PDA

View Full Version : What is the oxidation number of.



sarah1004
Nov 18, 2009, 04:39 PM
2KMnO_4\ + 10K_2S_2O_3 + 16HCl \rightarrow 2MnCl_2 + 5K_2S_4O_6 + 12KCl + 8H_2O

what is the oxidation number of

Mn in MnCl2 --> +2
Mn in MnO4^- ---->
S in K2S4O6 ----------> +10
Cl in HCl ------>+1

I couldn't do (Mn in MnO4^- ---->) this one and also i am not sure on third one either..
Please help me ohh.. do i have to multiply coefficant to each molecule before looking for oxidation number?

Perito
Nov 18, 2009, 08:17 PM
Mn in MnCl2. Cl is always -1. Therefore, Mn must be +2.

Oxygen is always -2

S is usually -2

H is always +1

Since Oxygen is always -2, the formal charge from the four oxygen atoms must be -2 x 4 = -8. MnO4- has a negative charge on it, Therefore -8 + 7 = -1, so its oxidation number must be +7.

Yes, you do have to multiply the coefficient (not coefficant) of each atom by its formal oxidation number

sarah1004
Nov 18, 2009, 09:37 PM
Mn in MnCl2 --> +2
Mn in MnO4^- ----> +7
S in K2S4O6 ----------> +13
Cl in HCl ------>+1

are these are correct?
but I cannot find Mn in MnO4^- this part on equation...

THanks

Perito
Nov 19, 2009, 04:45 AM
Mn in MnCl2 --> +2
Mn in MnO4^- ----> +7
S in K2S4O6 ----------> +13
Cl in HCl ------>+1

are these are correct?
but i cannot find Mn in MnO4^- this part on equation...



Why simply repeat the question?

Mn in MnCl2 is +2 because each Cl is -1. That is correct.

Mn in MnO4- is +7 That is correct.

S in K2S4O6. Sulfur is definitely NOT +13. That is not correct.

IF sulfur were +13, this is the calculation that you'd perform:

K: +1, 2 atoms, total formal charge = +2
O: -2, 6 atoms, total formal charge = -12
S: +13, 4 atoms, total formal charge = +52.

52 + 2 - 12 = 42. That is not a neutral molecule! Try again with K = +1 and O = -2.

Cl in HCl. Chlorine is definitely not +1 in this molecule. In this molecule, H has an oxidation state of +1. You can figure it out from there.

sarah1004
Nov 19, 2009, 10:51 AM
S in K2S4O6
If K is +1 and O is -2

+2 + -12 = -10

but that's S4.. so... It should be +2 or +3... I guess...
and Cl in HCl is +1

Thank

Unknown008
Nov 19, 2009, 10:57 AM
Actually, since it is thiosulfate we are dealing wit, you'll come up with an unusual answer. Actually doing the calculation:

K2S4O6 has a total charge of zero
K is +1, so 2K is +2
O is -2, so 6O is -12
Total of those two gives -10

That means S4 must me +10 for the whole to be 0.
So, that makes S be 10/4 = 2.5 (Surprising huh? A fraction!)

But actually, you have a peroxide bonding within the O atoms which have an oxidation state of -1 instead of -2. But that's not for now, but late on.

Next, HCl has no charge, so the charge of H and Cl must add up to zero.
H is +1.
If Cl is +1 too, the total makes 2+, which is not right!
So, what is the Oxidation state (or number) of Cl in HCl?

sarah1004
Nov 19, 2009, 11:02 AM
Cl is -1
I hope it is!

So, S is +1?
Thanks

Unknown008
Nov 19, 2009, 11:04 AM
At last! :)

sarah1004
Nov 19, 2009, 11:14 AM
Thank you!!

Can you help me on this..
A piece of a metal 34.9g was heated to about 95C and them added to 550g water with the initial temperature of 28.2C. If the final temperature of the water and the metal is 31.2C, calculate the specific heat of the metal.

Do we have any special formula?

I think its
Heat lost/mass of metal*change in temperature of metal.

so its like
X/34.9g*(31.2-28.2C)

I am not sure... but.. helpe

Unknown008
Nov 19, 2009, 11:23 AM
Hmm, nope. In such experiments, you don't know the specific heat capacity of the metal, which is the amount of energy required to raise the temperature of 1 kilogram of a substance by 1 degree Kelvin.

The formula is Q = mc\theta

Where m is the mass, c the specific heat capacity, theta the change in temperature, and Q the heat involved.

The specific heat capacity of water is usually given.

You have the mass of the water, the temperature rise of the water and presumably the specific heat capacity of the water.

You can find the amount of heat transferred from the metal to the water.

Then, using again the same formula, but using the values for the metal, you have the energy Q, the mass m, the change in temperature. Then, you can solve for c.