help with oxidation states

Hello, This is an example question But I just want to know how you would find the oxidation states for items in complex molecules like the ones in options C and D
A 2Cu2+ + 4I– 2CuI + I2
B [Fe(H2O)6]
3+ + Cl–
[Fe(H2O)5(Cl)]2+ + H2O
C [CoCl4]
2– + 6H2O [Co(H2O)6]
2+ + 4Cl–
D Mg + S MgS

Reply 1

mitostudent

Original post by mitostudent

Here's an image as that isn't too clear Screenshot 2024-08-01 163049.png

how would you approach finding the oxidation states for the complex ones?

Reply 2

UtterlyUseless69

Original post by mitostudent

Here's an image as that isn't too clear Screenshot 2024-08-01 163049.png

how would you approach finding the oxidation states for the complex ones?

Same rules as usual. The sum of all the oxidation states of all the atoms is the charge the species has.

In [Fe(H2O)6]^3+, the iron is +3 as water molecules are neutral (each O is in the -2 oxidation state and each H is in the +1 oxidation state) and so the water ligands can be thought of as having no effect on the charge.

In [Fe(H2O)5Cl]^2+, the iron is still +3. The charge decreases by 1 as one of the neutral ligands is replaced by a chloride ligand (oxidation state -1) and iron(III) isn't strongly oxidising enough to undergo a redox reaction with chloride ions. I suppose you could instead use an electronegativity argument instead if you weren't already aware of the relative oxidising power of iron(III), as iron is less electronegative than chlorine (compare their locations on the periodic table - Cl is on period 3 and further to the right than Fe, a period 4 element) and so Cl will assume a -ve oxidation state.

Using similar logic with the cobalt species, you will find that in either case, cobalt is +2 and chlorine is -1 on both sides of the equation, so that isn't redox either. It's a ligand exchange, which you will study in year 13.

Reply 3

UtterlyUseless69

Original post by mitostudent

Here's an image as that isn't too clear Screenshot 2024-08-01 163049.png

how would you approach finding the oxidation states for the complex ones?

The answer should be D by the way, as Mg is 0 as an element, but +2 in MgS.

If in doubt, eliminate the options that don't involve complex ions first and then make a guess - that way, your chances of getting the right answer should be significantly improved.

Reply 4

mitostudent

thanks for your response! I always get confused when there are transition metals involved 😄 I've noticed that you reply to a lot of my chemistry questions so this isn't strictly related but do you know where I (as a yr 12 student) could learn about molecular orbital theory and why atoms and molecules have different excitation states in a beginner friendly way? If you have any explanations for it it also helps 🙂

Reply 5

UtterlyUseless69

When trying to self-teach new concepts in chemistry, I would always start with chem libretexts, though each page is quite wordy. This step of learning the new concept is optional and you can always skip ahead to the next step, but I personally find it helpful.

https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_1e_(OpenSTAX)/08%3A_Advanced_Theories_of_Covalent_Bonding/8.05%3A_Molecular_Orbital_Theory

https://chem.libretexts.org/Courses/University_of_California_Davis/UCD_Chem_002CH/Text/UNIT_IV%3A_MOLECULAR_SPECTROSCOPY/20.3%3A_Excited_Electronic_States%3A_Electronic_Spectroscopy_of_Molecules (probably best to read after understanding the article linked above / having watched video explanations of MO theory)

Of course, written explanations can be quite slow to digest and so after getting a rudimentary understanding from chem libretexts, I would usually clarify my understanding of what I have just read using video explanations. The organic chemistry tutor seems to explain just about everything topic under the sun in an accessible way and so they are probably the first channel to check for video explanations. On the rare occasions the organic chemistry tutor has little to offer on the topic in question, you will need to look elsewhere. Khan academy also offers some explanations of undergraduate level topics explained in adequate detail, but in an accessible manner.

https://m.youtube.com/watch?v=6tB6E6R_XpQ

https://m.youtube.com/watch?v=1FQPXtN7MeI# (watch after making some sense of MO theory)

https://m.youtube.com/watch?v=rllHziqWlgU (since this is briefly addressed in the second article linked above)