Oxidation and Reduction

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle to start. The word oxidation sounds like it is all about oxygen, right? Now, potassium reacts with fluorine and gives potassium fluoride. No oxygen anywhere. No hydrogen either. And still, chemists say that potassium is oxidised. Oxidation, without any oxygen? Strange, isn't it? Today we will see exactly why. And look, this idea is the base of the whole redox and electrochemistry unit, and it comes up often in JEE questions. So, let's begin.

0:38The classical view: oxygen and hydrogen

So, let us start with the old, classical view. Oxidation means addition of oxygen, or addition of an electronegative element, an element like oxygen that pulls electrons strongly towards itself. Okay? It also means removal of hydrogen, or removal of an electropositive element, like a metal. Right? And reduction is just the reverse: removal of oxygen or an electronegative element, or addition of hydrogen or an electropositive element. Clear? Let's try one from the course. Copper oxide, C u O, plus hydrogen, H two, gives copper plus water, H two O, okay? The arrow means gives, and the small two below is a subscript, two atoms. Now, many students see the oxygen in copper oxide and say copper oxide is oxidised. Is that right? No, no, careful! Look at what changes. Hydrogen gains oxygen and becomes water, so hydrogen is oxidised. Copper oxide loses its oxygen, so copper oxide is reduced. And notice: both happen in the same reaction. With me?

1:46The electronic view: electrons move

Now, back to our puzzle. Potassium and fluorine: no oxygen, no hydrogen. So in that reaction, what is really moving from one atom to another? Electrons, exactly! In the modern, electronic view, oxidation is loss of electrons. On screen, e with a small minus sign written above is an electron. Got it? And reduction is gain of electrons. Really simple. Okay? Now think. Can a species lose electrons all by itself, with nobody there to take them? No! Electrons cannot just vanish. If one species loses them, another species must gain them. So oxidation and reduction always occur together, and every such reaction is called a redox reaction: reduction and oxidation, together. Clear?

2:36Oxidising agent and reducing agent

Now, two names that confuse many students. The oxidising agent. Just from the name, it gets oxidised, right? No, no, wait! This is the trap. The oxidising agent oxidises someone else, and to do that, it takes their electrons. So the oxidising agent, also called the oxidant, gains electrons, and is itself reduced. Okay? And the reducing agent, also called the reductant, gives its electrons away. It loses electrons, so it is itself oxidised. So remember: the agent does the job to the other species, and itself gets the opposite. Got it?

3:14Topper's trick: OIL RIG

Here is a trick so that you never mix these up. OIL RIG. Oxidation Is Loss, Reduction Is Gain, of electrons. Easy to remember, right? There is one more way to see it, with oxidation numbers, which come in the next part of this chapter. Oxidation raises the oxidation number, and reduction lowers it. With me? Look at our copper oxide reaction again. The arrow here means changes to. Hydrogen goes from zero to plus one: raised, so oxidised. Copper goes from plus two to zero: lowered, so reduced. Same answer as before. Clear?

3:51Example 1: Zn and Cu2+, find the oxidant and reductant

Now, your turn. Zinc metal goes into a solution of copper two plus ions, and we get zinc two plus ions and copper metal. The s in brackets means solid, and aq in brackets means aqueous, dissolved in water. The two plus written above is the charge on the ion, okay? Which species is oxidised, which is reduced, and which are the oxidant and the reductant? Pause the video, and try it first. Done? Okay, let's check. The zinc atom becomes the zinc two plus ion. To get a charge of plus two, it must lose two electrons, two e minus. Loss of electrons is oxidation. So zinc is oxidised. Those two electrons go to the copper two plus ion, which becomes copper metal. Gain of electrons is reduction. So copper two plus is reduced. With me? Now, the agents. Zinc is oxidised, so zinc is the oxidising agent, right? Oh no, careful! Remember the trap. Zinc loses electrons, so zinc is the reducing agent, the reductant. And copper two plus gains electrons, so copper two plus is the oxidising agent, the oxidant. OIL RIG, and the agent gets the opposite name. Clear?

5:08Example 2: why K is oxidised by F2 with no oxygen

Now let's solve our puzzle from the start. Two K, plus F two, gives two K F: potassium and fluorine give potassium fluoride. There is no oxygen or hydrogen here. So why is it still an oxidation of potassium? Pause the video, and think about electrons. Done? Okay, let's check. Each potassium atom gives away one electron and becomes K plus, the potassium ion, with a single plus charge written above. Loss of electrons, so that is oxidation. If you said that, correct! But electrons cannot vanish, remember? So who takes that electron? Fluorine, yes, exactly! Fluorine is the more electronegative element, so it pulls the electron away from potassium. With me? And here is the nice part. Even the classical view agrees! Potassium gains fluorine, an electronegative element, and gaining an electronegative element counts as oxidation. Okay? And with oxidation numbers, potassium goes from zero to plus one. Raised, so oxidised. So, oxidation without oxygen? Puzzle solved. Oxidation is really about losing electrons. Got it?

6:22Quick revision: loss, gain, oxidant, reductant, OIL RIG

So, quick revision. Oxidation is loss of electrons, reduction is gain, and they always happen together. The oxidant gains electrons and is reduced; the reductant loses electrons and is oxidised. And remember OIL RIG. Also, the oxidation number rises in oxidation, and falls in reduction. Now go and try the practice problems on this page. You will find them easy. All the best!