Chemistry - 3.2.5: Transition Metals

4 characteristics of transition metals

1. Complex formation

2. Formation of coloured ions

3. Variable oxidation state

4. Catalytic activity

With the transition elements, where are electrons lost from first when ions are formed?

4s orbital

Transition element

An element that forms at least one stable ion with an incomplete d-sub level of electrons

Why are scandium and zinc not transition elements?

They do not form ions with partly full d-shells (Sc³⁺ = 3d⁰, Zn²⁺ = 3d⁰)

Complex

A central metal atom or ion surrounded by ligands

Ligand

A molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons - they have lone pairs of electrons

Co-ordination number

The number of co-ordinate bonds to the central metal atom or ion

Possible shapes of complexes

Octahedral if co-ordination number = 6

Tetrahedral or Square planar if co-ordination number = 4

Linear if co-ordination number = 2

Aqua ions

Ions that form when the salt of a transition metal is dissolved in water, so the positively charged metal ion is surrounded by (usually 6) water molecules acting as ligands

Multidentate ligands

Molecules or ions that have more than one atom with a lone pair of electrons which can bond to a transition metal ion

3 examples of monodentate ligands

H₂O, NH₃ and Cl^-

Monodentate ligand

A molecule or ion that has one atom with a lone pair of electrons

2 examples of bidentate ligands

Ethane-1,2-diamine (H₂NCH₂CH₂NH₂) or ethanedioate ion C₂O₄^2-

Ethane-1,2-diamine abbreviation

en

Give an example of a mutlidentate ligand

EDTA^4-

Chelate

A complex ion with multidentate ligands

What are chelates used for?

To remove d-block metal ions from solution

The chelate effect

An entropically-driven process whereby multidentate ligands replace monodentate ligands in complexes

Why does the chelate effect take place? (3)

In a ligand substitution reaction,
- Enthalpy change is negligible (same number of bonds broken and formed)
- Entropy change is positive (more molecules in products)

So as ΔG = ΔH - TΔS,
- ΔG decreases and the reaction becomes more feasible

Compare the NH₃ and H₂O ligands (2)

Similar in size and uncharged

What happens to the co-ordination number when NH₃ and H₂O ligands are exchanged?

The co-ordination number does not change^-

Compare the Cl^-, NH₃ and H₂O ligands and explain the impact on the co-ordination number

Cl^- is larger than the other two (as uncharged) so fewer ligands can fit around the central metal ion - this means it has a co-ordination number of 4 rather than 6

What is haem?

An iron (II) complex with a multidentate ligand - has co-ordination number of 6.

What is haemoglobin’s function and how does it carry it out?

Transports oxygen around the body as oxygen forms a co-ordinate bond to the Fe²⁺ in haem

Why is carbon monoxide toxic?

It replaces the oxygen co-ordinately bonded to the Fe (II) in haemoglobin and binds more strongly to it

Describe the structure of haem

4 of the co-ordination sites are taken up by porphyrin (a tetradentate ligand), and another is taken up by a nitrogen of another protein

Give an example of a linear complex

Tollens’ reagent - [Ag(NH₃)₂]⁺

What type of isomerism can transition metal complexes form?

Geometrical isomers and optical isomers

What isomerism can octahedral complexes form?

Cis-trans isomerism with monodentate ligands and optical isomerism with bidentate ligands

With complexes, how do you determine which one is the cis isomer and which one is the trans isomer?

Cis = the ligands are next to each other
Trans = the ligands are on opposite sides of the central metal ion

Alternate name for cis isomer

Z isomer

Alternate name for trans isomer

E isomer

What isomerism can square planar complexes form?

Cis-trans isomerism

Cisplatin structure

How does colour arise?

Some of the wavelengths of visible light are transmitted / reflected into the eyes, while the remaining wavelengths are absorbed

Why are transition metal complexes coloured? (3)

1. The complex absorbs some wavelengths of light energy

2. This causes the d electrons move from a ground state to an excited state (i.e. to a higher energy level)

3. You therefore see the remaining wavelengths of light

What is the frequency of light related to? (2)

The energy difference between the d electron’s ground and excited state as well as the colour of light

What frequency of light is violet?

High frequency

What frequency of light is red?

Low frequency

What are the equations for the energy difference between the d electron’s ground and excited state?

ΔE = hv
ΔE = energy difference, h = Planck’s constant, v = frequency

OR

ΔE = hc/λ
ΔE = energy difference, h = Planck’s constant, c = speed of light, λ = wavelength

What does the colour of a transition metal complex depend on?

The energy gap ΔE

What does the energy difference (ΔE) depend on? (3)

1. The oxidation state of the metal ion

2. The co-ordination number

3. The ligand

What happens if the energy difference (ΔE) changes?

The colour of the complex changes

How does a simple colorimeter work?

It uses a light source and a detector to measure the amount of light of a particular wavelength that passes through a coloured solution

In colorimetry, what happens as the concentration of the solution increases?

The amount of light transmitted through the solution decreases

How can you use colorimetry to determine the concentration of an unknown solution? (3)

1. Make some known concentrations of the coloured solution and measure the absorbance of each one with a colorimeter

2. Plot a graph of absorbance against concentration

   1. Read unknown concentration from graph

Why can co-ordinate bonds form between transition metal ions and ligands? (2)

The ligand donates a lone pair - the transition metal ion accepts it

Write out the reactions involved in the redox titration of Fe²⁺ by MnO₄^- (3)

MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O
Fe²⁺ → Fe³⁺ + e^-

MnO₄^- + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O

How do you find the concentration of an oxidising / reducing agent?

By carrying out a redox titration

Ratio of Fe²⁺ to MnO₄^-?

5:1

Describe what happens in the redox titration of Fe²⁺ by MnO₄^- (3)

1. Add iron (II) sulfate tablets dissolved in sulfuric acid into a conical flask

2. Using a burette, gradually add potassium manganate solution (VII)

3. Stop when the colour of the mixture changes from colourless to pink

In the redox titration of Fe²⁺, why can’t you use hydrochloric acid instead of sulfuric acid?

Hydrochloric acid contains Cl- ions, which would also be oxidised by the MnO₄^- ions - the MnO₄^- ions should only be used to oxidise Fe²⁺ ions

In the redox titration of Fe²⁺, why is sulfuric acid used?

MnO₄^- ions do not oxidise sulfate ions

What affects the redox potential of a transition metal ion? (2)

The pH and the ligand

What happens if transition metal ions are in acidic solution?

They are reduced

How are transition metal ions with low oxidation states kept and why?

In acid solution to stabilise them against oxidation by air

What happens if transition metal ions are in alkaline solution and why?

Lower oxidation states of transition metal ions tend to be oxidised - because the solution is alkaline, there is a tendency to form negative ions and it is easier to lose electrons from negatively charged species than positively charged or neutral ones

What substitution occurs when transition metal ions are in alkaline solution?

One H₂O ligand is replaced by an OH^- for each positive charge of the metal ion

How are vanadium species in oxidation states IV, III and II formed?

By the reduction of vanadate (IV) ions by zinc in acidic solution

Write out the equations for the formation of each vanadium species with different oxidation states + the reduction of zinc. (4)

Zn → Zn²⁺ + 2e^-

VO₂⁺ + 2H⁺ + e^- → VO²⁺ + H₂O

VO²⁺ + 2H⁺ + e^- → V³⁺ + H₂O

V³⁺ + e^- → V²⁺

How is Tollens’ reagent used to distinguish between aldehydes and ketones?

Aldehydes reduce [Ag(NH₃)₂]⁺ to metallic silver:

[Ag(NH₃)₂]⁺ + e^- → Ag + 2NH₃

Write out the reactions involved in the redox titration of C₂O₄^2- by MnO₄^- (3)

MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O
C₂O₄^2- → 2CO₂ + 2e^-

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

Ratio of C₂O₄^2- to MnO₄^-?

5:2

2 types of catalysts

Heterogeneous and homogeneous

Heterogeneous catalysts

Catalysts that are in a different phase from the reactants

How do heterogeneous catalysts work? (3)

1. Normally, they’re solids and the reaction takes place on the solid surface

2. Reactants pass over the surface

3. The reaction takes place on active sites on the surface of the catalyst

How can heterogenous catalysts be made more efficient? (2)

1. Increasing their surface area

2. Spreading the catalyst onto an inert support medium to increase the surface-to-mass ratio

Why do heterogenous catalysts not last forever? (2)

1. Poisoning - the surfaces may become covered with unwanted impurities, blocking the catalsyt’s active sites

2. The catalyst may gradually be lost from the support medium

Give two examples of heterogeneous catalysts

1. Fe (s) in the Haber process

2. V₂O₅ in the contact process

How does V₂O₅ act as a catalyst in the contact process?

SO₂ + V₂O₅ → SO₃ + V₂O₄ (or 2VO₂)

2V₂O₄ + O₂ → 2V₂O₅

What is the contact process? Include the overall equation.

A process used to produce sulfuric acid:

2SO₂ + O₂ ⇌ 2SO₃

Homogeneous catalyst

A catalyst that is in the same phase as the reactants

Give an example of a homogeneous catalyst

Fe²⁺ in the reaction between I^- and S₂O₈^2-

How does Fe²⁺ act as a catalyst in the reaction between I^- and S₂O₈^2-? (2)

S₂O₈^2- + 2Fe²⁺ → 2SO₄^2- + 2Fe³⁺

2Fe³⁺ + 2I^- → 2Fe²⁺ +I₂

What is the overall equation in the reaction between I^- and S₂O₈^2-?

S₂O₈^2- + 2I^- → 2SO₄^2- + I₂

Why is a catalyst needed in the reaction between I^- and S₂O₈^2-? (3)

• Both the ions are negatively charged so would repel each other

• So there is a high activation energy

• The Fe ions are positively charged, so can attract each ion

Autocatalysis

When one of the products of the reaction is a catalyst for the reaction

Give an example of an autocatalysed reaction

The oxidation of ethanedioic acid by manganate (VII) ions

In redox reactions, what happens to Cr₂O₇^2- ions?

They are reduced to Cr³⁺

In redox reactions, what happens to Fe²⁺ ions?

They are oxidised to Fe³⁺

Why is a catalyst needed in the reaction between ethanedioic acid and acidified potassium dichromate solution? (3)

1. There is no/little catalyst at the start

2. There are two negative ions, which repel

3. So the reaction has a high activation energy

Sketch and describe a graph of how one of the reactants of an autocatalysed reaction (e.g. MnO₄^-) varies with time (2)

1. The slope increases as the catalyst forms

2. Then, the rate decreases as the MnO₄^- ions are used up

Why can transition metal ions be used as catalysts? (3)

1. They can exist in variable oxidation states

2. Include equations showing how each oxidation state is formed

3. This then lowers the activation energy (state effect)

How is the risk associated with the use of cisplatin as a drug minimised?

By being used in small amounts

How does the ligand / coordination number / oxidation state affect the colour of a complex? (3)

1. Different energies of d-electrons

2. Different wavelengths of light absorbed by the d-electrons

3. Different wavelengths of light transmitted

When carrying out colorimetry, why must the container for each sample have the same dimensions?

Absorption depends on the path length / distance travelled through solution

When carrying out colorimetry, why is a coloured filter used?

To select the wavelength most absorbed by the sample

Why might you use colorimetry instead of a titration? (2)

Quicker to analyse extracted samples than by titration / uses smaller volumes of solution

Draw an ethanedioate ion

Give 2 metal ions that do not change coordination number when in ligand substitution reactions with neutral ligands

Co²⁺ and Cu²⁺

Give 3 metal ions that change coordination number when in ligand substitution reactions where neutral ligands are replaced by charged ones (e.g. Cl^-)

Co²⁺, Cu²⁺ and Fe³⁺

Why are Cu / Cr complexes not coloured? (2)

1. It has a full d orbital

2. So cannot absorb frequencies of visible lights

In a ligand substitution reaction, why is the enthalpy change close to 0? (2)

1. The bonds being broken and formed are similar, so have similar enthalpy

2. The same number of bonds are being broken and made.

What process does cisplatin inhibit during cell division?

DNA replication