Download reactions of period 3 elements with water

A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 1
REACTIONS OF PERIOD 3 ELEMENTS WITH WATER
a) (i)
Sodium melts into a ball / heat is given out (1)
Effervescence / gas evolved (1)
Sodium skates across the surface (1)
Ignition may occur (1)
Maximum 3 marks
Quality of language: two or more sentences with correct spelling, punctuation and grammar in which the
meaning is clear (1).
(ii) 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) (1)
(iii) 13-14 (1)
(iv) Redox / oxidation-reduction (1)
b) (i)
Less reactive / less powerful reducing agents / higher ionisation energies / lower in electrochemical series /
less –ve Eê values (1)
(ii) Pass steam (1)
over the heated metal (1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 2
REACTIONS OF PERIOD 3 ELEMENTS WITH OXYGEN
a)
Na
Reaction conditions
Formula of oxide
(or lower oxide)
b) (i)
Al
Si
Ignite in air or O2
(1)
(1)
(1)
MgO
Al2O3
SiO2
(½)
(½)
(½)
(½)
(ii) 2Mg + O2 → 2MgO (1)
(iii) 2Al + 3O2 → 2Al2O3 (1)
(v) 2P + 3O2 → 2P2O3 (1)
S
Heat in O2
Na2O
(iv) Si + O2 → SiO2 (1)
P
Ignite in air or O2
4Na + O2 → 2Na2O (1)
(vi) S + O2 → SO2 (1)
Mg
P4O6 or
P 2O 3
(½)
Cl
SO2
(½)
(6)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 3
STRUCTURE AND BONDING OF PERIOD 3 OXIDES
a)
Na2O
MgO
Bonding
Ionic
(1)
Structure
Ionic lattice
(1)
Al2O3
SiO2
P4O10
SO2
Covalent
(1)
Macromolecular
(1)
b) From Na2O → MgO → Al2O3 there is an increase in m.p. (1)
Reason: ionic bonding becomes stronger (1)
(Or lattice enthalpy increases in magnitude (1))
because cations decrease in size (½)
and increase in charge (½)
(Or because surface charge density / charge:radius ratio increases (1))
SiO2 has the highest m.p. (1)
because all / many strong covalent bonds must be broken for it to melt (1)
Remainder have low m.p. (1)
because the only forces to be overcome on melting are relatively weak (1)
intermolecuar forces / van der Waals’ forces (1)
c) Melting point of MgO is higher than that of CaO (1)
Reason: the ionic bonding in MgO is stronger than that in CaO (1)
(Or the lattice enthalpy of MgO is greater than that of CaO (1))
because Mg2+ is smaller than Ca2+ / Mg2+ has a higher charge density (1)
Simple molecular
(1)
Cl2O
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 4
PERIOD 3 OXIDES WITH WATER
a) Oxides on LHS react / dissolve in water to give alkaline solutions / pH >7.0 (1)
e.g. Na2O / Li2O (1)
Na2O(s) + H2O(l) → 2NaOH(aq) (1)
Oxides further along a period are insoluble in water (1)
e.g. BeO / MgO / Al2O3 (1)
Oxides on RHS give acidic solutions / pH < 7.0 (1)
e.g. SO2 / SO3 / other acidic oxide (1)
SO2(g) + H2O(l) → H2SO3(aq) (1)
b) Acidity increases with oxygen content / oxidation number (1)
CO: no reaction with water (1)
pH = 7 (1)
CO2(g) + H2O(l) → H2CO3(aq) (1)
pH = 5-6 (1)
P4O6(s) + 6H2O(l) → 4H3PO3(aq) (1)
pH = 2-4 (1)
P4O10(s) + 6H2O(l) → 4H3PO4(aq) (1)
pH = 0-2 (1)
Note Examiners may accept equations without state symbols.
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 5
ACID - BASE CHARACTER OF PERIOD 3 OXIDES
a)
Na2O
Oxide
Acid - base character
MgO
Al2O3
SiO2
P4O6
P4O10
SO2
SO3
Cl2O
ClO2
Cl2O7
Basic (½) Basic (½) Amphoteric Acidic (½) Acidic (½) Acidic (½) Acidic (½)
(1)
b) MgO(s) + H2SO4(aq) → MgSO4(aq) + H2O(l) (1)
Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O(l) (1)
Al2O3(s) + 2NaOH(aq) + 7H2O(l) → 2Na[Al(OH)4(H2O)2](aq)(1)
Accept Na[Al(OH)4]
SO3(s) + 2NaOH(aq) → Na2SO4(aq) + H2O(l) (1)
Or SO3(s) + NaOH(aq) → NaHSO4(aq) (1)
Accept similar equations for SO2(g)
(Throughout, accept similar equations for other acids and bases. Examiners may accept equations without state symbols)
c) MgO
O2- + 2H+ → H2O(1)
O2- accepts protons (1)
and is therefore a base on the BrØnsted-Lowry theory (1)
Or donates lone pairs of electrons to H+ (1)
and is therefore a base on the G.N. Lewis theory (1)
SO3
SO3 + 2OH- → SO42- + H2O (1)
Or
SO3 + OH- → HSO4- (1)
SO3 accepts a lone pair of electrons from OH- (1)
and is therefore an acid on the G.N. Lewis theory (1)
(Accept similar explanation for SO3 + H2O, or for SO2 with either OH- or H2O)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 6
STABILITY OF GROUP 4 OXIDATION STATES
a) (i)
+2 (1)
+4 (1)
Signs must be given.
(ii) More stable state of tin is +4 (1)
More stable state of lead is +2 (1)
b) (i)
SnCl2
PbCl2
SnO2
PbO2
Reducing agent (1)
Neither (1)
Neither (1)
Oxidising agent (1)
(ii) Accept SnCl2 with KMnO4, K2Cr2O7, FeCl3, HgCl2, C6H5NO2, etc.
Correct reactant (1)
Correct product (1)
Accept PbO2 with conc HCl(aq), Na3[Cr(OH)6] etc.
Correct reactant (1)
Correct product (1)
c) +2 state increases in stability (1)
because this is essentially an ionic state / M2+ ions are concerned (1)
Ionisation energies decrease / ions are formed more easily on descending the group (1)
+4 state decreases in stability (1)
because energy is needed to promote an ns electron to a vacant np orbital (1)
and, for larger atoms, less energy is released when (weak) covalent bonds are formed (1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 7
HYDROLYSIS OF GROUP 4 CHLORIDES
a) (i)
SiCl4(l) + 2H2O(l) → SiO2(aq)(s) + 4HCl(aq) (1)
H
Cl (1)
O
OH
H
Si
(1)
Si
Cl
Cl
Cl
Cl
Cl
+ HCl
Cl
Repeated × 3
SiO2
-2H2O (1)
Si(OH)4 (1)
(ii) H2O cannot coordinate to the C atom / nucleophilic attack by H2O cannot occur (1)
because C does not have a vacant orbital in its outer shell / 2d orbital to accept a lone pair (1)
b) (i)
Observation A white suspension / cloudy solution (1)
Equation
SnCl2(aq) + H2O(l) ↔ SnCl(OH)(s) + HCl(aq) (1)
Prevention
By adding hydrochloric acid (1)
which disturbs equilibrium to the LHS (1)
(ii) PbCl2 has an ionic structure (1)
but SnCl2 is predominantly covalent (1)
With H2O acting as a nucleophile, substitution occurs (1)
Examiners may accept equations without state symbols.
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 8
DEFINITIONS AND ELECTRONIC CONFIGURATIONS
a) A d-block element is one with its highest energy electron in a d-orbital (1)
A transition element is one which can form one or more stable ions with a partially occupied d-subshell (1)
b) (i)
Yes (1)
Configuration is [Ar] 3d1 4s2 (1)
(ii) No (1)
Its only cation has the configuration [Ar] 3d0 4s0 (1)
c) (i)
3d
4s
4p
Cu
[Ar]
(1)
Cu2+
[Ar]
(1)
Cr
[Ar]
(1)
Cr 3+
[Ar]
(1)
(ii) Unusual feature
Their atoms have only one 4s electron (1)
while atoms of other transition elements have two 4s electrons (1)
Reason for copper
Configuration as shown rather than [Ar] 3d9 4s2 (1)
because of exceptional stability associated with a fully occupied 3d subshell (1)
Reason for chromium
Configuration as shown rather than [Ar] 3d4 4s2 (1)
because there is no mutual electrical repulsion in any orbital and the electron spin is parallel
/ [Ar] 3d5 4s1 is a more stable electronic configuration (1)
d) (i)
Iron(III) ion (1)
Manganese(II) ion (1)
Accept Fe3+ & Mn2+
(ii) Half-filled 3d subshell (1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 9
BONDING IN COMPLEX IONS
a) (i)
A molecule or anion (1)
which can donate one or more lone pairs of electrons to a cation (1)
(ii) Complex cation
Species with a positive charge (1)
formed from ligands bonded to a central cation (1)
[Ag(NH3)2]+ or any other (1)
Example
Complex anion
Species with a negative charge (1)
formed from ligands bonded to a central cation (1)
[Fe(CN)6]4- or any other (1)
Example
b) (i)
3+
H2O
H2O
III
OH2
Cr
H2O
(2) Deduct 1 mark if charge is missing
Octahedral shape (1)
OH2
H2O
(ii) Coordinate / dative covalent (1)
(iii) Lone pair of electrons on the O atom of a H2O molecule (1)
Cr3+ ion has vacant orbitals which can accept 6 pairs of electrons (1)
3d
(iv)
4s
4p
(1) for unpaired electrons
(1) for electron pairs
(1) for labels
[Ar]
Electrons
possessed by
Cr3+ ion.
Electrons originating from the ligands
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 10
COLOUR OF COMPLEX IONS
a)
ION
COLOUR
Cr3+(aq)
Mn2+(aq)
green
or purple
(pale) pink
Fe2+(aq)
(pale) green
Fe3+(aq)
yellow
or brown
b) [Cu(H2O)6]2+ → [CuCl4]2- or other example
Blue
Yellow or green (1) for formulae (1) for colours
c) (i)
3d
4s
4p
Ti4+ [Ar]
(1)
Cu+ [Ar]
(1)
Zn2+ [Ar]
(1)
Ti2+ [Ar]
(1)
(ii) Only Ti2+ (1)
(iii) Colour results from the absorption of one or more frequencies of white light (1)
in promoting d-electrons from a low energy orbital to a higher energy orbital (1)
Not possible if the 3d subshell is empty / rules out Ti4+ (1)
Not possible if the 3d subshell is full / rules out Cu+ & Zn2+ (1)
Only Ti2+ has a partially filled d-subshell (1)
Maximum 4 marks
Cu2+(aq)
blue
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 11
ISOMERISM
a) Test 1
Reagent
Observation with A
Observation with B
BaCl2(aq) / Ba(NO3)2(aq) (1)
White (½) precipitate (½)
No precipitate / solution remains clear (1)
Test 2
Reagent
Observation with A
Observation with B
b) (i)
A
AgNO3(aq) (1)
No precipitate / solution remains clear (1)
Cream / off-white / pale yellow (½) precipitate (½)
B
[Pt(NH3)6]4+
4+
NH3
H3N
[PtCl(NH3)5]3+
H3N
NH3
IV
Cl
(1)
NH3
NH3
C [PtCl2(NH3)4]2+
D
[PtCl3(NH3)3]+
2+
NH3
Cl
Cl
IV
Pt
Pt
Cl
Cl
+
NH3
NH3
IV
(1)
NH3
NH3
H3N
NH3
IV
Pt
Pt
H3N
3+
NH3
(1)
NH3
Cl
NH3
(1)
NH3
Accept cis or trans isomer
Accept fac or mer isomer
(ii) Dissolve equal masses / amounts of each in water (1)
Either Titrate a suitable volume of each with AgNO3(aq) (1)
Titres for A, B, C and D are in the ratio 4 : 3 : 2 : 1 (1)
Or
Add excess AgNO3(aq) to each, then filter, wash, dry and weigh precipitates (1)
Masses for A, B, C and D are in the ratio 4 : 3 : 2 : 1 (1)
Cl
c) (i)
Cl
NH3
II
NH3
II
Ni
Ni
H3N
(1)
Cl
trans (½)
Cl
(1)
NH3
cis (½)
(ii)
H3N
Cl
Cl
III
Cl
NH3
cis (½)
NH3
III
Co
Co
H3N
+
NH3
+
NH3
(1)
H3N
Cl
NH3
trans (½)
(1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 12
LIGAND EXCHANGE REACTIONS
a) Definition A reaction in which one or more coordinated water molecules (1)
is replaced / substituted by other ligands (1)
Reason 1
Relatively strong ligands tend to displace weaker ones / water is a weak ligand (1)
Reason 2
Ligand exchange is reversible / if a ligand is added in high concentration, equilibrium will be disturbed to
RHS (1)
Quality of language:two or more sentences with correct spelling, punctuation and grammar in which the meaning is clear (1).
b) (i)
Ionic equation
[Cu(H2O)6]2+(aq) + 6NH3(aq) → [Cu(NH3)6]2+(aq) + 6H2O(l) (1)
Colour change
From blue (½) to royal blue/purple (½)
Shape of resulting complex ion Octahedral (1)
(ii) Ionic equation
Colour change
[Cu(H2O)6]2+(aq) + 4Cl-(aq) → [CoCl4]2-(aq) + 6H2O(l) (1)
From blue (½) to green/yellow (½)
Shape of resulting complex ion Tetrahedral (1)
c)
If ligands are relatively large (1)
fewer of them can be accommodated around a cation (1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 13
DEPROTONATION THEORY
a) (i)
pH decreases from 7.0 (1)
(ii) Reasons
A proton (1)
is transferred from [Fe(H2O)6]3+ / H2O ligand (1)
to a molecule of solvent water (1)
Hence [H3O+] > [OH-] (1)
Maximum 3 marks
Equation [Fe(H2O)6]3+(aq) + H2O(l) ¾ [Fe(OH)(H2O)5]2+(aq)
(Examiners may accept equation without state symbols)
(iii) Type of reaction
Function of hydrated metal ions
Function of water molecules
b) (i)
+ H3O+(aq) (1)
Deprotonation / acid-base (1)
Acid (1)
Base (1)
[Fe(OH)(H2O)5]2+(aq) + H2O(l) ¾ [Fe(OH)2(H2O)4]+(aq) + H3O+(aq) (1)
[Fe(OH)2(H2O)4]+(aq) + H2O(l) ¾ [Fe(OH)3(H2O)3](s) + H3O+(aq) (1)
(Examiners may accept equations without state symbols)
(ii) OH- is a stronger base than H2O (1)
c) (i)
Solution is acidic (1)
[Cu(H2O)6]2+ behaves in a similar manner to [Fe(H2O)6]3+ (1)
(ii) FeCl3 has the lower pH (1)
Fe3+ has a higher charge than Cu2+ (1) and is smaller (1)
Or Fe3+ has a higher charge density than Cu2+ (2)
Hence Fe3+ increases polarisation of O⎯H bonds (1)
which weakens these bonds / facilitates loss of a proton (1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 14
REACTIONS OF CATIONS WITH OH- AND NH3
a) (i)
Both OH- and NH3 are bases (1)
(ii) Green precipitate (½)
White precipitate (½)
Green precipitate (½)
Brown precipitate (½)
Blue precipitate (½)
Green precipitate (½)
Blue precipitate (½)
Any 3
Accept full formulae
Cr(OH)3 (½)
Mn(OH)2 (½)
Fe(OH)2 (½)
Fe(OH)3 (½)
Co(OH)2 (½)
Ni(OH)2 (½)
Cu(OH)2 (½)
b)
Excess dilute NaOH(aq)
Excess NH3 (aq)
Example of a compound which will dissolve
Cr(OH)3 (1)
Co(OH)2 / Ni(OH)2 / Cu(OH)2
(Accept Cr(OH)3) (1)
Reason for dissolving
Cr(OH)3 is amphoteric (1)
NH3 is a ligand /
ammine formation (1)
Type of reaction occurring
Deprotonation / acid-base (1)
Ligand exchange / substitution (1)
Observation
Green solution (1)
CoII – pale brown solution /
NiII – blue solution /
CuII – dark blue solution (1)
Formula of the ion produced
[Cr(OH)4(H2O)2]- /
[Cr(OH)5(H2O)]2- /
[Cr(OH)6]3- (1)
[Co(NH3)6]2+/
[Ni(NH3)6]2+ /
Cu(NH3)4(H2O)2]2+ (1)
c) (i)
Aluminium hydroxide (1)
Lead(II) hydroxide (1)
Tin(II) hydroxide(1)
Tin(IV) hydroxide (1)
Beryllium hydroxide (1)
Any 2
(ii) [Al(OH)4(H2O)2]- (1)
[Pb(OH)4]2- (1)
[Sn(OH)4]2- (1)
[Sn(OH)6]2- (1)
[Be(OH)4]2- (1)
Any 2
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 15
OXIDATION STATES
a) (i)
+6
(ii) +6
(iii) +3
(iv) +5
(v) +5
(vi) +3
(vii) +4
(viii) +3
(ix) +6
(x) +2
(½) each x 10 = 5 marks
b) Energy levels of 4s and 3d electrons are very similar (1)
hence 3d as well as 4s electrons can be used in bonding (1)
Although more energy is required to remove more electrons (1)
this is often compensated by the release of latttice enthalpy (1)
or hydration enthalpy (1)
Maximum 3 marks
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 16
COLORIMETRY
a) (i)
Volume of 0.1 M CuSO4(aq)
20 cm3 (½)
3
Volume of 0.1 M NH3(aq) 80 cm (½)
(ii)
n (Cu2+) = 0.1 x 20/1000 = 0.002 mol (1)
n (NH3) = 0.1 x 80/1000 = 0.008 mol (1)
∴ mole ratio Cu2+ : NH3 = 1.4 (1)
(iii)
[Cu(NH3)4(H2O)2]2+ (1)
Or
[Cu(NH3)4]2+ (1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 17
LIGAND EXCHANGE AND STABILITY CONSTANTS
(a) (i)
Yellow colour is a complex with ammonia (1) because the ammonia is a more reactive ligand than water (not
sulfate)/ the ammonia complex is more stable the aqua complex (1).
Blue colour is a cyanide complex (1) because the cyanide is a more reactive ligand than ammonia / the cyanide
complex is more stable the ammonia complex (1).
No further reaction (1) because ammonia is a less reactive ligand than the cyanide (1).
Allow 1 mark for well constructed answer and use of three terms like: ligand, reactive, powerful,
displacement, complex, stable, aqua.
(ii) Mauve colour is an ethane-1,2-diamine complex (1) because the ethane-1,2-diamine is a more reactive ligand
than ammonia/ the ethane-1,2-diamine complex is more stable the ammonia complex (1).
No further reaction with the yellow cyanide complex (1) because ethane-1,2-diamine is a less reactive ligand than
the cyanide (1).
(b) (i)
No (1) the complex with ethane-1,2-diamine has a larger stability constant that the EDTA complex (1).
(ii) No (1) the complex with cyanide has a larger stability constant that the ammonia complex (1).
(iii) [Co(CN)6]3- (1) The cyanide is the more stable complex and is formed in preference to ammonia complex (1)
All the [Co(H2O)6]3+ has reacted with the cyanide (1)
(iv) [Co(NH3)]3+ (1) The [Co(NH3)]3+ is the more stable complex and is formed in preference to [Co(NH3)]2+
complex (1)
All the ammonia has reacted with the [Co(H2O)6]3+ (1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 18
d-ORBITAL SPLITTING AND LIGHT ABSORPTION
a) (i)
dx2y2 d x 2
Adding ligands
Energy
dx2y2 d x 2
Absorbing light
Cu2+ uncomplexed
d xy d xz d yz
d xy d xz d yz
Ground state
Excited state
dx2y2 d x 2
dx2y2 d x 2
(ii)
Adding ligands
Energy
Absorbing light
Fe3+ uncomplexed
d xy d xz d yz
d xy d xz d yz
Ground state
Excited state
(iii)
dx2y2 d x 2
Adding ligands
Energy
dx2y2 d x 2
Absorbing light
V3+ uncomplexed
d xy d xz d yz
Ground state
d xy d xz d yz
Excited state
-1 mark for every mistake. Maximum 6 marks.
b) (i)
No electrons in the 3d orbitals (1) No electrons available for promotion (1)
(ii) All 3 d orbitals are full (1) No empty higher level orbital for an electron to be promoted to(1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 19
VANADIUM CHEMISTRY I
3d
a)
V
V3+
4s
(1)
[Ar]
(1)
[Ar]
b) Oxidation Number
+2
+3
+4
+5
4p
Formula
[V(H2O)6]2+ (1)
([V(H 2O) 6]3+)
VO2+(aq) (1)
VO2+(aq) (1)
Colour
(Violet)
Green (1)
Blue (1)
Orange / yellow (1)
Name
(Vanadium(II))
(Vanadium(III))
Oxovanadium(IV) (1)
Dioxovanadium(V) (1)
c) Orange / yellow (1) → green (1) → blue (1) → green (1) → violet / lavender (1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 20
VANADIUM CHEMISTRY II
a) Shape
Octahedral (1)
Types of bonding Covalent (within the ligand) (1)
and dative covalent / coordinate (between central metal and ligand) (1)
b) (i)
Prediction Acidic (1)
Equation
(ii) Identity
Equation
c) (i)
[V(H2O)6]3+ + H2O ¾ [V(OH)(H2O)5]2+ + H3O+ (1)
Vanadium(III) hydroxide / V(OH)3 / [V(OH)3(H2O)3] (1)
[V(H2O)6]3+ + 3OH- → [V(OH)3(H2O)3] + 3H2O (1)
VO2+(aq) + 2H+(aq) + e- → VO2+(aq) + H2O(l) (2)
(ii) SO32-(aq) + H2O(l) → SO42-(aq) + 2H+(aq) + 2e- (2)
(iii) 2VO2+(aq) + SO32-(aq) + 2H+(aq) → 2VO2+(aq) + SO42-(aq) + H2O(l) (2)
(iv) KMnO4 / K2Cr2O7 / any other oxidising agent with Eê > + 1.0 V (1)
In the above equations, award (1) for formulae and (1) for balance.
Examiners may accept equations without state symbols.
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 21
CHROMIUM CHEMISTRY I
a) Equation
Explanation
b) Observation
Equations
[Cr(H2O)6]3+ + H2O ¾ [Cr(OH)(H2O)5]2+ + H3O+ (1)
The solution contains [Cr(H2O)6]3+ ions which are purple / violet / blue (1)
and [Cr(OH)(H2O)5]2+ ions which are green (1)
Green / grey-green (½) precipitate (½)
which dissolves / is soluble in excess NaOH(aq) (1)
to give a green (½) solution (½)
[Cr(H2O)6]3+ + 3OH- → [Cr(OH)3(H2O)3] + 3H2O (1)
[Cr(OH)3(H2O)3] + OH- → [Cr(OH)4(H2O)2]- + H2O (1)
Accept equations for the formation of [Cr(OH)5(H2O)]2- or [Cr(OH)6]3Examiners may require state symbols of complexes, e.g. [Cr(OH)4(H2O)2]-(aq)
c) Observation
Half-equation
d) (i)
From green / blue green (1) to blue (1)
[Cr(H2O)6]3+ + e- → [Cr(H2O)6]2+
Or Cr3+(aq) + e- → Cr2+(aq) (1)
Chromate(VI) ion (1)
(ii) Name
Equation
Dichromate(VI) ion (1)
2 CrO42-(aq) + 2H+(aq) → Cr2O72-(aq) + H2O(l) (2)
Award (1) for formulae and (1) for balance.
Examiners may accept an equation without state symbols.
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 22
CHROMIUM CHEMISTRY II
a)
Cr
Cl
H 2O
19.5
40.1
40.4
19.5/52
40.1/35.5 40.4/18
= 0.375
1.13
2.24
ratio
1
3
6
∴ empirical formula = CrCl3(H2O)6 (1)
m/100g
n
(1)
(1)
b) X = [Cr(H2O)6]3+ (1)
Hexaaquachromium(III) ion (1)
Y = [CrCl2(H2O)4]+ (1)
Tetraaquadichlorochromium(III) ion (1)
c) The cations have different ligands attached to them (1)
causing different degrees of splitting of energy levels of d orbitals (1)
d) In Y, the chloride ions / chlorine atoms can be close to each other (cis isomer) (1)
or across the structure from each other (trans isomer) (1)
+
H2O
H2O
OH2
III
Cl
H2O
cis
e) (i)
III
Cl
Cr
Cr
Cl
+
H2O
H2O
(1)
Cl
OH2
(1)
H2O
trans
[CrCl(H2O)5]2+ (Cl-)2 (1) Ionic charges are not essential
(ii) Two (1)
A2 Level
TOPIC 21 ANSWERS & MARK SCHEMES
QUESTIONSHEET 23
COBALT CHEMISTRY I
a) Transition element
An element which forms one or more stable ions (1)
with incompletely filled d-subshells (1)
Example
Co2+ [Ar] 3d7 / Co3+ [Ar] 3d6 (1)
Cationic complex
Species with an overall +ve charge (1)
formed by coordination of ligands to a central cation (1)
Example
[Co(H2O)6]2+ / [Co(NH3)6]2+ / [Co(NH3)6]3+ (1)
Anionic complex
Species with an overall –ve charge (1)
formed by coordination of ligands to a central cation (1)
Example
[CoCl4]2- / [Co(OH)4]2- (1)
b)
2+
H2O
H2O
II
OH2
II
12
Co
Co
H2O
H2O
c) Type of bonding
d) (i)
12
12
12
○
OH2
Feature
2-
Cl
Cl
(1)
○
○
Cl
Cl
(1)
Coordinate / dative covalent (1)
Lone pair of electrons (1)
Blue (½) precipitate (½)
which dissolves in / is soluble in excess conc. NH3(aq) (1)
to give a pale brown / red-brown (½) solution (½)
(ii) Replacement / substitution (1)
of one type of ligand by another (1)
[Co(H2O)6]2+ + 6NH3 → [Co(NH3)6]2+ + 6H2O (1)
e) Observation
Explanation
Solution becomes dark brown (1)
Oxidation of CoII to CoIII / formation of [Co(NH3)6]3+ (1)