7.2 Oxidation states

Question 1

What is an oxidation state

Answer 1

. Each element in a compound is given an oxidation state, which tells us how many electrons it has lost or gained, compared with the element on its own

Question 2

What oxidation state does an electron on its own have

In a compound, what do the sum of oxidation states equal

What do the sum of oxidation states sum to in a complex ion eg NH4+

Answer 2

zero

Zero because all compounds are neutral

They sum to the charge on the ion

Question 3

What does a positive number as an oxidation state mean

What does a negative number mean

Answer 3

It means the element has lost electrons eg Mg2+ has an oxidation state of +2

Shows that the element has gained electrons and has therefore been reduced. Eg Cl- has oxidation state -1.

Question 4

Some elements have the same oxidation state in all their compounds, however others may have different ones.

Give examples of the rules for..
. Hydrogen
. Group 1 and 2
. Aluminium
. Oxygen
. Flourine and chlorine

Answer 4

. Hydrogen is +1 except in metal hydrides eg NaH where it is -2

. Group 1 and 2 always have +1 for g1 and +2 for g2

. Aluminium always has a +3 charge

. Oxygen always has -2 except in peroxides where it is -1, and in the compound 0F2 where it is +2

. Fluorine is always -1
. Chlorine is -1 except for in compounds with F and 0 where it has positive values

Question 5

Find the oxidation states in

Phosphorus pentachloride PCL5

Answer 5

Cl has a 1- charge and there are 5 of them so that mens there is a -5 charge of chlorine so there must be a +5 chage of phosphorus so the sum of oxidation states is zero

Question 6

What are the oxidation states in nitric acid

HNO3

Answer 6

Oxygen will have a -2 charge and there are 3, so a -6 charge is created, whilst hydrogen has a 1+ charge which makes the sum -5 so far which means nitrogen must be +5.

Nitrogen has a positive oxidation state because it is combined with a more electronegative element, oxygen

Question 7

What are the oxidation states in black copper oxide

Cu2O

Answer 7

Oxygen has state -2, so two copper ions must equal +2 which means each copper is +1

Question 8

What do the roman numerals mean in compounds eg in CuO which is called copper(ii) oxide and Cu2O which is called copper (i) oxide

Answer 8

The roman numerals refer to the oxidation state, so in copper (ii) oxide, the copper has charge +2

Question 9

What is a superoxide

Answer 9

An unusual oxidation state