3.2.5 Transition metals

Question 1

what are the physical and chemical properties of transition metals?

Answer 1

• formation of coloured ions
• variable oxidation state
• catalytic activity
• can form complexes

Question 2

what is meant by ‘ligand’?

Answer 2

a molecule or ion that forms a co-ordinate
bond with a transition metal by donating a pair of
electrons

Question 3

what is meant by a ‘complex’?

Answer 3

central metal atom or ion surrounded by
ligands

Question 4

what is meant by ‘co-ordination number’?

Answer 4

the number of co-ordinate bonds to
the central metal atom or ion

Question 5

what is meant by a ‘Lewis acid’ and does the metal ion or the ligand act as one?

Answer 5

• electron pair acceptor
• metal ion

Question 6

what is meant by a ‘Lewis base’ and does the metal ion or the ligand act as one?

Answer 6

• electron pair donor
• ligand

Question 7

what is meant by a ‘bidentate ligand’ and give an example?

Answer 7

• they can form 2 co-ordinate bonds
• e.g. ethane-1,2-diamine; C2O4
  (2–) [oxalate]

Question 8

what is meant by a ‘multidentate ligand’ and give an example?

Answer 8

• can form 3 or more co-ordinate bonds
• e.g. EDTA (4-)

Question 9

what makes Cl- different to NH3 and H2O as ligands?

Answer 9

• NH3 and H2O are similar in size and uncharged, so the coordination number does not change when undergoing ligand substitution
• Cl- is much larger so the coordination number changes

Question 10

what is ‘Haem’, how is it formed and what is it used for?

Answer 10

• iron(II) complex with a multidentate ligand
• oxygen forms a co-ordinate bond to Fe(II) in
  haemoglobin, enabling oxygen to be transported in the
  blood

Question 11

why is carbon monoxide toxic to humans?

Answer 11

it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobinm, preventing it from being transported in the blood

Question 12

what is meant by the ‘chelate effect’?

Answer 12

• when bidentate and multidentate ligands replace monodentate
  ligands from complexes.

Question 13

what does the chelate effect do?

Answer 13

• positive entropy change in ligand substitution is favourable as a more stable complex is formed, meaning it’s better to have more moles on the right
• greater entopy change = more negative ΔG = more favourable
• this is achieved through the chelate effect

Question 14

why is the enthalpy change for ligand substitution so small?

Answer 14

the bonds being formed are very similar to the bonds being broken

Question 15

what is the criteria to undergo cis/trans geometric isomerism?

Answer 15

• must be octahedral with 2 types of ligands (6 monodentate ligands); e.g. [Cu(H2O)4(OH)2)]
• square planar (4 monodentate ligands; 2 of each type)

Question 16

what would a cis geometric isomerism look like e.g. [Cu(NH3)4Cl2]

Answer 16

the Cl group would be next to each other (90 degrees apart)

Question 17

what would a trans geometric isomerism look like e.g. [Cu(NH3)4Cl2]

Answer 17

the Cl would be opposite each other (180 degrees apart)

Question 18

what is the compound for cisplatin and how would you draw it and transplatin?

Answer 18

• Pt(NH3)2Cl2
• cisplatin -> Pt in the centre; NH3 and Cl will be on opposite sides (square planar shape)
• transplatin -> NH3 and Cl on same side

Question 19

what is the criteria to undergo optical isomerism?

Answer 19

• non-superimposable mirror images
• must be octahedral with 3 bidentate ligands

Question 20

react [Cu(H2O)6]2+ (aq) with NH3 (aq)

Answer 20

[Cu(H2O)6]2+ (aq) + 4NH3 (aq) ⇌ [Cu(H2O)2(NH3)4]2+ (aq) + 4H2O (l)

Question 21

what colour is [Cu(H2O)6]2+ (aq)

Answer 21

blue solution

Question 22

what colour is [Cu(H2O)2(NH3)4]2+ (aq)

Answer 22

deep blue solution

Question 23

describe what happens when you react [Cu(H2O)6]2+ (aq) with NH3 (aq) and why this occurs

Answer 23

• change in colour because there is a change in ligand
• no change in coordination number or shape because the ligands H2O and NH3 are neutral and similar in size

Question 24

react [Cu(H2O)6]2+ (aq) with Cl- (aq)

Answer 24

[Cu(H2O)6]2+ (aq) + 4Cl- (aq) ⇌ [CuCl4]2- (aq) + 6H2O (l)

Question 25

describe what happens when [Cu(H2O)6]2+ (aq) reacts with 4Cl- (aq) and why this happens

Answer 25

- change in colour due to change in ligand - change in coord. no and shape because 4 chloride ions replace 6 water molecules (chloride ions are bigger than H2O)

Question 26

what colour is [CuCl4]2+

Answer 26

yellow solution

Question 27

react [Co(H2O)6]2+ with NH3

Answer 27

[Co(H2O)6]2+ (aq) + 6NH3 (aq) ⇌ [Co(NH3)6]2+ (aq) + 6H2O (l)

Question 28

what colour is [Co(H2O)6]2+

Answer 28

pink solution

Question 29

what colour is [Co(NH3)6]2+

Answer 29

straw coloured solution

Question 30

what is meant by a homogeneous + hetergeneous catalyst

Answer 30

- homo - same phase as the reactant - hetero - diff. phase to the reactants

Question 31

process of a catalyst

Answer 31

- reactants adsorb onto the surface/active site of the catalyst - bonds weaken/reaction takes place - products desorb from the surface

Question 32

what happens after reactants are adsorbed?

Answer 32

they are desorbed (ie they leave the surface)

Question 33

what happens (in regards of adsorption and desorption) if the catalyst is too strong or weak? give an example of a strong and a weak catalyst.

Answer 33

too strong (e.g. tungsten) -> reactants cannot move around the surface; this prevents them from desorbing too weak (e.g. silver) -> reactants cannot get adsorbed

Question 34

how does catalyst poisoning cause the rate of reaction to stay the same?

Answer 34

reaction impurities -> blocks the active site -> prevents adsorption -> bonds of the molecules stay strong -> harder to break -> catalyst has no event on RoR

Question 35

what happens to costs when there are impurities in the reaction?

Answer 35

increase in chemical production costs because the catalyst needs to be replaced or cleaned regularly

Question 36

how do you calculate the energy absorbed by the electrons and what are the units

Answer 36

ΔE = hv = hc / λ - ΔE = energy gap (J) - h = Planck's constant (6.63 x10(-34)) - v = frequency of light absorbed (Hz) - c = speed of light (3x10(8)) - λ = wavelength of light (m)

Question 37

what are the colours of vanadium at different oxidation states?

Answer 37

- V(2+) - violet - V(3+) - green - VO(2+) - blue - - VO2(+) - yellow

Question 38

explain why an aqueous solution containing [Fe(H2O)6] 3+ ions has a lower pH than one containing [Fe(H2O)6]2+ ions. (3 marks)

Answer 38

- Fe3+ has a greater charge density/smaller than Fe2+ - Fe3+ is more polarising/polarises water molecules more - therefore more O-H bonds break/are weakened

Question 39

Explain why complexes formed from transition metal ions are coloured. (3 marks)

Answer 39

- absorb (some) frequencies of (visible) light - to promote/excite electrons in d-orbitals - remaining/complementary /frequencies/colours/energies of (visible) light are reflected/transmitted to give the colour seen