When balancing a redox reaction, you should follow these steps.
1. Split the reaction into two half reactions.
2. Balance all atoms other than oxygen and hydrogen.
3. Balance the oxygen by adding H2O molecules.
4. Balance the hydrogen by adding H+ ions.
5. Balance the charges by adding electrons.
6. Make the number of electrons equal.
7. Add the half reactions together.
8. Simplify as much as possible.
For this problem Cr is being reduced, and N is being oxidized, so we can write these half reactions as step 1, shown in the image below. Balancing Cr and N, we get step 2. To balance oxygen, we can add H2O to get step 3. Then to balance hydrogen, add H+ to get step 4. Add electrons to balance the charges on both sides to get step 5. Balance the electrons gained with electrons lost by multiplying the reactions by, in this case, a factor of 3 to get step 6 . Add the two half...
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