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Name
SECTION
TOTAL:
TA Name
Chemistry 130 (1,2)
Dr. J. F. C. Turner
Examination 2
2 hours
Please answer all parts of all questions and show all your work
You may use:
Non-programmable calculators
You may not use:
Any written materials (cards or notes etc) or any other electronic device
1.(a) Perchloric acid, HClO 4 , is a strong acid. Write down the balanced chemical equation for the dissociation of
perchloric in acid in water.
2 points
(b)
The acid constant for perchloric acid, pK A,HClO , is given by pK A,HClO =
perchloric acid.
4
4
10 .
Write down K A,HClO
4
for
4 points
(c)
Write down the equation that represents the autoprotolysis of water and the value of the autoprotolysis
constant, K W .
K W=
(d)
4 points
Write down the relationship between p H , p OH and pK W
2 points
(e)
Define the term 'strong acid'
2 points
(f)
Calculate p H and p OH for the following concentrations of aqueous perchloric acid
Concentration / mol L
1
pH
pOH
0.00001
0.001
0.1
1
6 points
(g)
Does water act as a buffer in this situation? Underline your answer.
Yes
No
2 points
(h)
Write down the balanced reaction between aqueous perchloric acid and aqueous potassium hydroxide.
4 points
(i)
What volume of a 0.05 M solution of potassium hydroxide, KOH aq , is required to completely react with 40 ml
of 0.1 M HClO 4 aq ?
6 points
(j)
What is the p H and p OH of the solution that results when perchloric acid reacts completely with potassium
hydroxide?
2 points
(k)
The acid constant for perchloric acid, pK A,HClO , is given by pK A,HClO =
name of the conjugate base for perchloric acid?
4
4
10 . Write down the formula and the
4 points
(l)
The acid constant for perchloric acid, pK A,HClO , is given by pK A,HClO = 10 . Using the relationship
pK A
pK B = pK W that relates the acid constant of an acid to the base constant for the conjugate base, calculate
pK B for the conjugate base of perchloric acid.
4
4
4 points
(m)
Is the conjugate base of perchloric acid a strong or weak base? Underline your choice
Strong
Weak
2 points
44 points
2(a)
Hypochlorite ion, OCl aq and iodide ion, I aq react in aqueous solution in the presence of hydroxide ion.
OH aq , according to the following reaction
OCl - aq
I- aq
OI- aq
Cl - aq .
Although hydroxide ion does not appear in the stoichiometric reaction above, it is involved in the mechanism,
which contains more than one elementary step. An experimental determination of the dependence of the rate law on
the concentrations of OCl aq , I aq and OH aq yielded the following data:
-
1
-
1
-
1
4
1
1
[OCl aq ]0 / mol l
[I aq ]0 / mol l
[OH aq ]0 / mol l
mol l s
0 / × 10
0.004
0.002
1.0
4.8
0.002
0.004
1.0
5.0
0.002
0.002
1.0
2.4
0.002
0.002
0.5
4.6
0.002
0.002
0.25
9.4
From these data, determine the orders of reaction for each of the ions and the overall reaction order.
Expt
1
2
3
4
5
Order of the reaction with respect to OCl aq =
Order of the reaction with respect to OH aq =
Order of the reaction with respect to I aq =
Overall order =
(b)
10 points
The first step in the mechanism, which is not the rate-determining step, for this reaction is the fast equilibrium
with the hypochlorous acid reacting further to give the products. If the concentration of OH aq is increased in
this equilibrium, predict the effect on this equilibrium using Le Chatelier's principle.
4 points
(c)
How does this dependence on Le Chatelier's principle appear in the rate law for this reaction?
2 points
(d)
Is hypochlorite ion, OCl
aq
, a strong or weak base? Underline your answer
Strong
Weak
2 points
pK A for hypochlorous acid, HOCl, is given by pK A,HOCl = 7.53 . Calculate p H and p OH for a 1.3 M
(e)
solution of hypochlorous acid.
6 points
(f)
Calculate the p H of a 0.75 M solution of sodium hypochlorite, NaOCl
aq
.
6 points
30 points
3.(a) Write down the equation for Raoult's Law, which relates the pressure of a solution to the components of the
solution
2 points
(b)
When may Raoult's law be applied?
4 points
(c)
Provide a definition of a perfect or ideal solution, in terms of intermolecular forces that are present in that
solution
4 points
(d)
An ideal or perfect solution contains 275 mLs of toluene, (C7H8) and 400 mLs of benzene (C6H6). Calculate
(i) the total pressure in Torr for this mixture,
(ii) the partial pressure of toluene and benzene for this mixture
(iii) the composition of the vapor above this mixture in terms of the mole fraction of benzene and the mole fraction
of toluene
1
1
given that the density of toluene is toluene =0.864 g mL , the density of benzene is benzene=0.867 g mL , the vapor
°
°
pressure of pure toluene is P toluene =28.4 Torr and the vapor pressure of pure benzene is P benzene=95.1 Torr . The
-1
relative atomic mass of carbon = 12.011 g mol and the relative atomic mass of hydrogen = 1.0079 g mol-1
Partial pressure of benzene =
Mole fraction of benzene in the vapor =
Partial pressure of toluene =
Mole fraction of toluene in the vapor =
Total pressure =
10 points
(e)
100 grams of cholesterol, C27H45OH is added to the toluene/benzene solution.
Under these conditions,
cholesterol has no measurable vapor pressure and the solution is still ideal. The relative atomic mass of oxygen =
15.996 g mol-1. What are the number total number of moles present in the resulting solution?
f)
4 points
For the new cholesterol/toluene/benzene solution calculate
(i) the total pressure in Torr for this mixture,
(ii) the partial pressure of toluene and benzene for this mixture
(iii) the composition of the vapor above this mixture in terms of the mole fraction of benzene and the mole
fraction of toluene
Partial pressure of benzene =
Mole fraction of benzene in the vapor =
Partial pressure of toluene =
Mole fraction of toluene in the vapor =
Total pressure =
10 points
34 points
4.(a)
, EA.
Write down the expression for the Arrhenius equation which relates the rate constant, k, to the activation energy
(b)
Write the Arrhenius equation in a form which relates ln k to the activation energy, EA.
2 points
2 points
The energy profile for an elementary reaction along a reaction path is shown below.
B
E
A
D
C
(c)
Which chemical species exist at A, B and C?
A =
B =
C =
6 points
(d)
What are the energies associated with D and E?
D =
E =
4 points
(e)
Is this reaction endothermic or exothermic?
(f)
For the forward reaction, write down the Arrhenius equation for the rate constant k1
2 points
2 points
(g)
For the reverse reaction, write down the Arrhenius equation for the rate constant k
to the activation energy that you use.
1
, paying careful attention
2 points
(h)
In terms of the forward and reverse rate constants, write down the equilibrium constant, K , for this reaction.
2 points
(i)
Sulfur dioxide and oxygen react at 973 K according to the following reaction
SO2
1
O
2 2
g
SO3
g
g
The equilibrium constant, K C at this temperature is K C = 20 . Calculate K C for the reaction
2 SO3
g
2 SO2
g
O2
g
8 points
(j)
Write down the general relationship between K C and K P for a gas phase reaction.
4 points
K
(k)
For the reaction in which sulfur trioxide decomposes to oxygen and sulfur dioxide, calculate
P , given that
R = 8.314 J mol K 1
4 points
(l)
Is a relationship described in part (j) possible for a reaction which occurs exclusively in the liquid or solid phase?
Explain your answer
4 points
42 points
End of Examination