Working from Lewis structures, assign oxidation numbers in the following molecules or ions: a. H

Working from Lewis structures, assign oxidation numbers in...

Key Concepts

Oxidation State Assignment Rules

There are specific rules used to assign oxidation numbers, such as assigning hydrogen typically as +1 (except in metal hydrides) and oxygen as -2 (with exceptions like peroxides or when bonded to fluorine). Additionally, the sum of the oxidation numbers in a neutral molecule must equal zero, and in a polyatomic ion, it must equal the ion's charge. These rules provide a framework for consistent electron bookkeeping.

Lewis Structures

Lewis structures are diagrams that represent the valence electrons of atoms within a molecule. They illustrate how atoms are bonded by showing shared pairs of electrons and lone electron pairs, which is essential for understanding the molecular framework before assigning formal oxidation numbers.

Oxidation Numbers

Oxidation numbers are hypothetical charges that atoms would have if all bonds were completely ionic. They help in tracking the transfer of electrons in chemical reactions, particularly in redox processes, by providing a systematic way to assign electron ownership in a molecule or ion.

Electronegativity

Electronegativity refers to the tendency of an atom to attract electrons in a chemical bond. In the context of oxidation number assignments, atoms with higher electronegativity are assumed to pull electrons closer to themselves, influencing the distribution of oxidation states across the structure.

Molecular Ions

Molecular ions are species that carry an overall positive or negative charge due to an imbalance between protons and electrons. When assigning oxidation numbers in these ions, it is crucial to ensure that the total sum of the assigned oxidation numbers matches the net charge of the ion, thus maintaining chemical consistency.

Transcript

00:01 When we're looking at carbon dioxide and we want to assign oxidation numbers, we're going to start with the more electronegative atom here, which is the oxygen.

00:16 And we're going to look at the anytime we're assigning oxidation numbers, you want to be remembering or looking at the specific rules that are used to assign oxidation numbers.

00:26 So oxygen has its own set of rules and there are some exceptions, but basically oxygen will almost always have an oxidation number of negative 2.

00:36 There are some exceptions, like when it's with a peroxide or when it's with a fluorine, but that's not the case for carbon dioxide.

00:43 So we will give it an oxidation number of negative 2 in this case.

00:52 We'll write that in superscript.

00:54 And then we want to multiply that negative 2 by how many atoms of oxygen we have, which is 2.

01:01 So you'll say 2 times negative 2.

01:03 That's a negative 4.

01:04 We're going to write that at the bottom just to kind of keep track of it all.

01:12 Okay, now since carbon dioxide is neutral, we want whatever.

01:16 Whatever the carbon oxidation state is plus the oxygen to equal zero.

01:21 So what plus negative 4 equals 0? that's a positive 4.

01:27 You divide that 4 by how many atoms of carbon you have, which is 1.

01:32 So 4 divided by 1 is 4.

01:39 So that's how we figure out that carbon has an oxidation number of 4.

01:43 An oxygen has an oxidation number of negative 2.

01:49 Let's take the ammonium ion for another example.

01:52 So in this case, it's not a neutral compound, but it's an ion that has a positive one charge.

02:12 So we're still going to look at the more electronegative ion here, which actually is going to be the nitrogen.

02:22 We actually, typically, whenever you see anything with a hydrogen, you'd want to start with the hydrogen first, because hydrogen has its own set of rules as well.

02:30 So hydrogen has an oxidation number of plus one in all compounds containing elements that are more electronegative than it.

02:40 But it has an oxidation number of negative one in compounds with the metal.

02:43 So this, in this case, it's not a compound with the metal.

02:47 It's a compound where the other element, the nitrogen, is more electronegative than the hydrogen.

02:55 So therefore, hydrogen is going to follow the rule where it has an oxidation number of plus one.

03:00 So since we're starting with the hydrogen, we'll go ahead and assign that.

03:08 That's a plus one.

03:11 We'll multiply that by how many hydrogens we have, which is four.

03:15 So that's going to give us a plus four.

03:21 And now, whatever the oxidation number of the nitrogen is, plus this overall charge of four for hydrogen has to equal a plus one.

03:32 Because this is not a neutral compound.

03:34 It's an ion with a plus one charge.

03:36 So we have to say what plus a four? 4 equals plus 1 and it's going to be a negative 3.

03:46 Okay, then you divide that by how many nitrogen atoms you have, which is 1.

03:51 So 3 divided by 1 is a 3.

03:59 It's a negative 3.

04:00 So nitrogen has an oxidation number of negative 3 and hydrogen has an oxidation number of plus 1.

04:07 Let's do another example.

04:23 In this case, we're going to look at the most intronegative ion first, which is the oxygen.

04:33 Oxygen has an oxidation number of negative 2 in this case because it's not with a peroxide.

04:40 It's not with a fluorine.

04:41 So it follows the general rule where it always has a negative 2.

04:49 Okay, we multiply that by how many atoms we have of it, which is 4.

04:54 So negative 2 times 4 is a negative 8.

05:05 I forgot to mention that this whole thing has an overall charge of negative 1.

05:14 So now in order to figure out the manganese, we're going to have to figure out what plus this negative 8 gives us an overall charge of negative 1 and that answer is a positive 7 now we have to divide that 7 by how many atoms of manganese we have which is 1 so 7 divided by 1 is 7 so this manganese has a oxidation number of 7 where the oxygen has an oxidation number of negative 2 let's take a sulfur i think this is disulfur trioxide, which has an overall charge of negative 2.

06:18 Okay.

06:19 Again, we're going to start with the more electronegative element here, which is oxygen.

06:25 And oxygen has this general rule where it's almost always a negative 2.

06:30 So that oxidation number will be negative 2.

06:37 We're going to multiply that by how many atoms of oxygen we have, which is 3.

06:42 So negative 2 times 3 is negative 6...

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