How many electrons are needed (and on what side) to balance the half-reaction:?

Question

2SO4^(2-) ----> S2O8^(2-)

I was thinking that adding 2 electrons to the left side wold balance it...am I way off??

Thanks!

Answer 1

You are backwards....

On the left side you have 2 moles of sulfate x 2 electrons per mole, for a charge of -4.

On the  right side, a single mole of persulfate, -2.

SO you need to add two electrons to the RIGHT side to have a charge of -4 on each.