Transition Metals

Question 1

Define ‘transition element’

Answer 1

An element that forms at least one stable ion with a part full d-shell of electrons.

Scandium only forms Sc3+; 3d0, and zinc only forms Zn²⁺; 3d10; they are d-block elements but not transition elements.

Question 2

What are the characeteristics of transition metal elements?

Answer 2

• Complex formation: transition elements form complex ions, where a transitional metal ion is surrounded by ions or other molecules (ligands) which bond to the transition element by co-ordinate bonds.
  E.g. [Cu(H₂O)₆]²⁺ is a complex ion formed when copper sulfate dissolves in water.
• Coloured ions: the majority of transition metal ions are coloured,
  e.g. Cu²⁺(aq) is blue.
• Variable oxidation states: transition metals have more than one oxidation state in their compounds:
  e.g. Cu(I) and Cu(II)
  They can therefore take part in many redox reactions.
  So variable as the energy levels of the 4s and 3d subshells are very close to one another; different no. of electrons can be gained/lost using similar amounts of energy.
• Catalytic activity/catalysis: many transition metals and their compounds show catalytic activity:
  e.g. iron is the catalyst in The Haber Process, Vanadium(V) oxide in the Contact Process and manganese(IV) oxide in the decomposition of hydrogen peroxide.

Question 3

What leads to the characteristics of transition metal elements?

Answer 3

All arises from an incomplete d sub-level in atoms or ions; a d-orbital can fit 10 electrons in, thus transition metals must form at least one ion between 1 and 9 electrons in d-orbital.

In general there are two 4s electrons across the period, with electrons being added to the inner 3d sub level.
This explains the overall similarity of the elements.

• Cr prefers to have one electron in each orbital of the 3d subshell and just one in the 4s subshell (more stability).
• Cu prefers to have a full 3d subshell and just one electron in the 4s subshell (more stability).

Question 4

Define the term ‘ligand’

Answer 4

A ligand is an ion/molecule with a lone pair of electrons that forms a co-ordinate bond with a transition metal; donating a pair of electrons to a central metal ion.

Question 5

What is a complex, and what does it involve?

Answer 5

• A complex is a metal ion surrounded by co-ordinately bonded ligands (covalent bond where both electrons in the shared pair come from the same atom; the ligand).

Question 6

What is the coordination number?

Answer 6

The co-ordination number is the number of co-ordinate bonds that are formed with the central metal ion; the number of co-ordinate bonds to ligands that surround the d-block metal.

Question 7

What types of ligands can arise?

Answer 7

• Unidentate ligands; ligands with one lone pair of electrons.
  E.g. H2O, NH3, Cl-
• Bidentate ligands; multidentate ligands with two lone pairs; they can each form two coordinate bonds with a metal ion.
  E.g. NH2CH2CH2NH2 and C2O4^2-
• Multidentate ligands; ligands with more than one lone pair which bonds to a transition metal ion.
  (Dentate; latin for ‘teeth’; can ‘bite the metal ion more than once)
  E.g. EDTA^4- has six lone pairs (hexadentate), forming six coordinate bonds with a metal ion.

Question 8

What type of ligand is the prosthetic group haem, of haemoglobin?

Answer 8

• Haem is an iron(II) complex with a multidentate ligand.
• The central Fe2+ has a coordination number of 6 (hexa-coordinated); 4 of the co-ordination sites are taken up by a ring system called ‘porphyrin’ which acts as a tetradentate ligand.
• The 4 lone pairs come from nitrogen atoms, which form said circle around the Fe2+.
• The molecule and the four nitrogen items together make up the multidentate ligand.
• A globin protein and either oxygen/water bind to the last two sites to form an octahedral structure.

Question 9

What shape complexes do small ligands usually form with a transition metal ion?

Answer 9

• 6 coordinate bonds means an octahedral shape.
• Transition metal ions form octahedral complexes with small ligands:
  e. g. H₂O and NH₃
• The ligands don’t all have to be the same.

Question 10

What shape complexes do larger ligands usually form with a transition metal ion?

Answer 10

• 4 coordinate bonds usually mean a tetrahedral shape.
• Transition metal ions commonly form tetrahedral complexes with larger ligands
  e. g. Cl-
• In a few complexes such as cisplatin, 4 coordinate bonds form a square planar shape.

Question 11

What shape complex does Ag+ form?

Answer 11

• Ag+ commonly forms the linear complex [Ag(NH₃)₂]⁺, as used in Tollens’ reagent.
• 2 coordinate bonds.

Question 12

Why do colour changes arise in transition metal complexes?

Answer 12

• Due to changes in oxidation state, coordination number, and ligand.
• Changes in coordination number always involves a change in ligand too.
• Changes in ligand can cause a colour change even if the oxidation state and coordination number remain the same.

Question 13

How does the colour of the transition metal complexes itself arise?

Answer 13

• Colour arises from electronic transitions from the ground states to excited states:

ΔE = hv

v = frequency of light absorbed
h = Planck’s constant

• In an isolated transitional metal atom, all the d-orbitals are of the same energy.
• But in a compound when ligands bond to the ions, some orbitals are given more energies than others, splitting the d-orbital into two different energy levels.
• Electrons occupy the ground state.
• Jumping to higher orbitals (excited states) requires energy from visible light.
• The amount of energy needed to make electrons jump depends upon the central metal ion and its oxidation state, the ligands and the coordination number, as these affect the size of the energy gap.
• Frequencies are absorbed when electrons jump; the rest are reflected and combine to make the complement colour; the colour you see, the combination of colours that are not absorbed.

Question 14

How does spectrometry aid determining the concentration of coloured ions?

Answer 14

• Spectrometry can be used to find the concentrations of transition metal ions, by measuring how much visible light it absorbs.

1. White light is hone through a filter, which is chosen to only let the colour of light through that is absorbed by the sample.
2. The light then passes through the sample to a colorimeter, which calculates how much light was absorbed by the sample.
3. The more concentrated a coloured solution is, the more light it wil absorb; can use this measurement to work out the concentration of a solution of transitional metal ions by..
4. Producing a calibration graph of known concentrations.

Question 15

Do transition elements show variable oxidation states?

Answer 15

• Yes, it is one of the many reasons why transition metal complexes have such a range of colours.

Question 16

How are Cr3+ and Cr2+ formed?

Answer 16

• Chromium ions are fomed by the reduction of Cr₂O₇^2- in acid solution.
• The dichromate ions are reduced by zinc and dilute acid;

Cr2O72- + 14H+ + 3Zn → 3Zn2+ + 2Cr3+ + 7H2O
orange green

• Zinc reduces Cr3+ further to Cr2+:

2Cr3+ + Zn → Zn2+ + 2Cr2+

• Cr2+ is unstable though so oxidises straight back into Cr3+ in the air; need to use inert atmosphere.

Question 17

What is the oxidation of Co2+ in alkaline solution by H₂O₂?

Answer 17

• 2Co²⁺ + H₂O₂ → 2Co³⁺ + 2OH^-

Question 18

What is the oxidation of Co²⁺ with air in ammoniacal solution?

Answer 18

• Place Co2+ ions in an excess of aqueous ammonia, which causes [Co(NH3)6]3+ to form.
• If there complex ions are left to stand in air, oxygen oxidises them to [Co(NH3)6]3+.

1. [Co(H2O)6]2+
   Add NH3
2. [Co(OH)2] OR [Co(H2O)4(OH)2]
   Add NH3
3. [Co(NH3)6]2+
   Allow to stand in air
4. [Co(NH3)6]3+

Question 19

Why do transition metals and their compounds make for good catalysts?

Answer 19

As they can change oxidation states by gaining or losing electrons within their d orbitals. Thus they can transfer electrons to speed up reactions.

They can acts as both hetergeneous and homogeneous catalysts; different and same phase.

Question 20

What are heterogenous catalysts?

Answer 20

• A heterogenous catalyst is in a different phase from the reactants.
• The reaction occurs at the surface of the heterogeneous catalyst
  e. g. gases are passed over a solid iron catalyst in the Haber Process

Question 21

What does increasing the surface area of a heterogenous catalyst do?

What can be used to increase surface area?

Answer 21

• Increasing the surface area of the catalyst increases the number of molecules that can react at the same time, increasing the rate of reaction.
• The use of a support medium can maximise the potential surface area of a catalyst;
  e. g. catalytic converters contain a ceramic lattice coated with a thin layer of rhodium; the rhodium acts as a catalyst helping to convert the waste gases to less harmful products.

The lattice maximises the sufrace area of the catalyst, making it more effective.
And, it minimises the cost of the catalyst as only a thin coating is needed.

Question 22

How does V₂O₅ acts as a catalyst in the Contact Process?

Answer 22

• Vanadium(IV) oxide oxidises sulfur dioxide to sulfur trioxide and is itself reduced to vanadium(IV) oxide.

SO2 + V2O5 → SO3 + V2O4

• The vanadium(IV) oxide is then oxidised back to vanadium(V) oxide by oxygen:

2V2O4 + O2 → 2V2O5

• Hence V2O5 is regenerated and unchanged.

Question 23

What is used as the catalyst in the manufacture of methanol from carbon monoxide and hydrogen?

Answer 23

Chromium(III) oxide; Cr₂O₃.

Question 24

What is the catalyst in the Haber process?

Answer 24

Iron, Fe.

Question 25

What effect can impurities have on catalysts?

Answer 25

• Catalysts can become poisoned by impurities and consequently have reduced efficiency; this has a cost implication,
  e. g. poisoning by sulfur in the Haber process (methane obtained from natural gas that contains impurities including sulfur), poisoning by lead in catalytic converters (can coat the surface, hence unleaded petrol ONLY)
• Impurities in the reaction may bind to the catalyst’s surface and block reactants from being adsorbed; catalyst poisoning.
• It reduces the surface area of the catalyst availible to reactants, slowing down the reaction.
• Cost increase due to less product being made in a certain time or with a certain amount of energy.
• Catalyst may even need replacing or regenerating; costs money.

Question 26

What is a homogenous catalyst and how does it work?

Answer 26

• A homogenous catalyst is in the same physical state as the reactants, and works by forming an intermediate species.
• The reactants combine with the catalyst to make an intermediate species, which thenr reacts to form the products and reform the catalyst.
• Causes the enthalpy profile diagram for such a reaction to have two humps, corresponding to the two reactions.
• The activation energy required to form the intermediates is lower than that needed to make the products directly from reactants.
• Intermediates formed at the first trough.

Question 27

How does Fe2+ homogenously catalyse the reaction between S2O82- and I-?

Answer 27

• Redox reaction between iodide ions and peroxodisulfate (S2O82-) is very slow as both ions are negatively charged; they repel each other, hence they’re unlikely to collide and react.
• Introducing Fe2+ really speeds it up as each stage of the reaction involves both a positive and a negative ion, so there’s no repulsion.
• Catalyst is regenerated.

Question 28

How does the (homogenous) autocatalysis of Cr2O42- and MnO4- by Mn2+ occur?

Answer 28

• Autocatalysis is when a product catalyses the reaction.
• Mn2+ is a product of the reaction and acts as a catalyst for the reaction, thus as the reactio progresses, the amount of product increases and the reaction speed up.

Question 29

Why is Fe2+ important to the function of haemoglobin?

Answer 29

• Fe2+ in haemoglobin allows oxygen to be carried in the blood; both water and oxygen can bind to the Fe2+ ion as ligands.
• Thus the complex can transport oxygen to where it’s needed, and then swap it for a water molecule.
• If carbon monoxide is inhaled, the haemoglobin swaps its water ligand for a carbon monoxide ligand, forming carboxyhaemoglobin as opposed to oxyhaemoglobin.
• Bad as CO is a strong ligand, doesn’t really exchange with water or oxygen ligands, meaning the haemoglobin can’t transport oxygen any more.
• CO poisoning straves the organs of oxygen; causing headaches, dizzness, unconciousness and even death if it’s not treated.

Question 30

What square-planar complex is used as an anticancer drug?

What are the benefits and risks?

Answer 30

• Cisplatin is a complex of platinum(II) with two chloride ions and two ammonia molecules in a square planar shape.
• Cisplatin is active against a variety of cancers, including lung and bladder cancer, preventing cancer cells from reproducing.
• However, it also prevents normal cells from reproducing, including blood and hair cells, thus causing hair loss, supressing the immune system (hence inreasing the risk of infection).
• Cisplatin may also cause damage to the kidneys.

Question 31

What is the active complex used in Tollens’ reagent?

Answer 31

• [Ag(NH3)2]+ is used in Tollens’ reagent to distinguish between aldehydes and ketones.
• Tollens’ reagent is prepared by adding just enough ammonia solution to silver nitrate solution to form a colourless solution containing the above complex ion.