## moongazer 3 years ago K2Cr2O7 + S8 ----> K2O + Cr2O3 + SO2 (the numbers are subscripts) find the oxidation number

1. Callisto

Oxidation number of ..?

2. moongazer

all elements there

3. Callisto

K -> Group I metal. Either 0, or +1. Here, it's an ion. So, what is the oxidation number of K?

4. Callisto

Btw, I think it's K2Cr2O7 on the left?

5. moongazer

yes, there is a typo :)

6. Callisto

Okay. What have you got for K?

7. moongazer

+1

8. moongazer

9. moongazer

I'll just post it :)

10. Callisto

Yup. For both sides, there is no oxidation number change for K. So, K on both sides are +1. Sure :)

11. moongazer

12. moongazer

Isn't it that if the subscript of O is greater than 2, it will have an oxidation number of -2 ?

13. Callisto

O has an oxidation number of -2 - You're right! :)

14. moongazer

but if "O" only it will have -1 and O2 it will have 0?

15. Callisto

No, not really. O is in group 6, it need 2 more electrons. So, usually, its oxidation number is -2 when it is bonded to other atoms. If it is an element (i.e. O2), its oxidation number is 0 as element in its own form. But remember, it's just 'usually' when its oxidation number is of -2. There are some exceptions. For instance, if I remember correctly, O in F2O is +2 since F is a more electronegative element and it has an oxidation number of -1. Since there are two F, oxidation number of O has to be +2 in order to get a net oxidation number of the compound equal to zero.

16. moongazer

ohh, thanks for the info. Here are my answers :) K2(+1)Cr2(+6)O7(-2) + S8(0) ----> K2(+1)O(-2) + Cr2(+3)O3(-2) + S(0)O2(0) The oxidation number is enclosed by parenthesis.

17. moongazer

Is it correct ?

18. Callisto

Almost. S(0)O2(0) <- not correct (for both)

19. moongazer

K2(+1)Cr2(+6)O7(-2) + S8(0) ----> K2(+1)O(-2) + Cr2(+3)O3(-2) + S(-2)O2(+1)

20. moongazer

I made the oxidation number of O2 the unknown. Is it correct now?

21. Callisto

Not really. For SO2, O is -2 (you can take a look at O first as it is more electronegative than S!) O2 is NOT an element in this case, so O doesn't have a oxidation number of 0!

22. moongazer

K2(+1)Cr2(+6)O7(-2) + S8(0) ----> K2(+1)O(-2) + Cr2(+3)O3(-2) + S(+4)O2(-2) I think this is correct already :)

23. Callisto

Nice :)

24. moongazer

I'll say something. I'll just fix the laptop :)

25. moongazer

26. Callisto

27. moongazer

Whew! The battery almost got dead. It keeps on saying plugged in, not charging.

28. moongazer

So it means, If you have a molecule with an oxygen,to find the oxidation number of the elements in it. You will identify first the more electronegative element and made the charge of it equal to its oxidation number. Then make the oxidation number of the other element the unknown.

29. moongazer

you will solve for the unknown oxidation number

30. Callisto

Hmm.. you can say so. But when you consider electronegativity of the element, that would be talking about the group V - group VII For metal, it usually has positive oxidation number. If I remember correctly, for group I metal, when it does not exist in element state, it should be of an oxidation number of +1, and looks like it works for most of the group II metal too. But for transition metals, that's another case...

31. Callisto

One little practice for you (if you want one lol) Find the oxidation number of H, O and Cl in the compound HOCl.

32. moongazer

H(+1)O(-2)Cl(+1)

33. Callisto

Yup :D

34. moongazer

I hope it is correct. :)

35. moongazer

Yey! thanks!

36. Callisto

Welcome :)

37. Callisto

There are three definitions....

38. moongazer

is it the loss of electrons and reactions directly with O2 ?

39. moongazer

transfer of electrons?

40. Callisto

Definition of oxidation: 1. addition of oxygen to a substance 2. a process in which a substance loses electrons 3. a process in which the oxidation number of an element in a substance increases.

41. moongazer

and reduction is the opposite of that?

42. Callisto

Yup...

43. moongazer

In my notes it says below oxidation it says: +O -H What does "H" have to do with oxidation and reduction ?

44. Callisto

-H?!

45. moongazer

idk I just saw in my notes. I was late during our class so I didn't know what is H there

46. moongazer

Anyway, thank you very much in your time and effort helping me. :)

47. Callisto

You're welcome.. Sorry that I couldn't help :(

48. moongazer

It's alright you helped me a lot. :)