Question

Which of the following changes requires a reducing agent?

Zn yields Zn2+
MnO2 yields Mn2+
2Br- yields Br2
SO2 yields SO42-

Answer 1

@aaronq ?

Answer 2

Reducing means GAINING electrons, as a result the oxidation state/number becomes more negative.

Answer 3

oh so if it is "gaining electrons is it Zn yields Zn2+ ?

Answer 4

recall that cations eg \(Zn^{2+}\) have lost electrons.

Answer 5

anions have gotten more eg. \(Cl^-\)

Answer 6

In order to do this question, you need to assign oxidation states to all the atoms.

Answer 7

isn't
Zn is being oxidized from 0 to +2 and thus requires a reducing agent

Answer 8

nope, if the question was asking "which one requires an oxidizing agent" then that would be the answer.

Answer 9

hmm oh okay so how do i know which changes requires a reducing agent ?

Answer 10

you need to determine which of these goes from a higher (more positive) oxidation state to a lower (more negative) oxidation state. So, it boils down to assigning oxidation states to all the atoms.

Answer 11

SO2 yields SO42-
okay is this one right because it goes from a higher other words more positive oxidation to a low more negative oxidation ?

Answer 12

whats the oxidation state of S on each side of the equation?

Answer 13

O

Answer 14

+2
+6
?? i really don't know :/

Answer 15

SO2
Oxygen is (almost) always -2, because there are 2 of them and the compound is neutral, Sulfur has to be +4 to balance out the charges.

-4+x=0
x=+4

SO42-

here there are 4 oxygens, each one -2, but there is also a negative charge on the compound.

-8+x=-2
x=+6

Sulfur went from +4 to +6

Answer 16

Try \(\sf MnO_2 \rightarrow Mn^{2+}\)

Answer 17

MnO2 --- +4

Mn2+ ---- +2

Answer 18

yep! you should specify that those are the oxidation states of Manganese, but i get what you mean. Now assess, did Mn get oxidized or reduced?

Answer 19

reduced

Answer 20

yep! thats it

Answer 21

Omg thank you soo much!!

Answer 22

no problem! here are a couple of rules and examples when assigning oxidation states,

Rules (most often true, except in rare occasions):
Group 1 always +1
Group 2 always +2
Oxygen always -2
Group 7 always -1
Noble gases, elemental or homonuclear (the same element) molecules are zero

The rest you have to figure out by subtracting or adding to achieve whatever charge the compound bears.

eg. H2O
H is group 1, so H = +1, there are 2 of them
O must balance the charge so, O=-2

HCl
H=+1, Cl=-1

MgSO4,
you have to know that SO4^2- is a polyatomic ion, treat that as it's own separate thing:
O=-2
Total contribution of O=-2(4) = -8
^4 oxygen atoms

Charge on ion=-2, so S has to fulfil the rest which is 6, so S=+6

back to MgSO4, so we treat SO4 as entity and it bears a -2 charge,
Mg therefore has to be +2 to balance the charges.

Answer 23

oh ok! Thanks again! :D

Answer 24

no probs!