CHEMICAL

REACTIONS
Grade VIII

14-07-2014
https://flexbooks.ck12.org/cbook/ck-12-chemistry-
flexbook-2.0/section/2.16/related/lecture/what-is-a-
chemical-reaction
A chemical reaction is the process of breaking chemical bonds
of the reacting substances (reactants) and making new
bonds to form new substances (products) with the
absorption or release of energy, usually in the form of heat
and light

C(s) + O2(g) CO2(g)

2H2(g) + O2(g) 2H2O(g)

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 A chemical bond is the attractive force that holds
the atoms of a molecule together in a compound

 In a chemical reaction, the substances which react

together are called reactants whereas the new
substances formed are called products

 Reactants Products
(Starting materials) (New substances)
Characteristics of chemical reactions:
i) Evolution of gas:
(a) Zn + H2SO4 ZnSO4 + H2
https://www.youtube.com/watch?v=D5kTJ0SkATs
(b) 2KClO3 Strong Heating 2KCl + 3O2
(c) Na2SO3 + 2HCl 2NaCl + H2O + SO2
(Suffocating smell)
ii) Change of colour:
(a) Fe + CuSO4 FeSO4 + Cu
(Blue) (Green) (Red Deposit)
(b) CuSO4 + H2S CuS + H2SO4
(Blue) (Black Solid)
(c) Pb(NO3)2 2PbO + 4NO2 + O2
(White) (Pale yellow) (Reddish brown)

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iii) Formation of precipitate:
(a) Pb(NO3)2(aq) + 2KI(aq) PbI2 (ppt) + 2KNO3(aq)
(Colourless) (Colourless) (Yellow) (Colourless)

(b) AgNO3(aq) + NaCl(aq) AgCl + NaNO3(aq)

(White ppt.)

(c) FeSO4(aq) + 2NaOH(aq) Fe(OH)2 + Na2SO4(aq)

(Dirty green ppt.)
(d) BaCl2(aq) + H2SO4(aq) BaSO4 + 2HCl(aq)
(White ppt.)

https://www.youtube.com/watch?v=2EQznGPZY5A

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iv) Change of state
(a) NH3 (g) + HCl (g) NH4Cl(s)
(b) H2S(g) + Cl2(g) 2HCl(g) + S(s)

v) Change in energy

Exothermic Endothermic
(Heat is released) (Heat is absorbed)
e.g. Burning of a fuel Decomposition of calcium
carbonate

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Conditions necessary for a chemical change:
i) Close Contact
2Na + 2H2O 2NaOH + H2
https://www.youtube.com/watch?v=ODf_sPexS2Q
ii) Solution
NaCl(aq) + AgNO3(aq) AgCl + NaNO3(aq)
iii) Heat
2Pb(NO3)2 2PbO + 4NO2 + O2
Fe + S FeS
https://www.youtube.com/watch?v=p8uRb-8Po3U

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iv) Light
e.g.- Photosynthesis
v) Electricity
Electric
2H2O Current 2H2 + O2
vi) Pressure
4500C
N2 + 3H2 2NH3
200 atm
Positive Catalyst

vii) Catalyst
Negative Catalyst

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➢Combination reactions

➢Decomposition reactions

➢Displacement reactions

➢Double-displacement reactions

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❖ What is direct combination or synthesis?
A reaction in which two or more substances (elements or compounds)
combine together to form a new substance is called a combination reaction.

B + C AB C

where B and C represent elements or compounds

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• Examples:
• H2(g) + Cl2(g) 2HCl(l)
• Mg(s) + Cl2(g) MgCl2(s)
• Fe(s) + S(s) FeS (s)
• MgO(s) + H2O(l) Mg(OH)2 (aq)

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❖ What is decomposition reaction?
• A reaction in which a substance is broken
down into two or more simpler substances is
known as decomposition reaction.
• A decomposition reaction is opposite of
combination and takes place only when some
energy in the form of heat, light or
electricity is supplied to the reactant.

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A B C A + B + C

where A, B, C represent an element or compound

❖ Various types of decomposition reactions

1. Thermal decomposition reaction
o Decomposition caused by heating
2. Electrolytic decomposition (electrolysis) reaction
o Decomposition reaction caused by electricity
3. Photodecomposition reaction
o Decomposition reaction caused by light

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• Examples:
• ZnCO3(g) ZnO(s) + CO2(g)
• 2H2O (l) electrolysis 2H2(g) + O2(g)
• CaCO3(s) CaO(s) + CO2 (g)
• 2H2O2(l) UV 2H2O(l) + O2(g)

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❖ What is displacement reaction?

• It is a chemical change in which a more

active element displaces a less active
element from its salt solution.

X + Y Z Y + X Z
where X represents an element and Y-Z is a
compound/salt in aqueous medium

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• Examples:
• Zn(s) + 2HCl(dil) ZnCl2(aq) + H2(g)
• 2KBr(aq) + Cl2(aq) 2KCl(aq) + Br2(aq)
• CuSO4(aq) + Zn(s) Cu(s) + ZnSO4(aq)
• Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

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❖ What is double-displacement reaction?
• This is a type of chemical change in which
two compounds in a solution react to form
two new compounds by mutual exchange of
radicals.

W X +Y Z Y X + W Z

where W-X and Y-Z represent a compound/salt in

aqueous medium
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• Examples:
• AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)
precipitate

• Ni(NO3)2 (aq) + 2NaOH (aq) Ni(OH)2(s) +2 NaNO3(aq)

precipitate

• 2KBr(aq) + BaI2 (aq) 2KI(aq) + BaBr2(aq)

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❖ What is precipitation reaction?
• The reaction in which one of the products
formed is an insoluble substance and is thrown
out of the solution as a solid (called precipitate)
is called precipitation reaction.
• The precipitate formed is indicated by a
downward arrow( ).
• Examples:
AgNO3(aq) + NaCl(aq) ➔ AgCl(s) + NaNO3(aq
precipitate
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• Examples:
• AgNO3(aq) + KCl(aq) AgCl + KNO3(aq)
precipitate

• Pb(NO3)2 (aq) + 2KI(aq) PbI2 + 2KNO3(aq)

precipitate

• BaCl2(aq) + Na2SO4(aq) BaSO4 + 2NaCl(aq)

precipitate

• LiBr(aq) + AgNO3(aq) LiNO3(aq) + AgBr

precipitate

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 NEUTRALIZATION REACTION

❖ What is neutralization reaction?

• The reaction between an acid and a base that
forms salt and water is referred to as reaction
of neutralization.
• Example:
NaOH + HCl ➔ NaCl + H2O

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❖ What do you mean by exothermic and
endothermic reaction?

• A reaction which is accompanied by the evolution of heat is

known as exothermic reaction whereas reaction
accompanied by absorption of heat is known as endothermic
reaction.

✓ Melting of ice is an endothermic reaction

✓ Freezing of water is an exothermic reaction

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• Example:
• H2O(s) + heat H2O(l) (endothermic)
• C(s) + H2O(v) + heat CO(g) + H2(g) (endothermic)

• CH4(g) + 2O2(g) CO2(g) + 2H2O(g) + heat (exothermic)

• H2(g) + O2(g) H2O(g) + heat (exothermic)

Exothermic Endothermic
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reaction reaction