1 Chemistry Tutorial REDOX REACTIONS by Dr John G Wright Press PgDn or left click for the next slide The Wright Stuff

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ChemistryChemistry Tutorial TutorialChemistryChemistry Tutorial Tutorial

REDOX REACTIONSREDOX REACTIONSbyby

Dr John G WrightDr John G Wright

Press PgDn or left click for the next slide

The Wright Stuff

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Oxidation and ReductionOxidation and Reduction Oxidation Numbers (this set of 18 slides)Oxidation Numbers (this set of 18 slides) DisproportionationDisproportionation Balancing Half EquationsBalancing Half Equations Electrochemical CellsElectrochemical Cells ProblemsProblems

The individual topics can be viewed as separate slide shows. The individual topics can be viewed as separate slide shows. If a slide show is missing from the web page, it means it is If a slide show is missing from the web page, it means it is being updated and improved. (Or perhaps I just haven’t being updated and improved. (Or perhaps I just haven’t finished writing it yet!)finished writing it yet!)

Redox ReactionsRedox Reactions

The tutorials are divided into five main sections, each covering a separate topic concerning redox reactions, plus a problems section.

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Oxidation and ReductionOxidation and Reduction It is assumed that you have read the first tutorial on Oxidation It is assumed that you have read the first tutorial on Oxidation

and Reduction and are familiar with the meaning of these and Reduction and are familiar with the meaning of these terms, especially the electron-based definitions.terms, especially the electron-based definitions.

Students on introductory level courses may only need to read Students on introductory level courses may only need to read the previous section on Oxidation and Reduction, but those on the previous section on Oxidation and Reduction, but those on advanced courses, such as A-level and above, will need to advanced courses, such as A-level and above, will need to study both tutorials, with the emphasis being placed on the study both tutorials, with the emphasis being placed on the modern electron-based definition of oxidation and reduction. modern electron-based definition of oxidation and reduction.

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Oxidation NumbersOxidation NumbersThis section is about calculating the oxidation state or This section is about calculating the oxidation state or oxidation number of an element in a compound. It uses a set oxidation number of an element in a compound. It uses a set of rules which require little or no previous knowledge of the of rules which require little or no previous knowledge of the chemical being examined. (The oxidation number is similar to chemical being examined. (The oxidation number is similar to the valency of the element but has a + or - sign.)the valency of the element but has a + or - sign.)

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Oxidation NumbersOxidation Numbers

The oxidation number of an element is the charge which the The oxidation number of an element is the charge which the atom would have if the element was acting as an ion in the atom would have if the element was acting as an ion in the species being studied.species being studied.

This can seem a bit strange at times, especially when you This can seem a bit strange at times, especially when you know that the compound you are examining is covalent. But it know that the compound you are examining is covalent. But it enables us to easily work out whether an element is oxidised or enables us to easily work out whether an element is oxidised or reduced in a reaction. We just compare the oxidation number reduced in a reaction. We just compare the oxidation number before and after the reaction and work out whether it has before and after the reaction and work out whether it has gained or lost electrons.gained or lost electrons.There are a series of simple rules we use to calculate the There are a series of simple rules we use to calculate the oxidation number of an element.oxidation number of an element.

First, the definition.

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Oxidation NumbersOxidation Numbers The rules are given in order of importance. If you reach a rule The rules are given in order of importance. If you reach a rule

which gives you data to work from and a later rule appears to which gives you data to work from and a later rule appears to contradict the earlier result, just ignore it, you use the earlier contradict the earlier result, just ignore it, you use the earlier result. I.e. later rules do not overrule earlier decisions that result. I.e. later rules do not overrule earlier decisions that you have already made. you have already made.

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Oxidation NumbersOxidation Numbers The oxidation number of an uncombined element is zero. The oxidation number of an uncombined element is zero.

I.e. the element in the element itself is zero.I.e. the element in the element itself is zero. The algebraic sum of the oxidation numbers of all the atoms in The algebraic sum of the oxidation numbers of all the atoms in

a compound equals zero.a compound equals zero. The algebraic sum of the oxidation numbers of all the atoms in The algebraic sum of the oxidation numbers of all the atoms in

an ion equals the charge on the ion. an ion equals the charge on the ion.

Algebraic sum means take account of the sign of the charge. Algebraic sum means take account of the sign of the charge. Learn these rules.Learn these rules.

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Fluorine is always ALWAYS -1 in its compounds.Fluorine is always ALWAYS -1 in its compounds. Group I metals are always ALWAYS +1 in compounds.Group I metals are always ALWAYS +1 in compounds. Group II metals are always +2 in their compounds.Group II metals are always +2 in their compounds. Oxygen is almost always -2 in compounds, except in peroxides Oxygen is almost always -2 in compounds, except in peroxides

(H(H22OO22, ROOR, etc.) where it is -1. (Or when overruled by one of , ROOR, etc.) where it is -1. (Or when overruled by one of

the above rules.)the above rules.) Halides are often -1, but other numbers are often possible.Halides are often -1, but other numbers are often possible. Hydrogen can be +1 or -1. (If it is the first element given in a Hydrogen can be +1 or -1. (If it is the first element given in a

formula, it is usually +1, if given second or third, it may be -1. formula, it is usually +1, if given second or third, it may be -1. If in doubt, the most electronegative element is +ve and the If in doubt, the most electronegative element is +ve and the other element will be -ve.)other element will be -ve.)

But where do you start? We find that some atoms always have But where do you start? We find that some atoms always have the same oxidation number in their compounds or ions. As the same oxidation number in their compounds or ions. As stated earlier, the first rules overrule the later rules.stated earlier, the first rules overrule the later rules.

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Calculate the oxidation number of Mn in KMnOCalculate the oxidation number of Mn in KMnO44..

K and O are in our table of known oxidation numbers, and K and O are in our table of known oxidation numbers, and KMnOKMnO44 is a compound. is a compound.

K is +1 (Group I metal) and oxygen is almost always -2.K is +1 (Group I metal) and oxygen is almost always -2.

K + Mn + (4 x O) = 0 (that’s a zero)K + Mn + (4 x O) = 0 (that’s a zero)

So +1 + Mn + (4 x -2) = 0So +1 + Mn + (4 x -2) = 0

i.e. +1 + Mn - 8 = 0i.e. +1 + Mn - 8 = 0

i.e. Mn - 7 = 0i.e. Mn - 7 = 0

so Mn = +7 (We say +7 and not 7, because it is a charge.)so Mn = +7 (We say +7 and not 7, because it is a charge.)

Easy? It’s just simple arithmatic, not chemistry.Easy? It’s just simple arithmatic, not chemistry.

Let’s look at these rules in action in a simple problem.

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Calculate the oxidation number of chlorine in the ion ClOCalculate the oxidation number of chlorine in the ion ClO33--

Oxygen comes before chlorine in our known table, so it’s rule Oxygen comes before chlorine in our known table, so it’s rule takes preference, and it has a charge of -2.takes preference, and it has a charge of -2.

Cl + (3 x O) = -1 ( the -1 is the charge on this ion)Cl + (3 x O) = -1 ( the -1 is the charge on this ion)

Cl + (3 x -2) = -1Cl + (3 x -2) = -1

Cl - 6 = -1Cl - 6 = -1

Cl = +5 Cl = +5 (Again, it’s +5 and not 5 as it’s the charge on the “ion”.)(Again, it’s +5 and not 5 as it’s the charge on the “ion”.)

Here’s another example, this time involving an ion.

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Oxidation NumbersOxidation Numbers Calculate the oxidation number of phosphorus in POClCalculate the oxidation number of phosphorus in POCl22FF

Oxygen is almost always -2, chlorine is usually -1 and fluorine Oxygen is almost always -2, chlorine is usually -1 and fluorine is always ALWAYS -1. As P is the unknown, we can reason is always ALWAYS -1. As P is the unknown, we can reason that although Cl can have other values, then -1, the commonest that although Cl can have other values, then -1, the commonest value, is the one to use (otherwise the problem is unsolvable).value, is the one to use (otherwise the problem is unsolvable).

P + O + (2 x Cl) + F = 0P + O + (2 x Cl) + F = 0

P - 2 - 2 -1 = 0P - 2 - 2 -1 = 0

P - 5 = 0P - 5 = 0

P = +5P = +5

But phosphorus can have other values for it’s oxidation But phosphorus can have other values for it’s oxidation number, depending on the compound under investigation.number, depending on the compound under investigation.

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Oxidation numbersOxidation numbers

What is the oxidation number for P in NaHWhat is the oxidation number for P in NaH22POPO33??

Na = +1, H = +1 usually, and O = -2 almost always.Na = +1, H = +1 usually, and O = -2 almost always.

Na + 2H + P + 3O = 0Na + 2H + P + 3O = 0

+ 1 + 2 + P - 6 = 0+ 1 + 2 + P - 6 = 0

P - 3 = 0, so P = +3P - 3 = 0, so P = +3 What is charge on the Cr in KWhat is charge on the Cr in K22CrCr22OO77 ? ?

K and O are in the table of fixed values.K and O are in the table of fixed values.

2K + 2Cr + 7O = 02K + 2Cr + 7O = 0

+ 2 + 2Cr - 14 = 0+ 2 + 2Cr - 14 = 0

2Cr - 12 = 0, so 2Cr = +12, and Cr = +62Cr - 12 = 0, so 2Cr = +12, and Cr = +6

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Oxidation NumbersOxidation Numbers To avoid any confusion when an element can have several To avoid any confusion when an element can have several

oxidation numbers, the oxidation number is usually mentioned oxidation numbers, the oxidation number is usually mentioned in the compound’s name, written as Roman numerals in brackets in the compound’s name, written as Roman numerals in brackets after the element to which it refers. after the element to which it refers. In names like “elementate(X)”, the number refers to “element” In names like “elementate(X)”, the number refers to “element” and not the associated oxygens.and not the associated oxygens.

So if we look at the examples we’ve just done, we get the So if we look at the examples we’ve just done, we get the following names:-following names:-

KMnOKMnO44 potassium manganate(VII)potassium manganate(VII)

NaClONaClO33 sodium chlorate(V)sodium chlorate(V)

POClPOCl22FF phosphorus(V) oxydichlorofluoridephosphorus(V) oxydichlorofluoride

NaHNaH22POPO33 sodium dihydrogenphosphate(III)sodium dihydrogenphosphate(III)

KK22CrCr22OO77 potassium dichromate(VI)potassium dichromate(VI)

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Oxidation numbersOxidation numbers Check for yourself that the numbers in the following Check for yourself that the numbers in the following

compounds’ names are the same as the oxidation number of compounds’ names are the same as the oxidation number of the element with which they are associatedthe element with which they are associated

LiNOLiNO33 lithium nitrate(V), PtCl lithium nitrate(V), PtCl44 platinum(IV) chloride platinum(IV) chloride

NaNONaNO22 sodium nitrate(III), NaBrO sodium nitrate(III), NaBrO44 sodium bromate(VII) sodium bromate(VII)

POClPOCl33 phosphorus(V) oxychloride phosphorus(V) oxychloride

CrClCrCl33 chromium(III) chloride, TiO titanium(II) oxide chromium(III) chloride, TiO titanium(II) oxide

TiOTiO22 titanium(IV) oxide titanium(IV) oxide

Did you get them all right?Did you get them all right?

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Consider the equation below.Consider the equation below.2FeCl2FeCl22 + Cl + Cl22 2FeCl 2FeCl33

The oxidation number of the iron changes thus:The oxidation number of the iron changes thus:FeFe2+ 2+ Fe Fe3+3+ ( + e ( + e-- of course) of course)

sometimes written as Fe(II) sometimes written as Fe(II) Fe(III) + e Fe(III) + e--

i.e. Fei.e. Fe2+2+ has been oxidised. This example is easy to spot as an has been oxidised. This example is easy to spot as an example of oxidation, but sometimes it’s a bit harder.example of oxidation, but sometimes it’s a bit harder.

An important use of oxidation numbers is in the recognition ofoxidation and reduction. They let us quickly see when anelement has gained or lost electrons, and hence been oxidisedor reduced. If the oxidation number becomes more positive, oxidation has occurred, becoming more negative means that reduction has occurred.

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II-- IO IO33--

Is this oxidation or reduction?Is this oxidation or reduction?

Calculating the oxidation numbers reveals all.Calculating the oxidation numbers reveals all.

In IIn I--, the iodine is -1, while in IO, the iodine is -1, while in IO33-- it is +5. (Don’t take my it is +5. (Don’t take my

word for it, calculate the oxidation numbers yourself.) So the word for it, calculate the oxidation numbers yourself.) So the iodide ion has lost electrons, and been oxidised to +5.iodide ion has lost electrons, and been oxidised to +5.

II-- IO IO33-- + 6e + 6e- -

which could also be written aswhich could also be written asI(I) I(I) I(V) + 6e I(V) + 6e- -

Consider a reaction where the following change occurs (the other reagents are not important at this stage).

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3MnO3MnO22 + KClO + KClO33 + 6KOH + 6KOH 3K 3K22MnOMnO44 + KCl + 3H + KCl + 3H22OO

Mn = +4 Mn = +4 Mn = +6 Mn = +6 Cl = +5Cl = +5 Cl = -1Cl = -1i.e Mn has lost electrons and been oxidised, whilei.e Mn has lost electrons and been oxidised, whileCl has gained electrons and been reduced.Cl has gained electrons and been reduced.

4FeCO4FeCO33 + O + O22 2Fe 2Fe22OO33 + 4CO + 4CO22

Fe = +2 Fe = +2 Fe = +3 Fe = +3 O = 0 O = 0 O = -2 O = -2 C = +4 C = +4 C = +4 C = +4i.e. Fe has lost electrons and been oxidised, while the i.e. Fe has lost electrons and been oxidised, while the oxygen has gained electrons and been reduced. The carbon hasn’t oxygen has gained electrons and been reduced. The carbon hasn’t changed. Check these oxidation numbers for yourself.changed. Check these oxidation numbers for yourself.

Here are a few more examples showing the use of oxidationnumbers to discover whether oxidation or reduction occurs.

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The EndThe End I hope you have enjoyed this tutorial. It is the second in a I hope you have enjoyed this tutorial. It is the second in a

series of tutorials on oxidation and reduction. The tutorials series of tutorials on oxidation and reduction. The tutorials become progressively more advance, but unless it says so on become progressively more advance, but unless it says so on the frist couple of pages, are all intended for A-level students.the frist couple of pages, are all intended for A-level students.

I also hope you have increased your understanding of I also hope you have increased your understanding of chemistry, learned something useful about chemistry and that chemistry, learned something useful about chemistry and that it will increase your marks in examinations.it will increase your marks in examinations.

Bye for now, Dr John G Wright, Bye for now, Dr John G Wright, The Wright StuffThe Wright Stuff www.sky-web.net www.sky-web.net

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1 Chemistry Tutorial REDOX REACTIONS by Dr John G Wright Press PgDn or left click for the next slide The Wright Stuff