Redox

Question 1

What are the oxidisation state rules?

Answer 1

• sum of all oxidation states in a compound is 0 (e.g.MgO)
• sum of all oxidation states in an ion is equal to the charge on that ion.
• oxidation state of all uncombined elements is 0 (e.g. C, O2)
• oxidation state of oxygen in a compound is usually -2, however is a peroxide (anything with O2 2-) oxidation state is -1.
• oxidation state of hydrogen in a compound is usually +1, however in a metal hydride (metal joined to hydrogen) oxidation state of hydrogen will be -1.
• when in a compound, group 1 have oxidation state of +1, group 2 is +2 and group 3 is +3.
• unless asked to find the oxidation state of a group 7 element, can assume that it will be -1.

Question 2

What does the roman numeral represent in a compound or ion?

Answer 2

Represents the oxidisation number .
E.g. iron (II) = +2e Iron (III) = +3e
This is useful for transition metals.

Question 3

What are oxyanions?

Answer 3

Negative ions that contain the element oxygen bonded to at least one other element. Names usually end in -ate.
E.g. NO2- = nitrate(III) = oxidation state of N= +3

Question 4

What is a redox reaction?

Answer 4

A redox reaction is one in which both oxidation and reduction takes place.
Oxidation and reduction can be defined either by considering changing in oxidation number or electron transfer.

Question 5

What changes to the oxidation number occur when oxidised or reduced?

Answer 5

Oxidised, oxidation state increases.
Reduced, oxidation state decreases.
E.g. Na oxidised from 0 —> +2
Cl reduced from 0 —> -1

Question 6

What happens to electrons when oxidised and reduced?

Answer 6

Oxidation loses electrons. ( e.g. Mg —> Mg 2+ + 2e- )
Reduction gains electrons ( e.g. Cl2 + 2e- —> 2Cl- )

Question 7

What are oxidised and reduced ions/elements called?

Answer 7

Oxidation is a reducing agent as it has helped something else be reduced by donating electrons (e.g. Mg is a reducing agent.)
Reduction is a oxidising agent as it has helped something else be oxidised by removing electrons. (E.g. Cl2 is an oxidising agent.)

Question 8

What happens when metals and acid react? (Redox reaction)

Answer 8

Metal oxidised form + metal ion
Hydrogen in acid reduced forming hydrogen as a gas