Chem 150
Problem Set Electrochemistry and Thermochemistry
1. Given below is a sketch of a Voltaic Cell. Name the two electrodes:
The copper electrode is the
The silver electrode is the
The u-shaped glass tube filled with KNO3 connecting the two half cells is called the
Write down the reactions that occur on both electrodes as well as the overall cell reaction.
Cathode reduction:
Anode oxidation:
Overall:
Calculate the cell voltage E°cell. Assume both half cells to be in their standard state.
Finally, draw an arrow in the diagram above to indicate in which direction electrons flow between the
electrodes.
2. Draw a standard hydrogen electrode and connect it to a half cell consistent of a zinc road immersed
in a 1 M zinc nitrate solution. Label the electrodes anode and cathode. Show equations for the reaction
on both electrodes and give the overall cell reaction as well as the cell Voltage.
3. Which is by far the best material to be used in the manufacture of a salt water fishing wheel?
stainless steel
zinc plated steel
chromium plated brass
4. What is anodic protection? Give two examples.
5. Balance the following redox reaction.
H+ + Ag + HNO3 → Ag+ + NO + H2O
6. Given below is a sketch of a Voltaic Cell. One half cell consists of a platinum wire dipping in to a
solution that is 0.5 M in Cu2+ and 0.1 M in iodide whilst the other half cell is made up of 0.5 M aq.
sulfuric acid in which a platinum wire is immersed that is continuously purged with sulfur dioxide under
standard pressure (1 atm). Ka1 for H2SO4 is >>1, ka2 is 1.1 x 10-2.
0.5 M H2SO4 (aq)
Cu2+(aq 0.5 M), I- (aq 0.1 M), CuI(s)
Label the two electrodes as anode and cathode.
Write down the overall cell reaction
Calculate the cell voltage. (see end of problem set for relevant reduction half potentials)
7. Draw a diagram for the entropy of a pure substance as a function of temperature (0-500 K). Assume
two phase transitions in this temperature range (melting and boiling).
8. Calculate the enthalpy of formation of Methanol (CH3OH(l)) from standard enthalpies of formation of
CO2 (ΔHf° = -393.5 kJ/mol), H2O (ΔHf° = -285.8 kJ/mol) and the standard enthalpy of combustion for
methanol (-726.56 kJ/mol).
9. Calculate the standard enthalpy of combustion of methane (CH4) from the standard enthalpy of
formation of methane (ΔHf° = -74.8 kJ/mol), CO2 (ΔHf° = -393.5 kJ/mol) and H2O (ΔHf° = -285.8 kJ/mol)
10. Complete the following phrase:
The Gibbs free energy for a spontaneous process is ______________________
11. Which of the following (if any) are state functions (depend only on the final and initial state of a
system and not on the path used to get there):
Heat, work, inner energy, enthalpy, entropy, Gibbs free energy
12. Name the three systems below. Describe the differences between them in terms of the flow of heat
and matter.
13. This question is related to the experiment Bromination of Acetone where we reacted acetone and
bromine (in the presence of acid) according to the equation bellow:
CH3C(O)CH3 + Br2 → CH2BrC(O)CH3 + HBr
We measured the time it took for the bromine to completely disappear. We ran 4 experiments at room
temperature, one in a cold water bath at 11 C and one in a hot bath at 41 C and observed that the rate
of the reaction depends on the temperature.
A valid question that arose during one of the lab periods was the following. Since this reaction is
spontaneous it is probably exothermic and releases heat. Wouldn’t this heat effect our measurements?
Try to answer the question with the following data I found on our reactants and products. Note that the
HCl we added does not show up in the net stoichiometric reaction. It functions as a catalyst and is not
consumed during the reaction.
species: (gas phase data!)
CH3C(O)CH3(g)
Br2(g)
CH2BrC(O)CH3(g)
HBr(g)
ΔfH (in kJ/mol)
-217.1
+30.9
-181
-36.3
Assume that ΔH for the gas phase reaction is similar to ΔH for the reaction in solution. This will not
produce a huge error in dilute solution with one exception. HBr will react further in water and ionize
completely according to:
HBr(g) + H2O →H3O+ + Br-
ΔH = -85 kJ/mol
Hint: This heat must be added to the heat of the reaction between the acetone and the bromine!
Assume that the heat capacity of you mixture is that of water (4.184 J/gK) and neglecting the
calorimeter constant for your conical flask calculate the temperature increase of your 50 g solution.
Advanced Problem:
D.W. Mitchell, Industrial and Engineering Chemistry, September 1949, p 2027-2031
It is difficult to measure the heat of formation of magnesium nitride from the elements. This reaction
has to be carried out at high temperatures in a furnace. How can we possibly get room temperature
data (e.g. standard state ΔfH )
Mitchell determined the enthalpy (from here on referred to as heat) of formation of Magnesium nitride
(6) at room temperature and 1 atm by measuring the heat of solution of magnesium nitride in
hydrochloric acid (1) and comparing it to the heat of solution of magnesium in hydrochloric acid (2), the
heat of formation of ammonia (3), The heat of condensation of ammonia (4) and the heat of solution of
ammonia in hydrochloric acid (5)
(1) Mg3N2(s)+ 8HCl (aq) → 3MgCl2(aq) + 2NH4Cl(aq)
ΔH1 = -287,894 cal/mol
(2) Mg(s)+ 2HCl (aq) → MgCl2(aq) + H2(g)
ΔH2 = -111,322 cal/mol
(3) 1.5H2(g) + 0.5N2(g) →NH3 (g)
ΔH3 = -11,036 cal/mol
(4) NH3 (g) → NH3 (l)
ΔH4 = -5,005 cal/mol
(5) NH3 (l) + HCl (aq) → NH4Cl(aq)
ΔH5 = -16,043 cal/mol
(6) 3Mg(s) + N2(g) → Mg3N2(s)
ΔH6 = ?
Calculate the enthalpy (heat) of formation of magnesium nitride (6) in kJ/mol and compare it to the
enthalpy of formation of magnesium oxide (-601.7 kJ/mol).
Formulas and constants
ΔH° = ΣvpΔHf°(products) – ΣvrΔHf°(reactants)
Ecell = E°cell -
x log Q
Reduction half potentials which are not on your handout:
Cu2+(aq) + I-(aq) + e- → CuI(s)
+ 0.86 V
SO42- (aq) + 4H+ (aq) + 2e- → SO2 (g) + 2H2O (l)
+ 0.170 V