Page 1 - Microsoft Word - Lesson note-2(Redox reaction)

SUBJECT – CHEMISTRY

CHAPTER – REDOX REACTIONS

SUBTOPIC – Balancing of chemical reactions

 By oxidation number method:
Rules:

(i)Write the skeletal ionic equation.
(ii)Assign oxidation number to the atoms in the equation.
(iii)Find the increase and decrease in oxidation number and make them
equal by multiplying with suitable integers.
(iii)Balance other atoms except hydrogen and oxygen.
-
+
(iv)Add H ions in acidic medium and OH ions in basic medium to make
ionic charges equal.
(v)Count H-atoms and add appropriate no. of water molecules to
achieve the balanced equation.
2-
2-
3+
2
Example-(i) Balance Cr2O7 + SO3 → Cr + SO4 in acidic medium.
2-
3+
+
Ans-Balanced equation is Cr2O7 + 3SO3 + 8H → 2Cr + 3SO4 + 4H2O
2-
2-
-
-
-
Example-(ii) Balance MnO4 + Br → MnO2 + BrO3 in basic medium.
-
-
-
-
Ans- Balanced equation is 2MnO4 + Br + H2O → 2MnO2 + BrO3 + 2OH
 By Ion-electron or half reaction method:
Rules:
(i) Write the skeletal ionic equation.
(ii)Separate the equation into two half reactions i.e. oxidation half and
reduction half reactions.
(iii)Balance other atoms except hydrogen and oxygen.
(iv)In acidic medium add H2O molecules in the deficient of Oxygen to
balance Oxygen atoms and H ions to the side of deficient in hydrogen.
+
(v)In basic medium add H2O molecules to the excess side of Oxygen and
OH to the other side. If hydrogen is still unbalanced, add OH ion for
-
-
each excess hydrogen on the same side and one H2O to the other side.
(vi)Add electrons(e ) to the side deficient in electrons to equalise the
-
charge on both sides.
(vii)Multiply one or both the half reactions by a suitable number so that
the number of electrons become equal in both the equations.
(viii)Add the two balanced half reactions and cancel any term common
to both the sides to get the balanced equation.