Transition Metals

What is a transition metal?

An element that has an incomplete d sub-level in either the atom or one of the common ions

Give 4 properties of transition metals.

They form complexes

They form coloured ions

Variable oxidation states

Good catalysts

What is a complex?

A molecule consisting of a central metal ion with ligands bonded to it

What is meant by a ligand?

A species that has a lone pair of electrons and can co-ordinately bond to a metal ion

What is a unidentate/monodentate ligand?

A ligand that only forms one coordinate bond with a metal ion

Give some common monodentate ligands.

Cl^-, H₂O,NH₃

What is a bidentate ligand?

A ligand that forms 2 coordinate bonds to a metal ion

Give the names and structures of the 2 required bidentate ligands.

ethane-1,2-diamine

ethanedioate ion

What is a multidentate ligand?

A ligand that forms more than 2 coordinate bonds with a metal ion

Name 2 multidentate ligands.

EDTA and porphyrin

How many co-ordinate bonds does EDTA form?

6

How many co-ordinate bonds does porphyrin form?

4

What is the co-ordination number of a linear complex?

2

What is the bond angle in a linear complex?

180*

When are linear complexes most commonly formed?

In silver (I) complexes

What is the co-ordination number of a square planar complex?

4

What is the bond angle in a square planar complex?

90*

When are square planar complexes formed?

With Pt²⁺ and Ni²⁺ ions

What is the co-ordination number of a tetrahedral complex?

4

What is the bond angle in a tetrahedral complex?

109.5*

When are tetrahedral complexes formed?

With larger ligands like Cl^- ions

What is the co-ordination number in an octahedral complex?

6

What is the bond angle in an octahedral complex?

90*

When are octahedral complexes formed?

With smaller ligands

What is the purpose of haemoglobin?

To transport oxygen around the body

Why is carbon monoxide toxic to humans?

It bonds more strongly to the Fe²⁺ ion in haemoglobin than oxygen, preventing the transport of oxygen around the body

In what complex shapes does cis-trans isomerism occur?

Octahedral and Square planar

Draw the structure of cisplatin.

Draw the structure of transplatin.

Draw the cis and trans isomers of an octahedral complex?

With what complex shape does optical isomerism occur?

Octahedral

When does octahedral isomerism occur in octahedral complexes?

When there are 3 bidentate ligands

Draw 2 optical isomers of an octahedral complex?

How are the d-orbitals arranged in a free transition metal ion?

They are degenerate (have the same energy)

What happens to the d orbitals in a transition metal ion when ligands are attached?

They split so some are at a higher energy than others (normally a 2-3 split)

Explain why we see coloured compounds in transition metal compounds?

Electrons in the lower energy d sub-level absorb light of a specific frequency and are excited

All other frequencies of light are reflected (which we see)

Why is visible light absorbed by d-electrons?

The energy gap between the levels is in the visible light region

Give the equation for the energy absorbed by electrons to excite them.

Energy = h x frequency

Give the value of planck’s constant.

6.63 × 10^-34

What factors effect the colour of a transition metal compound?

The type of metal

The oxidation state of the metal

The ligands

The co-ordination number/ shape of the complex

What is colorimetry?

An analytical technique that uses the absorption of visible light to determine the concentration of a substance (by measuring absorbance)

What is the relationship between concentration and absorption?

They are directly proportional

How can a calibration curve be used to determine the concentration of a unknown substance?

Find the absorption value on the graph and connect to the corresponding concentration

Why do transition metals have variable oxidation states?

They can lose both s and d electrons

Give the 3 main oxidation states of manganese.

MnO₄^- (Mn = +7)

MnO₂ (Mn = +4)

Mn²⁺

In what compound is the manganate ion commonly found?

KMnO₄

What is the standard state of manganese oxide (MnO₂)?

solid

What colour is the Mn²⁺ ion?

colourless

Give some properties of potassium (per) manganate.

Purple liquid

Strong oxidising agent in acidic conditions

Why are most transition metal redox titrations self-indicating?

There is a significant colour change when the metal ion changes oxidation state

Why is H₂SO₄ commonly used to provide the acidic conditions for a redox titration?

It provides sufficient H+ ions and does not react with the reactants or products

What is always the role of manganate ions in a redox titration?

Oxidising agent

What is the role of Fe²⁺ ions in a redox titration?

Reducing agent

Why is ethanoic acid not used as the acid for redox titrations?

It does not supply enough H+ ions as it is a weak acid

Give the colour change in a redox reaction between Fe²⁺ and MnO₄^- ions.

Colourless to (permanent) pink

Give an overall equation for a redox titration between Fe²⁺ and MnO₄^- ions and state the molar ratio of Fe:MnO₄.

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

5:1

Write an equation for the redox reaction between MnO₄^- and C₂O₄^2- ions and give the molar ratio for C₂O₄^2-:MnO₄^-.

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

5:2

What is the molar ratio when both types of reducing ions are involved in the titration?

5:3

Why are transition metals good catalysts?

They have variable oxidation states, which allow them to form intermediates

They provide a surface for reactants to adsorb

State the 2 types of catalysts?

Homogeneous and heterogeneous

What is a homogeneous catalyst?

Where it is the same state as the reactants

What is a heterogeneous catalyst?

Where it is a different state to the catalyst

Name the catalyst used in the contact process.

Vanadium (V) oxide

Is the contact process heterogeneous or homogeneous?

Heterogeneous

What is the overall equation for the contact process?

SO₂(g) + 1/2O₂ —> SO₃(g)

Give the 2 steps in the catalysed route of the contact process.

1. V₂O₅ + SO₂ —> SO_{3 +} V₂O₄

2. V₂O₄ + 1/2O₂ —> V₂O₅

Describe the general process for heterogenous catalysis.

1. Reactants adsorb to active sites

2. Bonds weaken in reactant molecules and they are brought closer together in the right orientation

3. The reaction occurs

4. Products desorb from the surface

What happens when the adsorption is too strong?

The product cannot desorb from the surface and reactants cannot move around

What happens when the adsorption of the catalyst is too strong?

The reactants cannot be adsorbed

What is catalytic poisoning?

When certain substances block active sites in the catalyst, slowing the rate of reaction

Give the overall equation for the reduction of peroxydisulfate.

S₂O₈^2- + 2I^- —> I₂ + 2SO₄^2-

Give the 2 half equations involved in the reduction of peroxydisulfate.

2I^- —> I₂ + 2e-

S₂O₈^2- + 2e- —> 2SO₄^2-

Why is the Ea very high for the reaction between peroxydisulfate and iodide?

They are both negative ions, so there is repulsion between them

Why are Fe²⁺ and Fe³⁺ good catalysts for the reaction between peroxydisulfate and iodide?

They are positive, so attract the negative ions, and they have variable oxidation states, allowing them to form intermediates

Give the 2 equations for the alternative catalysed route for the reaction between peroxydisulfate and iodide?

2Fe³⁺ + 2I^- —> I₂ + 2Fe²⁺

2Fe²⁺ + S₂O₈^2- —> 2Fe³⁺ + 2SO₄^2-