transition elements

Question 1

Fe2+ -> Fe3+

Answer 1

use MnO4 2-
purple to colourless

Question 2

Fe3+ -> Fe2+

Answer 2

use I-
orange to green

Question 3

Cr3+ -> Cr2O7 2-

Answer 3

use hot alkaline H2O2
green to orange

Question 4

Cr2O7 2- -> Cr3+

Answer 4

use Zn
orange to green

Question 5

Cr3+ -> Cr2+

Answer 5

use Zn
green to blue

Question 6

Cu2+ -> Cu+

Answer 6

use I-
pale blue solution to brown solution with white precipitate

Question 7

Cu+ -> Cu2+

Answer 7

use acid
white precipitate to brown precipitate in pale blue solution

Question 8

Cu2+ with dropwise NaOH

Answer 8

blue solution to blue precipitate
Cu2+ (aq) + 2OH- (aq) -> Cu(OH)2 (s)

Question 9

Cu2+ with excess NH3

Answer 9

dark blue solution
[Cu(H2O)6] 2+ + 4NH3 -> [Cu(H2O)2(NH3)4] 2+ + 4 H2O

Question 10

Fe2+ with dropwise NaOH

Answer 10

pale green solution to green precipitate
Fe2+ (aq) + 2OH- (aq) -> Fe(OH)2 (s)

Question 11

Fe3+ with dropwise NaOH

Answer 11

yellow solution to rust-brown precipitate
Fe3+ (aq) + 3OH- (aq) -> Fe(OH)3 (s)

Question 12

Mn2+ with dropwise NaOH

Answer 12

pink solution to pale-brown precipitate
Mn2+ (aq) + 2OH- (aq) -> Mn(OH)2 (s)

Question 13

Cr3+ with dropwise NaOH

Answer 13

violet solution to grey-green precipitate
Cr3+ (aq) + 3OH- (aq) -> Cr(OH)3 (s)

Question 14

Cr3+ with excess NH3

Answer 14

purple solution
[Cr(H2O)6] 3+ + 6NH3 -> [Cr(NH3)6] 3+ + 6H2O

Question 15

Cu2+ with excess HCl

Answer 15

blue solution to yellow solution
[Cu(H2O)6] 2+ + 4Cl- -> [CuCl4] 2- + 6H2O

Question 16

what is a transition element?

Answer 16

a d-block element that forms an ion with an incomplete d sub-shell

Question 17

which d-block elements aren’t transition elements?

Answer 17

Zn forms Zn2+ which has a complete d sub-shell
Sc forms Sc3+ which has an empty d sub-shell

Question 18

ligand

Answer 18

a compound/ion that donates a pair of electrons to a central transition metal ion to form a coordinate bond

Question 19

types of ligand

Answer 19

monodentate = donates one pair of electrons
bidentate = donates two pairs of electrons
multidentate = donates multiple pairs of electrons

Question 20

common monodentate ligands

Answer 20

H2O
NH3
Cl-
OH-
SCN-
CN-

Question 21

common bidentate ligands

Answer 21

[(COO)2] 2-
NH2CH2CH2NH2

Question 22

square planar

Answer 22

coordination number of 4
bond angle 90
eg cis-platin [PtCl2(NH3)2]

Question 23

what are optical isomers?

Answer 23

stereoisomers that are a pair of non-superimposable mirror images of each other

Question 24

stereoisomerism

Answer 24

stereoisomers are compounds that have the same structural formula but different arrangement of atoms in space
cis isomer = groups next to each other
trans isomer = groups opposite each other

Question 25

what is ligand substitution?

Answer 25

a reaction in which one ligand in a complex ion is replaced by another

Question 26

haemoglobin and ligand substitution

Answer 26

at the centre of each haem complex is an Fe2+ ion
the Fe2+ ion binds reversibly with oxygen (O2) by accepting a lone pair from the O2 molecule to form a coordinate bond
CO binds irreversibly with haemoglobin so oxygen can’t be transported

Question 27

properties of transition elements

Answer 27

variable oxidation states
formation of coloured ions
effective catalysts

Question 28

what is cis-platin used for?

Answer 28

binds to the DNA in fast-growing cancer cells
this alters the DNA structure in the cancerous cells, preventing cell division

Question 29

what is a racemic mixture?

Answer 29

a mixture containing equal amounts of the two optical isomers
this has no effect on plane polarised light, as the rotations cancel each other out

Question 30

how do we distinguish between optical isomers?

Answer 30

using a polarimeter
optical isomers rotate plane-polarised light differently