Download - Unit 4 2004 Jan
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Unit 4 January 04 Markscheme
Q1
(a) (i) 4Al + 3O2 2Al2O3(1)
Ignore state symbols
Ionic/ electrovalent/ ionic with covalent character(1) 2
(ii) S + O2 SO2(1)
Allow equations based on S2 or S8Covalent(1) 2
(b) (i) Amphoteric / acid and base (1)
H+
in one equation, OH-in another(1)
ORMolecular equations using HCl and NaOH (1 max)
Al2O3 + 3H2O + 2OH
2Al(OH)4
OR............. + 6OH
2Al(OH)63
OR equation with AlO2
as product (1)
A12O3 + 6H+ 2Al
3++ 3H2O (1)
State symbols not required
Equations using Al(OH)3 (1 max) 4
(ii) Acidic (1)
SO2 + 2OH SO32+ H2O
OR SO2 + OH
HSO3
OR any correctly balanced equation with SO2 and H2O as reactants
and appropriate combinations of SO32
, HSO3
; H+
and
H3O+
as products (1) 2
(c) (i) PCl3 + 3H2O 3HCl + H3PO3(2)
Species (1), balancing (1), allow P(OH)3 as product 2
(ii) A pair of electrons/ lone pair from the oxygen in a water molecule
(1)
cannot form a bond with/ donate electrons to the carbon atom (1)because the C atom has no available orbital / no 2d / carbon is a
small atom surrounded by chlorine atoms /chlorine atoms are large
and surround the carbon atom (so the attack is sterically hindered)
sense of relative size needed, steric hindrance is not enough (1)
Therefore a CCl bond has to be broken (first), which is strong /
Ea too high (1) 4
(d) Reaction II as the lead +2 state is more stable than the lead +4
OR vice versa in terms of tin
OR +2 oxidation state becomes more stable down the group (1)
PbCl4 / Pb 4+ is oxidising but SnCl4 / Sn 4+ is not (1)
For this mark a reference to the relative oxidising power of PbCl4 and
SnCl4 is needed 2
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[18]
Q2
(a) (i) Propanoyl chloride 1
(ii) Ammonia (1)Phosphorus pentoxide/ phosphorus(V) oxide/ P2O5 / P4O10(1)
Name or formulae acceptable, but if both given, both must be
correct 2
(iii) Bromine (1)NOTaq/gas
(conc) aq NaOH / KOH (1)
Heat (under reflux) (1) This mark is conditional on both reagents
being stated 3
(b) (i) (Substance) AOR correct name (1)
C2H5COCl + H2O / C2H5COOH + HCl (1) 2
(ii) (Substance) ZOR correct name (1)
C2H5NH2 + H2O / C2H5NH3+
+ OH
(allow C2H5NH3OH)
(1)
Each equation can score 2 marks if no substances are stated. 2[10]
Q3
(a) Restricted rotation / lack of free rotation around C=C (1)
NOTcannot rotate
There are two different groups on each carbon of C=C / four different
groups around two carbon atoms (1) 2
(b) Potassium dichromate (1)
If given oxidation state must be correct
dilute H2SO4 / H2SO4 solution (1)
(Heat and) distil off (citral as it is formed) (1)
IF KMnO42 maxie 2nd
and 3rd
marks 3
(c) (i) Brown / orange / yellow colourless / decolourises / disappears 1
(ii) Yellow/ orange/ red precipitate / crystals / solid 1
(iii) Red precipitate / crystals/ solid 1
(d) (i) C6H11(CH3)(Br)CCH2CHO OR C6H11(CH3)HCCH(Br)CHO
brackets essential
C H C C C H O C H C C C H O1 1 1 1
3 3
6 6
B r B rH H
H HH C H CO R
1
(ii) C6H11(CH3)C=CHCH(OH)CN brackets essential
OR
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C HC C
C
1 1
3
6
H
O H
HC H
C N
NOT with bond going to N 1
(iii) C6H11(CH3)C=CHCH(OH)CH3brackets essential
OR
C HC C
C
1 1
3
3
6
H
O H
HC H
C H
ALLOW mixture of displayed and structural formulae in (i), (ii) and
(iii) 1
[11]
Q4
(a) (i) The heat (energy) / enthalpy change when 1 mol of gaseous atoms
(1)
NOTenergy on its own
is formed from the element (1) 2
(ii) Cl2
(g) Cl (g) 1
(iii)
S r ( g ) 2 C l ( g )
2 C l ( g )
S r ( g )
S r ( g )
S r ( s ) + C l ( g ) S r C l ( s )2 2
+
2 +
H f
Cycle marks:
1 markfor cycle showing all balanced species
1 markfor all state symbols correct
Allow Sr(g) Sr2+
(g) as a single stage
Calculation marks:
Hf= sum of other terms and / or
829 = +164 +550 +1064 + 2 x ( +122) + 2 x ( 349) + LE
or LE= 82916455010642 x(+122) 2x(349) 2
ie 1 markfor multiplyingHa of Cl by 2
1 markfor multiplying EA of Cl(g) by 2
LE = 2153 (kJ mol1
) (1)
The answer markisconsequential on whether they multiply neither
or
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just one ofHa and EA by 2 , butnotconsequential if they have
wrong
signs. 5
(b) (i) + 737 = HfSrCl + 122 ( 829) OR correct cycle (1)
HfSrCl = 214 (kJ mol
1
) (1) 2(ii) As the reaction (left to right) is endothermic (1)
a decrease in temperature will decrease the value ofK(1)
thus the equilibrium position will shift to the left (1) 3
(iii) Kp= psrCl pCI(1)MUST be stated
moles of SrCl = moles of Cl (1) This can be stated or implied
pSrCl = pCl = 4.2 = 2.1 (atm) (1)
Kp = 2.1 atm 2.1 atm = 4.4(l) (1) atm2(1)
Units are NOT consequential on wrong Kp 5
[18]
Q5
(a) (i) NH3 base and NH4+
acid (1)
H2O acid and OH
base (1)
OR
linking(1)
acid and base correctly identified(1) 2
(ii) Starting pH of (just above) 11 (1)
Graph showing vertical line between pH 4 and 6With vertical section 35 units in length (1)
at a volume of HCl of 20 cm3(1)
Final pH of between 1 and 2 (1) 4
(iii) Named indicator consequential on vertical part of their graph (1)
Because all of its range is within the vertical part of the graph /
pKind 1 is within vertical part of graph / it changes colour
completely/ stated colour change (MO: yellow red; BB: blue
yellow; PP: pink colourless) within the pH of the vertical part of
the graph (1) 2
(b) (i) Ka= [ ] [ ][ ]223
HNONOOH
+
square brackets essential 1
(ii) [H+] = [NO2
] or [H
+]2= Ka [HNO2] (1)
[H+] = (Ka 0.12) = 0.00751 mol dm
3(1)
pH = log [H+] = 2.12/2.1 (1)
ALLOW any correct conversion of [H+l into pH provided the
answer is less than 7 3
(iii) Moles NaNO2 = 1.38/69 = 0.020 (1)
[NO2] = 0.020 / 0.10 = 0.20 (mol dm3)
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[H+] =
salt][
acid][a
K
=20.0
120.01070.4 4 = 2.82 10
4(1)
pH = log 2.82 104
= 3.55 /3.6 /3.5 (1) 4
(iv) In a buffer both [acid] and [salt] must be large compared to the
added H+
or OH-
ions (1)but in NaNO2 alone [ HNO2] is very small (1)
OR
to remove both H+
and OH
there must be a large reservoir of
both NO2
ions and HNO2 molecules (1)
which there are a solution of NaNO2 and HNO2 but not in NaNO
alone (1) 2[18]