23.4 Variable oxidation states of transition elements

Question 1

Why can transition metals have a greater variety of oxidation states in different compounds

Answer 1

. A typical transition metal can use its 3d electrons as well as its 4s electrons in bonding.

This can be compared to eg group 2 which can only lose its outer 2 electrons to become a 2+ charge

Question 2

Give the pattern for the oxidation states of transition metals

Answer 2

. All transition elements form +1 and +2 oxidation states

Only the lower states of transition metals actually exist as simple ions
Eg Mn2+ ions exist but not Mn7+ which is found in the (MnO4)- ion

Question 3

Give the electron configuration of nickel and Ni2+ and explain reasoning for this

Answer 3

Ni: 1s2 2s2 2p6 3s2 3p6 4s2 3d8

Ni2+ : 1s2, 2s2, 2p6, 3s2, 3p6, 3d8

So when the 2 electrons are lost, the 4s sub shell always empties before the 3d because once full it has more energy than the 3d shell so places itself above the 3d shell.

Question 4

Give electron configuration of iron and Fe2+

Answer 4

Fe: 1s2 2s2 2p6 3s2 3p6 3d6 4s2

Fe2+: 1s2 2s2 2p6 3s2 3p6 3d6

Question 5

Lots of the reactions in transition metal chemistry are redox reactions where the metals are either oxidised or reduced

Give the example of irons two stable oxidation states: Which one is more stable

Give reaction of Fe2+ with chlorine and its products

Answer 5

Fe2+ and Fe3+

. Fe3+ is more stable, eg Fe2+ is oxidised by oxygen in the air
. It can also happen by chlorine:

Fe2+(aq) + Cl2(g) —> 2Fe3+ + 2Cl-

Here, chlorine is the oxidising agent as its oxidation number drops from 0 to -1 as is gains an electron, whilst Fe2+ loses an electron .

Question 6

How does potassium manganate (VII) (KMnO4) act in acidic solution

Answer 6

It acts as an oxidising agent in solution that containins H+ ions

. This is because it is oxidation state +7 so has lost all its valence electrons so will try and attract them back from other species, to reduce itself.

So as a result will oxidise Fe2+ to Fe3+

During this reaction, the oxidation state of the manganese drops from +7 to +2

Question 7

How would you write the equation for kMnO4 and Fe2+

Answer 7

.First do half equation for MnO4- to Mn2+

MnO4-(aq) + 8H+ +5e- –>Mn2+ + 4H2O

.Then do half equation for Fe2+ to Fe3+

Fe2+ —> Fe3+ + e-
5Fe2+ –> 5Fe3+ + 5e-

Then multiply the Fe equation by 5 to balance the number of electrons so they can cancel out

Combine the equations by putting everything on the left side of arrow of both equations on the left and doing the same for the right. The charged electrons will cancel out.

MnO4- + 8H+ + 5fe2+ –>
Mn2+ + 4H2O + 5Fe3+

Question 8

What are redox titrations

Answer 8

If you wish to measure the concentration of an oxidation or reducing agent , it is similar to an acid base titration

Question 9

What colour is Fe2+ ion
What colour is Fe3+ ion

Answer 9

Fe2+ is pale green
Fe3+ is pale violet

Question 10

What colour is MnO4- ion

What colour is Mn2+ ion

Answer 10

Intense purple

Pale pink

Question 11

How would you do the redox titration between FeSO4 and KMnO4

What are the colour changes

Answer 11

Using a burette, add the KMnO4 (aq) to a solution containing Fe2+ ions which is already acidified with sulphuric acid

The purple colour disappears as the MnO4- ions are converted to Mn2+ which is pale pink.
Once enough MnO4- ions have been added to react with Fe2+ ions, one more drop of KMnO4- will turn the solution purple

This is the end point of the titration

Question 12

Why can’t you use HCl instead of sulphuric acid in the KMnO4 and FeSO4 reaction

Answer 12

. HCl contains Cl- ions which are oxidised by MnO4- ions

This would affect the titration because the manganate ions should only be used to oxidise Fe2+ ions.

Question 13

How can you use potassium dichromate (VI) for titrations
K2(Cr2O7)

Do the equations for it with iron

Answer 13

. It can also titrate against Fe2+ ions as it contains (Cr2O7)2- ions
All aqueous

(Cr2O7)2- + 14H+ 6e- —> 2Cr3+ + 7H2O

Fe2+ —> Fe3+ + e- multiply by 6
6fe2+ —> 6Fe3+ + 6e-

Combine:
(Cr2O7)2- + 14H+ + 6Fe2+ —>
2Cr3+ + 7H2O + 6Fe3+

Question 14

What ions do potassium dichromate turn into when reduced by the iron ions

Answer 14

Cr3+ ions

Question 15

Why is sulphuric acid added to these titration reactions

Answer 15

To provide the H+ ions required

Question 16

What indicator is used for the potassium dichromate and iron titration

Answer 16

sodium diphenylaminesulfonate

Turns from colourless to purple at end point

Question 17

How does the PH of a solution affect which oxidation state of a metal is reduced

Answer 17

A metal with a high oxidation state is reduced in acidic solution.
So gains electrons

A metal with a low oxidation state of a transition metal occurs in more alkaline solutions

Question 18

Why do transition metals with low oxidation states get oxidised in alkaline solution more

Why do transition metals with high oxidation states get reduced in acidic solution

Answer 18

. There is a tendency to form negative ions in alkaline solution
Because oxidation is the loss of electrons, so this is easier from negatively charged species than for positively charged or neutral ones

. Because they are electron deficient so it is more easy for them to gain electrons

Question 19

What would the typical transition metal species be in acid solution

M represents transition metals

Answer 19

M(H2O)6 2+ as it is positively charged

Question 20

What would the typical transition metal species be in neutral solution

M represents transition metals

Answer 20

M(H2O)4(OH)2 as it is neutral

Question 21

What would the typical transition metal species be in alkaline solution

M represents transition metals

Answer 21

M(H2O)2(OH)4 2- as it is negative

Question 22

How would you oxidise a transition metal to a high oxidation state

Answer 22

. Add an alkali
Then add an oxidising agent