Chapter 22 - Transition Elements

Question 1

Define transition elements.

Answer 1

Transition elements are d-block elements that form one or more stable ions with partially filled d-subshell.
(Sc and Zn excluded)

Question 2

Explain why transition metals have a higher melting point than s-block metals.

Answer 2

Since both 3d and 4s electrons are involved in metallic bonding of transition metals, the total ionisation energy will be higher, and more energy is required to overcome the metallic bonding of transition metals. S-block metals only have 1 or 2 electrons in 4s, so less energy is required.

Question 3

Explain why transition metals have a higher density than s-block metals.

Answer 3

Transition metals have a smaller atomic radius but higher atomic mass than s-block metals; the structure will be more closely packed and thus there will be greater number of atoms per unit volume.

Question 4

Explain why transition metals can have variable oxidation states unlike s-block metals.

Answer 4

Transition metals have close similarity in energy of the 4s and 3d electrons. (Once 4s electrons are removed, some or all 3d electrons may also be removed without much more energy.)
In contrast, in s-block metals, once the outermost electrons in the s orbital are removed, subsequent removal of electrons would be from the inner quantum shell, which requires too much energy.

Question 5

How does the number of available oxidation states vary among transition metals?

Answer 5

Increases from Ti to Mn and decreases from Mn to Cu. After Mn (3d5), pairing of d electrons occur and there is a decrease in the number of electrons available for bond formation, resulting in the decrease in the number of oxidation states exhibited by the elements with more than 5 d-electrons.

Question 6

How does oxidation state relate to redox reactions?

Answer 6

Transition metal ions in a high oxidation state tend to be oxidising agents, while transition metals in low oxidation state tend to be reducing agents.

Question 7

Explain why transition elements and their compounds can act as homogeneous catalysts.

Answer 7

They can exhibit variable oxidation states.

Question 8

Explain why transition elements and their compounds can act as heterogeneous catalysts.

Answer 8

Availability of energetically accessible, partially filled 3d orbitals to accept/donate electron pairs for adsorption of reactant molecules on the catalyst surface.

Question 9

Define complex.

Answer 9

A complex is a molecule or ion formed by a central metal atom or ion surrounded by one r more ligands.

Question 10

Explain 2 factors of transition elements that gives them the tendency to form complex ions rather than simple ions.

Answer 10

1) cations of transition elements have an incomplete 3d subshell and small ionic radius gives them high charge density. This results in a high polarising power, producing a strong tendency towards forming dative covalent bonds with ligands.
2) transition metal cations have vacant 3d (and 4s and 4p) orbitals which can be used to accommodate the lone pair of electrons from the ligands, resulting in dative bond formation.

Question 11

Define ligands.

Answer 11

A ligand is an ion or molecule with one or more lone pair of electrons available to be donated into the vacant orbitals of the transition metal atom or ion.

Question 12

How can ligands be classified according to the number of dative bonds it forms with the central atom?

Answer 12

Monodentate ligands: bond using the electron pair of a single donor atom
Bidentate ligands: bond using electron pairs of two donor atoms
Polydentate ligands/chelating agents: bond using electron pairs on more than one donor atom (including bindentate)

Question 13

What is the co-ordination number?

Answer 13

The number of dative bonds around the central atom/ion.

Question 14

Why can the aqua complexes of transition metal cations undergo hydrolysis in water?

Answer 14

Due to the cation’s high charge density, the H2O ligands are attracted very strongly to the cation, causing the O–H bond to weaken. H2O from water attracts H from the O–H, causing O–H to break and H3O+ is formed.

Question 15

Define ligand exchange reaction.

Answer 15

It is when the ligands in a complex ion is exchanged wholly or partially for other ligands. The reaction will happen readily if the new complex formed is more stable than the original complex, but will still be largely reversible.

Question 16

Describe the ligand exchange reaction in Haemoglobin.

Answer 16

Fe2+ is located in the centre of a plane of 4 nitrogen atoms in a haemoglobin. A haemoglobin molecule picks up an O2 molecule, which form a dative bond with the iron to form oxyhaemoglobin. The binding is reversible and enables haemoglobin to carry oxygen around and release it where needed.
CO can undergo ligand exchange with oxygen and binds it with the haemoglobin to form carboxyhaemoglobin. This binding with iron is irreversible, thus making carboxyhaemoglobin a very stable complex. As a result, a relatively small quantity of CO can inactive a substantial fraction of haemoglobin for oxygen transport.

Question 17

What are 2 reasons an object has a particular colour?

Answer 17

1) it reflects or transmits light of that colour. Thus, if an object absorbs all wavelengths except green, the reflected/transmitted light enters our eyes and is interpreted as green.
2) it absorbs light of the complementary colour. Thus, if an object absorbs only red light, the remaining mixture of reflected/transmitted wavelength enters our eyes and is interpreted as green (complement of red).

Question 18

How does the splitting of degenerate (have same energy) d orbitals in octahedral complex occur?

Answer 18

In the presence of ligands in an octahedral field, the ligands approach the metal ion along the mutually perpendicular x, y and z axes. As the ligands approach, their electron pairs repel electrons in the five d orbitals of the metal ions, causing all 5 orbitals to increase in energy. However, the d electrons are repelled unequally due to the different orientations of the d orbitals, causing the five d orbitals to be no longer degenerate, but are split into 2 groups with an energy gap (∆E) between them.

Question 19

How does the presence of ligands cause transition elements to have colour?

Answer 19

In the presence of ligands, the partially filled degenerate 3d orbitals of a transition metal ion are split into two different energy levels with a small energy gap (∆E) between them. The energy gap falls within the visible region of the electromagnetic spectrum. The electron at the lower energy level will absorb energy from the visible spectrum and promote to one of the higher energy orbitals. Such a d-d transition is responsible for the colour of the complex ion, where the colour observed is the complement of the colours absorbed.

Question 20

What are 2 factors affecting the colour of transition metal complexes?

Answer 20

1) elements with no d electrons or fully filled d orbitals have no colour.
2) different ligands have different effects on the splitting of d orbitals. The stronger the ligand, the bigger the energy gap (∆E) and the smaller the wavelength absorbed. (The energy of the light is inversely proportional to its wavelength)