Rutgers Camden - Department of Chemistry
Chem 346 – Spring 2011
Midterm Exam #2
Section 1- Concepts and Definitions
(50 % of the total for this Exam, 5 points each)
1. Use the phase diagram for methanol below to answer the questions below.
a) If the pressure of the system is fixed at a pressure of 400 psia, raising the temperature from 273 K to 573 K would result in a transition from ______to ______.
b) At 473 K, the liquid phase of methanol is stable down to a pressure of about ______psia.
c) If the system is at supercritical point at T0 and P0 as shown, by increasing or decreasing which parameters can the supercritical phase be obtained?
2. The Figure below shows schematically the distribution of chemical compound in the closed vessel at points A, B, C and D, with bottom of the reactor being the liquid and top – the vapor phase.
Use the correct operators (= < > ≤ ≥ ≠ ≈) to place into the generalized expression
A B C D
to relate the chemical potentials for that compound at these points A, B, C and D.
3. In the Figure below, the dependence of chemical potential of pure substance is shown as function of which thermodynamic variable as below:
a) pressure;
b) temperature;
c) molar volume;
d) molar entropy;
e) none of the above.
4. One of the most central concepts discussed this Semester relates to the changes observed for a particular thermodynamic quantity. The driving force for a phase change of the substance, the change of the chemical system towards equilibrium and the electromotive force as learnt in Electrochemistry are all the examples where the _____ is minimized. Circle the correct answer below.
Helmholtz energy Internal energy Gibbs energy Entropy Enthalpy
5. For the reversible chemical reaction involving gases A and B:
A <=> 2 B
use the Le Chatelier principle to decide the following: when the pressure of the whole system is decreased from 2 atm to 1 atm, the amount of compound B present in equilibrium will:
a. Decrease;
b. Increase;
c. Remain constant.
6. Two ideal gases A and B are being mixed. Sketch below the following versus the molar fraction xA of gas A:
a. Gibbs energy of mixing Δmix G;
b. Entropy of mixing Δmix S.
Be sure to indicate the correct sign of the Δmix G and Δmix S, i.e. positive, zero or negative.
7. For the dilute solutions, provide up to four examples of physical phenomena observed experimentally that represent the colligative properties of the solvent or the solute. Indicate if these are the properties of the solvent or of the solute that are being changed.
8. For the reversible chemical reaction:
2A <==> B,
write the mathematical expressions for the following:
a. For the ideal conditions (non-interacting substances), write the reaction quotient Q expressed in molar fractions x of these substances;
b. For the non-ideal conditions, reaction quotient Q expressed in activities of these substances;
c. For the non-ideal conditions, reaction quotient Q expressed in molar fractions x of these substances.
9. For galvanic cell shown below, complete the erased terms in the Figure below to correctly assign the standard notations for the electrodes (above) and electrochemical processes (below) taking place at them.
10. For chemical reaction taking place in the electrochemical device, assign the correct symbols for ΔGr and electromotive force E in the Figure below.
Possible symbols: = ≥ ≤ ≠ ≈
Example: E = 0.
Section 2- Problems (50 % of the total for this exam, 12.5 points each)
1. At 25 °C, the density of a 50 % by weight solution of ethanol in water is 0.914 gram per cm3. Given the partial molar volume of water in this solution is 17.4 cm3/mol, calculate the partial molar volume of the ethanol.
2. Consider a container of volume 10.0 L that is divided into two compartments of equal size. In the left compartment, there is nitrogen at 1.0 atm and 25 °C. In the right compartment, there is hydrogen at the same temperature and pressure. Calculate the entropy and Gibbs energy of mixing when the partition between two compartments is removed. Assume that these gases are perfect.
3. Using the following reaction and thermodynamic data at 25 °C as below, calculate the value of constant K of thermodynamic equilibrium at 25 °C and 85 °C.
2H2O2 (aq) <==> 2H2O (l) + O2 (g)
Compound / Δf° H, kJ/mol / Δf° G, kJ/molH2O2 (aq) / -191.17 / -120.35
H2O (l) / -285.83 / -237.13
O2 (g) / 0 / 0
4. Dinitrogen tetroxide N2O4 is 18.46 % dissociated at 25 °C and 1.00 bar in the equilibrium:
N2O4 (g) <==> 2NO2 (g)
Calculate the:
a. K at 25 °C;
b. Δr Gø;
c. K at 100 °C.
Use the constant enthalpy Δr Hø =+57.2 kJ/mol over the temperature range of interest.
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