CHEM - 1405 CHAPTER - 7
CHEMICAL REACTIONS
Physical and Chemical Change
A physical change is the one where there is no change in the chemical composition of the
substance. Example, boiling of water, melting of ice etc
A chemical change is the one which is always accompanied by a change in the
composition of the substance. For example, burning of coke, decomposition of water into
hydrogen etc
Evidences for chemical reaction
During chemical reaction, one or more of the following changes will take place.
 A gas is released
 An insoluble solid (precipitate) is formed
 A color change is observed
 Heat energy change is noticed
Chemical Equations
A chemical equation represents the complete chemical change using the symbols and formula of
the substances involved.
Balancing of Chemical Equation
Equalizing the number of atoms of each element on either side of the equation is called
balancing of chemical equations. Reactants are written on the left hand side and products
are written on the right hand side of the equation. Balancing of equations is in accordance
with the Law of Conservation of Mass.
For balancing, we need to adjust the stoichiometric coefficients by putting suitable
coefficients. Never change the subscripts while balancing the equation.
CH4 + 2O2
CO2 + 2H2O
2C2H6(g) +
7O2(g)
4CO2(g) + 6H2O(g)
Chemical equation must have an equal # of atoms of each element on either side.
Types of Chemical Reaction
1. Combination reactions
Two or more substances combine to form a single product.
H2 + O2
H2 O
2. Decomposition reactions
The breakdown of a compound to two or more simple compounds
KClO3
KCl + O2
3. Combustion reactions
Reaction of a substance with oxygen, usually with the liberation of heat and light along with a flame
CH4 + O2
CO2 + H2O
4. Single displacement reaction
An atom or an ion is replaced by another atom or ion in this reaction.
CuSO4 + Zn
ZnSO4 + Cu
5. Precipitation Reactions.
These reactions result in the formation of insoluble substances known as the precipitate.
Precipitation reactions usually involve ionic compounds.
AgNO3(aq) + KCl ( aq)
AgCl (s) + KNO3 (aq)
(Precipitate)
6. Metathesis Reactions (Double Displacement reactions)
Reactions involving the exchange of ions in solutions are called metathesis reactions or
double displacement reactions.
K2SO4(aq) + BaCl2 ( aq)
BaSO4 (s) + 2KCl (aq)
Pb(NO3)2(aq) + 2KI(aq)
PbI2 (s) + 2KNO3(aq)
7. Neutralization reactions
Reaction between an acid and a base to form salt and water is a neutralization reaction.
This is a special type of double displacement reaction.
NaOH + HCl
NaCl + H2O
The Activity Series
A series of metals arranged in the order of their ability to undergo reaction. This series is the
activity series.
Reactive metals are the metals at the top of the activity series.
Less active metals are the metals at the bottom of the activity series.
A metal can be displaced from its salt solution by an another metal which is more active.
If we place Zn into a solution of AgNO3, then Ag is displaced because Zn is more active than Ag in
the activity series:
Zn(s) + 2AgNO3
Zn(NO3)2 + 2Ag(s)
The metals above H2 displace hydrogen gas from water or dilute acids.
Solubility Guidelines for Common Ionic Compounds in H2O
Ionic compounds soluble in H2O
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Compounds containing alkali metal ions and ammonium salts
Nitrates and Acetates
Sulfates (exception Ca, Sr, Ba, Pb)
Chlorides, Bromides, Iodides (exception Pb, Hg (I), Ag)
Ionic compounds insoluble in H2O
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Hydroxides (exception Li, Na, K, Rb, Ca, Sr, Ba)
Phosphates, carbonates and Sulfides (exception Li, Na, K, Rb, NH4+)
Compounds containing S2–
Compounds containing OH– (Exception: compounds of NH4+ and the alkali
metals and alkaline earth metals)