Chem 5 Jeopardy!

Equilibria

100

(3 min) If 0.450 M sodium azide (NaN₃) is added to water at 25ºC. If the K_b of hydrazoic acid (HN₃) is 1.9 x 10^-5, please determine the resulting pH.

pH = 9.19

200

(3 min) Determine the K_a of the unknown acid “HA” given that the K_b of SO₃^2- is 9.80 x 10^-8 and the equilibrium constant for the reaction below is 5.1 x 10^-12.

HA + SO₃^2- → HSO₃^- + A^-

K_a = 5.00 x 10^-5

300

(5 min) A 400 mL solution of hypochlorous acid (HClO) is titrated with 2.00 M potassium hydroxide (KOH). If 150 mL of KOH were required to reach the equivalence point, please determine the pH of the solution when 65 mL of KOH were added. The K_a of hypochlorous acid is 3.0 x 10-8.

pH = 7.407

Thermo

100

(2 min) If the enthalpy of combustion of some hydrocarbon “X” is -2877 kJ/mol and the entropy of combustion is -1.488 kJ/Kmol, please calculate the range of temperatures at which this reaction is spontaneous.

T <= 1933 K

200

(3 min) Please calculate the Gibbs free energy change of the following reaction if the initial partial pressures of SO₂, O₂, and SO₃ are 0.50 atm, 1.60 atm, and 1.20 atm (respectively) and the reaction has an equilibrium constant of 7.900 x 10⁸ at 527ºC.

2 SO₂ (g) + O₂ (g) ⇋ 2 SO₃ (g)

 DeltaG_rxn = -127.77 kJ/mol

300

(5 min) The following reaction is conducted in a constant pressure (“coffee cup”) calorimeter at atmospheric pressure (1 atm) and room temperature (25ºC) with 36.033 g carbon. If the heat capacity of the calorimeter is 6450 J/K and the reaction temporarily heats up the calorimeter by 34.795ºC, calculate the change in internal energy and the molar enthalpy of the reaction at 25ºC:

C (s) + 2 H₂ (g) → CH₄ (g)

 Delta H^o_rxn = -74.81 kJ/mol

 Delta U = -221.95 kJ

EChem

100

(3 min) Balance the following redox reaction in basic conditions:

PbO (s) + ClO₃^- (aq) → Cl₂ (g) + PbO₂ (s)

5 PbO + H₂O + 2 ClO₃^- → Cl₂ + 2 OH^- + 5 PbO₂

200

(4 min) State the oxidation numbers for each atom in the below half-reactions that make up an electrochemical cell. What is the Gibbs free energy (in standard conditions) of the cell reaction if dichromate (Cr₂O₇^2-) is oxidized? Is this value concurrent with our qualitative expectations based on the reduction potentials?

Cr₂O₇^2- (aq) +14 H₃O⁺ (aq) + 6 e^- → 2 Cr³⁺ (aq) +21 H₂O (l)  E^o_red = 1.33 V

PbSO₄ (s) + 2 e^- → Pb (s) + SO₄^2- (aq)    E^o_red = -0.357 V

Cr⁶⁺ ; O^2- ; H⁺ ; Cr³⁺ ; Pb²⁺ ; S⁶⁺ ; Pb⁰   

 DeltaG^o_rxn = 976.62 kJ

Given that the species with the higher reduction potential was oxidized, we would expect that this redox reaction will be nonspontaneous. The positive change in free energy supports that qualitative expectation.

300

(5 min) A Galvanic cell is constructed according to the half-reactions and balanced redox reaction below. If the 0.500 L anode solution (at 25ºC) is initially at pH = 10 and Cu⁺ is added as 0.500 mol NaCl, please determine the voltage generated by the cell.

Mn(OH)₃ (s) + e^- → Mn(OH)₂ (s) + OH^- (aq)        E^o_red = -0.40 V

Cu⁺ (aq) + e^- → Cu (s)    E^o_red = 0.522 V

Mn(OH)₂ (s) + OH^- (aq) + Cu⁺ (aq) → Mn(OH)₃ (s) + Cu (s)

 Delta E_cell = 0.685 V

Mystery

100

(2 mins) A derivation of the Henderson-Hasselbalch equation from the K_a expression of a generic weak acid “HA”. When should you use this equation?

Ka = ([H^+][A^-])/([HA])

pKa = -log(Ka) = -log(([H^+][A^-])/([HA])) = -log(H^+) - log([A^-]/([HA]))

pKa = pH - log([A^-]/([HA])) = pH + log([HA]/([A^-]))

pH = pKa - log([HA]/([A^-]))

200

(3 min) Assuming you could calculate the heat and entropy change of a system during some isothermal process, please (qualitatively) explain how you would calculate the entropy of the surroundings in i) a reversible process and ii) an irreversible process.

∆S_surr,_rev = q_surr/T= -q_{sys, rev}/T = -∆S_{sys, rev}

∆S_{surr, irrev} = q_surr/T = -q_{sys, irrev}/T

Heat flow is a process function, so q_rev =/ q_irrev

300

(7 mins) A galvanic cell is constructed from the following two half-reactions:

ClO₄^- (aq) + 2 H₃O⁺ (aq) + 2 e^- → ClO₃ (aq) + 3 H₂O (l)    E^o_red = 1.19 V

I₂ (s) + 2 e^- → 2I^- (aq)        E^o_red = 0.535 V

The reaction is conducted in standard conditions with the exception of the concentration of H₃O⁺. Instead of adding 1.00 M H₃O⁺, 1.00 M of a weak acid (“HA”) is added. If the cell produces 0.312 V at these initial concentrations, please determine the K_a of the weak acid. (R = 8.314 J/Kmol, F = 96,485 C/mol)

K_a = 2.53 x 10^-12

1000

Final Jeopardy! Category: Electrolytic Cells.

You have 1 minute to determine your wager.

An electrolytic cell is constructed to isolate lithium metal. At the anode, hydrogen gas is bubbled at a partial pressure of 1 atm through a buffered solution past a platinum electrode. At the cathode, a platinum electrode is submerged in a 3M solution of LiCl.

The anode is buffered by a weak acid HA with pKa 4.75, which is prepared by addition of 3 equivalents of HA with 1 equivalent of its conjugate base A^-.

To power this electrolytic cell, you may choose between four batteries of voltage 2V, 3V, 4V, or 5V. What is the minimum amount of energy required to isolate 2g of lithium metal, assuming the concentrations of Li⁺ and H₃O⁺ at the cathode and anode respectively do not change appreciably throughout the reaction? Assume standard temperature.