Cambridge International A Level Chemistry Answers to self-assessment questions

Answers to SAQs
Chapter 24 c Eʅ = +1.33 V + (–0.77 V) = +0.56 V
The positive value indicates that the reaction
1 a i Ti 1s2 2s2 2p6 3s2 3p6 3d2 4s2 as written is feasible and its relatively large
value suggests that the reaction is likely to
ii Cr 1s2 2s2 2p6 3s2 3p6v3d5 4s1
occur (although values of Eʅ tell us nothing
iii Co 1s2 2s2 2p6 3s2 3p6 3d7 4s2 about the rate of a reaction).
iv Fe3+ 1s2 2s2 2p6 3s2 3p6 3d 4s0 d 6
v Ni2+ 1s2 2s2 2p6 3s2 3p6 3d8 4s0 e i 0.0153 × 0.001 = 0.000 015 3 mol
vi Cu+ 1s2v2s2 2p6 3s2 3p6 3d10 4s0 ii 0.000 015 3 × 6 = 0.000 091 8 mol
b For scandium the only observed oxidation iii 0.000 091 8 –3

state is +3, so the electronic configuration 0.025 = 0.003 67 mol dm

of Sc3+ is 1s2 2s2 2p6 3s2 3p6 4s0. This ion has 4 a i +3
no d electrons, so does not satisfy the ii +2
definition of a transition element. The
only ion of zinc is Zn2+, with the electronic iii +3
configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s0. This iv +3
ion has a completely filled, not a partially v +2
filled, d subshell – so zinc is not a transition b [Ni(EDTA)]2–
element. c ethanedioate ion (ox) and
c The +7 oxidation state involves all of the 3d ethane-1,2-diamine (en)
and 4s electrons in manganese.
d Oxidation state of vanadium in a (VO2+) = +5; 5 a i 6
b (VO2+) = +4; c (V3+) = +3; d (V2+) = +2. ii H 2N
e i +4 as this involves all the 4d and 5s
Cl NH2
electrons, leaving the noble gas electronic
configuration of krypton. Co
ii ZrO2
Cl NH2
2 a The outer electron is further from the
H2N
attractive force of the nucleus in calcium
than in cobalt, and cobalt has a higher iii They are mirror images, which are not
nuclear charge. superimposable.
b density = mV b 2– 2–
Nickel atoms have a higher Ar, and a lower NC Cl NC Cl
volume, than calcium atoms, so the density Ni Ni
of nickel is higher.
NC Cl Cl CN
3 a Fe2+(aq) → Fe3+(aq) + e–
Cr2O72–(aq) + 14H+(aq) + 6e– cis-isomer trans-isomer
→ 2Cr3+(aq) + 7H2O(l)
b 6Fe2+(aq) → 6Fe3+(aq) + 6e– 6 a +2
Cr2O72–(aq) + 14H+(aq) + 6e– b [CoCl4]2–(aq) + 6H2O(l) → [Co(H2O)6]2+(aq) + 4Cl–(aq)
→ 2Cr3+(aq) + 7H2O(l) [[PtCl2(NH3)2]2–(aq)]
Cr2O72–(aq) + 6Fe2+(aq) + 14H+(aq) 7 a i
[[PtCl4]2–(aq)] [NH3(aq)]2
 → 2Cr3+(aq) + 6Fe3+ + 7H2O(l) [[Cr(H2O)4]+(aq)]
ii
[[Cr(H2O)6]2+(aq)] [Cl–(aq)]2
Cambridge International AS and A Level Chemistry © Cambridge University Press 2014
 Cambridge International A Level Chemistry Answers to self-assessment questions

[[Ni(NH3)4(H2O)2]2+(aq)] b Zn2+ ions have electronic configuration

iii
[[Ni(H2O)6]2+(aq)] [NH3(aq)]4 [Ar]3d104s0. If d-orbital splitting were to occur
b i [Fe(H2O)6]3+(aq)]: in a complex ion containing Zn2+, each of the
H2O 3d orbitals would contain two electrons, and
would therefore be fully occupied. Visible
H 2O OH2 light could not be absorbed in promoting an
electron from a lower energy 3d orbital to a
Fe
higher energy 3d orbital.
H2O OH2

H2O

ii SCN– has a higher value of Kstab than H2O.

So the position of equilibrium is shifted to
the right.
iii [Fe(H2O)5SCN]2+(aq)
iv Yes; a colour change is likely / possible.
F– has a higher value of Kstab than SCN–.
So F substitutes for SCN (and for water)
because the position of equilibrium is
shifted to the right.

8 a orbitals at the same energy level

b The ligands in a complex cause the
d orbitals to split, forming two sets of non-
degenerate orbitals. The difference in the
energy (ΔE) between the non-degenerate
d orbitals corresponds to the energy of part
of the visible spectrum of light. So when
light travels through a solution or a solid
containing the complex, an electron from
one of the three lower non-degenerate
orbitals absorbs that amount of energy
(ΔE) and jumps into one of the two higher
non-degenerate orbitals. This leaves the
transmitted light coloured.
c

3d

Ni2+ ...3d8

9 a Sc3+ ions have electronic configuration

[Ar]3d04s0. If d-orbital splitting were to occur in
a complex ion containing Sc3+, there would be
no electrons in the three 3d orbitals of lower
energy, so visible light would not be absorbed
in promoting an electron from a lower energy
3d orbital to a higher energy 3d orbital.

Cambridge International AS and A Level Chemistry © Cambridge University Press 2014