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Unformatted text preview: CH302 Practice Exam 1TA style Answer Key 1. Rearrange the Gibbs free energy equation ( G = H- T S) to solve for the temperature at a phase transition. a. T = ( H- G)/ S b. T = H/ S c. T = - G d. T = - G/ S At a phase transition delta G = 0. The temperature dependence at a phase transition can be determined from thermodynamics. 2. What will happen to vapor pressure when non-volatile solute A is added to a pure solution B? a. Vapor pressure of A increases b. Vapor pressure of B decreases c. Vapor pressure doesn't change d. Boiling point decreases of A inrcreases. e. Boiling point of B decreases Adding a non-volatile solute (its vapor pressure is essentially zero) to any solvent, will decrease the measured vapor pressure of the solvent, by a factor equal to the solvents new mole fraction. 3. Which of the following is a possible combination of values for H lattice , H hydration and H solution . All values are in kJ mol-1 . a. -12, -22, -34 b. 67, 9, 58 c. 31, -18, 49 d. 47, -43, 4 H lattice is always + and H hydration is always -. Their sum is H solution , which can be positive or negative. 4. Which of the following is always true for phase diagrams? a. The slope of the melting curve is always positive. b. The slope of the condensation curve is always negative. c. There are exactly 3 phases for all chemicals. d. When going from left to right on the graph, pressure and temperature both increase along the condensation curve. e. The critical point is located above 400 K. Melting curves can have a negative slope (ie water). The condensation curve always has a positive slope. Many chemicals have numerous solid phases, and some even have more than one distinct liquid phase. The critical point can occur at a variety of temperatures. 5. At what temperature will water boil on top of a mountain if the normal boiling point is 100 C at 1 atm. (You should not need to look at this graph to answer the question) a. Below 100 C b. At 100 C c. Above 100 C d. Water will not boil on top of a mountain. Pressure is lower at higher altitudes. Following the evaporation curve on the phase diagram from 1 atm to a lower pressure you see that temperature decreases. 6. Consider a 27 g sample of ice at 1 atm. Initially, the sample is frozen at -20 C. How much heat must be added to the sample for it to become a liquid at 78 C. Heat capacity of ice: 2 J/gK. Heat capacity of water: 4 J/gK. H of melting: 330 J/g a. 9.5 kJ b. 18.4 kJ c. 5.3 kJ d. 10.6 kJ q = mc T (for the ice) + m H (for melting) + mc T (for the water) q = 27 g * 2 J/gK * 20 K + 27 g * 330 J/g + 27 g * 4 J/gK * 78 K q = 18.4 kJ 7. As the temperature of a solvent (increases/decreases), the solubility of (some/all) gases increases....
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This note was uploaded on 08/24/2011 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.
- Spring '07