ESSC CHEM 105 FINAL MOCK EXAMINATION
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Student Name:
Student ID:
Instructions:
1. Answer all questions. Show all work for full masks.
2. Non-programmable calculators are permitted.
3. Include units in all numerical answers. Constants and formulas are on the last
page.
Hess’ Law and standard enthalpies of formation
Q1. Diamond and graphite are two crystalline forms of carbon. At 1 atm and 25°C,
diamond changes to graphite so slowly that the enthalpy change of the process must be
obtained indirectly. Using equations from the numbered list below, determine ΔH for
C (diamond) ⟶ C (graphite)
(1)
(2)
(3)
(4)
C (diamond) + O2 (g) ⟶ CO2(g)
2CO2(g) ⟶ 2CO(g) + O2(g)
C (graphite) + O2 ⟶ CO2 (g)
2CO(g) ⟶ C (graphite) + CO2(g)
ΔH = -395.4 kJ
ΔH = 566.0 kJ
ΔH = -393.5 kJ
ΔH = -172.5 kJ
Second and Third Laws: entropy and spontaneity
Q2. The oxidation of sulfur dioxide is an important step in the production of sulfuric acid:
2SO2 (g) + O2 (g) ⟶ 2SO3 (g)
At 298K, the thermodynamic data for this reaction are: ΔG = -141.6 kJ, ΔH = -198.4 kJ,
and ΔS = -187.9 kJ/K.
(a) Determine whether the reaction is spontaneous at 25 °C (298K). Predict how the
spontaneity of the reaction changes as the temperature increases.
(b) Assuming ΔH and ΔS remain constant with temperature (no phase change
occurs), determine whether the reaction is spontaneous at 900°C (1173 K).
(c) Estimate the temperature (in K) at which the reaction becomes non-spontaneous.
Q3. Predict which substance has greater molar entropy. Explain.
(a) Butane CH3CH2CH2CH3 (g) or 2-butene CH3CH=CHCH3(g)
(b) Ne (g) or Xe (g)
(c) CH4(g) or CCl4 (l)
Free energy, work, and equilibrium
Q4. Acetylene is produced commercially by the partial oxidation of methane. At 1500°C
and pressures of 1-10 bar, the yield of acetylene is about 20%. The major side product is
carbon monoxide, and some soot and carbon dioxide also form.
(a) At what temperature is the desired reaction spontaneous:
2CH4 + ½ O2 ⟶ C2H2 + 2H2 + H2O
(b) Acetylene can also be made by the reaction of its elements, carbon (graphite) and
hydrogen. At what temperature is this formation reaction spontaneous?
(c) Why must this reaction mixture be immediately cooled?
Application of electrochemistry
Q5. A chromium plating process operates at a constant current of 3.20 A using an
aqueous Cr³⁺ solution. How long (in minutes) will it take to deposit 2.40 g of Cr(s)?
Enthalpy, calorimetry, and bond energies
Q6. Gasoline is a mixture of hydrocarbons, but the heat released when it burns is close
to that of octane, C₈H₁₈(l) (ΔH°f = −250.1 kJ/mol). Research is underway to use H₂ from
the electrolysis of water in fuel cells to power cars instead of gasoline.
(a) Calculate ΔH° when 1.00 gal of gasoline (d = 0.7028 g/mL) burns to produce carbon
dioxide gas and water vapor.
(b) How many liters of H₂ at 25°C and 1.00 atm must burn to produce this quantity of
energy?
(c) How long would it take to produce this amount of H₂ by electrolysis with a current of
1.00 × 10³ A at 6.00 V?
(d) How much power in kilowatt-hours (kW·h) is required to generate this amount of H₂?
(1 W = 1 J/s, 1 J = 1 C·V, and 1 kW·h = 3.6 × 10⁶ J.)
(e) If the cell is 88.0% efficient and electricity costs $0.123 per kW·h, what is the cost of
producing the amount of H₂ equivalent to 1.00 gal of gasoline?
Q7: Determine the standard cell potential and the cell potential under the stated
conditions for the electrochemical reactions described here. State whether each is
spontaneous or nonspontaneous under each set of conditions at 298.15 K.
(a)
(b) The cell made from an anode half-cell consisting of an aluminum electrode in 0.015
M aluminum
nitrate solution and a cathode half-cell consisting of a nickel electrode in 0.25 M
nickel(II) nitrate solution.
(c) The cell comprised of a half-cell in which aqueous bromine (1.0 M) is being oxidized
to bromide ion (0.11 M) and a half-cell in which Al 3+ (0.023 M) is being reduced to
aluminum metal.
Q8: Determine ΔG and ΔG° for each of the reactions in the previous problem.
Q9: What happens to (becomes more negative or more positive) for the following
chemical reactions when the partial pressure of oxygen is increased?
Q10: Write the balanced cell reaction for the cell schematic below, calculate the standard cell
potential, and note whether the reaction is spontaneous under standard state conditions.
Cu(s)∣Cu2+(aq)∥Au3+(aq)∣Au(s)