REDOX balancing reaction Cr2O7 + NO-2 Cr3+ + NO3 (acid medium)
You have too many omissions in the equation. That should e [Cr2O7]^2-, I have no idea what NO-2 is (probably [NO2]^-), [NO3]^-
Here is a good site that will tell you to assign oxidation states to each element and use that to balance redox equations.
If you have more trouble after that, repost but take care to get the charges and formulas correct.
http://www.chemteam.info/Redox/Redox.html
To balance the redox reaction Cr2O7 + NO2- -> Cr3+ + NO3- in an acidic medium, we can follow the steps outlined below:
Step 1: Assign oxidation numbers.
Assign oxidation numbers to each element in the reaction. Chromium (Cr) is initially in the +6 state and is reduced to +3. Nitrogen (N) is initially in the +3 state and is oxidized to +5.
Cr2O7: Cr(IV), O(-II)
NO2-: N(III), O (-II)
Cr3+: Cr(III)
NO3-: N(V), O(-II)
Step 2: Determine the half-reactions.
Identify the two half-reactions: reduction and oxidation.
Reduction half-reaction: Cr(VI) -> Cr(III)
Oxidation half-reaction: N(III) -> N(V)
Step 3: Balance the atoms other than oxygen and hydrogen.
Balance the atoms involved in the half-reactions, excluding oxygen and hydrogen.
Reduction: Cr2O7 -> 2Cr3+
Oxidation: 3NO2- -> 3NO3-
Step 4: Balance oxygen atoms.
Add water (H2O) molecules to the side lacking oxygen atoms.
Reduction: Cr2O7 + 14H+ -> 2Cr3+ + 7H2O
Oxidation: 3NO2- -> 3NO3-
Step 5: Balance hydrogen atoms.
Add hydrogen ions (H+) to balance the hydrogen atoms on both sides of the equation.
Reduction: Cr2O7 + 14H+ -> 2Cr3+ + 7H2O
Oxidation: 3NO2- + 4H+ -> 3NO3-
Step 6: Balance charge.
Add electrons (e-) to one side of each half-reaction so that the charges are balanced.
Reduction: Cr2O7 + 14H+ + 6e- -> 2Cr3+ + 7H2O
Oxidation: 3NO2- + 4H+ -> 3NO3- + 2e-
Step 7: Multiply the half-reactions.
Multiply each half-reaction appropriately so that the number of electrons is equal in both reactions.
Reduction: 2Cr2O7 + 14H+ + 6e- -> 4Cr3+ + 7H2O
Oxidation: 6NO2- + 8H+ -> 6NO3- + 4e-
Step 8: Combine the half-reactions.
Add the two balanced half-reactions together.
Overall reaction: 2Cr2O7 + 14H+ + 6NO2- + 8H+ -> 4Cr3+ + 7H2O + 6NO3- + 4e-
Step 9: Cancel out common species.
Cancel out common species on both sides of the reaction.
Overall reaction: 2Cr2O7 + 14H+ + 6NO2- -> 4Cr3+ + 7H2O + 6NO3-
This balanced redox reaction in an acidic medium is:
2Cr2O7 + 14H+ + 6NO2- -> 4Cr3+ + 7H2O + 6NO3-