Enter…. Oxidation, reduction and oxidation numbers

Enter…

Oxidation, reduction and oxidation numbers

Oxidation and reduction go together

like acid and base….in as much

as you cannot have one without the other.

The following pages contain some

background to this topic, and then

some exercises based upon these concepts.

Notes and hints are available !

A glossary and Periodic Table are

available through the appropriate button…

The Nobel Prize for Chemistry in 1992 was awarded to Rudolph A. Marcus in recognition

of his contributions to the theory of electron transfer reactions in chemical systems…….in other words “redox chemistry!”

Plants are very clever chemists! They sit there at ambient temperature and pressures makingthe most amazing colours and perfumes!

In photosynthesis the plants use light energy to form energy-rich compounds from water

and carbon dioxide. The decisive reactions take place in a reaction

centre in the cell. The reactions are unbelievably fast…..just look at the next slide!

Brightly coloured crystals and solutions contain metal ions in different valance states, and light of

a particular frequency is absorbed to produce the observed colour.

Redox chemistry has many practical applications:

• ..here are just some…

• ..imagine life without the portable power pack in your phone…………………..

Safety lights….

When the rod is bent a glass ampoule is broken and the liquids from the vial and rod are mixed. This chemical reaction produces “cold light”.

This phenomenon is an example ofchemiluminescence, a complicated process

involving electron transfer steps.

The firefly also makes use of chemiluminescenceto attract a mate!

Introduction to oxidation number

Redox reactions are characterised by electron transfer.

The assignment of oxidation numbers to atoms is

an electron bookkeeping device that can help us

to keep track of electron ownership.

There are some rules to apply in order

to establish the value of ON for an atom.

Do remember that it is only ATOMS that

can have an oxidation number, or to be

said to be in a particular oxidation state.

Oxidation number defined..

The oxidation number of an atom

in a chemical species is defined to

be the charge that the atom would

have if the bonding in the species

were considered to be wholly ionic.

Rules for assigning oxidation number [O.N.]

1 Atoms in their elemental form have an oxidation number of ZERO [0]

2 Oxygen is assigned a value of [-2] except in peroxides [O2

2-] where it is [-1] and superoxides [O2-] where it is [-1/2]

3 In neutral species the sum of the O.N. must be zero

4 In charged species the sum of the O.N. must equal the charge on the species

5 Hydrogen is [+1] except when in combination with metals when it become [-1]

6 In deciding the sign of the atom’s O.N. remember the definition of O.N. in the first place: decide what the most probable ionic charge would be based upon the atoms’ relative positions on the Periodic Table.

Rules for assigning oxidation number [O.N.]

1 Atoms in their elemental form have an oxidation number of ZERO [0]

2 Oxygen is assigned a value of [-2] except in peroxides [O2

2-] where it is [-1] and superoxides [O2-] where it is [-1/2]

3 In neutral species the sum of the O.N. must be zero

4 In charged species the sum of the O.N. must equal the charge on the species

5 Hydrogen is [+1] except when in combination with metals when it become [-1]

6 In deciding the sign of the atom’s O.N. remember the definition of O.N. in the first place: decide what the most probable ionic charge would be based upon the atoms’ relative positions on the Periodic Table.

Questions follow….

Common oxidation numbers

Which of the following elements would give compounds in which the oxidation state of its atoms would be [+2]

Na

Ca

F

O

glossary PT Rules page

• No. Go back to the question and check the glossary or Periodic table slides…

• Yes. Calcium is in Group II of the Periodic Table so would form ionic compounds in which the charge would be +2

Calcium

• ‘fraid not. Fluorine is in Group VII so will form compounds in which the atom adopts a -1 charge, so the ON of fluorine atom will be [-1]

• No. Oxygen is in Group 6. Think about the charge on the oxide ion…

• In many compounds and ions oxygen atoms are assigned a value of [-2]. Remembering that for neutral molecules the sum of the oxidation numbers has to be zero, what is the oxidation number of Phosphorus atom in P2O3?

• 3• 4• 5

glossary PT Rules page

• Correct. The sum of the oxidation numbers must be zero for this neutral molecule, and each oxygen atom has an oxidation number of [-2]

• No …. Check the definitions pages and the Periodic Table….

• Wrong…think again…

• For polyatomic ions the sum of the oxidation numbers has to be the charge on the ion. So, find the oxidation number of phosphorus atom in HPO4

2-.

• 3

• 5

• 7

Glossary PT Rules page

• No…hydrogen is given an ON of [+1]…

• Yes. In this case the sum of the oxidation numbers has to be [-1], the same as the charge on the ion.

• No. Phosphorus is in Group 5 so the maximum ON has to be [+5]. This is a general rule

Classifying reactions

Chemists find it very useful to group reactions

that are similar in some way according to

classification schemes. Any one reaction

maybe classified in a variety of ways.

One way is to decide whether oxidation

numbers of atoms in products are different

to those in the reagents. If this is so then

the reaction is classified as REDOX.

Half equations are those which represent

EITHER an oxidation OR a reduction, and

as such will therefore contain electrons.

When added together correctly these will

give the overall redox equation, in which the

electrons will cancel, so not appear.

This section of the programme will cover half equations

first, and then move onto full redox systems.

Balancing half equations in acidic solutions

• Oxidants and reductants in redox equations may be polyatomic species where only one of the atoms changes in oxidation number.

• Consider this: • BrO- Br -

• ON of Br atom initially is [+1], and finally is [-1]. This therefore is a reduction so electron symbol appears left hand side:

• e- + BrO- Br –

• The change form [+1] to [-1] requires “ electrons:• 2e- + BrO- Br –

• The oxygen atom on the left is balanced with water:

• 2e- + BrO- Br - + H2O

• Finally the hydrogen atoms are balanced on the left with hydrogen ions:

• 2H+ + 2e- + BrO- Br - + H2O

• A final check indicates that both atoms and overall charge balances both sides of the equation.

Questions follow

• Look at the following reaction:

• NO NO3-

• Identify the oxidation numbers of the N atom before and after reaction to decide whether this is and oxidation or a reduction.

• Oxidation

• Reduction

• [+2] [+5] The oxidation number goes UP so “yes” this IS an oxidation…

• Check again the changes in oxidation number

• How many electrons are needed to bring about the following change, and is this a reduction or an oxidation?

• BrO3− Br2

10 electrons reduction

5 electrons oxidation

10 electrons oxidation

5 electrons reduction

• Look again at oxidation numbers for bromine…

• Yes. The bromine atom has an oxidation number of +5 in the ion BrO3

− and zero in the element Br2. There are two atoms in the latter, EACH requiring 5 electrons to bring about the REDUCTION.

• In which species is the oxidation state of the underlined atom the highest?

• MnO42−

• BrO2−

• SO32−

• VO2+

• Remember that the oxidation number of oxygen is pre-assigned at [-2], and that the sum of oxidation numbers must be the same as the charge on the ion..

• Yes. MnO42− is the manganate (VI) ion

a breather…..

• The oxidation number does not always need to be an integer…in which species below is the oxidation number of sulphur fractional?

• S2O32-

• S2O42-

• S2Cl2

glossary1 Oxidation is the loss of electron[s] or

an increase in oxidation number

2 An oxidising agent is therefore

an electron[s] acceptor

3 When an oxidising agent has done

its job it will have become reduced, so an atom’s

oxidation number will have decreased within it

4 Reduction is the gain of an electron[s]

5 A reducing agent is therefore an electron[s] donor

6 When a reducing agent has done its job it will have been

oxidised an atom’s oxidation number within it will have been increased

7 The OXIDATION NUMBER of an atom is the formal charge

that it would carry if the bonding in the chemical species

were to be considered to be wholly ionic

• The sum of oxidation numbers must be the same as the charge on the ion, or zero for a neutral molecule….

• Yes…in this ion the oxidation number is 2.5 (difficult to write in Roman numerals!)

• Does the number of electrons transferred indicate the “strength” of an oxidant or a reductant?

yes

no

Can’t say

• Wrong! Reductants and oxidants can transfer only one electron but be more “potent” or “stronger” than ones which transfer two or more….

• Wrong! Different oxidants or reductants have different “strengths” in different situations, yet can transfer the same numbers of electrons…

• “Can’t say” is actually the correct answer!

• The choice of reductant or oxidant depends on many factors…amongst these being the RATE at which the redox reaction occurs. I suppose it comes down to what we mean by “strength”…

Continue…

...and finally…

By now you will hopefully feel confident

about assigning oxidation numbers

to atoms in molecules and ions;

using them to identify oxidation or reduction

half equations and combining these

to give balanced redox equations.