Problem Set 2

There's a key posted for the problem set we did in class that was titled "Problem Set #2" on my Gen Chem webpage:
http://www.drbodwin.com/teaching/genchem.php
Direct link to the key:  http://www.drbodwin.com/teaching/problemsets/c150gps02k.pdf

Don't forget, there are also old exams posted at:
http://www.drbodwin.com/teaching/examarchive.php

Other questions, let me know…

Oxidation Numbers

Oxidation numbers describe the balance between electrons and protons on an atom, whether that atom is happily floating around all by itself or part of a massive molecule. Oxidation numbers can be determined two different ways: by using rules, or by looking at structure. Let's look at the rules first.

Oxidation Numbers by the Rules:
1. For neutral, uncombined elements, Ox# = 0. Examples: Fe(s), H₂(g), Hg(l), Ne(g)
2. For monoatomic ions, Ox# = charge. Examples: Fe²⁺(aq) {Ox# = +2}, P^3-(g) {Ox# = -3}
3. Oxygen is almost always Ox# = -2, except in O₂ {Ox# = 0, Rule #1} and peroxides {Ox# = -1}
4. Hydrogen is almost always Ox# = +1, except in H₂ {Ox# = 0, Rule #1} and hydrides {Ox# = -1}
5. The sum of the Ox#s on all the atoms in a polyatomic molecule or ion is equal to the charge on the whole polyatomic molecule or ion.

Let's look at a redox reaction and assign Ox#s by the rules:

Cl₂(g) + 2 O₂(g) ↔ 2 ClO₂(g)

Cl₂(g) : Rule #1, Ox# = 0

O₂(g) : Rule #1, Ox# = 0

ClO₂(g) : Rule #3, oxygen is Ox# = -2.

ClO₂(g) : Rule #5, (Ox# Cl) + 2(Ox# O) = 0 (the charge on a neutral molecule)

(Ox# Cl) + 2(-2) = 0

(Ox# Cl) = +4

So in this redox reaction, each Cl is going from 0 to +4, losing 4 electrons, Losing Electrons is Oxidation; and each O is going from 0 to -2, gaining 2 electrons, Gaining Electrons is Reduction.

For many substances, it's actually easier to assign Ox#s by looking at the structure. The process is very similar to finding Formal Charge, the electrons are just assigned a little differently. Formal Charge assigns electrons as if all bonds are purely covalent, meaning that all bonding pairs of electrons are split with one electron given to each atom in the bond. Oxidation Number assigns electrons as if all bonds are purely ionic, meaning that all of the bonding electrons go to the more electronegative element in the bond.

Oxidation Number by the Structure:
1. Draw a good Lewis Structure
2. Assign all bonding electrons to the more electronegative element in the bond
3. Compare the electrons assigned to each atom to the valence electrons of the neutral element

Again, let's look at an example, in fact, let's look at the sameexample as above, ClO₂(g). Drawing a good Lewis Structure:

This is a very interesting molecule, it violates the octet rule andit has an unpaired electron. Looks like it would be pretty reactive. Looking at the electronegativities, oxygen is more electronegative than chlorine, so all of the bonding electrons will be assigned to oxygen, giving each oxygen 8 assigned electrons and the chlorine 3 assigned electrons.

Neutral oxygen has 6 valence electrons, we've assigned 8 electrons to oxygen, so the oxidation number for oxygen in this molecule is -2, just like we predicted using the rules. Neutral chlorine has 7 valence electrons, we've assigned 3 electrons to chlorine, so the oxidation number for chlorine in this molecule is +4, again, just like we predicted using the rules. Both methods work. Why would you ever use structures when the rules work? Try looking at hydrogen peroxide, H-O-O-H, or a more complex molecule like glucose. The rules don't always give the best picture of what's happening in a molecule, and this can ultimately make it harder to predict reactivity or other behaviors.

Redox - Definitions

If you want to understand chemistry, you have to follow the electrons. If electrons are transferred during a chemical reaction (as opposed to just being rearranged...), then a reduction-oxidation process is taking place. To help keeping track of the electron transfer in redox processes, we can use a couple acronyms/mnemonics related to the definitions of reduction and oxidation.

OIL / RIG - “Oxidation Is Losing electrons” / “Reduction Is Gaining electrons”

LEO / GER - “Losing Electrons is Oxidation” / “Gaining Electrons is Reduction”

Hmm, why is “reduction” associated with gainingelectrons? Remember, electrons are negatively charged, so gaining electrons increases the number of negatively charged particles associated with an atom which reduces its net charge. But are we really looking at “charge” to determine redox chemistry? Sometimes it seems like it, but other times the charge doesn't seem to line up with the processes. We really have to look at oxidation number, which is related to charge, but a with some subtle differences. One way to distinguish charge and oxidation number is that “charge” can be used to describe the net overall balance between electrons and protons in a system that might contain multiple atoms, but “oxidation number” describes the balance between electrons and protons for each individual atom in a structure regardless of its size. Oxidation numbers sound important enough for their own post, look for it soon.

Another thing that can cause some mix-ups is the term “oxidation” or {in verb form} “oxidize”. {Or for the British English spellers in the crowd, “oxidise”.} The element oxygen is very often involved in redox reactions. Is oxygen usually undergoing oxidation or reduction? You know you want to say oxidation, the words look so similar... But if we start with molecular oxygen, O₂(g), it's almost always going to gainelectrons. Gaining Electrons is Reduction. GER, indeed! The important thing to remember here is that reduction and oxidation are ALWAYS coupled processes. You can't have one without the other. This leads to some other terminology...

The process of one substance undergoing oxidation causes something else in the system to be reduced. The substance that is being oxidized is the reducing agent or reductant because it is causing reduction to take place.
The process of one substance undergoing reduction causes something else in the system to be oxidized. The substance that is being reduced is the oxidizing agentor oxidant because it is causing oxidation to take place.

So if oxygen is usually being reduced, it is a good oxidizing agent.