Chemistry 105 Practice
Midterm #2
[8] 1.
Provide a sketch for
the voltaic cell
Pb/Pb2+(aq)
(1.00M) // MnO4-(aq) (1.00 M), Mn2+ (aq)
(1.00M), H+(aq) (1.00M)/Pt
a) Identify
the anode and the cathode, show the direction of
electron flow and the direction of ion migration through the salt bridge. Give the balanced overall cell reaction and
calculate the cell voltage at standard conditions.
b)
Determine DG°298K and the equilibrium
constant for the electrochemical reaction.
c) Calculate the cell potential at 25°C if all concentrations of metal ions are
kept the same, but the pH of the right hand cell is increased from 0.00 to
4.00.
[6] 2.
Consider the following electrolytic cell:
Pt
/ HBr (aq) (1.00 M) // Cu(NO3)2
(aq) (1.00 M) / Pt
The circuit was closed and a constant current was
passed through the cell for 2.50 h. The
platinum cathode, which had an initial mass of 22.038 g
weighed 27.594 g after the electrolysis run.
The weight for the platinum anode was the same before and after the
electrolysis.
a)
Write down all of the half-reactions that could occur at the anode and at the
cathode.
b)
During the electrolysis, a colourless gas formed at the anode. Identify the gas and calculate what volume it
would occupy at STP.
[5]
3. The cumyl radical,
C9H11, dimerizes at room temperature to give (C9H11)2.
2 C9H11
(aq) ® (C9H11)2
(aq)
The
reaction is second order in C9H11. The rate constant for the dimerization
reaction at 25°C is 0.16 M-1s-1.
a)
Calculate the concentration of (C9H11)2 (aq)
at t=16.00 minutes in a solution that was initially 0.0800 M in C9H11
(aq).
b)
How many minutes will it take for 50% of the initial 0.0800 M C9H11
(aq) to dimerize in this solution?
[6]
4. The reaction rate for a hypothetical
process increases by a factor of four (4) when the temperature is raised from
15°C to 25°C.
a)
Find the activation energy in kJ/mol.
b)
Which of the following would affect the numerical value for the rate constant, kobs?
Circle your choice(s).
i) a decrease in
temperature
ii) reducing the
reactant concentration by one half
iii) adding a
catalyst
iv) changing the
solvent from water to an organic solvent, such as benzene
[5]
5. For the reaction:
Br- + OCl- ® Cl- + OBr-
The
empirical rate law is Reaction Rate = 125 s-1[Br-][ OCl-][
a)
Show that the reaction mechanism given below is consistent with the
experimental rate law.
b)
Calculate the rate constant for the RDS given that k1/k-1
= 3.0 X 10-6 M-1.
OCl-
+ H2O === HOCl +
HOCl +
Br- === Cl- +
HOBr (slow)
HOBr +
[5]
6. Answer each of the following short
answer questions:
a)
In a voltaic cell, the anode is connected to the ________________ terminal
(positive, negative).
b) The maximum work that can be done by an
electrochemical cell is _______________.
c) The half-life of first order reaction is (independent/dependent)
_________________ of/on the initial reactant concentration.
d) For a dead cell with the equation:
V(s) +
Pd2+ (aq) ® V2+
(aq) +
Pd(s)
Ecell
= ________________ V.
e) An aqueous solution of CoCl2 (aq)
(pH=7) was electrolyzed between two Pt electrodes. What gas will be formed at the anode?
_________________
[6] 7. The following voltaic cell was constructed at
a local electrochemistry research laboratory:
Mg/Mg2+(aq) (1.00 M) // Ag+ (aq) (1.00 M) / Ag (s)
Each
half cell has a volume of 0.250 L.
If
the cell delivered a current of 0.250 A for 12.0 h at 25°C,
a)
Calculate the concentration for each cation in its
respective compartment at t=12.0 h.
b)
Calculate the cell voltage at t=12.0 h.