Advanced Higher ChemistryAdvanced Higher Chemistry

Unit 1Unit 1

Oxidation states of the Oxidation states of the

transition metalstransition metals

Oxidation statesOxidation states

In ionic compounds the oxidation In ionic compounds the oxidation number is equal to the charge on the number is equal to the charge on the ion in a compound e.g. in Feion in a compound e.g. in Fe2+2+(Cl(Cl--))22 iron is in oxidation state +2 and in iron is in oxidation state +2 and in FeFe3+3+(Cl(Cl--))33 it is in oxidation state +3. it is in oxidation state +3.

An element is said to be in a An element is said to be in a particular oxidation state when it has particular oxidation state when it has a specific oxidation number.a specific oxidation number.

Oxidation Numbers : RulesOxidation Numbers : Rules Free or uncombined element = 0Free or uncombined element = 0 Single atom ions = charge on ionSingle atom ions = charge on ion Hydrogen = +1 (except metal hydrides = Hydrogen = +1 (except metal hydrides =

-1)-1) Oxygen = -2 (except peroxides = -1)Oxygen = -2 (except peroxides = -1) Fluorine = -1Fluorine = -1 The sum of all oxidation numbers in a The sum of all oxidation numbers in a

molecule = 0molecule = 0 The sum of all oxidation numbers in a The sum of all oxidation numbers in a

polyatomic ion = charge on ionpolyatomic ion = charge on ion

ExerciseExercise

Now try the exercise on page 14 of Now try the exercise on page 14 of the Unit 1(c) notes.the Unit 1(c) notes.

Oxidation NumbersOxidation Numbers

Oxidation numbers are used to Oxidation numbers are used to determine whether or not an determine whether or not an oxidation or reduction reaction has oxidation or reduction reaction has occurred.occurred.

Increase in oxidation number = Increase in oxidation number = oxidationoxidation

Decrease in oxidation number = Decrease in oxidation number = reductionreduction

Example Example

Is the conversion of CrIs the conversion of Cr22OO772-2- to Cr to Cr3+3+ an an

oxidation or reduction?oxidation or reduction?

CrCr22OO772-2- : : CrCr3+3+ : +3 : +3

(2 x Cr) + (7 x O) = -2(2 x Cr) + (7 x O) = -2

(2 x Cr) + (7 x -2) = -2(2 x Cr) + (7 x -2) = -2

2 x Cr + (-14) = -22 x Cr + (-14) = -2

2 x Cr = +12 2 x Cr = +12

Cr = +6Cr = +6 +6>+3 = reduction+6>+3 = reduction

Oxidising and reducing Oxidising and reducing agentsagents

It is generally true that compounds It is generally true that compounds containing metals in high oxidation containing metals in high oxidation states tend to be oxidising agents states tend to be oxidising agents whereas compounds containing whereas compounds containing metals in low oxidation states tend metals in low oxidation states tend to be reducing agents.to be reducing agents.

ExerciseExercise

Now try the exercise on page 15 of Now try the exercise on page 15 of the Unit 1(c) notes.the Unit 1(c) notes.

Transition MetalsTransition Metals

Transition metals can have several Transition metals can have several oxidation states.oxidation states.

The oxidation state is often +2 but the The oxidation state is often +2 but the 3d electrons can also be easily lost 3d electrons can also be easily lost leading to the other oxidation states. leading to the other oxidation states. (see table p16)(see table p16)

Different ions in different oxidation Different ions in different oxidation states have different stabilities.states have different stabilities.

A change in oxidation state leads to a A change in oxidation state leads to a colour change. (see table p16)colour change. (see table p16)

ExerciseExercise

Now try the exercise on page 16 of Now try the exercise on page 16 of the Unit 1(c) notes.the Unit 1(c) notes.