Advanced Higher Chemistry Unit 1 Oxidation states of the transition metals

Oxidation states n In ionic compounds the oxidation number is equal to the charge on the ion in a compound e.g. in Fe 2+ (Cl - ) 2 iron is in oxidation state +2 and in Fe 3+ (Cl - ) 3 it is in oxidation state +3. n An element is said to be in a particular oxidation state when it has a specific oxidation number.

Oxidation Numbers : Rules n Free or uncombined element = 0 n Single atom ions = charge on ion n Hydrogen = +1 (except metal hydrides = -1) n Oxygen = -2 (except peroxides = -1) n Fluorine = -1 n The sum of all oxidation numbers in a molecule = 0 n The sum of all oxidation numbers in a polyatomic ion = charge on ion

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

Oxidation Numbers n Oxidation numbers are used to determine whether or not an oxidation or reduction reaction has occurred. n Increase in oxidation number = oxidation n Decrease in oxidation number = reduction

Example n Is the conversion of Cr 2 O 7 2- to Cr 3+ an oxidation or reduction? Cr 2 O 7 2- : Cr 3+ : +3 Cr 2 O 7 2- : Cr 3+ : +3 (2 x Cr) + (7 x O) = -2 (2 x Cr) + (7 x -2) = -2 2 x Cr + (-14) = -2 2 x Cr = +12 Cr = +6+6>+3 = reduction

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

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

Transition Metals n Transition metals can have several oxidation states. n The oxidation state is often +2 but the 3d electrons can also be easily lost leading to the other oxidation states. (see table p16) n Different ions in different oxidation states have different stabilities. n A change in oxidation state leads to a colour change. (see table p16)

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