20 PbSO, (combination)
2: Sulphur dioxide and
oxygen combine under Hold a piece of
magnesium ribbon over d
certain conditions to form sulphur trioxide. flame. It burns with a white light.
dazzling
250,8)+O(e) >2s0,(s forming magnesium oxide.
Refer page 30. 2Mg+O, 2Mg0 (synthesis)
(24
 2. Decomposition (iii) a
compound can break up to form two or more
PKe chemical reaction in which a compound splits new compounds.

into two or
moresimpler substances (elements or Examples I: The compound calcium carbonate
compounds) is called decomposition reaction on
strong heating decomposes to form two
compounds, calcium oxide and carbon dioxide.
In a decomposition reaction, usually a single
reactant decomposes without the presence of a second
CacOs) neat
1000°C
CaO(s) + COg)
the action of heat is sufficient calcium calcium carbon
substance and very often carbonate] oxide] dioxide)
to cause the reaction to take place. AB decomposes to
Ferrous sulphate undergoes
give products
C, D, etc. thermal
decomposition to give feric oxide, sulphur dioxide
AB> C +D+.. and sulphur trioxide.
Decomposition breaking up of a compound either
is the
2FeSO,(s) Fe,0,5)+ SOg) + SO,)
into elements or simpler compounds, such that these [White] Brown
productsdo not recombine to)form the original compound. Ferrous sulphate crystals are actually ferrous
occur in the presence of
Decomposition may sulphate heptahydrate (FeS0, 7H,O). These crystals are
heat orlight, or bythe passage of an electric current. green in colour, on heating in a test tube they change
Heat, light and electricity are different forms of to white and then a brown solid (Fe,0,) is formed.
energy which cause breaking of bonds in the The gas coming out is a mixture of sulphur dioxide
molecules of reactants to yield simpler products,) (SO.) and sulphur trioxide (SO,) having a suffocating
In a decomposition reaction: and pungent smell of burning sulphur.
() a compound breaks up into two or more elements. (A decomposition reaction that is brought abou
Examples 1: The compound mercuric oxide, by heat without any recombination on cooling is
when heated, decomposes to form two elements, known as thermal decomposition.
mercury and oxygen.
Thermal decomposition of metal compounds
2Hg0 () 2Hg ( + 0, (8)
Decomposition of compoundsof metals by heat are
mercuric oxide] mercury] [oxygen
Note h e symbol a (delta) js used to signify based on their reactivity. Compounds of reactive metals
that heat has caused the reaction. are more stable, they do not decompose easily, while
V2: Whensilverchloride (white) is exposed to less reactive metal compounds decompose readily.
sunlight, it undergoes photochemical decomposition ) Metal hydroxides
forming a grey metal, silver and a greenish yellow Hydroxides
gas, chlorine. Metal hydroxides are stable
2AgCl(s)Sunnght 2Ag(s)
+Clh Na to heat.
silver chloride [white] Silver grey chlorine]
V3: When electriceurrent is passed through Decompose on heat1ng to
Ca
acidulated water, the latter decomposes into hydrogen Mg form metal oxide and
and oxygen. Al water vapour.

electric
24,00 2H2 (e)
current 0 (g) Zn Ca(OH), Ca0+H,O
(n, andcompound
a can break up to form both clements Zn(OH), ZnO H,O
compounds. Pb(OH) PbO+ H,O
Pb CuOH),CuO+H.O
Example: The compound potassium nitrate, on Cu-
heating, decomposes to produce a compound, Yields metal, oxygen and
Hg
potassium nitrite, and an element, oxygen. water vapour.
heat
2KNO,() 2KNO,(6) +O,8) Ag 4AgOH 4Ag+0+21,0
potassium nitrate potassium nitrite] [oxygen

( 25
 ii) Metal carbonates
Decomposition reactions in our body
Carbonates Digestion of food by our body is an
an eexarmple
of a decomposition reaction.
Stable to heat and
Na soluble in water. The starch present in the food we
eat
decomposes into glucose and sugar. Proteins under
Ca Decompose on heatin decomposition to form amino acids. Fats and i
Mg With decreasing vigour
to fom metal
are decomposed to fatty acids and finally oxid
oxide
and carbon dioxide. by respiration into carbon dioxide and water

[O,
MgCO, MgO+ CO
Suarch Glucose
co+H.0+Ere
Pb
ZnCO ZnO+ CO Experiments To show decomposition reactions
CuCO, CuO +CO ons.
Cu 1. Take somelead nitrate crystals in a test tube
a
Hg Decompose on heating to form metal, heat them. The crystals first melt and, on
oxygen and carbon dioxide. furth
heating. give out both nitrogen dioxide, a reddi
brown gas, and oxygen. A yellow solid
Ag 2Ag,CO 4Ag +0, +2C0, (e
monoxide) is left behind in the test tube.
i) Metal bicarbonates (metal 2Pb(NO,), 2PbO+4NOo, 1 +0,1
hydrogen carbonate)
2. Put some zinc carbonate in a test tube fitted
Metal bicarbonates metal with
hydrogen carbonates
or
a cork and a bent
glass tube. On
decompose to give metal carbonates, water vapour heating, carbon
and carbon dioxide.
dioxide is given out, which will turm lime
wate
milky. The residue, i.e. zinc oxide, is yellow wher
2NaHCO Na,CO3 +
H,0 +cO hot, but it turns white on cooling.
sOdium bydrogen carbonate sodium carbonate
ZnCO, ZnO + CO,
Ca(HCO) CaCO3 + HO +CO
3. Heat
calcium bydrogen carbonate calcium carbonate orange-coloured ammonium dichromate in
a test tube. Upon heating. it swells and
MgHCO) MgCO + H0 + CO,
magnesium bicarbonate magnesium carbonate
decomposes with flashes of
light, evolving
nitrogen and water vapour and a green solid
(iv) Metal nitrates chromium oxide is left behind.
(NH,),Cr0, > Cr,0, +4H,0 f+ N,
Nitrates ammonium chromium
dichromate oxide
K On heating, they melt
Na and decompose to give
4. When hydrated copper(II) sulphate is heated in
a test tube, the blue-coloured
metal nitrite and oxygen. crystals change
2KNO, 2KNO +O into white anhydrous salt. The change may e
Ca Decompose on heating represented by the following equation:
Mg to form metal oxide,
Al nitrogen dioxide and CuSO SH,O(s) CuSO(s) + SH,Og)
Oxygen. hydrated salt anhydrous salt
(blue) (white)
n
2Ca(NO,) 2Ca0+4NO, +O2 However, anhydrous copper(11) sulphate may b
the blue hydrated form by
Fe 2Zn(NO)2 2Zn0+ 4NO,+O changed to
sample of the anhydrous salt and adding a few
a
taking
Pb
2Pb(NOg) 2PbO+4NO,+0
Cu 2Cu(NO,), 2Cu0+4NO,+ O, drops of water to it (this is the test to detect the
Hg Decompose on heating to Form metal, presence of water).
nitrogen dioxide and xygen.
CuSO(s) +SH,O() » CuSO,5H,O(s)
anhydrous salt hydrated salt
Ag 24gNO, 2Ag+2NO, +0 (white) (blue)

26
 that we have Experiments To show displacement reactions.
So far. most of the reactions in a certain
considered proceed quite detinitely
1. Take a solution of copper sulphate in a beaker,
to identify the reactants add a few pieces of zinc and stir with a glass
direction, and it is possible
therealso exists a rod. The blue colour of the solution gradually
and the products. However.
reactions in which the direction of fades, and soonthesolution becomes colourless.
group of
the At the same time, reddish brown particles of
chemical change can be reversed by changing
conditions under which the reaction is taking place. copper settle down in the beaker. Here, zinc
reactions are called reversible reactions. displaces copper from its compound copper
Such
of copper sulphate
hydrated sulphate.
Thus, this change
is a reversible CuSO4 +Zn ZnSO, +Cul
to anhydrous copper sulphate
reaction, and the equation for the reaction is 2. In a test tube, take some dilute sulphuric acid
CusOSH,Os) CuS0g(5) + SH,O() and drop a small piece of magnesium ribbon.
To show that the reaction is reversible, we put
Brisk effervescence takes place, and hydrogen
is evolved, which burns with a pop sound on
in between the reactants and
the sign " bringing a burning match stick near the mouth
the products. of the test tube. Here, magnesium displaces
the latter is
If steam is passed over red hot iron, hydrogen from sulphuric acid.
converted into magnetic oxide, and
partially Mg +HSO MgSO, +H
hydrogen is released. If, on the other hand, hydrogen
is passed over the heated oxide, it partially 3. Pass chlorine gas through a solution of
changes back to the metal along with steam. potassium iodide. The colourless solution
3Fe + 4H,O Fe,0 + 4H2 turns yellow brown as iodine is formed. Here,
In either case, the reaction ends with a mixture chlorine displaces iodine from potassium
iodide.
of reactants and products.
2KI +Ch 2KCI + 1
Experinents: To show thermal dissociation
From the above examples, it can be noticed that
Heat some solid ammonium chloride in a test
in metals, zinc is more active (reactive) than copper,
tube. Two colourless gases, anmmonia and hydrogen
chloride, are produced. As these gases move up to
and magnesium is more active compared to
the upper part of the test tube which is cooler, they hydrogen. In non-metals, chlorine is more active as
compared to iodine.
combine to form ammonium chloride, which
appears as a white sublimate on the upper cooler Activity (reactivity) series of elements
side of the test tube.
Metals Non metals
NHCI NH, 1+ HCI
A reversible decomposition reaction brought Potassium K | Most active metal

about only by heat isthermal dissociation. Sodium Na

Fluorine F | Most active
2. On heating, nitrogen tetraoxide changes to Calcium Ca
Chlorine C
nitrogen dioxide, a reddish brown gas. On Magnesium Mg| Bromine Br
Aluminium Al
cooling, nitrogen dioxide changes into the lodine Iy Least active
onginal compound, nitrogen tetraoxide. Zinc Zn
heat Iron
N,O cool
2NO, Lead Pb
Note: The more reactive
Hydrogen] H]|
Displacement
i s a chemical change in which a more active Cu
element displaces the less
Copper reactive element from its
elemeny-displaces a less active element from itS salt Mercury Hg
salt solution.
solution. Silver Ag
ABC AC + B Gold Au
Element C displaces, element B from the Platinum Pt y Least active metal

compound AB.
27
 (b) Neutralization: h e reaction
By taking similar examples, the activity series
given above can be prepared. acid and a base that Joms salt and betwe
water
reaction of neutralisa
4. Double decomposition referred to as a
place because
(C
The reaction takes he
This is a type of chemical change in which two
ion(H*) from the acid
combines with he hy
compounds in a solution react to form two new base to form watery
ion(OH-) from the
compounds by utual exchange of radicals. Double NaOH+ HCI NaCI H.o
decomposition reaction is also called double (acid) (salt)
(water
displacement reaction. (base)
OR
AB +CDAD +CB Na* OH+ H* C Na* Cl + H o
These reactions are of wo types: (a) OH
precipitation lonic form
reactions and (b) neutralization reactions.
the common ions, Nat
(a) Precipitation reaction : A chemical reaction Cancelling C and
in which two only change is the combination of H* and o
compounds in their aqueous state react
to form insoluble salt (a precipitate) as one of
an to form un-ionized water,
the products is known as a ie. H+OH> H,0
precipitation reaction.
For example: In neutralisation reaction, a soluble base
BaCl, (aq) + Naz$O, (aq) BasO,(s) + 2NaCl (aq) or an insoluble base reacts with an acid tot.
o form
white ppt. and water.
CuSO, (aq) + HaS (g) CuS(s) + H,SO4 (aq) Neutralisation of a soluble base (alkali) wiîth
black ppt. KOH+HNO3 KNO, +H,0
Precipitation is the formation of a solid (insoluble (Alkali) (Acid) (Salt)
2NaOH + H,S0, Na,SO, + 2H,0
product), either
when two solutions are mixed, or (Alkali) (Acid) (Salt)
when a gas is bubbled into a solution. NH,OH + HCI NH,CI +H,0
(Alkali) (Acid) (Salt)
Experiments : To show double decomposition Neutralisation of an insoluble base with an acid
reactions. PbO+ 2HNO, Pb{N03)> +H,0
1. Take a solution of silver nitrate in a test tube (Base) (Acid) (Salt)
and add dilute hydrochloric acid or a solution CuO+H,S0, CuSO +H,0
of sodium chloride. A white, curdy precipitate (Base) (Acid) (Salt)
is formed. PD(OH), + 2HNO, > Pb(NO3)2 + 2H,0
AgNO + HCI AgCI+ HNO (Base) (Acid) (Salt)
AgNO, + NaCI > AgCI+ NaNO Cu(OH)+ H,SO4 CuSO, +2H,0
(Base) (Acid) (Salt)
2. Fill one-third of a test tube with dilute sulphuric As a chemical process, neutralization h
acid, and add to it a solution of barium chloride. many uses
A thick white precipitate is immediately formed.
(a) When someone is stung by a bee, formic
BaClh +HSO, BaSO4 t+ 2HCI enters the skin and causes pain, which canb
Note Double decomposition reactions may also
occur with evolution of a gas.
relieved by rubbing the spot with slaked lina
baking soda, both of which are basic in nds
Example: and thus neutralize the acid.

On adding dilute sulphuric acid or dilute (6) If our stomach glands secrete excess HCL
hydrochloric acid to metal sulphide we get metal experience pain, which is relieved by taking m
sulphate or metal chloride and an offensive (rotten magnesia (magnesium hydroxide) or soluia
egg) smelling gas H^S is evolved. sodium hydrogen carbonate, both of which ae
FeSts)+ HSO, (aq) -> Fes0,(aq) + H,ST in nahure. On the other hand, if there is a deho
of HCl, some or the other suitable
ZnS(s)+ 2HCI (ag) ZnCI, (aq) + H,ST organe
is taken in dilute form to make up for 1.
28
 somewhat acidic. and thus
(c) Acid that is accidentally spilled on to our clothes (d) If the soil is
certain crops, slaked
can be neutralized with ammonia solution. unfavourable for growing
acid.
However if acid is spilled on our skin, we should neutralize the excess
lime is added to
immediately wash that area with water.

EXCERCISE 2(B)
1. Complete the following statements. (c) PbO, + SO2 > PbSO,
(a) The chemical change
involving iron and hydrochloric ( 2KCI0, 2KC1+ 30
acid illustrates a
... eaction.
(g) 24,0 2H,0+0
b) In the type of reaction called .. **************
(h) KNO, + H,$O, > HNO, + KHSO,
two compounds exchange their positive and negative
radicals. (i) CuO +H> Cu +Hz,0.
(C) A
catalyst either .. g) CaCO3 C a 0 + CO2
. Of the rate of a
chemical change but itself
of the reaction.
remans. .. at the end (k) NH,CI NH, + HCI
(1) PbO+ 2HNO, > Pb(N0,) +2H,0
(d) On heating. hydrated copper
sulphate changes its colour (m) AgNO + NaCI - AgCl+ NaNO
from.
2. When hydrogen burns in 11. Match the following
oxygen, water is formed; when
electricity is passed through water, hydrogen and oxygen (a) Zn(s)+ H,SO (aq) (i) Photochemical
are given out. Name the
type of chemical changes involved ZnSO,aq)+H,(g) decomposition
in the two cases.
(b) 2AgCIs) i) Thermal
3. Explain the following chemical changes by giving one Sunligh 2Ag(s)+Cl,(g) decomposition
example of each.
(a) Double decompo (c) 2KCI (i) Displacement reaction
on (b) Thermal dissociation
(c) Reversible reaction (d) Displacement Electricity, 2K+Cl,
4. (a) What is synthesis ? d) 2Hg0(6) (iv) Electrolytic
(b) What kind of chemical reaction is synthesis ?
Support your answer with an example.
2Hg(s)+0,(g) decomposition
12. Multiple choice:
Decomposition brought about by heat is known as thermal
(a) Which of the following is not a characteristic of a
decomposition. What is the difference between thermal
dissociation and thermal decomposition. chemical change?
i) Itis ireversible
6. (a) Define neutralization reaction with an example. (i) Nonet energy change is involved
b) Give a balanced equation for this reaction.
(ii) New substance is formed
(c) Give three applications of neutralization reactions.
(iv) Involves absorbtion or liberation of energy
What do you understand by precipitation reaction ? Explain (b) Areaction of a type : AB +CDAD+CD, involves
with an example.
) no chemical change
8. (a) What are double displacement reactions ?
(i) decomposition of AB and CD
b) Give an example of double displacement reaction, (ii) exchange of ions of AB and CD
where a gas is evolved.
(iv) combination of AB and CD
9. (a) What is a decomposition reaction ?
(c) The reaction:
(b) Decomposition reactions can occur by (i) heat BaClfag)+ H,SO,(aq) -> BaS0,(s) +2HCl(aq) is
i) electricity and (ii) sunlight.
) displacement reaction
Give two balanced equations for each.
(ii) neutralisation reaction
10. State the type of reactions each of the following equations (i) decomposition reaction
represent and balance the ones that are not balanced. (iv) double displacement reaction
(a) Cl +2KBr 2KCl+ Br (d) Thermal decomposition of sodium carbonate will
(b) NaOH + HCI NaCl+ H,O produce
(c) 2Hg0 2Hg +O, i) carbon dioxide (i) oxygen
(d) Fe +CuSO > FeSO, +Cu (ii) sodium hydroxide (iv) no other product

(29
 2.4 ENERGY hydroxide (slaked
REACTIONS
CHANGE IN
CHEMICA alkalinecalcium
produced boils the
water and a
hissinu \ime).
In every chemical change or chemical reaction, produced. This is
called slaking of lime sOung
change in energy is
involved, i.e. there is a
difference
CaO+ H,0 > Ca(OH) + Hleat
wh
between he chemical energies of the reactants and lquicklime|
slaked lime
the products. This tem
energy can be in the form of heat, 4: Formation of water: When
light, sound and clectricity. burnt in oxygen, water is formed and hydrog
Every substance has a fixed amount released. neal o
of stored
energy which is in the form of 2H,0 + Heat OX
and is referred to as its potential energy 2H, +O
chemical energy. Note: The sign A indicates heat.
A
chemical reaction involves the breaking up
chemical bonds between atoms resulting in of S: Formation of ammonia : Nitroee
of energy in the form of heat, and
absorption with hydrogen in the presence of cataluet
simultaneous
formation of nds with release of energy. These two
divided) iron at 450°C to S00°C, and finey
above1
di
types of energies are different from each other, i.e.
atmospheres of pressure to form ammonia
onia. e
Cne
there is either a surplus or a deficit of N+3H,A 2NH, + Heat
energy during
the reaction. Therefore, in a chemical
reaction, energy 6: Neutralisation reactions are
also
is either absorbed or released.
reactions.
exothemi
Depending upon the energy released or NaOH+ HCI NaCl
> +
H,0 + Hleat
absorbed, chemical changes or chemical reactions are (base) (acid) (salt) (water)
of two types: Note : Our bodies need energy to
make possik
1. Exothermic 2. Endothermic the reactions that take place in our cells.
reactions allow us to carry Out our everyday These
1. Exothermic reaction: A chemical reacton in
We need energy to stay alive and we
activihe
which heat (a form of energy) is given out is get this enerov
from food. During digestion, food is
called an exothermic reaction. It causes a rise n broken down
temperature.
into simpler substances. For
example, the
In an exothermic reaction, total
carbohydrates in ice, potatoes and bread are broken
energy content down to form glucose.
of reactants is more than that of the products.
The combustion of glucose with
Examples 1: When carbon burns in oxygen to Oxygen in the
cells of the body provides energy :
form carbon dioxide, lot of heat is
a
produced. CgH206 +
60, 6CO +
6H,0 Energy +

C +02 CO, + Heat This reaction is exothermic and it is known

carbon oxygen] [carbon dioxidel by a
special name respiration.
2 The combustion of natural gas is an
Respiration, rusting, and burning of coal, petrol,
important source of energy for homes and industry.
Natural gas is mainly methane. Its complete kerosene, cooking gas, etc., are some common
exothermic reactions.
combustion produces carbon dioxide and water
vapour. 2. Endothermic reaction : A chemical reactuon n
which heat is absorbed is called an endothermir
CH+ 202 CO +2H,0 reaction. It causes a fall in temperature.
Substances such as methane, which undergo
combustion readily and give out a large amount of Thus, such reactions cannot be sustained
energy are known as fuels. without supply of energy from an exterma
SOurce.
3: When water is added to quicklime, a lot of
In an endothermic reaction, total energy conteu
heat energy is produced with the formation of of products is more than that of the reactanis

30
 Examples I : Formation of carbon disulphide: This reaction is essential for life on
When carbon is heated with
earth. It
sulphur at high takes place in the green leaves of
plants containing
temperature, liquid carbon disulphide is formed.
chlorophyll requires energy from
and It is sunlight. a
C+2S Cs, photochemical reaction.
2 When nitrogen and oxygen are heated The green pigment
chlorophyll is essential for
together to a temperature of about 3000°C, nitric this reaction, because it traps energy from the sun.
Oxide gas is formed. This is an endothermic reaction.
2 Decomposition of silver nitrate
N2 +O Heat 2NO (
(3000°C) 2AgNO Sunlight 2Ag +2NO, +0,
3: Calcium carbonate
decomposes into carbon ELECTROCHEMICAL REACTION
dioxide and calcium oxide when it is heated to a
is reaction that with
temperature of about 1000°C; this is also an a occurs absorption of
endothermic reaction. electrical energy.

CaCO () heat Ca0 (s)+ CO (8)

Examplesl: Fused potassium chloride, on
calcium
(1000°C) calcium oxide] [carbon- passing current through it, breaks into charged
carbonate dioxide particles (ions) of potassium and chloride..
PHOTOCHEMICAL REACTION
KCI electric K* + C1
It is a reaction that occurs with absorption of light Current

energy. Acidulated water breaks into hydrogen and

oxygen when electric current is passed through it.
Examplesl: Photosynthesis :
24,0Clectric

6CO +12 H,0 ChloropnyiCgH120, + 6 H,0 +60 current 2H2 +0,

sunlight

CHAPTER AT A GLANCE)
A chemical reaction is the process of breaking the chemical bonds of reactants and formation of new
bonds to form new products.
Chemical changes occur by (i) mixing the substances in solution state (i) heat (ii) light (iv) electricity
(V) pressure (vi) the presence of a catalyst.
Chemical reaction is confirmed by (i) evolution of gas (ii) change of colour (ii) formation of
precipitate (iv) change of state.
A reaction in which two or more substances combine to form a new substance is called a combination
reaction Fe +S Fes.
A reaction in which a substance is broken down into two or more substances is called a decomposition
reaction: 2Hg0 2Hg +Op
A reaction in which one part of a molecule is replaced by another is called a displacement reaction
CuSO + Fe > FeSO, +Cu.
A reaction in which two reacting molecules exchange their corresponding ions, is called a double
displacement reaction: NaCl + AgNO, AgCI + NaNO.
A reaction in which one of the products formed is an insoluble substance that is thrown out of the
solution solid (precipitate) is called
as a precipitation reaction: Na,SO, +BaCl BaSO, + 2NaCi

31
 waler oniy Is Called d a neu
neutralisation
a
sult and
ne reaction between an acid and a base that forms **
reaction.
Na,SO, + 2H,O
,SO, + 2NaOH Water
alt
Acid Base rexctlon.
exothermc
which heat is liberated (or given out) is called
an
reacton in
+ Heat
C(s)+ O,(g)> cO,(g)
A reaction in which heat is absorbed is called an endothermic reucon
2C(s) 0,(g) A, 2CO()

EXCERCISE 2(C)
(b) Electrochemical reaction.
1. What is a chemical change ? Give two examples of
case.
Give one example in each
chemical changes.
10. Complete and balance the tollowing reactions
2. Why is energy involved in a chemical change ?
(aq)
3. What do you understand by a "ehemical reaction' ? (a) NaCl (aq) +AgNO
4. Give an example of a reaction where the following are
(b) Pb(NO3)2 + KI > (c) CuCO 4
involved
(a) Evolution of heat (b) Absorption of heat
(d) Pb(NO,) (e) NH +0,
(c) High pressure is required
in the following cases ?
11. What do you observe
5. State the main characteristics of chemical reactions. Give
heated.
at least one example in each case. (a) Lead nitrate is
to sunlight.
6. Give an example of each of the following chemical (b) Silver chloride is exposed
is exposed to sunlight.
(c) Hydrogen peroxide
changes.
(a) A reaction involving
(d) HS gas is passed through copper sulphate solutia
(i) change of state (ii) formation of precipitate (e) Barium chloride is added to sodium sulphate soluta

An exothermic and an endothermic reaction () Water is added to quicklime.

(b)
is added to silver nitre
involving carbon as one of the reactants. (g) Sodium chloride solution
(c) A reaction where colour change is noticed. solution.

Give two
7. Define exothermic and endothermic changes. 12. Name:

examples in each case. (a) a carbonate which does not decompose on heating
exothermic reactions
8. State the effects of endothermic and (b) a nitrate which produces oxygen as the only gas.
on the surroundings. compound which produces carbon dioxide beat
on
(c) a

9. Define (d) a nitrate which produces brown gas on heating

(a) Photochemical reaction

32