ligand substitution reactions

Question 1

define ligand substitution reaction

Answer 1

a reaction where the ligands around a complex ion are substituted by other ligands

Question 2

what happens when a ligand substitution reaction is complete with no change in coordination number

Answer 2

all ligands are substituted by new ones. coordination number is unchanged because number of coordinate bonds is the same

Question 3

1. Write an equation for the reaction between hexaaqua cobalt (II) ions with an
   excess of ammonia (NH3)

(all water ligands are substituted)

2. The complex formed turns brown when left in the air for a period of time,
   why?

Answer 3

1. [Co(H2O)6]2+ + 6NH3 ⎯→ [Co(NH3)6]2+ + 6H2O
2. Cobalt (II) oxidises to cobalt (III) due to oxygen in the air [Co(NH3)6]3+

Question 4

what happens when a ligand substitution reaction is complete with change in coordination number

Answer 4

all ligands are substituted by new ones

often new ligands are a different size so can accommodate different number of ligands

Question 5

1. HCl produces Cl- ions in solution that can act as ligands

Write an equation for the reaction of [Cu(H2O)6]2+ with concentrated HCl

2. What has happened to the co-ordination number? Why?

Answer 5

1. [Cu(H2O)6]2+ + 4Cl- ⎯→ [CuCl4]2- + 6H2O
2. Co-ordination number has decreased (from 6 to 4), because Cl- ligands are
   larger and only 4 can fit around Cu2+

Question 6

what happens when a ligand substitution reaction is incomplete with no change in coordination number

Answer 6

only some of the ligands are substituted, but overall the number of co ordinate bonds is the same

Question 7

Write an equation for the reaction of hexaaqua copper (II) ions with an excess
of ammonia

Answer 7

[Cu(H2O)6]2+ + 4NH3 ⎯→ [Cu(H2O)2(NH3)4]2+ + 4H2O

Co-ordination number unchanged

Incomplete substitution

Question 8

what is the chelate effect

Answer 8

multidentate ligands form thermodynamically stable complexes

Question 9

when [Cu(H20)6] 2+ reacts with excess NH3

1. what product is formed
2. what is the colour change

Answer 9

1. [Cu(H2O)2(NH3)4]2+
2. blue to royal blue

Question 10

when [Co(H2O)6]2+ is reacted with excess NH3

1. what product is formed
2. what is the colour change

Answer 10

1. [Co(NH3)6]2+
2. red to red brown
   dark brown when left in air

Question 11

when [Cu(H2O)6]2+ reacts with concentrated HCl

1. what product is formed
2. what is the colour change

Answer 11

1. [CuCl4]2-
2. blue to green

Question 12

when [Co(H2O)6]2+ reacts with concentrated HCl

1. what product is formed
2. what is the colour change

Answer 12

1. [CoCl4]2-
2. red to blue

Question 13

when [Fe(H2O)6]3+ reacts with concentrated HCl

1. what product is formed
2. what is the colour change

Answer 13

1. [FeCl4]-
2. brown to yellow

Question 14

Write an equation for Gibbs’ Free-Energy

Give the meaning of each term

What is the significance of the value for ΔG?

Answer 14

ΔG = ΔH - T ΔS

ΔG = Gibbs’ Free-energy. If less than 0, reaction feasible

ΔH = enthalpy change

T = temperature

ΔS = entropy change

Question 15

Write an equation for the reaction of [Cu(H2O)6]2+ with EDTA4-

Answer 15

[Cu(H2O)6]2+ + EDTA4- ⎯→ [Cu(EDTA)]2- + 6H2O

Question 16

[Cu(H2O)6]2+ + EDTA4- ⎯→ [Cu(EDTA)]2- + 6H2O

1a. Explain why the entropy change for this reaction is very high

1b. Explain why the enthalpy change for this reaction is almost 0 (very small)

2. What does this mean about ΔG?

Answer 16

2 species in reactants produce 7 species in the product, therefore ΔS is very
positive

6 Cu-O bonds broken, 2 Cu-N and 4 Cu-O bonds made. They have a similar
bond energy so ΔH is very small

ΔG is very negative. There is a strong thermodynamic stability to the product
(thermodynamic driving force). This is called the Chelate effect