Oxidation Number

Transcript

0:00Introduction

Hello everyone! Let's start with a small puzzle. There is a deep blue compound of chromium, C r O five, called chromium peroxide. What is the oxidation number of chromium in it? Easy, right? Five oxygens, each minus two, that makes minus ten. So chromium must be plus ten. But wait. Chromium has only six valence electrons. How can it be plus ten? Something is wrong here, isn't it? By the end of this video, you will know exactly what. Oxidation numbers come up often in JEE, and the whole redox chapter rests on them. So, let's begin.

0:40The basic rules

So, an oxidation number is a number we give to every atom, using a few simple rules. First question. Oxygen gas, O two. The small two at the bottom is a subscript, and it counts the atoms. What is the oxidation number of each oxygen atom here? Zero, yes! Oxygen here is an element in the free state, not combined with any other element. And every element in the free state has oxidation number zero. The same goes for P four, S eight, and sodium metal, N a, in the brackets. All zero. Got it? Next, a monatomic ion: a single atom carrying a charge. Its oxidation number is simply equal to its charge, okay? Now the most useful rule of all. Add up the oxidation numbers of all the atoms. For a neutral compound, the sum is zero. For an ion, the sum equals the charge on the ion. Every calculation today runs on this one line. Clear? Fluorine is the simplest: minus one in all its compounds, always. Remember this one. Right? And the metals. In their compounds, alkali metals are plus one, and alkaline-earth metals are plus two. With me?

1:54Hydrogen and oxygen: watch the exceptions

Now hydrogen. Hydrogen is usually plus one, okay? But look at N a H, sodium hydride. Sodium is an alkali metal, so it is plus one. And the sum must be zero. So what is hydrogen here? Minus one, exactly! In metal hydrides, hydrogen is minus one: sodium hydride, calcium hydride C a H two, and lithium aluminium hydride, L i A l H four. Right? Now oxygen. Usually minus two. But in peroxides, oxygen is minus one: hydrogen peroxide H two O two, sodium peroxide, and barium peroxide. Check H two O two yourself: two hydrogens make plus two, so the two oxygens share minus two, minus one each. Clear? Two rarer ones. In the superoxide K O two, potassium is plus one, so each oxygen is minus one half. And in the ozonide K O three, each oxygen is minus one third. Yes, fractions! These are averages over the oxygens of the ion, and we will come back to this idea in a moment. Okay? Now, oxygen difluoride, O F two. Oxygen is minus two, so... wait. Fluorine is always minus one, remember? So who wins here, oxygen or fluorine? Fluorine wins, always! Two fluorines give minus two, so oxygen is plus two in O F two. And in O two F two, the two fluorines give minus two, shared by two oxygens: plus one each. Oxygen with a plus sign! If you only memorise oxygen as minus two, this is the trap. Got it?

3:37Average or actual? The big trap

Alright, now the most important idea of this video. When we use the formula, we only get the average oxidation number of that element. Why does that matter? Let's see. Take F e three O four. Let x be the oxidation number of iron. Three x, plus four times minus two in brackets, equals zero. The double arrow means so: x is plus eight over three. Can a single iron atom really be plus eight over three? No! F e three O four is really F e O and F e two O three together; the dot just joins the two parts. So one iron is plus two, and two irons are plus three. Two plus three plus three is eight, over three atoms: eight over three. That is only the average. Okay? So remember this line. Averages can be fractions, but an actual oxidation number never is. Clear? And here is the second warning sign. Sulphur and chromium can go up to plus six at most, their group maximum. But the formula gives sulphur plus eight in H two S O five, and chromium plus ten in C r O five. Sounds familiar? Yes, our puzzle! An average above the group maximum is the signal of a peroxide link, two oxygens bonded to each other. When you see it, stop trusting the formula, and find the actual values from the structure. We will do exactly that in example two. With me?

5:06Example 1: Cr in K2Cr2O7, K2CrO4, CrO2Cl2; Mn in KMnO4

Now, example one, an easy one. Find the oxidation number of chromium, C r, in potassium dichromate K two C r two O seven, in potassium chromate K two C r O four, and in C r O two C l two. And of manganese, M n, in K M n O four and K two M n O four. Pause the video and try it first. Done? Okay, let's check. Dichromate first. Two potassiums give plus two. Seven oxygens give minus fourteen. So plus two, plus x, minus fourteen, equals zero, and chromium is plus twelve, right? No, no, careful! There are two chromium atoms. So it is two x, not x. Two x equals twelve, and x is plus six. This slip is very common. Always count the atoms! Okay? Now potassium chromate. Two potassiums, plus two. Four oxygens, minus eight. So x is plus six again. Same chromium, same plus six. Right? Next, C r O two C l two. The two oxygens give minus four, and the two chlorines here are minus one each, so minus two. x minus four minus two equals zero, so x is plus six. Clear? Now manganese. In K M n O four, potassium is plus one, and the four oxygens are minus eight. So manganese is plus seven. And in K two M n O four, there is one more potassium, so plus two. Plus two, plus x, minus eight, equals zero: x is plus six. See? One extra potassium, and manganese drops by one. Got it? So, chromium is plus six in all three compounds, and manganese is plus seven and plus six.

6:57Example 2: Cr in CrO5 is +6, not +10 (peroxide links)

Now example two, and this is our puzzle from the start. Find the oxidation number of chromium in C r O five, the blue chromium peroxide. Pause the video and try it. Hint: think about the warning sign we just learnt. Done? Okay, let's check. The formula average first: x, plus five times minus two, equals zero. So x is plus ten. But chromium has only six valence electrons, so plus six is its maximum. Plus ten is impossible! So, what does this signal? Peroxide links, yes, exactly! In the structure, chromium holds one oxygen by a double bond, the two little lines. And there are two peroxide groups, where two oxygens are joined to each other by the single line. The double-bonded oxygen is minus two. Each peroxide oxygen is minus one, and there are four of them. With me? So, x, plus minus two in brackets, plus four times minus one, equals zero. That is x minus six equals zero, so x is plus six. Chromium is plus six, not plus ten. Puzzle solved! The formula gave only an average; the structure gave the real answer. Perfect!

8:16Quick revision: rules, exceptions and the peroxide signal

So, quick revision. Free elements are zero, and the sum is zero for a compound, or the charge for an ion. Fluorine is always minus one; hydrogen is minus one in metal hydrides; oxygen is minus one in peroxides. And if an average goes above the group maximum, find the peroxide link in the structure. Now go and try the practice problems on this page. You are ready for them. All the best!