Balancing Redox Equations

Balancing Redox Equations• Many redox reactions are too complex to

simply balance by inspection we must ensure that the e- lost and gained balance

• There are 2 methods to balance redox rxn:– Half reaction– Using oxidation numbers

Half Rxn Method

We’ll balance:Sn + HNO3 SnO2 + NO2 + H2O

• Step 1: Assign oxidation numbers to each element to identify which is oxidized and which is reduced.

• Step 2: write the half rxn, using only elements that change in oxidation number. Then add e- to balance the charges.

• Step 3: Multiply each reaction by the lowest whole number so that e- lost = e- gained.

• Step 4: Transfer the coefficients into the original reaction.

• Step 5: Balance the remaining elements by inspection.

• Step 6: Check the equation to see that it is balanced.

• Balance the equation I2 + Cl2 + H2O HIO3 + HCl

• BalanceK2Cr2O7 + FeCl2 + HCl CrCl3 + KCl + FeCl3 + H2O

Balancing Acid/Base Solutions• We follow the same steps as before, but leave

the H and O until the end, and then balance by adding H+, OH- or H2O according to:

• In acid solutions, we add H+ and H2O as needed

• In basic solutions we add OH- and H2O as needed

Cr2O72- + SO3

2- Cr3+ + SO42- (in acid solution)