Would I use half reactions to balance: Cl_2 --> Cl- + ClO_3- ?

Yes. This is a redox equation and the special name is disproportionation. You have Cl2 in which one atom is oxidized and the other is reduced.

Yes, you can use half reactions to balance the given equation: Cl_2 -> Cl- + ClO_3-.

To balance this equation using half reactions, you need to split it into two separate reactions: the oxidation half reaction and the reduction half reaction.

First, identify the element that undergoes oxidation and the one that undergoes reduction in the overall reaction.

In this case, chlorine (Cl_2) is being reduced to chloride ion (Cl-), so we can write the reduction half reaction as follows:

Cl_2 -> 2 Cl- (1)

Next, we need to identify the other element that undergoes oxidation. In this case, chlorine (Cl_2) is being oxidized to chlorine dioxide ion (ClO_3-). To balance the number of chlorine atoms, we should have 3 Cl atoms on the reactant side. Therefore, we write the oxidation half reaction as follows:

3Cl_2 -> ClO_3- (2)

Now we can balance the electron transfer between the two reactions. The reduction half reaction (1) needs 2 electrons, while the oxidation half reaction (2) does not involve any electron transfer. Therefore, we need to multiply the oxidation half reaction by 2 to balance the electron transfer:

2(3Cl_2) -> 2(ClO_3-) (3)

Finally, add the two balanced half reactions together:

2(3Cl_2) + 2Cl_2 -> 2(ClO_3-) + 2Cl-

Now the equation is balanced.

To summarize, to use half reactions to balance the equation Cl_2 -> Cl- + ClO_3-, you can follow these steps:
1. Identify the oxidation and reduction half reactions.
2. Balance the number of atoms other than oxygen and hydrogen in each half reaction.
3. Balance the number of oxygen atoms by adding water (H2O) to the side of the equation that is deficient in oxygen.
4. Balance the number of hydrogen atoms by adding H+ ions to the side of the equation that is deficient in hydrogen.
5. Balance the charge by adding electrons (e-) to the side of the equation that is deficient in charge.
6. Multiply each half reaction by appropriate factors to balance the electrons transferred.
7. Add the balanced half reactions together and simplify if necessary.