Substitution Reactions

Question 1

Ligand Substitution or Exchange Reaction

What does this usually cause?

Answer 1

Where one ligand can be swapped for another.

A colour change

Question 2

Effect on the complex ion if the ligands are of similar size and same charge

Answer 2

The co-ordination number of the complex ion doesn’t change, and neither does the shape

Question 3

Colour and shape change in cobalt from H2O to NH3 ligands

What happens if the ammonia isn’t in excess?

Give formula?

Answer 3

Octahedral pink in H2O

to

Octahedral deep blue in NH3.

A blue precipitate forms.

Cu (OH)2 (H2O)4

Question 4

Effect of complex ion if the ligands are different sizes

Answer 4

There’s a change in:

• Co ordination number
• Change of shape

Question 5

Colour and shape change in cobalt from H2O to Cl ligands

Answer 5

Octahedral pink in H2O

to

Tetrahedral blue in Cl

Question 6

Colour and shape change in copper from H2O to Cl ligands

Answer 6

Octahedral pale blue in H2O

to

Tetrahedral yellow in Cl

Question 7

Colour and shape change in copper from H2O to Cl ligands

Answer 7

Octahedral purple in H2O

to

Tetrahedral yellow

Question 8

When is the change in ligands easy to reverse?

Answer 8

If the new ligands form stronger bonds with the central metal ion than the old ligands did

Question 9

Multidentate ligands and reversibility of substitution reactions

Answer 9

Multidentate ligands form more stable complexes than monodentate ligands.

So the change in ligands is hard to reverse

Question 10

Enthalpy change for a ligand substitution reaction and describe

Answer 10

When ligand substitution/exchange reactions occur, co-ordinate bonds are broken and formed.

The strength of the bonds being broken is often very similar to the strength of the the new bonds being made.

So the enthalpy change for a ligand exchange reaction is usually very small

Question 11

Chelate Effect and Entropy

Answer 11

Explains increase in stability.

When monodentate ligands are substituted with bidentate or multidentate ligands, the number of particles in solution increases.

The more particles, the greater the entropy.

Reactions that result in increases in entropy are more likely to occur

Question 12

Why is it difficult to reverse certain substitution reactions?

Answer 12

Because reversing them would cause a decrease in entropy