Wize High School Grade 12 Chemistry Textbook > Electrochemistry
Balancing Redox Reactions: Oxidation State Method
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Balancing Redox Reactions Using the Oxidation Step Method
Steps:
1) Assign oxidation states to each element
2) Determine which element is being oxidized (loses electrons, oxidation state
increases
) and which element is being reduced (gains electrons, oxidation state decreases
)3) Determine how much the oxidation #s change by (ex. if N goes from +2 to +3 there was a change of 1). This tells us the number of electrons transferred per atom.
4) For the atoms that are oxidized and reduced, look at the reactants side of the chemical formula. Calculate the number of electrons transferred per reactant (look at subscripts!)
5) Calculate the simplest whole number coefficients that will balance the total number of electrons transferred. Place those coefficients only for the reactants.
6) Balance the rest of the chemical equation without changing the coefficients we placed in step 3
7) Balance O atoms using H2O
8) Balance H atoms using H+
In basic solution only:
9) Add OH- to both sides equal in number to the number of H+ present.
10) Combine OH- and H+ to form H2O. Cross out any H2O if it appears on both sides of the arrow.
Example: Balance the following redox reaction in acidic conditions:
HNO3(aq) + H3AsO3(aq) --> NO(g) + H3AsO4(aq) + H2O(l)
1) Assign oxidation states:
+1,+5,-2 +1,+3 ,-2 +2,-2 +1,+5,-2 +1,-2
2) What is being oxidied and reduced?
N went from +5 to +2 -->gain electrons=reduction
As went from +3 to +5--> lost electrons=oxidation
3) How much did the oxidation #s change by? Aka how many electrons are transferred per atom?
N oxidation state changed by 3
As oxidation state changed by 2
4) Determine how many electrons are transferred per reactant (look at chemical equation)
N is in the reactant HNO3. This reactant only has 1 N in total . Therefore the reactant transfers a total of 3 electrons
As is in the reactant H3AsO3. This reactant only has 1 As in total . Therefore the reactant transfers a total of 2 electrons.
5) Choose simplest coefficients that will balance the # of electrons and only put coefficients in front of the reactants
HNO3 transfers a total of 3 electrons
H3AsO3 transfers a total of 2 electrons
The smallest whole number that 3 and 2 go into is 6:
HNO3 3 electrons x2=6 electrons
H3AsO3 2 electrons x3 =6 electrons
That gets us: 2HNO3 + 3H3AsO3 --> NO + H3AsO4 + H2O
6) Balance the rest of the equation without changing the coefficients we already placed.
2HNO3(aq) + 3H3AsO3(aq) --> 2NO(g) + 3H3AsO4(aq) + H2O(l)
15 Os on left and 15 on right so no need to add H2O to balance O.
11 Hs on left and 11 on right so no need to add H+ to balance H+.
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Balance the following redox reaction in basic conditions using the oxidation state method.
ClO3-(aq) + I2(aq) -> Cl-(aq) + IO3-(aq)