Question

help!! oxidation numbers!!
AgNO3 + KI ==> AgI + KNO3

2PbS + 3O2 ==> 2PbO + 2SO2

Answer 1

Do you have the rules in front of you now?

Answer 2

yeah but I'm still really unclear on them

Answer 3

Ok. Let's start with KI.

Answer 4

K is in group 1, so its oxidation number is 1.

Answer 5

Make sense?

Answer 6

yep

Answer 7

does that make I -1?

Answer 8

Sweet. Yup!

Answer 9

and am I correct saying NO3 is -1 so Ag is +1?

Answer 10

Yes, the charge on NO3 is -1, so the charge and oxidation number for Ag is +1. :)

Answer 11

Can you find the rest of the oxidation numbers for AgNO3?

Answer 12

there's more? @JoannaBlackwelder

Answer 13

Yup, you just found the oxidation number for Ag. You still have N and O to find.

Answer 14

wait which side are you talking about? so far I have:
Ag: +1
NO3: -1

Answer 15

oh and
K: +1
I: -1

Answer 16

Those are the charges, but for oxidation numbers, each element gets its own oxidation number.

Answer 17

KI is good. :)

Answer 18

ohhhh ok so what is NO3 then?

Answer 19

Oxygen is always -2 (unless it is a peroxide, which it is not).

Answer 20

oh ok

Answer 21

So, the N has to be +5 for the NO3 ion to add to -1.

Answer 22

why? what would ag be?

Answer 23

Ag is +1 because it is bonded to the NO3 ion which has a charge of -1.

Answer 24

oh ok

Answer 25

oh ok I see where you got the +5 now

Answer 26

what about the other side? How can I tell?

Answer 27

:)

Answer 28

@JoannaBlackwelder

Answer 29

Try the KNO3 first.

Answer 30

well is NO3 still -1?

Answer 31

That is the charge of the ion, but not the oxidation number.

Answer 32

how do I know if anything has changed then?

Answer 33

O is still -2

Answer 34

always? no matter which side of the reaction?

Answer 35

Unless it is a peroxide (which is almost never). :)

Answer 36

got it! ok so what else stays the same?

Answer 37

Hydrogen (H) has oxidation number +1 with nonmetals and –1 with metals.
Fluorine (F) only has oxidation number, –1.
Oxygen (O) has oxidation number -2, except in peroxides where it is –1.
The alkali metals (Group 1) have 1 valence. This is lost to achieve noble gas configuration, and so alkali metals have oxidation number +1.
The alkaline earth metals (Group 2) have 2 valence electrons, and so have oxidation number +2.
Group 13 elements have 3 valence electrons, and so have oxidation number +3.
Group 14 elements have 4 valence electrons. They tend to have oxidation number +4 with nonmetals and oxidation number –4 with metals and H.
The pnictogens (Group 15) has 5 valence electrons. They gain electrons to achieve noble gas configuration, so they usually have oxidation number –3.
The chalcogens (Group 16) have 6 valence electrons, and typically have oxidation number –2.
The halogens (Group 17) have 7 valence electrons, and so have oxidation number –1.
In the transition metals Ag has oxidation number +1, Zn and Cd have oxidation number +2, and Sc, Y and La have oxidation number +3.
Groups 14 to 17 have several oxidation number when they form oxoacids.

Answer 38

oh boy ok

Answer 39

so since I is in the 17th group does it always have an oxidation number of -1?

Answer 40

I know it is a lot, but it is honestly better to have a lot of rules than a little.

Answer 41

Almost always.

Answer 42

true true

Answer 43

Unless it is in a oxoacid (which is almost never)
http://chemteacher.chemeddl.org/services/chemteacher/index.php?option=com_content&view=article&id=154

Answer 44

haha so many exceptions!

Answer 45

Yeah, but they are pretty uncommon. :)

Answer 46

ok so in this case, I is still -1 and K is still +1

Answer 47

Right. :)

Answer 48

great. and N is -3?

Answer 49

because N changed in the last one depending on what the other numbers were

Answer 50

How do you get that?

Answer 51

because N is in the 15th group

Answer 52

"The pnictogens (Group 15) has 5 valence electrons. They gain electrons to achieve noble gas configuration, so they usually have oxidation number –3."

Answer 53

They do usually have oxidation number -3, but the oxidation numbers also have to sum to 0 since the compound is neutral.

Answer 54

yeah that's what I thought which is yet another reason for me to be confused. Ok so how do I find it?

Answer 55

I need to find Ag and N in the products

Answer 56

Same way as last time.

Answer 57

NO3 has charge of -1 and O has oxidation number of -2.

Answer 58

So, N has to be +5 to make them sum to -1.

Answer 59

why -1?

Answer 60

Because NO3 has charge -1

Answer 61

why does the sum have to be -1?

Answer 62

Because the oxidation numbers have to sum to the charge of the molecule.

Answer 63

hmm ok

Answer 64

How about Ag in the products?

Answer 65

it still has a +1 charge because I has a -1 charge and the charge of AgI is 0

Answer 66

right?

Answer 67

yup :)

Answer 68

awesome! Ok let me try the last one on my own and I'll tell you what i get

Answer 69

Sounds good. :)

Answer 70

2PbS + 3O2 ==> 2PbO + 2SO2

Pb: +4
S: -4
O: -2

==>

Pb: +4
O: -2
S: +4
O: -2

Answer 71

Elements are always 0.

Answer 72

The rules are for compounds.

Answer 73

oh right so

Pb: +4
S: -4
O: 0

==>

Pb: +4
O: -2
S: +4
O: -2

Answer 74

How did you gt Pb: +4, S:-4 in the reactants.

Answer 75

pb is in the 14th and it is paired with a nonmetal so it should be +4 right?

Answer 76

Looks right to me :)

Answer 77

But in the products, Pb would be 2+ because O is -2.

Answer 78

Pb just tends to be 4+ but O is always 2-.

Answer 79

gotcha! so it's
Pb: +4
S: -4
O: 0

==>

Pb: +2
O: -2
S: +4
O: -2