Deck 10: Transition elements

Question 1

what is a transition element

Answer 1

d block element that can form one or more stable ions with a partially filled d subshell

Question 2

which first row d block elements are not transition elements

Answer 2

Sc and Zn

Sc only forms the stable ion of Sc3+ which has an empty d sub shell
Zn only forms the stable ion of Zn2+ which has a fully filled d subshell

Question 3

why is copper still a transition element

Answer 3

copper forms Cu+ with a fully filled d subshell however, it also forms Cu2+ which has a partially filled d subshell, hence it is still a transition element

Question 4

atomic radius trend for first row transition elements

Answer 4

relatively constant

• nuclear charge increases across the period
• but the electrons are added to the 3d subshell, so the increase in number of 3d electrons provide more shielding between the nuclear and the outer 4s electrons
• increase in shielding offsets the increase in nuclear charge considerably so electrostatic attraction between the nuclear and outer 4s electron increases minimally

Question 5

IE trend for first row transition elements

Answer 5

relatively constant

• electron removed from the 4s orbital
• across period, proton number increases and nuclear charge increases
• but the electrons are added to the 3d subshell, so the increase in number of 3d electrons provide more shielding between the nuclear and the outer 4s electrons
• increase in shielding offsets the increase in nuclear charge considerably so electrostatic attraction between the nuclear and outer 4s electron increases minimally
• energy needed to remove the 1st electron is almost constant

Question 6

compare mp and bp of s block and transition elements

Answer 6

higher for transition elements

• the valence 3d and 4s electrons are close in energy
• hence both 3d and 4s electrons are available for delocalisation into the sea of electrons
• with a greater number of delocalised electrons and higher charge density of the cations, the metallic bonding is stronger in the transition elements
• for the s block, only s electrons are delocalised

Question 7

why melting point of Ti is higher than Ca

Answer 7

both are metals with metallic bonding

Ti is a transition elements and both its 3d and 4s electrons are available for delocalisation into the sea of electrons since the 3d and 4s electrons are close in energy

with a greater number of delocalised electrons, Ti has stronger metallic bonding than Ca, which needs more energy to overcome

Question 8

transition elements or s block denser

Answer 8

transition elements
cos smaller atomic radius but larger atomic mass

density increases across the period for transition elements

Question 9

chemical properties of transition elements vs s block (4)

Answer 9

transition
- form compounds with a variety of oxidation states (s block fixed OS)
- form coloured compounds and ions (s block usually white solid, colourless aq solution)
- elements and their compounds often show catalytic activity (s block little or none)
- great tendency to form stable complexes (s block much less tendency)

Question 10

why transition elements can show variable oxidation states

Answer 10

close similarity in energy of the 3d and 4s electrons so that both are available for bond formation

Question 11

when are there lower and higher oxidation states for transition elements (brief)

Answer 11

lower: ionic compounds
higher: covalent compounds (bonded to more electronegative atoms)

Question 12

explain low and higher OS for transition elements

Answer 12

low:
- successive IEs increase gradually
- eg. Cr can form Cr2+ or Cr3+ by losing 2/3 electrons
- but for s block, there is a large increase between 2nd and 3rd IE because its from inner shell

high:
- eg. Cr6+ in CrO42- where all the 3d and 4s electrons are utilised in covalent bond formation

Question 13

how to find highest possible OS

Answer 13

no. of unpaired d electrons + 4s electrons

Question 13

acidity/basicity of oxides of elements in low and high oxidation states

Answer 13

low OS: basic
high OS: acidic

Question 14

why is there a decrease in E˚ from Mn3+/Mn2+ to Fe3+/Fe2+
(+1.54V to +0.77V)

Answer 14

because the addition of an electron to Fe3+ leads to a 3d6 configuration where there is inter electronic repulsion between the paired electrons in the same 3d orbital so the reduction of Fe3+ is less favoured

extra: reduction to Fe2+ still favoured because there is stabilisation from the reduction of the high charge density

Question 15

what feature of 1st row transition elements enable them to function effectively as heterogeneous catalysts

Answer 15

the availability of partially filled 3d subshell allows for ready exchange of electrons to and from reactant molecules, facilitating the formation fo weak bonds with the reactant molecules

• avail of 3d electrons for bond formation w reactants
• avail of low lying vacant orbitals which can accept e pairs from reactants

Question 16

hetero catalysis explanation brief

Answer 16

1. diffusion
2. adsorption
   - bond within reactant are weakened, reducing activation energy
   - reactants brought into close contact with correct orientation for reaction and there is increased conc of reactant at surface of catalyst
3. reaction (bonds break, new formed)
4. desorption
5. diffusion of product away leaving vacant active sites

Question 17

what feature of 1st row transition elements enable them to function effectively as homo catalysts

Answer 17

ability to exist in diff oxidation states and the relative ease of conversion from one OS to another

facilitate the formation and decomposition of the intermediate formed from the transition metal catalysts and the reactants

Question 18

definition of a complex

Answer 18

contains a central metal atom/ion linked to one or more surrounding ions/molecules

Question 19

definition of a ligand

Answer 19

an ion/molecules which contains at least one atom bearing a lp of electrons which can be donated into a low lying cavant orbital of a centra metal atom/ion forming a complex

Question 20

what do the different parts of a ligand work as

Answer 20

ligand: lewis base (electron pair donor)

central metal atom: lewis acid

Question 21

what is the criteria for a complex

Answer 21

the number of bonds formed with the metal ion must exceed the oxidation state of the metal ion

Question 22

what is the coordination number of the central metal ion

Answer 22

total number of coordinate bonds that the central metal ion forms with ligands

Question 23

charge of CN

Answer 23

-1

CN-

Question 24

types of ligands (brief)

Answer 24

monodentate ligand polydentate - bidentate - tridentate - hexadentate

Question 25

monodentate ligands and examples

Answer 25

ligands which can only form one coordinate bond with a central metal ion coordination no. = no. of ligands Cl-, OH-, CN-, H2O, NH3, Ctriplebond O

Question 26

bidentate ligands

Answer 26

ligands which can form 2 coordinate bonds simultaneously with the central metal atom/ion

Question 27

hexadentate ligands

Answer 27

ligands which can form 6 coordinate bonds simultaneously with the central metal atom/ion eg. EDTA ligand

Question 28

why do transition metal ions show high tendency to form complexes

Answer 28

transition metal ions have low lying vacant orbitals 3d, 4s 4p to accommodate the lp of electrons from the ligands to form coordinate bonds the transition metal ions are relatively small and highly charged - hence they have high charge density to attract the lp of e from the ligands - high charge density and hence polarising power produces a strong tendency towards covalent bond formation with ligands

Question 29

formation of aqua complexes

Answer 29

for group 1, the metal cations form ion dipole interactions but for group 2 or 3 or transition metal ions with higher charge density, transition metal ions exist as aqua complexes - each oxygen atom donates a lp of electrons to a low lying vacant orbital of the transition metal ion to form a coordinate bond - overall charge usually 2+ or 3+

Question 30

acidity of aqua complexes eg FeCl3 (aq)

Answer 30

high charge density of Fe3+ ion makes it strongly polarising distorts the electron cloud of the H2O molecules bonded to it, weakening the O-H bonds and enabling these H2O molecules bonded to it to become proton donors free water molecules in the solution act as bases and the following eqm is established ACIDIC ENOUGH TO REACT W NA2CO3 TO PRODUCE CO2 AND HYDROXIDE PPT

Question 31

reaction when Na2CO3 is added to Fe3+ (aq)

Answer 31

Question 32

stereochem of complexes

Answer 32

Question 33

colour of these complexes

Answer 33

Question 33

what the observed colour

Answer 33

the complement of the absorbed colour (opposite on the colour wheel)

Question 34

explain the occurence of colour

Answer 34

- when light energy is absorbed by a substance, an electron in the atom of that substance is promoted from an orbital of lower energy to one of higher energy (ground to excited state) - the different electronic transitions involve the absorption of different wavelengths and if the absorbed wavelengths are in the visible region of the spectrum, then the substance appears coloured

Question 35

why is sodium chloride solution colourless

Answer 35

the excitation of an electron from the lower energy 2p orbital to the higher energy 3s orbital requires a wavelength of light beyond the range of the visible spectrum radiation from the visible spectrum does not provide sufficient energy to promote electrons in Na+ ions to even the lowest of their possible excited electronic states

Question 36

explain the crystal field theory

Answer 36

1. in an isolated metal ion , the 5 3d orbitals are degenerate 2. for an octahedral complex, 6 ligands approach the central metal ion along the x, y and z axes 3. this results in the electrostatic repulsion between the electrons in the 3d orbital and the lp on the ligands, increasing potential energy 4. hence the energy of an electron in a 3d orbital of the complex is higher than in the isolated atom 5.the 3dx2-y2 and 3dz2 orbitals have the greatest electron density along the axes but for the rest the greatest electron density points between the axes 6. the repulsion between electrons in the 3dx2-y2 and 3dz2 and the lp of the ligands are the greatest 7. the 5 3d orbitals are affected differently by the approach of the ligands so it causes the orbitals to split into 2 energy levels with an energy gap between them and the degeneracy of the orbitals is destroyed

Question 37

explain the significance of the energy gap

Answer 37

- the energy gap is formed between the 2 sets of d orbitals - corresponds to the energies of electromagnetic radiation in the visible region of the spectrum - transition metal compounds are coloured because radiation in the visible region of the spectrum is of the right quantity of energy to promote electrons from d orbitals of lower energy to one of higher energy (d-d transitions)

Question 38

Why is aqueous solution of NiCl2 coloured (LSPEC)

Answer 38

- In aq solution, Ni2+ exists as an octahedral complex [Ni(H2O)6]2+ - the presence of the H2O ligands causes the splitting of the 5 originally degenerate 3d orbitals in the Ni2+ ion into 2 sets of slightly different energy levels - since the 3d subshell is partially filled, the electrons from the lower energy d orbitals can absorb energy corresponding to certain wavelengths from the visible spectrum and get promoted to the higher energy d orbital - such d-d transitions are responsible for the colour observed in NiCl2 - the colour observed is the complement of the colour absorbed

Question 39

what affects the colour of the transition metal complexes

Answer 39

1. identity of the metal and its oxidation state - different no. of d electrons and repel the electrons of the ligands to different extends hence the energy gap between the split d orbitals are different 2. nature of ligands bonded - diff ligands split the d orbitals of the metal ion into 2 sets of slightly different energy levels to different extents - different transition metal complexes absorb different wavelengths of light from the visible spectrum for d-d transitions - diff colours corresponding to the complements of the diff colours absorbed are observed

Question 40

splitting in tetrahedral vs octahedral complexes

Answer 40

for tetrahedral, the ligand approaches the central metal ion between the axes for octahedral, approach along the axes for tetrahedral, the 3d xy, yx and xz orbitals experience the greatest electrostatic repulsion from the ligand lone pairs and have higher energies than the 3dx2-y2 and 3dz2 opposite for octahedral

Question 41

define stability constant

Answer 41

Kstab of a complex ion is the eqm constant for its formation the greater the value of K stab, the more stable the complex ion is relative to the aqua complex containing the same metal ion

Question 42

what is this reaction called

Answer 42

ligand exchange reaction

Question 43

what does the high Kstab of this reaction mean

Answer 43

- eqm lies very much right - the [Ni(NH3)6]2+ ion is more stable than the [NiH2O)6]2+ ion - NH3 is a stronger ligand than H2O - NH3 forms a stronger dative covalent bond than the Ni2+ ion - there will be the replacement of the H2O ligands by the NH3 ligands when excess NH3 is added

Question 44

just read

Answer 44

LIGAND FORMING THE COMPLEX OF HIGHER KSTAB WILL REPLACE THOSE FORMING THE COMPLEX OF LOWER KSTAB

Question 45

what does larger log Kstab mean

Answer 45

means larger K stab which means complex is more stable

Question 46

read

Answer 46

Question 47

read

Answer 47

Question 48

look at presentation of K stab and all

Answer 48

Question 49

structure of haemoglobin

Answer 49

- metal ion is iron 2+ - 4 of the coordination sites are taken up by nitrogen from a porphyrin which is a tetradentate ligand - the 5th site is taken up by a nitrogen atom - last site can be reversibly bonded with oxygen

Question 50

how does haemoglobin get destroyed

Answer 50

when better ligands than oxygen bond strongly to the Fe2+ and destroys the oxygen carrying capacity of haemoglobin eg. carbon monoxide and cyanide ions

Question 51

just read

Answer 51

Question 52

just look ligand exchange reactions

Answer 52

Question 53

just look (summary)

Answer 53