THE COLLEGE OF THE BAHAMAS
FINAL EXAMINATION KEY
SEMESTER 01-2010
FACULTY OF PURE AND APPLIED SCIENCES
SCHOOL OF CHEMISTRY, ENVIRONMENTAL & LIFE SCIENCES
X NASSAU
FREEPORT
EXUMA
ELEUTHERA
DATE AND TIME OF EXAMINATION: Tuesday, April 20, 2010, 7 pm
DURATION: 3 HOURS
COURSE NUMBER:
CHEM 225
COURSE TITLE:
College Chemistry II
STUDENT NAME:
STUDENT NUMBER:
LECTURER’S NAME
INSTRUCTIONS TO CANDIDATES: This paper has 12 pages and 3 sections. Please follow the
instructions given with each section.
SECTION I: Multiple Choice Questions
Select the SINGLE best alternative in each of the following cases and indicate your answer by marking the
corresponding letter on the answer sheet provided.
C −3
2
1) For the equilibrium:
D −2
H2(g) + I2(g) l 2HI(g)
3
the equilibrium constant is 794 at 25°C. Hence
E only obtained by experiment.
the equilibrium constant for the reaction:
½H2(g) + ½I2(g) l HI(g)
6) The reaction between propanone (CH3COCH3)
is closest to:
and iodine in acidified aqueous solution is
A 28
described by the following equation:
5
B 6.3 × 10
CH3COCH3(aq) + I2(aq) →
C 397
CH2ICOCH3(aq) + H+(aq) + I-(aq)
D 1588
The reaction is first order with respect to both
propanone and hydrogen ion, but zero order
E 1.3 × 10-3
with respect to iodine. Which of the following
conclusions may be drawn from these
2) For a reaction which occurs in a single
observations?
elementary step, the equilibrium constant is
A The reaction must occur in more than one
related to the rate constants of the forward and
elementary step.
back reactions by:
B Iodine is involved in the rate-determining
kf
A K=
step.
kb
C Increasing the concentration of hydrogen ion
B K = k f kb
decreases the rate of reaction.
D Propanone is not involved in the ratek
C K= r
determining step.
kb
E Increasing the concentration of iodine
D K = k f − kb
increases the rate of the reaction.
E K = k f + kb
7) The rates of chemical reactions depend strongly
on temperature. Which one of the following
3) The rate equation for the reaction
statements best explains this observation?
aA + bB → cC + dD is:
A At high temperatures the activation energy
A rate = k[A][B]
of the reaction is lower.
a
b
B rate = k[A] [B]
B At high temperatures almost all molecules
c
d
are present as activated complexes.
[C] [ D]
C rate =
a
b
C At high temperatures the concentrations of
[A] [B]
reactants is higher.
1
1
c
d
[C] [ D ]
D At high temperatures the partial pressures of
D rate =
1
1
the reactants are higher.
[A] a [B] b
E At high temperatures a much larger
E can only be obtained by experiment.
proportion of molecules have sufficient
energy to react.
4) Most reactions:
A are zero-order with respect to products.
8) In which one of the following reactions (which
B are first order with respect to products.
take place in aqueous solution) is water acting
as an acid?
C are second order with respect to products.
D do not involve products.
A H2O + HCl l H3O+ + ClE have an order with respect to products that
B H2O + CH3COOH l H3O+ + CH3COOcan only be discovered by experiment.
C H2O + C2H5NH2 l C2H5NH3+ + OHD H2O + HSO4- l H3O+ + SO425) Consider the following reaction:
E 4H2O + Cu2+ l [Cu(H2O)4]2+
N2(g) + 3H2(g) → 2NH3(g)
The rate of disappearance of hydrogen divided
by the average rate of appearance of ammonia
is:
A 3
2
B 2
3
CHEMISTRY 225 SEMESTER 01-2010 FINAL EXAMINATION CONTD.
9) The decomposition of dinitrogen pentoxide in
B Increasing the total volume.
tetrachloromethane solution may be represented
C Increasing the concentration of reactants.
by the equation:
D Reducing the temperature of the system.
2N2O5 → 4NO2 + O2(g)
E Removing one of the products.
colourless
brown
The nitrogen dioxide is soluble in
tetrachloromethane whilst the oxygen is not.
Measurement of which one of the following
physical properties could NOT be used for
determining the rate of this reaction?
A Electrical conductivity of the solution.
B Volume of oxygen evolved.
C Absorbance of light by the solution.
D Mass of the solution.
E Pressure of oxygen evolved.
14) Given that the Ka values of the following five
acids decrease in the order: HI > HCl >
C6H5COOH > CH3COOH > HPO42-, which one
of the following salts would be expected to give
the highest pH value when in 0.1M solution?
A NaCl
B NaI
C C6H5COONa
D CH3COONa
E Na3PO4
10) The experimentally determined rate equation for
the reaction represented by the equation:
BrO3-(aq) + 5Br-(aq) + 6H+(aq) →
3Br2(aq) + 3H2O(aq)
+ 2
is: Rate = k[BrO3 ][Br ][H ]
If the concentration of each of the reactants is
doubled, which one of the following statements
regarding the rate of reaction is true?
A The rate is doubled.
B The rate is tripled.
C The rate is increased four-fold.
D The rate is increased eight-fold.
E The rate is increased sixteen-fold
11) At a certain temperature the partial pressures of
carbon monoxide, chlorine and carbon oxide
dichloride (COCl2) in the equilibrium mixture
defined by the equation:
CO(g) + Cl2(g) l COCl2(g)
were 2, 4 and 48 atm respectively. What is the
numerical value of Kp?
A 0.167
B 6
C 24
D 54
E 86
***************************
12) Consider the following system at equilibrium:
CaCO3(s) l CaO(s) + CO2(g)
∆H > 0
Which one of the following operations would
displace the equilibrium position to the right?
A Adding more CaCO3(s) to the system at
constant pressure.
B Adding more CaO(s) to the system at
constant pressure.
C Adding more CO2(g) to the system at
constant pressure.
D Heating the system.
E Adding an inert gas at constant volume.
13) Which one of the following changes the value of
the equilibrium constant for a reaction?
A Reducing the total pressure.
15) The Kb value for a certain base X- is 10-6 at
25°C. The Ka value for HX at the same
temperature is therefore:
A 108
B 10-6
C 106
D 10-8
E 6
16) Solutions of aluminium sulfate in water are
weakly acidic because:
A Water molecules bound to the aluminium
ion lose protons more readily than free water
molecules.
B Sulfuric acid is not a very strong acid.
C The second proton of sulfuric acid is lost
with some difficulty.
D Aluminium ions react readily with protons.
E Such solutions are usually impure.
17) Which one of the following statements
regarding the ionic product of water (Kw) is
NOT true?
A Kw is the equilibrium constant for the
reaction:
2H2O(l) l H3O+(aq) + OH-(aq)
B Kw is decreased by the addition of an acid to
an aqueous solution.
C Kw is altered by a change in temperature.
D Kw is constant for any dilute aqueous
solution at constant temperature.
E Kw is approximately 10-14 at 25°C.
18) A certain weak monoprotic acid has a pKa value
of 4.8. What is the pH of a buffer solution
which is 0.20 M in the weak acid and 0.20 M in
the sodium salt of the acid?
A 2.8
B 3.8
C 4.8
D 5.8
E 6.8
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CHEMISTRY 225 SEMESTER 01-2010 FINAL EXAMINATION CONTD.
19) Dilute sodium hydroxide solution gradually
Cu+(aq) + e- → Cu(s)
Eʅ = +0.52v
2+
+
becomes more dilute on standing. Which one of
Cu (aq) + e → Cu (aq)
Eʅ = +0.17v
the following statements is the best explanation
The standard electrode potential in volts for the
for this?
disproportionation:
A Sodium hydroxide is deliquescent.
2Cu+(aq) → Cu2+(aq) + Cu(s) is:
B Sodium hydroxide reacts with carbon
A -0.69
dioxide from the air.
B -0.35
C The solution tends to evaporate.
C +0.52
D Sodium hydroxide reacts with oxygen from
D +0.69
the air.
E +0.35
E Sodium hydroxide decomposes on standing.
20) Which one of the following statements
regarding a 10-8 M solution of hydrochloric acid
in pure water is correct?
A The solution has pH less than 7.
B The solution has a pH greater than 8.
C Such a solution cannot be prepared.
D The solution is a buffer solution.
E The solution is extremely acidic.
21) Which one of the following substances is the
strongest acid in aqueous solution?
A HF
B HCl
C HBr
D HI
E H2O
22) Which one of the following aqueous ions is the
strongest acid?
A Na+(aq)
B K+(aq)
C Mg2+(aq)
D Al3+(aq)
E Ag+(aq)
Questions 26) to 31) concern the following
experimental situation.
Six equilibrium mixtures (1) were made up with
various volumes of 0.10 M iron(III) nitrate solution,
dilute nitric acid and 0.10 M silver nitrate solution.
The solutions were stirred for 15 minutes until a
precipitate (2) formed, and then filtered into
separate test tubes. 5 cm3 aliquots were each mixed
with about 1 cm3 of 1 M iron(III) nitrate (3) and
titrated (4) with 0.010 M KSCN(aq). The titre
values were used to calculate the concentrations of
the various species present at equilibrium and the
equilibrium constant calculated.
26) The equilibrium studied, referred to in (1),
involves an equilibrium between
A nitrate ions, hydronium ions and nitric acid
molecules.
B iron(III) ions, thiocyanate ions and
thiocyanato-iron(III) complex.
C iron(III) ions, iron(II) ions, silver ions and
silver.
D iron(III) ions and iron(II) ions.
E silver ions, iron(II) ions, nitrate ions and
silver.
27) Precipitate (2) was
A silver nitrate
B silver chloride
C silver thiocyanate
D silver
E iron(III) thiocyanate
23) Which one of the following species is
amphiprotic?
A HCO3B H2CO3
C CO32D O2
E Fe2+
24) The e.m.f. of the cell:
Pt|H2(g) |HCl(aq)⌟CuSO4(aq)|Cu
is INDEPENDENT of which one of the
following?
A temperature
B concentration of hydrochloric acid
C concentration of copper(II) sulfate solution
D size of the platinum electrode
E pressure of hydrogen
28) The iron(III) nitrate (3) was added in order to
A oxidise any silver still present to silver ions.
B react with excess thiocyanate ions to form a
coloured complex.
C catalyse the reaction between titrant and
titrand in (4).
D prevent the forward reaction in the
equilibrium from occurring.
E prevent the precipitation of silver
thiocyanate.
25) Standard electrode potentials for the gain of one
electron by the ions Cu+(aq) and Cu2+(aq) are as
follows:
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CHEMISTRY 225 SEMESTER 01-2010 FINAL EXAMINATION CONTD.
29) The precipitate (2) was filtered off in order to
33) A catalyst is added to the equilibrium system:
A prevent the equilibrium concentration of
2SO2(g) + O2(g) l 2SO3(g) ∆H = -196 kJ mol-1
silver ions being changed during the titration
E
(4).
34) The temperature is increased for the equilibrium
B make the solution clear so that the end-point
system:
in (4) could be seen more clearly.
N2(g) + 3H2(g) l 2NH3(g) ∆H = -92.4 kJ mol-1
C prevent solid matter interfering with the
B
equilibrium reactions.
35) Hydrogen is added to the equilibrium system:
D remove excess silver ions from the solution.
H2(g) + I2(g) l 2HI(g)
∆H = +51.8 kJ mol-1
E collect it for weighing.
D
****************************************
30) Using ONLY the titre value in (4), together with
the concentration of the potassium thiocyanate
36) The oxidation of ammonia produces nitrogen
solution and the aliquot volume, one may
and water by the reaction:
calculate
4NH3 + 3O2 → 2N2 + 6H2O
A the equilibrium concentration of nitrate ion
If the rate of formation of N2 is 2.0 Ms-1, then
B the equilibrium concentration of iron(III)
the rate at which
ion.
A H2O is formed is 3.0 Ms-1
C the equilibrium concentration of silver ion.
B NH3 reacts is 8.0 Ms-1
D the initial concentration of silver ion.
C NH3 reacts is 1.0 Ms-1
E the initial concentration of iron(III) ion.
D O2 reacts is 0.67 Ms-1
E O2 reacts is 3.0 Ms-1
31) Nitric acid was added in order to
A neutralise any base which might have been
37) According to the Brønsted-Lowry definition, an
added accidentally.
acid is a substance which donates a
B prevent formation of hydroxo-complexes of
2+
A hydrogen atom.
iron(III) such as [FeOH] .
B hydrogen ion.
C oxidise iron(II) to iron(III).
C hydrogen molecule.
D redissolve any metals which might be
precipitated.
D hydride ion.
E keep the total concentration of nitrate ions
E hydroxide ion.
constant throughout.
Questions 32) to 35) concern the effect on the
equilibrium constant and the yield of product(s),
caused by changes made to an equilibrium system.
EFFECT ON FINAL YIELD
OF PRODUCTS
EFFECT
ON K
A
increases
increases
B
decreases
decreases
C
decreases
no change
D
increases
no change
E
no change
no change
Select from A to E the pair of effects produced
when the stated change is made to the equilibrium
system shown. Each letter may be used once, more
than once, or not at all.
32) The pressure is increased by decreasing the
volume in the equilibrium system:
2SO2(g) + O2(g) l 2SO3(g) ∆H = -196 kJ mol-1
D
38) Which does NOT constitute an acid/base
conjugate pair?
A H3O+/ H2O
B NH3/ NH2NH4+/ NH3
C
D H2SO4/ SO42E HNO2/ NO239) Which statement is usually true of an acid/base
indicator?
A It is neither an acid nor a base.
B It is either a weak acid or a weak base.
C It always changes colour at pH 7.
D It always changes colour at a pH above 7.
E It always changes colour at a pH below 7.
40) A 0.10 M solution of a weak acid has a pH of
3.0. Its % ionization is closest to:
A -3
B 10-3
C 0.10
D 1
E 3
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CHEMISTRY 225 SEMESTER 01-2010 FINAL EXAMINATION CONTD.
SECTION II: Short Answer Questions
Attempt all of the following questions and write your answers in the spaces provided on the question paper.
You may need the following information:
R = 8.31 J mol-1K-1 = 0.0821 L atm mol-1 K-1, Faraday’s constant, F = 9.65×104 C mol-1
1) The equilibrium constant, Kp, for the dissociation of dinitrogen tetroxide to nitrogen dioxide is 0.14 at
298 K. The reaction can be represented by the equation:
N2O4 (g) l 2NO2(g).
a) Find the equilibrium partial pressure of each gas when 0.030 mol of N2O4 is admitted into a 3.00 dm3
vessel at 298 K and the system reaches equilibrium.
(6)
b) Find the total pressure of the system at equilibrium.
(1)
c) What effect will the addition of 0.01 mol of an inert gas to the container have on the equilibrium
position? Give a reason for your answer.
(2)
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CHEMISTRY 225 SEMESTER 01-2010 FINAL EXAMINATION CONTD.
2) Use the following information wherever necessary.
Kw = 1.0×10-14 @ 25°C
Ka(CH3COOH) = 1.7×10-5 @ 25°C
Ka(CH2ClCOOH) = 1.3×10-3 @ 25°C
Ka(NH4+) = 5.6×10-10 @ 25°C
a) Explain the difference in acid strength between ethanoic acid
and chlorethanoic acid
(3)
b) Calculate the Kb of ammonia @25°C.
(2)
c) Calculate the pH of a 0.020 M solution of ammonium chloride.
(5)
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CHEMISTRY 225 SEMESTER 01-2010 FINAL EXAMINATION CONTD.
d) Explain why phenolphthalein (pKa = 9.4) would be useless as an indicator in the titration of NH3(aq)
with HCl(aq). (HCl in the burette.)
(2)
3) The rate law for the reaction P + 2Q → products is R =k[P]2[Q] with a rate constant of 1.4×10-4M-2 s-1.
a) Find the rate of the reaction the instant 30.0 cm3 of a 0.20 M solution of P are mixed with 20.0 cm3 of a
0.25 M solution of Q.
(3)
b) What is the value of the rate constant if the concentration of P is doubled and the concentration of Q is
kept constant?
(1)
1.4×10-4M-2s-1 (rate constant is not affected by concentration, only by temperature)
−
EA
c) The Arrhenius equation states that k = Ae RT .
i) The term e-Ea/RT represents a certain proportion of molecules in a reacting system. Describe this
proportion fully.
(1)
ii) The graph given below was drawn from data obtained by measuring the time (t) taken for a given
amount of P to be used up at various temperatures.
Ln(t) against 1/T
6.5
6
5.5
ln(t)
5
4.5
4
3.5
3
2.5
0.0031 0.0032 0.0033 0.0034 0.0035 0.0036 0.0037
1/t /K-1
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CHEMISTRY 225 SEMESTER 01-2010 FINAL EXAMINATION CONTD.
E
Given that ln( t ) = A + const. , find the activation energy of the reaction.
RT
(3)
4) Use the following table of standard redox potentials wherever necessary.
Eʅ/V
Reduction
-
-
Cl2(aq) + 2e → 2Cl (aq)
Fe3+(aq) + e- → Fe2+(aq)
2H+(aq) + 2e- → H2(aq)
Fe3+(aq) + 3e-→ Fe(s)
Fe2+(aq) + 2e- → Fe(s)
+1.36
+0.77
0.00
-0.036
-0.44
a) Iron(II) chloride can be prepared by the reaction of hydrochloric acid on iron metal.
i)
Write an ionic equation for the reaction and calculate the Eʅ value for the associated cell.
(2)
ii) Use Eʅ values to explain why iron(III) chloride is not formed in this reaction, but is formed by the
action of excess chlorine solution on iron(II) chloride.
(4)
iii) Draw a fully-labelled diagram to show the experimental set-up of the standard Fe3+/Fe2+ half-cell.(4)
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CHEMISTRY 225 SEMESTER 01-2010 FINAL EXAMINATION CONTD.
RT
b) Given the Nernst Equation, E = Eʅ ln Q
nF
Find the e.m.f. of the non-standard cell:
Fe(s) | Fe2+(aq, 0.010 M) ⌟ H+(aq, 0.020 M) | H2(g,1.5 atm) | Pt(s)
(4)
c) Acidified permanganate (MnO4-) oxidizes oxalate (C2O4H2) to carbon dioxide and water in aqueous
solution, whilst itself being reduced to Mn2+. Derive a balanced ionic equation.
(5)
SECTION III
Answer EITHER of the following questions in the spaces provided on the question paper. (If you answer both,
only the first will be marked.) These questions are worth 15 marks each.
1) For the reaction
2NO(g) + H2(g) → N2O(g) + H2O(g)
the following experimental rate data are collected in three experiments carried out at the same temperature.
INITIAL [NO] /M
INITIAL [H2] /M
INITIAL RATE OF N2O
FORMATION /M min-1
0·60
0·37
0·18
1·20
0·37
0·72
1·20
1·11
2·16
a) Write a rate law for the reaction.
(2)
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CHEMISTRY 225 SEMESTER 01-2010 FINAL EXAMINATION CONTD.
b) Why is it necessary to carry out the three experiments at the same temperature?
(1)
c) Calculate the rate constant for the reaction.
(2)
d) Calculate the initial rate of formation of N2O when the concentration of NO is 0.50 M, and the
concentration of H2 is 0.40 M.
(2)
e) Calculate the initial rate of consumption of NO when the initial rate of formation of N2O is
2.16 M min-1.
(2)
f) A suggested mechanism for the reaction is:
H2 + NO → HNO + H
(1) - slow
H + NO l HNO
(2)
2HNO → H2O + N2O
(3)
i)
Show that the proposed mechanism is consistent with the overall equation for the reaction:
2NO(g) + H2(g) → N2O(g) + H2O(g)
(2)
ii) Identify two intermediates.
(2)
iii) What rate law is implied by this mechanism?
(1)
iv) Given your answers to parts (a) and f(iii) above, what can you say about the proposed mechanism?(1)
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2) The solubility product of Ca(OH)2 is 5·5×10-6 at 25ÿC.
a) Find the molar solubility of Ca(OH)2 in water at 25ÿC.
(4)
b) Calculate the molar solubility of Ca(OH)2 in 0.050 M Ca(NO)3(aq) at 25°C.
(3)
c) Calculate the molar solubility of Ca(OH)2 solution at pH 12.00 at 25°C.
(3)
d) Explain the difference in solubility between cases (a) and (b) in terms of Le Chatelier’s Principle.
(3)
e) EDTA (ethylenediamine tetraacetate) forms a soluble complex with calcium ions. Explain how would
the addition of EDTA affect the solubility of Ca(OH)2.
(2)
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