BrO3- (aq) + NO2(aq) → Br-(aq) + NO3-(aq) Balance this reaction in acidic conditions?

BrO₃⁻ (aq) + NO₂(aq) + 4 H⁺→ Br⁻(aq) + NO₃⁻ + 2 H₂O

Explanation:

The law of conservation of matter states that since no atom can be created or destroyed in a chemical reaction, the number of atoms that are present in the reagents has to be equal to the number of atoms present in the products.

Then, you must balance the chemical equation. For that, you must first look at the subscripts next to each atom to find the number of atoms in the equation. If the same atom appears in more than one molecule, you must add its amounts.  

The coefficients located in front of each molecule indicate the amount of each molecule for the reaction. This coefficient can be modified to balance the equation, just as you should never alter the subscripts.

By multiplying the coefficient mentioned by the subscript, you get the amount of each element present in the reaction.

Then, taking into account all of the above, you can determine the amount of elements on each side of the equation:

Left side: 1 Br, 5 O and 1 N

Right side: 1 Br, 3 O and 1 N

If the reaction occurs in an acidic medium:

So, balancing the oxygen:

BrO₃⁻ (aq) + NO₂(aq) → Br⁻(aq) + NO₃⁻ + 2 H₂O

Left side: 1 Br, 5 O and 1 N

Right side: 1 Br, 5 O, 1 N and 4 H

Then, balancing the hydrogens:

BrO₃⁻ (aq) + NO₂(aq) + 4 H⁺→ Br⁻(aq) + NO₃⁻ + 2 H₂O

Left side: 1 Br, 5 O, 1 N and 4 H

Right side: 1 Br, 5 O, 1 N and 4 H

You can see that you have the same amount on each side of the reaction. So the reaction is balanced.