🔵 TOPIC 15 - Transition Metals

Question 1

Why do transition metals show variable oxidation numbers?

Answer 1

The energy difference between the [ (n-1)d and ns ] orbitals is very small, so the energy required for the different ionisation energies is very similar.

Question 2

What is a ligand? Give some examples.

Answer 2

An atom, ion or molecule that donates a lone pair of electrons to a central metal ion (ie. Forms a dative [coordinate] bond)
For example: H2O, OH-, NH3, Cl-

Question 3

What is a complex ion? Give an example.

Answer 3

A central metal cation surrounded by ligands (may also be just called a ‘complex’ if no overall charge) (formed by dative bonds)
For example: [Cu(H2O)6]^2+

Question 4

What is a transition metal complex?

Answer 4

A transition metal cation OR atom surrounded by ligands

Question 5

What is a coordination number?

Answer 5

The number of dative bonds in the complex - not necessarily the number of ligands (eg. if bidentate/ multidentate ligands involved)
Eg/ octahedral complexes with mondentate ligands have 6-fold coordination

Question 6

What are bidentate/ multidentate ligands?

Answer 6

Ligands which form two (for bidentate) or multiple (for multidentate) dative covalent bonds with the central metal atom - they are able to do this as they have two (or more for multidentate) lone pairs of electrons

Question 7

H2O, NH3 and -OH are all common ligands involved in octahedral complexes. Why do they act as monodentate ligands?

Answer 7

NH3 has one lone pair, H2O has two lone pairs, and -OH has three lone pairs but they each only use one lone pair to form a dative bond due to the size of the ligand and repulsion caused by the electron pairs (they are all approximately the same size)

Question 8

How do the ionic radii of transition metals ions differ from the ionic radii of non-transition metal ions, and what effect does this have?

Answer 8

Transition metal ions tend to have smaller radii than non-transition metal ions in the same period. This enables them to attract ligands (electron-rich species) more strongly, including water molecules present in aqueous solutions (attracted so strongly it forms 6 bonds in a octahedral structure)

Question 9

How would you name these ligands in a complex:
Water (H2O:), hydroxide (-:OH), ammonia (:NH3), chloride (:Cl-), cyanide (:CN-)

Answer 9

H2O —> aqua
OH- —> hydroxo
NH3 —> ammine (Notice NH3 is amMine, but NH2 in amine)
Cl- —> chloro
CN- —> cyano

(Notice that the names for ligands with a negative charge end in -o)

Question 10

Name [Fe(H2O)6]2+

Answer 10

Hexaaquairon (II)

Question 11

Name [Cu(NH3)4(H2O)2]^2+

Answer 11

Tetraamminediaquacopper (II)

Notice that as it is a positively charged complex ion, the metal is just named as it is (if it was negative complex ion, the copper would be cuprate)

Question 12

Name [Al(OH)4]-

Answer 12

Tetrahydroxoaluminate (III)

Notice that as it is a negatively charged complex ion the metal has an -ate ending

Question 13

Name [Fe(CN)6]^4-

Answer 13

Hexacyanoferrate (II)

Question 14

Write the complex ion formula for pentaaquahydroxochromium (III)

Answer 14

[Cr(OH)(H2O)5]^2+

Question 15

Why is there a lack of colour in some aqueous ions and other complex ions?

Answer 15

• the ION may have completely filled (3)d ORBITALS (eg. Zn^2+) - 3d orbitals are still split, just no gaps to promote an electron into, so no visible light is absorbed
  OR
• the ION may have no electrons in their (3)d ORBITALS (eg. Sc^3+) - orbitals are still split, just no electrons to promote, so no visible light is absorbed

Question 16

Explain why aqueous ions and other complex ions are coloured

Answer 16

• the (3)d ORBITALS are split into two levels with slightly different energies
• when ELECTRONS in the lower energy level absorb energy (from the visible light spectrum) they can be promoted to the higher energy level
• the COMPLEMENTARY colour to the colour absorbed is TRANSMITTED (not emitted) (ie. the unabsorbed light) (Eg. If red [low freq., long wavelength] is absorbed, blue is transmitted)

Question 17

What affects the colour of a transition metal?

Answer 17

The colour is determined by the difference in energy between the d-orbitals (the bigger the energy difference, the more energy the electron absorbs, so absorbs light with high frequency and short wavelength). This energy difference between the d-orbitals is changed when there are changes in:
- oxidation number
- ligands (size and type)
- coordination number (no. of dative bonds)
- shape of the complex

(So a transition metal will appear different colours in different complexes)

Question 18

Briefly describe the structure of haemoglobin

Answer 18

• an iron (II) complex (Fe^2+)
• contains a multidentate ligand
• square planar

Question 19

Explain what haemoglobin does in the body, and write some (word) equations showing this

Answer 19

• when blood passes through the lungs, haemoglobin collects oxygen molecules - the oxygen molecule acts as a ligand by using one of its lone pairs to form a dative bond with one of the Fe^2+ ions
• the strength of the dative bond between oxygen and haemoglobin is not very strong, so any CO breathed in is very likely to replace the oxygen already bound to the haemoglobin in a ligand substitution (/exchange) reaction

Haemoglobin + oxygen <—> oxyhaemoglobin (reversible reaction as only a weak bond is formed so easily broken)
Haemoglobin + carbon monoxide —> carboxyhaemoglobin (not reversible as a strong bond formed so hard to break)

Question 20

What are the advantages and disadvantages of the weak dative bond between the oxygen and haemoglobin (oxyhaemoglobin)?

Answer 20

Advantages: oxygen molecule can easily be released when it is needed
Disadvantages: a much stronger dative bond can form between carbon monoxide and the (Fe2+ of the) haemoglobin (carboxyhaemoglobin) (CO can act as a ligand due to lone pair of electrons on its CARBON atom)

Question 21

Why is carbon monoxide toxic?

Answer 21

It prevents oxygen from being transported around the body (a ligand substitution reaction occurs as CO replaces the oxygen already bound to the haemoglobin)

Question 22

What is a heterogeneous catalyst? What state are they usually? Give an example of a heterogenous catalyst.

Answer 22

A catalyst that is is a different phase to the reactants.
Are often solids.
Eg. In the Haber process a solid iron catalyst is used to speed up the reaction between hydrogen and nitrogen gas

Question 23

What is a homogenous catalyst? What state are they usually? What extra ‘step’ is there?

Answer 23

A catalyst in the same phase as the reactants.
Are often in aqueous solution.
The catalysed reaction will proceed via an intermediate species.

Question 24

Explain the surface adsorption theory, and which type of catalyst this is related to?

Answer 24

This relates to a heterogenous catalyst (which are often solid):
- aDsorption - one of the reactants aDsorbs onto the active site on the SURFACE of the catalyst (if an effective catalyst, the molecules can move about the surface bonding to different active sites)
- reaction - this weakens the covalent bonds in the reactants so that reactions occur more easily (increases rate of reaction)
- desorption - the product (a new product) detaches from the surface

Question 25

Why are heterogenous catalysts often used as powders?

Answer 25

The reaction occurs on the surface of the catalyst, so surface area needs to be at a maximum for the catalyst to be effective

Question 26

Explain the role of catalysts in catalytic converters

Answer 26

(Heterogenous catalysts)
- adsorption - CO (toxic) and NO (oxidised in atmosphere to NO2 which irritates respiratory system and forms acid rain) aDsorb onto the active site on the surface of the (heterogeneous) catalyst (platinum/rhodium/palladium)
- reaction - the CO and NO (covalent) bonds are weakened which allows reactions to occur
- desorption - as a result CO2 and N2 are desorbed (detached from the surface)

Question 27

What happens when heterogeneous catalysts are ‘poisoned’ with impurities (eg. Sulfur impurities poison the catalyst in the Haber process)? What problems does this cause in industry?

Answer 27

When poisoned by impurities (the impurities adsorb onto the catalyst surface occupying the active sites), the active sites are blocked so less surface area available for the reaction (preventing adsorption of the reactants). (The impurities are less likely to desorb from the surface). When this happens the catalyst has no effect on the rate of reaction which leads to increased chemical production costs in industry as the catalyst has to be cleaned or replaced regularly