Introduction to Transition Elements

Question 1

Define transition element.

Answer 1

A transition element is a d block element that forms at least one stable ion with a partially filled d subshell.

Question 2

Explain why even though Scandium and Zinc are d block elements they are not considered as transition elements.

Answer 2

Sc3+:[Ar] amd Zn2+: [Ar] 3d10
Sc3+ has no d electrons and Zn2+ has a completely filled d subshell. Hence, these ions do not have partially-filled d subshells.

Question 3

Explain why atomic radii of the first raw transition elements are smaller than those of the period 4 s block elements K and Ca.

Answer 3

Moving from s block to d block, nuclear charge increases. Despite electronsbeign added to the inner 3d orbitals, the 3d electrons are ineffective in shielding the 4s electrons from the nucleus, due to the diffused 4-lobed shape of the d orbitals. Effective charge increases and hence the electrostatic forces of attraction between the nucleus and valence electrons increase, and atomic radii in the first row of transition elements is hence smaller.

Question 4

Explain why atomic radii of transition elements are relatively invariant (relatively constant).

Answer 4

Across the period 4 transition elements, nuclear charge increases, since electons are being added to the inner 3d orbitals, shielding effect also increases, the increase in shielding effect almost cancels the increase in nuclear charge. Hence, effective nuclear charge increases very gradually and the electrostatic forces of attration between the nucleus and valence electrons increases only slightly, thus, atomic radii remain relativeley constant across the period 4 transition elements.

Question 5

Explain why the ionisation energies of transition elements are relatively invariant (relatively constant).

Answer 5

Across the period 4 transition elements, nuclear charge increases, since electons are being added to the inner 3d orbitals, shielding effect also increases, the increase in shielding effect almost cancels the increase in nuclear charge. Hence, effective nuclear charge increases very gradually and the electrostatic forces of attration between the nucleus and valence electrons increases only slightly. Energy required to remove the valence electrons increases only slightly.

Question 6

Explain why transition elements have higher melting and boiling points.

Answer 6

Both s block elements and transition elements have giant metallic lattice structures held together by strong metallic bonds. In transition elements, the sea of delocalised electrons is contributed by 3d and 4s electrons since the energy difference between the 3d and 4s orbitals is small. In s block elements, only one or two 4s electrons can be delocalised. Thus, transition elements have higher melting and boiling points than s block elements.

Question 7

Explain transition elements are denser than s block elements.

Answer 7

Transition metal cations have relatively smaller atomic radii and higher atomic mass compared to s block elements, hence they have more closely packed structures due to their stronger metallic bonding. More atoms of the d block elements are packed in a unit volume compared to the s block elements within the same period. Thus, transition elements are denser than s block elements.

Question 8

Why can transition elements variable oxidation states?

Answer 8

Transition elements possess variable oxidation states due to the small energy level difference between the 3d and 4s orbitals. Different number of 3d and 4s electrons can be lost to form stable ions or utilised in bonding to form compounds of different oxidation states. s blocl elements are restricted to oxidation numbers of +1 (group 1) and +2 (group 2) respectively and further removal of inner shell electrons will involve too much energy.

Question 9

How do we predict the maximum oxidation state of a transition element?

Answer 9

The maximum oxidation state a transition element can have is the number of 4s electrons plus the number of unpaired 3d electrons.

Question 10

Describe the characteristics (ie basicity/acidity) of transition element or ions.

Answer 10

Compounds of transition elements with low oxidation states are usually ionic, the lower oxides are hence basic.
Compounds of transition elements with high oxidation states are usually covalent, the highe oxides are hence acidic. This is because at higher oxidation states, transition element cations have high charge density and are highly polarising, hence they form covalent compounds and ions.
Oxides of transition elements in intermediate oxidation states are usually amphotheric,

Question 11

Describe the relative stability of oxidation states.

Answer 11

In Period 4 transition elements, stability of the +2 oxidation state relative to the +3 oxidation state increases across the series.

The relative stability of the oxidation states are reflected by their standard electrode potential.

Question 12

How does electrode potential reflect the relative stability of differing oxidation states of transition elements?

Answer 12

The more negative the electrode potential value, the less spontaneous for reduction to take place. The more decreasingly likely a transition element ion is oxidised to another ion of a different oxidation state, the less stable the product of the oxidisation is. (ie original transition element is less reactive towards oxidation)

Question 13

What does positive and negative electrode potential say about the stability of the ion?

Answer 13

Negative E values indicates that the reduction from M3+ to M2+ is not thermodynamically favourable. Hence M3+ is more stable wrt to M2+ and M2+ would be easily oxidised and is a good reducing agent.

Positive E values indicate that the reduction from M3+ to M2+ is thermodynamically favourable. Hence M2+ is moe stable wrt M3+ and M3+ would be easily reduced and hence M3+ would be a good oxidising agent.

Question 14

Define complex.

Answer 14

A complex consists of a central metal atom or ion surrounded by other ions or molecules called ligands bonded to the central atom/ion by dative covalent bonds.

Question 15

Explain why d-block elements have a high tendency to form complexes.

Answer 15

Due to their relatively small size and high charge, transition metal ions have high charge density and hence high polarising power to attract ligands. Transition metals have low-lying vacant d-orbitals which accept the lone pair(s) of electrons on ligands via dative covalent bonds.
Furthermore, s-block elements are larger and have a lower polarising power and tend to become involved in ion-dipole atraction rather than covalency.

Question 16

How do you calculate the net charge on the complex?

Answer 16

Oxidation number of metal cation + (Charge of ligand x number of ligands)
Complexes can be cationic, neutral or anionic.

Question 17

Explain what is co-ordination number in complexes.

Answer 17

It indicates the number of ligand groups around the central atom or ion.

Most common is 4 and 6 but 2 is widespread and is important in the chemistry of copper(I) and silver(I) compounds.

Question 18

Define ligands.

Answer 18

A ligand can be either a neutral molecule or an anion that contains at least one atom bearing a lone pair of electrons to be used in dative covalent bond formation with a metal atom or ion

All liagnds are Lewis bases as they are lone pair donors.

Question 19

Explain denticity in ligands.

Answer 19

Denticity is how a ligand can be classified according to the number of dative bonds they form with the central metal atom or ion.

Usually we work with bidentate (2 dative bonds) and hexadentate (6 dative bonds)

Question 20

Describe the Crystal Field Theory (d-orbital splitting).

Answer 20

1. In an isolated atom or ion, the 5 d-orbitals are degenerate. In an octahedral complex, ligands are modelled as 6 point negative charges that surround the positively charged atom or ion with 2 d-orbitals pointing directly at the ligands and 3 not pointing directly at the ligands.
2. Each ligand forms a dative bond with the transition metal ion via lone pair of electrons along the x,y and z axes.
3. When they approach the transition metal ion along the x, y and z axis there is iner-eelctonic repulsion between the lone pair of electrons from the ligand and the electrons in the d orbitals of the transition metal, causing the energies of these d-rbital electrons to increase to different extents. (x^2-y^2 and z^2 orbitals experience greater repulsion)
4. Hence the x^2-y^2 and z^2 orbitals experience higher energy levels than the xy, xz and zy orbitals

Dont need memorise just understand can alr.

Question 21

Explain how colours are observed due to d-d transitions.

Answer 21

Due to the presence of ligands splitting the d-orbitals into two energy levels, an electron in the lower energy d-orbital absorbs certain wavelength of light energy from the visible region of the electromagnetic spectrum and is promoted to a higher energy d-orbital. This is called d-d transition. The remaining wavelengths ae transmitted and the complementary colours are observed.

Question 22

Why are complexes of Sc(III), Cu(I) and Zn(II) not coloured?

Answer 22

For d-d transition to takeplace, at least one of the d-orbitals must be occupied by an electron and the d-subshell must not be fully filled.
Sn(III): [Ar] The 3d orbitals are empty and there are no electrons for d-d transition
Cu(I) and Zn(II): [Ar]3d10 The 3d orbitals are fully occupied with electrons and it is not possible for d-d transition to occur.

Question 23

How do we determine the colours of the complexes?

Answer 23

The energy gap between the two groups of d-orbitals determines the wavelength of visible light absorbed.
change in E is directly proportional to inverse wavelength
For large E, short wavelength absprbed hence complementary colour observed.
For small E, long wavelength absorbed hence complementary colour obseverved.

Question 24

Describe why (complex) is (a colour).

Answer 24

(Complex) has a partially filled 3d subshell. In (complex) the presence of (ligands) splits the 3d orbitals of the (metal ion) into two different energy levels. A 3d electron in the lower energy level absorbed light energy corresponding to the wavelegths for (complementary colour) and is promoted to a higher energy 3d orbital. The remaining wavelengths are transmitted and the complementary (colour) is observed.

Question 25

State the factors affectign energy gap.

Answer 25

1. Identity of metal and its oxidation state.Generally, energy gap increases with oxidation state which reflects the electrostatic nature of the d-orbital splitting. The higher the charge density fot eh metal ion, the stronger the sttraction for the ligands, and hence the stronger the repulsion, hence the a higher energy gap. 2. Shape of complex.(FYI) 3. Nature of ligands. Magnitude of energy gap changes as the strength of repulsion between d electrons and point charges changes. As such energy gap measures the strength of crystal field. Weak field ligands -> smaller energy gap and vice versa

Question 26

What is ligand exchange?

Answer 26

In a complex ion, the ligand can be replaced by a different ligand, which can usually change the colour of the solution.

Question 27

State the factors affecting ligand exchange.

Answer 27

1. Strength of the ligand. In the presence of stronger ligands, weaker logands in a complex ion are replaced by the stronger ligands. 2. Concentration of ligands. In the presence of a large concentration of a different ligand, ligand exchange can occur for a complex ion.

Question 28

Define and describe how heterogenous catalyst speeds up the reaction.

Answer 28

The catalyst and reactants are in different physical phases. For heterogenous catalysis to occur, the reactant molecules are adsorbed onto the active sites on its surface through the formation of weak bonds between the reactant molecules and the surface catalyst atoms. Adsorption weakens the covalent bonds within the reactant molecules, therby reducing the activation energy for the reaction. Adsorption increases the concentation of reactant molecules at the catalyst surfacte and allows the reactant molecules to come into close contact with proper orientation for reaction. ## Footnote Catalyst is usually in solid form while reactants are liquids and gases.

Question 29

Why are transition metals and their ions effective heterogenous catalysts?

Answer 29

They have partially filled 3d orbitals which allows the reactant particles to be adsorbed onto the catalyst surface.

Question 30

Desfine and describe how homogenous catalyst speeds up the reaction.

Answer 30

The catalyst and reactants are in the same physical phase. A homogenous catalyst provides an alternative pathway with a lower activation energy by forming an intermediate which is later consumed in the reaction. The catalytic activity of a homoegnous catalyst depends on its ability to exist in variable oxidation states.

Question 31

Why are transition metals and their ions effective homogenous catalysts?

Answer 31

They can exist in different oxidation states and undergo conversion from one oxidation state to another relatively easily.