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Answer Scheme Term 2 Trial Exam 2015
No Description Marks
1
2
3 4
5 6 7 8 9
10
11 12 13 14 15
Section A
A C3H4 +
O2 3CO2 + 2H2O
+184 0 3(-394) 2(-286) Hc = [3(-394)+2(-286)] [+184+0] = -1938
kJ mol
-1
C 2H+ + 2e- H2 ; I mol of H2 needs 2F electric current Q = It =
8 x 100 x 60 = 48,000 C
No. of moles of H2 = 48,000 9.65 x 104 x 2 = 0.25 mol
Volume of H2 = 0.25 x 24.4 = 6.1 dm3 + O2 = 9.15
D C Since both electrodes are the same metal, Eo = 0 V
Ecell =Eo +
= 0 +
+0.028 V
B D D B B C MgSO4.xH2O MgSO4 + xH2O
MgSO4 H2O
Mass (g) 2.1 4.3 2.1 = 2.2
No of moles 2.1 120 = 0.0175 2.2 18 = 0.122
Ratio 1 7
D B A D D
15
16. (a)
(b)
Section B Lattice energy is the energy released when one mole of
an ionic compound is formed from its gaseous ions.
15
Na(s) + Br2(g)
Na(g) + Br2(g)
Na+ (g) + Br2(g)
Na+ (g) + Br (g) + e
-1
Na+ (g) + Br (g)
NaBr (s) -329 kJ mol-1
+ 108 kJ mol-
1
+ 495 kJ mol-
1
+ 112 kJ mol-1 -325 kJ mol-1
Lattice energy
Energy
-
(c)
17. (a)
(b)
17. (c)
108 + 495 + 112 + (-325) + lattice energy = 329 .. Lattice
energy = 719 kJ mol-1 Cl2 + 2Br 2Cl + Br2 E
o = 1.36 1.07 = + 0.29 V . A positive Eo value means Cl2 is able
to oxidize Br to Br2. Chlorine reacts with water to form HOCl and
HCl that are colourless.
Cl2(aq) + H2O(l) HCl(aq) + HOCl(aq)
When exposed to sunlight, HOCl undergoes decomposition to form
HCl and O2. 2HOCl(aq) 2HCl(aq) + O2(g) This will shift the above
equilibrium to the right causing more Cl2 to react with water and
its colour fades. 3Cl2(aq) + 6OH(aq) 5Cl(aq) + ClO3(aq) + 3H2O(l)
Chlorine undergoes disproportionation in hot alkali to form
chlorate(I) and chlorate(V).
18.(a)(i)
(ii)
(b)
Section C The potential difference obtained when a half-cell of
an element is connected to a hydrogen standard electrode at 298K
and 1 atm.
K+(aq) + e K(s) ; Eo = - 2.92 V Cu2+(aq) + 2e Cu(s) ; Eo = +
0.34V Br2 (g) + 2e 2Br(aq) ; E
o = + 1.07 V The strength of the reducing power in descending
order is K, Cu, Br (OR K is a stronger reducing agent than Cu &
Cu is a stronger reducing agent than Br ion) The strength of the
reducing agent depends on the ability to release e-. The more
negative the standard electrode potential (E), the higher the
tendency of the metals or the anions to release electrons. The
strength of an oxidising agent depends on the ability to accept e-.
The more positive the standard electrode potential (Eo), the higher
the tendency of the cations or the non-metals to accept electrons.
. The strength of the oxidising power in descending order is Br2,
Cu
2+, K+. (OR Br2 is a stronger oxidising agent than Cu2+ ion
while Cu2+ ion is a stronger oxidising agent than K+ ion).
NaCl(aq) Na+(aq) + Cl(aq)
In dilute sodium chloride solution : At the anode, two possible
reactions that can take place are : 2Cl(aq) Cl2(aq) + 2e
- E0 = - 1.36 V 2H2O(l) O2(g) + 4H
+(aq) + 4e- E0 = - 1.23 V Since the Eo for the second reaction
is less negative, it will occur in preference to the first
reaction. Thus, water is oxidised to oxygen gas. In concentrated
soium chloride solution: In this case, Cl is oxidised in preference
to H2O. This is because the ease of a species to be discharged at
an electrode increases with concentration. Thus, chlorine is
produced as it is more concentrated.
30 1 1 1 1 1 1 1 1 1 1
-
(c)(i) (ii)
(iii) (iv)
2Zn + O2 2ZnO / 2Zn + O2 + 2H2O 2Zn(OH)2
............................................ Q = It = 2.1 x 105 x 1
= 2.1 x 105 C
......................................................................
O2 + 2H2O +4e 4OH 4F = 4(96500) C = 1 mol
2.1 x 105 C = (2.1 x 105) (4 x 96500) = 0.54 mol
......................................................................
Aluminium-air battery The cell can be recharged / No air pollutants
are produced.
1 1 1 1 1
19. (a)
19. (b)
(c)
The melting point increases in the order of P, Na, Al, Si. P
exists as P4 molecules, in a simple covalent molecular structure.
The bonds between the molecules are weak van der Waals forces. Na
and Al have giant metallic lattice structures. The bonds are strong
metallic bonds hence more energy is needed to break the bonds. The
metallic bond in Al is stronger than in Na because Al has 3 valence
electrons compared to only 1 in Na. Si has a giant covalent
structure bonded by strong covalent bonds. Sodium oxide is a basic
oxide, thus reacts with acid.
Na2O(s) + 2H+ (aq) 2Na+ (aq) + H2O (l)
Aluminium oxide is an amphoteric oxide which can react with both
acid and alkali. .
Al2O3 (s) + 6H+ (aq) 2Al3+ (aq) + 3H2O (l)
[accept using HCl /H2SO4 for equation, must be balanced] Silicon
dioxide, SiO2 and phosphorus pentoxide, P2O5 are acidic oxides,
thus no reaction with acid. Sodium carbonate .
2NaOH(aq) + CO2 (g) Na2CO3(s) + H2O (l)
1 1 1 1 1 1 1 + 1 1 1 1 1 1 1 1
20. (a)
(b)(i)
(ii)
The reaction proceeds as following : R-CH=CH2 + H2 R-CH2CH3
Nickel used in this reaction acts as a heterogeneous catalyst. .
Nickel is a transition element that provides a suitable surface for
the adsorption of the reacting molecules, RCH=CH2 and H2 by making
use of its outermost d orbitals. The adsorption brings the reacting
molecules closer to each other and at the same time weakens the
intramolecular covalent bonds. On top of that, it also holds the
molecules in the correct orientation for new bonds to be formed.
All these cause the rate of reaction to increase. When the bonds
are totally broken & the new bonds formed, the products are
released from the surface of nickel to provide a new surface for
the adsorption of other reacting molecules. Complex ion is an ion
formed when a central metal ion is bonded to a group of ions or
molecules through coordinate bonds. A ligand is an ion or molecule
that has lone-pair electrons that can be donated to a metal central
ion to form coordinate bonds. . Name :
Ethylenediaminetetraacetatocuperate(II) ion / (EDTA)copper(II)
Shape : Octahedral It is hexadentate ligand that forms six
coordinate bonds per molecule with the central Cu2+ ion.
1 1 1 1 1 1 + 1 1 + 1 1 2 1 1 + 1
