Types of Redox Reactions; Oxidising and Reducing Agents

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle to start. Potassium permanganate is an oxidising agent, and never a reducing agent. But hydrogen peroxide can oxidise, and it can also reduce. It can even react with itself: one part of it is oxidised, and another part is reduced! How can one substance do both jobs? Interesting, right? Today we sort redox reactions into types, and find out who can play which role. This comes up often in JEE, so let's begin.

0:33Combination and decomposition

So, first type: combination, where substances join to make one product. Carbon plus oxygen gives carbon dioxide. The arrow means gives, and the small two written below is a subscript: two atoms, okay? On the right, the small arrow means changes to: carbon goes from zero to plus four, oxygen from zero to minus two. Oxidation numbers change, so it is redox. Clear? Next, decomposition: one compound breaks apart. Potassium chlorate gives potassium chloride and oxygen. The big numbers in front are coefficients, how many units of each. Chlorine drops from plus five to minus one: reduced. Oxygen rises from minus two to zero: oxidised. Redox again. With me? So every decomposition is redox, right? No, no, careful! Heat calcium carbonate, Ca C O three, and you get calcium oxide plus carbon dioxide. Calcium stays plus two, carbon stays plus four, and oxygen stays minus two. Nothing changes, so this decomposition is not redox at all. Got it?

1:43Displacement: one element pushes another out

Now, displacement: one element pushes another out of its compound. First, a metal. Zinc plus copper sulphate gives zinc sulphate plus copper. Zinc goes from zero to plus two, oxidised. Copper goes from plus two to zero, reduced. Okay? Second, hydrogen displacement. Two sodium plus two water gives two sodium hydroxide, plus hydrogen gas. Sodium goes from zero to plus one, and hydrogen comes down from plus one to zero. Right? And third, halogen displacement. Chlorine plus potassium iodide gives potassium chloride plus iodine. Chlorine goes from zero to minus one, reduced, and iodide goes from minus one to zero, oxidised. Chlorine pushes iodine out. Clear?

2:32Disproportionation and its reverse

Now the most interesting type. Disproportionation: one element, in one oxidation state, is both oxidised and reduced. Some atoms go up, some go down. With me? Remember our puzzle? Here it is. Two H two O two gives two H two O plus O two: hydrogen peroxide gives water and oxygen. Oxygen in hydrogen peroxide is minus one. What is it in water, and what is it in oxygen gas? Yes, exactly! In water, oxygen is minus two: down, so reduced. In oxygen gas, it is zero: up, so oxidised. The same oxygen, from the same minus one state, does both. Puzzle solved! Okay? One more. Chlorine plus two O H minus, the hydroxide ion, gives Cl minus, plus Cl O minus, plus water. The small minus sign written above is the charge on the ion, right? Chlorine starts at zero, goes to minus one in chloride, and to plus one in hypochlorite. Up and down: disproportionation again. The reverse is comproportionation: two states of one element meet in the middle. Hydrogen sulphide plus sulphur dioxide gives sulphur and water. Sulphur at minus two goes up to zero, and sulphur at plus four comes down to zero. Clear?

3:56Who can be an oxidant, a reductant, or both

Now, the other half of our puzzle. In potassium permanganate, K M n O four, manganese is plus seven, the highest state manganese can have. So can it go any higher? No! It can only gain electrons, only be reduced. So an element in its highest oxidation state is an oxidant only. Same for potassium dichromate, chromium at plus six; nitric acid, nitrogen at plus five; and perchlorate, chlorine at plus seven. Okay? Now flip it. In its lowest state, an element can only go up, only be oxidised. So it is a reductant only. Hydrogen sulphide, sulphur at minus two; ammonia, nitrogen at minus three; and iodide, at minus one. Right? Now tell me. What if the element sits somewhere in the middle? Yes, exactly! It can go either way. Sulphur dioxide: sulphur at plus four, between minus two and plus six. Hydrogen peroxide: oxygen at minus one, between minus two and zero. Also nitrous acid, chlorine gas and hypochlorite, at plus three, zero and plus one. These act as oxidants and as reductants. Got it? Here is a topper's trick for disproportionation questions. Only a species in an in-between state can disproportionate, because it must go up and down at once. So fluorine cannot: it has no positive states, so from zero it can only go down. With me?

5:27Example 1: classify four reactions, spot the non-redox one

Now, your turn. Classify these four reactions. Part a: lead nitrate, where the brackets hold the nitrate group taken twice, breaks into lead oxide, nitrogen dioxide and oxygen. Part b: iron plus hydrochloric acid gives iron chloride and hydrogen. Part c: nitrogen dioxide plus water gives nitric acid and nitrous acid. And part d: silver ion plus chloride ion gives silver chloride. Pause the video, and try it first. Done? Okay, let's check. Part a: nitrogen goes from plus five in lead nitrate to plus four in nitrogen dioxide, reduced. Oxygen goes from minus two to zero in oxygen gas, oxidised. One compound breaking up, with oxidation numbers changing: redox decomposition. Correct! Part b: iron goes from zero to plus two, and hydrogen comes down from plus one to zero. Iron pushes hydrogen out of the acid: displacement of hydrogen. Easy, right? Now part c. Find the oxidation number of nitrogen in N O two, in HNO three, and in HNO two. Plus four, plus five, and plus three. So nitrogen at plus four goes up to plus five, and down to plus three, both at once. Up and down from one state: that is disproportionation! Clear? And part d. Two ions join and make one product, silver chloride. So it is a combination redox reaction, right? Oh no, careful! Silver is plus one before and after. Chloride is minus one before and after. No oxidation number changes, so it is not redox at all. It is just a precipitation reaction. So always check the oxidation numbers first, then name the type. Got it?

7:22Example 2: Cl2 in hot concentrated alkali gives chlorate

Now a harder one. Write the disproportionation of chlorine in hot, concentrated alkali, and the change in oxidation numbers. Hint: chlorine goes down to chloride, and up to chlorate. Pause the video, and try it. Done? Okay, let's check. Three Cl two, plus six O H minus, gives five Cl minus, plus one Cl O three minus, the chlorate ion, plus three water. Six chlorine atoms on each side, and a total charge of minus six on each side. Right? Now, what is the oxidation number of chlorine in chlorate? Call it x, okay? x, plus three times minus two for the three oxygens, equals minus one, the charge on the ion. So x equals plus five. The double arrow means so, or which gives. Clear? So one chlorine atom goes from zero up to plus five. That is oxidation: it loses five electrons, five e minus, where e minus stands for an electron. With me? And the other five chlorine atoms go from zero down to minus one. Each gains one electron, so five electrons in all. That is reduction. Okay? Look, five lost and five gained, a perfect match. That is why we get five chloride ions for every one chlorate ion. Beautiful, isn't it?

8:46Quick revision: redox types, oxidant only, reductant only

So, quick revision. The types: combination, decomposition, displacement, disproportionation, and its reverse, comproportionation. Highest state: oxidant only. Lowest: reductant only. In between: both. And if no oxidation number changes, it is not redox, however it looks. Now go and try the practice problems on this page. You can do them. All the best!