POK OI HOSPITAL
TANG PUI KING MEMORIAL COLLEGE
SECOND
TERM EXAMINATION ( 1997-1998 )
S6 CHEMISTRY I
Class : 6 S Date : __________________
Name : ________________ Time allowed: 180 minutes
Class Number : ________________ ( P.1 - P.15 /38 )
Instructions to students:
1. There are THREE sections in this paper, Section A, Section B and Section C.
2. Section A carries 60 marks, Section B carries 20 marks and Section C carries 20
marks. The total score is 100 marks.
3. All questions are COMPULSORY. Write your answers in the spaces provided in this
Question/ Answer Book.
4. A Periodic Table is printed on the last page of the question paper.
________________________________________________________________________
Section A
1. (a) Dichromate(VI) ion, Cr2O72- (aq) reacts with iodide ion, I- (aq) in an acidic
solution. When 25.0 cm3 potassium dichromate(VI) was treated in an acidic
solution with excess potassium iodide solution, the iodine liberated required
18.6 cm3 of 0.10 mol dm-3 sodium thiosulphate solution for complete reaction.
(i) Write a balanced equation for the following reaction:
(1) dichromate(VI) ion reacts with iodide ion in the acidic solution
(2) iodine reacts with sodium thiosulphate solution
(ii) Calculate the concentration of potassium dichromate(VI) solution.
(b)
|
Indicator |
Range
of changing colour |
|
A |
3.4 - 5.4 |
|
B |
5.4 - 7.8 |
|
C |
7.9 -9.9 |
|
D |
9.8 - 11.3 |
|
E |
10.5 - 12.5 |
In the titration of HA against BOH, which of the about indicator should be used,
explain your choice quantitatively.
Given : Ka of HA is 1.8 x 10-5 mol dm-3, BOH is assumed to be completely
ionized in dilution and both titrations are about 0.2 M.
Kw = 1.0 x 10-14 mol2 dm-6
2. (a) Explain briefly why you would regard each of the following statements as either
true or false.
(i) sp2 hydridization orbitals are often occupied by 1 ¡¨s¡¨ electron and 2 ¡¨ p¡¨
electrons.
(ii) If A-A, and B-B are covalent compounds then the bond energy of A-B is the
average of bond energy of A-A and B-B.
(iii) The electrons in “s¡¨s¡¨ orbitals are always more stable than those in “p¡¨p¡¨ orbitals
with the same principal quantum number.
( 3 marks )
(b) (i) Of the following chemical species write the electronic configurations of those
that are isoelectronic:
Ne, Al, S2-, Cl-, K+, Ar
(ii) Amongst the isoelectronic species in (i), predict with reasons the most inert
one.
(
3 marks )
(c) This question is concerned with the changes in temperature and electrical conductivity
when a 0.1 M solution of sodium hydroxide is added to an acid.
(i) On the axes in the above figure, if the acid being titrated is 25 cm3 of 0.1 M
hydrochloric acid, sketch the relationship between the volume of alkali added and
each of the properties measured.
(ii) On the same axes provided use broken lines to indicate the results you expect to
obtain when 0.1 M ethanoic acid were used instead of hydrochloric acid.
3. (a) Classify the following molecules in the linear, trigonal planar, pyramidal, tetrahedral or
trigonal bipyramidal.
BCl3,
NCl3, CO2, HCN, CO32- , NO3-
, PCl5
Linear:
Trigonal planar:
Pyramidal:
Tetrahedral:
Trigonal bipyramidal:
( 4 marks )
(b) (i) Draw the structure of standard hydrogen electrode.
(ii) When the hydrogen electrode is maintained at the standard conditions it is
arbitrarily assigned zero. State these conditions and give the ways to achieve
these in the laboratory.
(iii) Write the cell diagram for the standard hydrogen electrode.
4. (a) Calculate the composition of the vapour above a liquid mixture of methanol and
ethanol containing equal mass of the two at temperature T.
Given: the mixture behaves as an ideal solution,
molecular mass of methanol = 32 ,
molecular mass of ethanol = 46,
at T, vapour pressure of methanol = 80 kPa and
vapour pressure of ethanol = 45 kPa.
(b)(i) What is the Raoult's’sLaw?
(ii) Heat is evolved when chloroform, CCl4 is mixed with propanone. The graph
below is a plot of the temperature rise against percentage composition by
volume:
(1) How can you investigate the variation of temperature rise with the
percentage by volume of chloroform and propanone?
(2) How would you account for the evolution of the heat in terms of the
intermolecular forces of the molecules?
( 3 marks )
(c) For the following equilibrium,
N2O4 (g) === 2NO2 (g)
Given that the equilibrium constant is 2.5 x 104 Nm-2 at 1 atm ( 1.01 x 105 Nm-2 )
and 25 ¢XC.
(i) Calculate the degree of dissociation.
(ii) Calculate the partial pressure of NO2.
5. (a) (i) State Hess¡¦s’s Law.
(ii) The following is a table of some standard molar enthalpy changes of
formation.
|
Substance |
Hf
¢X
(298K) / kJ mol-1 |
|
ClF (g) |
-63.4 |
|
Cl (g) [ i.e 1/2 Cl2 (g) ® Cl (g) ] |
121.7 |
|
F (g) [ i.e 1/2 F2 (g) ® F (g) ] |
79.0 |
Use these values to calculate the molar bond dissociation enthalpy of Cl-F.
( 3 marks )
(b) Briefly explain each of the following terms:
(i) rate of reaction
(ii) rate constant
(iii) order of reaction
( 3 marks )
(c) Consider the energy diagram shown below for a certain reaction which take place
with three steps.
(i) What is the general name for X and Y respectively?
(ii) Which is the rate-determining step? Explain your answer.
(iii) Is the reaction endothermic or exothermic? Explain your answer.
6. (a) Give the IUPAC names for the following compounds.
( 3 marks )
(b)
|
Isomers |
melting
points / K |
boiling
points / K |
|
A. CH3-CH2-CH2-CH2-CH3 |
138 |
310 |
|
B. CH3-CH2-CH-CH3 CH3 |
113 |
302 |
|
CH3 C. CH3-C-CH3 CH3 |
253 |
284 |
The above data are the melting points and boiling points of the isomers having
formula C5H12. Explain the trend of
(i) melting points
(ii) boiling points
(c) By using Fischer projection, give all the stereoisomers of 2,3,4-trichloropentanol.
7. (a) The following experimental set up is to separate substance A and B of boiling
points 138 and 273 ¢XC respectively, and obtain substance A at reasonable purity.
(i) Give the name of the process.
(ii) Give the advantages in using this method for purification and also state its
limitations. What is the use of the glass tubing?
(iii) How would you proceed to further purity the sample from contaminant after
this process?
(b) The following sketches are the arrangement of boiling the mixture and distillation
to obtain the product.
(i) Give the names of A, B and C.
(ii) Why a water bath is used?
(iii) Use arrows to show the inlet and outlet of water.
(iv) How is it possible to tell when the distillation is completed?
¡@
( 4 marks )
8. In the experiment to determine the standard enthalpy change of neutralization of
sodium hydroxide and sulphuric acid, 50 cm3 of 0.40 M sodium hydroxide were
titrated, thermometrically, with 0.5 M sulphuric acid. The results were plotted as
follows:
(i) Define the standard enthalpy change of neutralization¡¦.
(ii) What is a thermometric titration?
(iii) How do you account for the shape of the graph?
(iv) (1) Calculate a value for the enthalpy of neutralization of sodium hydroxide
with sulphuric acid. ( The specific heat capacity of water is 4.2 J K-1 g-1 )
(2) What assumptions have you made in your calculations?
( 10 marks )
Section C
Marks will be allocated approximately as follows:
chemical knowledge 50%
organization 30%
presentation ( including proper use of English ) 20%
Equations, suitable diagrams and examples are expected whereas appropriate.
The examiners are looking for the ability to analyse, to evaluate and to express ideas
clearly.
9. Discuss the effect of changes of temperature, pressure and concentration of reactants
on
the reversible reaction:
H2 (g) + I2 (g) === 2HI (g)
It is given that the activation energies for the forward and reverse reaction are 174 and
185 kJ mol-1 respectively.
( 20 marks )
_______________________________________________________________________
Chemical knowledge :
Organization :
Presentation :
___________________________
END OF PAPER
POK OI HOSPITAL
TANG PUI KING MEMORIAL COLLEGE
SECOND TERM
EXAMINATION ( 1997-1998 )
S6 CHEMISTRY II
Class : 6S Date : __________________
Name : ________________ Time allowed: 180 minutes
Class Number : ________________ ( P.1 - P.7 /38 )
Instructions to students:
1. Answer ALL questions.
2. Write your answers on the answer book provided.
3. The total score is 100 marks.
4. A Periodic Table is printed on the last page of this paper.
________________________________________________________________________
1. (a) When solutions of iodine and propanone are mixed with dilute sulphuric acid, the
reaction:
occurs at a moderately speed. In order to determine the order of the reaction with
respect to I2, a reaction mixture was prepared from:
4 cm3 of 2.0 mol dm-3 propanone
2 cm3 of 0.01 mol dm-3 iodine
2 cm3 of 2.0 mol dm-3 sulphuric acid
2 cm3 of water
(i) Explain why the initial concentration of propanone used exceeds greatly than
that of iodine.
(ii) Suggest two techniques which would be suitable for determining the reaction
rate of this reaction, indicating why each is suitable in this particular case.
(iii) A graph of [I2] against time was plotted. Explain briefly how the reaction rate
at various time can be obtained from the graph.
(iv) To determine the order of the reaction with respect to I2, a further graph must
be plotted. Draw the expected shape of this graph if the reaction were of first
order. Label the axes of this graph clearly.
( 7 marks )
(b) (i) Write down the principle of carbon-14 dating.
(ii) Carbon-14 dating shows that a piece of ancient wood gives 10 counts per
minute per gram of carbon ( cpmg-1 ) compared with 15 cpmg-1 of carbon
from a sample of new wood. The half-life of 14C is 5600 years. What is the
age of ancient
wood?
( 8 marks )
(c) Consider the following hypothetical mechanism for a reaction:
Step 1 : A + B ------k1-------> E + F
k1 = 1.1 x 10-6 mol-1 dm3 s-1
Step 2 : A + E ------k2-------> H
k2 = 2.4 x 10 -2 mol-1 dm3 s-1
Step 3 : A + F ------k3-------> G
k3 = 7.6 x 10 -2 mol-1 dm3 s-1
Step 4 : H + G + B ------k4-------> C + D
k4 = 5.3 x 10 -1 mol-2 dm6 s-1
(i) Write the overall reaction.
(ii) State the order of elementary step 4.
(iii) Which one is the rate-determining step for the overall reaction? Explain
briefly.
(iv) What is the order for the overall reaction? Explain your answer.
(v) Write the rate law for the overall reaction in terms of disappearance of A.
( 5 marks )
2. (a) (i) The bond lengths ( in nanometres ) and also the dipole moments ( in Debye
units ), of the gaseous hydrogen halides are shown below:
|
|
HF |
HCl |
HBr |
HI |
|
Bond
length |
0.092 |
0.127 |
0.141 |
0.161 |
|
Dipole
moment |
1.91 |
1.05 |
0.80 |
0.42 |
(1) What is meant by ¡§dipole moment¡¨?
(2) Comment briefly on the reasons for the decrease in dipole moment from
HF to HI.
(ii) (1) State whether the following molecules would have a dipole moment or
not, giving reasons.
CH3Cl,
CCl4, BCl3, NH3
(2) CO2 has no dipole moment, while SO2 has quite a large one. What
difference in structure does this suggest? ( 8 marks )
(b) (i) Explain the terms ¡§ mass number¡¨ and ¡§atomic number¡¨.
(ii) (1) Describe briefly the principle of the mass spectrometer for measuring mass
number.
(2) The following is a mass spectrometer trace for a compound.
Suggest a value for the relative molecular mass of the compound and indicate
two elements which might
contain.
( 7 marks )
(c)
Consider the tabulated information and answer the questions follow:
|
Boron |
Aluminium |
||
|
fluoride, BF3 |
chloride, BCl3 |
fluoride, AlF3 |
chloride, AlCl3 |
|
gas |
gas |
solid |
Solid |
|
b.p. : -110 ¢XC |
b.p. : 12 ¢XC |
sublimes at 1291 ¢XC |
sublimes at 180 ¢XC |
|
Element |
B |
Al |
F |
Cl |
|
Electronegativity ( Pauling ) |
2.0 |
1.5 |
4.0 |
3.0 |
|
Atomic radius / nm |
0.090 |
0.130 |
0.072 |
0.099 |
(i) Aluminium chloride sublimes quite readily below 200¢XC and in the gas phase
then exists largely as a dimer Al2Cl6 (g), but aluminium fluoride sublimes at
a much higher temperature.
(1) Draw the structure of the dimer and explain why the electronic structure
of monomer make such dimerization.
(2) Explain why the properties between AlCl3 and AlF3 have such great
differences.
(ii) Explain why fluoride and chloride of boron show quite similar physical
properties, but they show no tendency to dimerize.
( 5 marks )
3. (a) The following is the experimental procedure for determining the equilibrium
constant of the reaction between silver nitrate solution and iron(II) sulphate
solution.
Fe2+ (aq) + Ag+(aq) ==== Fe3+(aq) + Ag (s)
Step 1 : 100.00 cm3 of 0.200 M AgNO3 and 100.00 cm3 of 0.24 M
FeSO4(NH4)2SO4 are mixed in a dry bottle.
Step 2 : The mixture is allowed to stand for one hour until an equilibrium is
attained and then the mixture is filtered.
Step 3 : 25.00 cm3 of the filtrate is titrated with KNCS and requires 6.20 cm3 of
0.10 M KNCS for complete reaction. [ NCS- (aq) reacts with Ag+ (aq)
before forming a complex with Fe3+(aq) ]
(i) Calculate the initial concentration of Ag+ (aq) and Fe2+ (aq) immediately
after mixing and before any reaction has occurred.
(ii) Is this titration required an indicator? Explain briefly.
(iii) What are the equilibrium concentrations of Ag+ (aq), Fe2+ (aq) and
Fe3+ (aq)?
(iv) Calculate the equilibrium constant, Kc.
(10 marks)
(b) The following data show how the pH changes during a titration when aqueous
0.100 M NaOH is added to 25.00 cm3 aqueous ethanoic acid.
|
Volume of 0.100 M NaOH / cm3 |
0.0 |
2.5 |
5.0 |
10.0 |
15.0 |
17.5 |
20.0 |
21.5 |
25.0 |
35.0 |
|
pH |
2.9 |
4.0 |
4.3 |
4.7 |
5.2 |
5.5 |
6.4 |
11.2 |
12.0 |
12.4 |
(i) Use the data to plot a titration curve on the graph paper.
(ii) State the pH range for the equivalence point and explain why the curve has the
shape obtained in your graph.
(iii) Calculate the initial concentration of the ethanoic acid.
(iv) Using any suitable pH value to calculate Ka for ethanoic acid.
( 10 marks )
4. (a) The following is a list of standard electrode potentials.
X I2 (aq), 2I- (aq)½Pt (s) +0.53V
Y Cu+ (aq)½Cu (s) +0.53V
Z Fe2+ (aq)½Fe (s) -0.44V
(i) Define the term standard electrode potential.
(ii) (1) Write down the cell diagram of the cell which could be formed from
electrodes X and Y.
(2) Which electrode is positive?
(3) Calculate the e.m.f. of the cell.
(4) Would the e.m.f. of the cell be increased, decreased or remain the same by
increasing the concentration of iodine relative to the concentration of
iodide? Give your reasoning. ( 8 marks )
(b) The following diagram is the structure of an alkaline dry cell.
When it is producing an electric current, the following changes occur at the two
electrodes.
Anode : Zn(s) reacts to form ZnO (s)
Cathode : MnO2 (s) reacts to form Mn2O3 (s)
(i) Write half equations for the reactions at the anode and at the cathode, and the
equation for the overall reaction that occurs in the alkaline dry cell.
(ii) Write the cell diagram for the cell, using the IUPAC convention.
(iii) Suppose that an alkaline dry cell were manufactured using cadmium metal
rather than zinc. What effect would this have on the cell e.m.f.?
ZnO (s)½Zn (s) E¢X = -1.22 V
Cd(OH)2 (s)½Cd(s) E¢X = -0.76 V
(iv) The most promising batteries employ lithium as the anodic metal. What
advantages might be realized by using lithium rather than
zinc? ( 7 marks )
(c) A student tried to show that zinc can prevent iron from rusting even iron is not
completely covered by zinc. He then carried out an experiment as show below:
Write down the observation in this experiment, and explain your answer briefly,
giving equation(s) whenever
appropriate.
( 5 marks )
5. (a) (i) (1) Define the partition coefficient.
(2) State two factors that influence the partition of a solute between two
immiscible solvents?
(ii) The partition coefficient of a substance Y between ethoxyethane and water
is 3, Y being more soluble in ethoxyethane. 100 cm3 of an aqueous solution
containing 10g of Y is extracted with
(1) 100 cm3 of ethoxyethane
(2) four successive quantities of 25 cm3 of ethoxyethane.
Calculate the mass of Y left in the aqueous phase in each
experiment. ( 7 marks )
(b) Sulphur dioxide has its triple point at -72.7 ¢XC and 16.3 torr. Solid SO2 is more
dense than liquid SO2.
(i) Sketch the pressure-temperature phase diagram for SO2.
(ii) What would be observed when SO2 at -75 ¢XC and 70 torr had its external
pressure slowly reduced to 5 torr at constant temperature? Draw a straight line
on the diagram to show your answer.
(iii) What is the significance of the triple point?
(iv) What is the practical difference between the triple point of a substance occurs
at a pressure greater than 1 atm and less than 1 atm?
( 8 marks )
(c) The boiling point - composition diagram for ethanol / ethyl ethanoate system at
1 atm pressure is shown below.
A mixture containing 0.8 mole fraction of ethanol is distilled completely at 1 atm
pressure.
(i) What is the composition of the first vapour to come off?
(ii) Can the above solution be separated completely by fractional distillation?
Explain briefly.
(iii) Is the deviation from Raoult¡¦s law positive or negative?
(iv) Does the mixing of the two liquids result in an increase or a decrease in the
intermolecular attraction as compared with that exists in separate pure liquids?
( 5 marks )
END OF PAPER