5: Coloured Ions, Variable Oxidation States and Redox Titrations

Question 1

What happens to energy of d orbitals when ligands bond to the transition metal ion?

Answer 1

Orbitals no longer have the same energy, so some become higher in energy and others drop

Question 2

Explain how transition metal ions display colour.

Answer 2

Energy is absorbed by electrons, exciting them to a higher energy level in d orbitals, and this energy is seen as visible light. The wavelength reflected is seen as visible light and other wavelengths are absorbed.

Question 3

Explain how different transition metal complexes display different colours.

Answer 3

They absorb all wavelengths of light except the colour that you can see, which is being reflected.

Question 4

Give an equation linking Planck’s constant, speed of light and energy.
Give units

Answer 4

Energy = h x f

h = Planck’s constant (J s-1)
f = frequency of light absorbed (Hz)

Question 5

Give an equation linking energy, Planck’s constant, speed of light and wavelength.

Answer 5

Energy = Planck’s x speed of light / wavelength

Question 6

Why do scandium and zinc appear as white?

Answer 6

They are not true transition metals, therefore their d orbital cannot split.

Question 7

State 3 ways in which a transition metal complex can be changed to alter its colour.

Answer 7

Change coordination number
Change oxidation state
Change ligand

Question 8

Explain why aqueous solutions containing [CuCl4)2- ions are yellow.

Answer 8

They reflect yellow light and absorb all other wavelengths of visible light.

Question 9

Give an ionic equation for the conversion of [CuCl4]2- to an unknown complex ion using excess water.

Answer 9

[CuCl4]2– + 6H2O → [Cu(H2O)6]2+ + 4Cl–

Question 10

Give an ionic equation for the conversion of [CuCl4]2- to an unknown compound using excess ammonia and water.

Answer 10

[CuCl4]2– + 4NH3 + 2H2O → [Cu(NH3)4(H2O)2]2+ + 4Cl–

Question 11

Give 3 uses for variable oxidation states.

Answer 11

Testing for organic functional groups
Redox titrations
Catalysis

Question 12

Explain the change in oxidation state that occurs in a positive test for aldehydes using Tollens.

Answer 12

[Ag(NH3)2]+ reduces so silver has an oxidation state of 0.

Question 13

Give 2 things that effect how easy it is to change the oxidation state of transition metals.

Answer 13

pH and the ligands present

Question 14

Will transition metals oxidise better in acidic or alkaline conditions?

Answer 14

Alkaline

Question 15

Will transition metals reduce better in acidic or alkaline conditions?

Answer 15

Acidic

Question 16

Give the reagent and conditions needed to reduce VO2+

Answer 16

Acidic, Zinc metal

Question 17

Give an equation for the reduction of 2VO2 + into 2VO2+
Give the oxidation state of 2VO2+ and 2VO 2+

Answer 17

2VO2 + + Zn + 4H+ -> 2VO 2+ + Zn2+ +2H2O
Oxidation state of 2VO2+ is +5
Oxidation state of 2VO 2+ is +4

Question 18

Give a redox equation for the reduction of 2VO 2+ into 2V 3+
Give the oxidation state of V3+

Answer 18

2VO 2+ + Zn + 4H+ -> 2V3+ + Zn2+ + 2H2O
Oxidation state of V3+ - +3

Question 19

Give an ionic equation for the conversion of 2V3+ into 2V2+
Give the oxidation state of 2V2+

Answer 19

2V3+ _ Zn -> 2V2+ + Zn2+
Oxidation state of 2V2+ is 2

Question 20

Why do transition metal titrations not always require an indicator?

Answer 20

When some transition metals get oxidised/reduced, they change colour which shows the end point of the titration.

Question 21

Give the redox equation for a transition metal titration of oxidising agent MnO4- with reducing agent Fe2+ to produce Mn2+ and Fe3+

Answer 21

MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 4H2O + 5Fe3+

Question 22

Give the redox equation for a transition metal titration of oxidising agent MnO4- with reducing agent C2O4 2- to produce Mn2+ and CO2

Answer 22

2MnO4- + 16H+ + 5C2O4 2- -> 2Mn2+ + 8H2O + 10CO2

Question 23

What is the reacting ratio for Fe2+:MnO4-?

Answer 23

5:1

Question 24

What is the reacting ratio for C2O4 2-:MnO4-?

Answer 24

2.5:1

Question 25

Describe the colour change seen when MnO4- reacts with Mn2+.

Answer 25

Purple to pink.

Question 26

Describe how a calibration graph can be produced and used to find he concentration of the iron (III) complex.

Answer 26

Measure absorbance for a range of concentrations using a colourimeter
Plot graph for absorbance against conc
Read value for conc measured against absorbance from the graph.

Question 27

When Na2C2O4(aq) is added to a solution containing [Fe(H2O)6]
3+ ions, a reaction
occurs in which all six water ligands are replaced by ethanedioate ions.
Explain why the replacement of the water ligands by ethanedioate ions is favourable.
In your answer refer to:
* the enthalpy and entropy changes for the reaction
* how the enthalpy and entropy changes influence the free-energy change for the
reaction.

Answer 27

Stage 1 - Enthalpy
1a Delta H negligible
1b make & break same number of bonds
1c make & break same type of bonds /
bonds have similar enthalpies
Stage 2 - Entropy
2a increase in entropy
2b increase in particles in solution / from 4 to
7 particles
Stage 3 - Gibbs Free Energy
3a Delta G = Delta H –TdeltaS
3b Delta G negative for forward reaction
3c correct discussion of why Delta G is negative
based on Delta H and TdeltaS