Unit 3: Electrochemistry
Chemistry
Grade 9
Chapter 1: Oxidation-Reduction Reactions
Outline:
1. Oxidation number
2. Redox Reactions
1. Oxidation number
 Definition: The oxidation number indicates how many electrons an atom has lost, gained or shared when it
bonds to other atoms.
 Assigning oxidation numbers:
Case
Atoms in an Element
Monoatomic ions
Oxidation Number
0
Same as the ionic charge
Atoms in a compound
According to the group
Oxygen
2. Oxygen in peroxide H2O2
3. Hydrogen with non-metals
Hydrogen with metals
-2
-1
+1
-1
Example
Na, Mg, O2, N2, O3, Hg…
Na 1+ : +1
Cl 1- : -1
O2- : -2
Al3+ : +3
Grp I : +1 ex: NaCl
Grp II: +2 ex: Ca2Cl
Al : +3 ex: Al(OH)3
Fl: -1 ex: CaF2
CO, CO2, SO2
H2O2
NH3, H2O, CH4, HCl
CaH2, LiH
NOTE:
The sum of oxiadation states in a neutral compound is equal to zero.
The sum of oxiadation states in a polyatomic ion is equal to its charge.
(The oxidation number of an atom is multiplied by its subscript)
Example:
to find oxidation number of Mn:
+1 +x -2(4) = 0
so, x= +7
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Unit 3: Electrochemistry
Chemistry
Grade 9
to find oxidation number of C :
x(2) -2(4) = -2
so, x= +3
2. Redox Reactions
Reduction
half
reaction:
gain of
electrons
Oxidation
half
reaction:
loss of
electrons
Redox
Reaction:
exchange of
electrons
NOTE: Terms to memorize!!
oxidixing
agent
oxidant
reducing
agent
reduced
Reactant that GAINES electrons in the redox
reaction by reduction half reaction
redutant
oxidized
Reactant that LOSES electrons in the redox reaction
by oxidation half reaction
Zn
 How to determine the redox reaction?
HCl
Example: Zn + 2HCl  ZnCl2 + H2
1) Assign the oxidation number for each atom in the balanced
reaction.
2
Unit 3: Electrochemistry
Chemistry
Grade 9

2) Predict if the reaction is a redox reaction via transfer of electrons.
When oxidation number increased  more positively charged
 loss of electrons  oxidation
When oxidation number decreased  more negatively charged
 gain of electrons  reduction

3) Calculate the number of electrons lost or gained which is the absolute value of the subtraction of the 2
oxidation numbers per atom.
4) Write the half oxidation and half reduction reactions.
Half oxidation rxn: Zn  Zn2+ + 2es
Half reduction rxn: H+ +1es  H2
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Unit 3: Electrochemistry
Chemistry
Grade 9
5) Balance the half reactions.
Half oxidation rxn: Zn  Zn2+ + 2es (balanced)
Half reduction rxn: 2H+ +2es  H2 (2es per H2 molecule)
Hint: Sum of charges in the reactant should be equal to the sum of charges in the product.
6) Check if the number of electrons lost is equal to number of electrons gained.
If not, the number of electrons exchanged is the LCM of both electrons lost and gained.
7) Add the half reactions to get the redox reaction.
Redox reaction: Zn + 2H+  Zn2+ + H2
Note: Cl- didn't interfere in the redox reaction. It’s called a spectator ion.
So, spectator ions don't interfere in the redox rxn i.e. their oxidation number doesn't change.
Note:
oxidant and reductant
are in direct contact
energy released is in
the form of heat
oxidant and reductant
are NOT in direct
contact
energy released is in
the form of electricity
Redox Reactions
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