Balance the following equation for a half reaction that occurs in acidic solution. Use e– as the symbol for an electron.
Mo^3+ -> MoO2^2+
Here you can find all you ever wanted to know about redox equations.
http://www.chemteam.info/Redox/Redox.html
Here is a hint to help get you started.
Mo changes from +3 on the left to +6 on the right.
To balance the given half reaction, Mo^3+ -> MoO2^2+, in acidic solution, you need to follow the steps below:
1. Write the unbalanced equation: Mo^3+ -> MoO2^2+
2. Identify the elements being oxidized and reduced. In this case, Mo is being oxidized, and its oxidation state increases from +3 to +6, while oxygen (O) is being reduced, and its oxidation state decreases from 0 to -2.
3. Add water (H2O) molecules to balance the oxygen atoms. Since there are four oxygen atoms on the right side (in MoO2^2+), you need to add four water molecules on the left side.
Mo^3+ -> MoO2^2+ + 4H2O
4. Balance the hydrogen atoms next. Since there are eight hydrogen atoms on the right side (in 4H2O), you need to add eight protons (H+) on the left side to balance them.
Mo^3+ + 8H+ -> MoO2^2+ + 4H2O
At this point, the hydrogen and oxygen atoms are balanced. However, the charges are not equal on both sides of the equation.
5. Balance the charges by adding electrons (e-). In this case, the charge of Mo^3+ is +3, while the charge of MoO2^2+ is +2. To balance the charges, you need to add one electron on the right side.
Mo^3+ + 8H+ + e- -> MoO2^2+ + 4H2O
Finally, verify that the equation is balanced. Count the atoms and charges on both sides to ensure they are equal. In this case, there are equal numbers of Mo, H, O, and the charges are balanced, so the equation is balanced.
The balanced half reaction in acidic solution is:
Mo^3+ + 8H+ + e- -> MoO2^2+ + 4H2O