Chem 11 Jeopardy!

QM and MO Theory

100

(2 min) The work function of copper is 7.53 x 10^-19 J. Calculate the energy of an electron ejected at half of the maximum velocity by irradiation with a wavelength of 77.2 nm. The mass of an electron is 9.11 x 10^-31 kg.

4.56 x 10^-19 J

200

(3 mins) Solve for the radial and angular nodes for a hydrogen 3px orbital.

 

r = 6a₀ = 3.174Å

Angular nodes:

theta = 0 and π,  

phi = 2 and 3π2

300

(6 min) Consider an electron in a 2D box which is 16.0 nm by 12.0 nm. The electron can emit 3 distinct wavelengths. Find the location of all nodes and the maxima and minima of the wave function.

Quantum state: [1,2] 

Node: y = 6 nm

Maxima: {8,3}

Minima: {8,9}

Acid/Base EQ

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

Thermodynamics

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) Calculate the Gibbs free energy change of the following reaction at 527ºC 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 carried out in a constant pressure calorimeter at atmospheric pressure (1 atm) and room temperature (25ºC) with 36.033 g of carbon and excess hydrogen gas. 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

Electrochemistry

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 CuNO₃, 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

Kinetics

100

(2 min) Radon-222 has a half-life of 4 days. How long does it take for 83% of radon-222 from a sample to decay?

8.83x10⁵ s = 10.23 days

200

(3 mins) Salivary amylase is present in the mouth and breaks down complex carbohydrates into simpler sugars in chemical digestion. Consider the reaction of 2.00 M of a starch polymer with 5 x 10^-6 M salivary amylase. Salivary amylase releases substrate at one-eighth the rate it releases product, and binds substrate at 100,000 times the rate it releases substrate. If k₂ is 4 x 10³ s^-1, determine the rate of reaction.

2.00 x 10^-2 M/s

300

(5 min) Given the following elementary steps, prove that the rate of the overall reaction is equal to the rate indicated below. You may assume steady state conditions for all intermediates:

A + B → C

2B + C → D

C → E

D + E → F

(k_1[A][B]^2sqrt(k_2k_3))/(k_2[B]^2+k_3)

Mystery

100

(2 mins) Derive 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) Calculate the entropy change from heating 1 mol N₂ from 50K to room temperature. ∆H_vap = 5.58 kJ/mol, ∆H_fus = 0.360 kJ/mol, and the solid, liquid, and gas c_P values are 2.00, 1.43, and 1.04 J/gK (respectively). Nitrogen melts at -210.ºC and boils at -195.8ºC. Assume constant pressure.

∆S = 138.4 J/K

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: Concentration Cell.

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 standard reduction potential of Li⁺ is -3.04 V.

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 2.0 g 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.