Given the balanced equation representing a reaction : 2Fe+ 3Cu^2+ --> 2Fe^3+ +3Cu

Question

When the iron atoms lose six moles of electrons, how many moles of electrons are gained by the copper ions?
(1) 12 moles
(2) 2 moles
(3) 3 moles
(4) 6 moles

how would i even start solving a problem like this?

Answer 1

It isn't a hard question if you remember that in a balanced equation the number of mols electrons lost ALWAYS equals the mols gained.
There is a second but more work way of doing it like this.
Oxidation state Fe on the left is 0. On the right is +6 (2*3+ = 6+) and going from 0 to 6+ Fe must have lost 6e.

Cu on the left is 6+ (3*2+= 6). On the right Cu is 0. Going from 6+ to 0 Cu must have gained 6e.
How 'bout that?

Answer 2

thanks so much. so you would always count up the oxidation states and see how much one side needs to equal the other?

Answer 3

That's right. In fact that is the whole concept behind balancing redox equations. You count up the electrons lost from one side to the other; count up electrons gained from one side to the other, then multiply by whatever numbers you need to make electrons gained equal electrons lost. That MAKES the reduction and oxidation parts balance and with those numbers in the bank so to speak you can easily balance the non-redox portion.

In addition, for this problem, they already tell you how many were lost so you don't even need to count them up. You know straight away that is 6 gained.

Answer 4

now I understand. thank you (: