Second Midterm From Last Fall's Chem 12 Course

Second midterm from last fall's Chem 12 course (plus one from the first midterm) — 8. from Ex1 (a) Write a balanced equation for combustion of liquid benzene, C6H6. Remember, combustion of a hydrocarbon is a reaction with oxygen to produce water and carbon dioxide. (b) Use the enthalpy of formation data given below to calculate the enthalpy of combustion of benzene. (c) How much energy is released by the combustion of 5 g of benzene? (MW 78 g/mol) (d) If this energy could be used to heat 1 kg of water (Cp = 4.184 J g–1 °C–1), at 25 °C, with 75% efficiency, what would the final temperature of the water be? ΔH°f (kJ/mol) Benzene (C6H6, liq.) H2O (g) CO2 (g) CH4 (g) CO (g) +49.0 –242.0 –393.5 –75 –110.5 1. Sodium metal reacts violently with water to produce H2 gas and NaOH (aq). (a) Write a balanced equation for the reaction. (b) From the table of standard reduction potentials, select two reactions that combine to give the reaction above, and calculate ε° for the overall reaction in part a. (c) Use your result from part b to calculate ΔG°, per mol of Na, at 298K. (d) Use S° and ΔHf° values in the tables provided to calculate ΔS° and ΔH°, per mol of Na. (e) Use your ΔH° and ΔS° values from part d to calculate ΔG° at 298K. 2. A Galvanic cell can be constructed by using the two half-reactions below. CrO42– ———> Cr3+ Ag+ ———> Ag ε° = 1.330 V ε° = 0.800 (acidic conditions) (a) Combine the two half-reactions and write a balanced equation for the overall cell reaction under acidic conditions. (b) What is ε° for the cell? (c) Sketch the Galvanic cell that you would construct from these two half-reactions. Be sure to indicate the electrode materials and what ions are present in each solution. (d) On your drawing, label the anode and cathode, show the direction of electron flow through the wire. If your cell includes a salt bridge, let's assume it's filled with aq KNO3, and indicate the direction of ion flow within the salt bridge. (e) Describe briefly what, if anything, would happen if the electrodes were carefully removed (still connected) and switched. 3. The formation of solid cadmium hydroxide from aqueous Cd2+ and hydroxide ions is represented below. Cd2+ (aq) + Cd(OH)2 (s) 2 HO– (aq) The reaction can be studied electrochemically, as you did in lab with the Ag/AgCl cell. The two relevant half-reactions are: Cd2+ (aq) + 2 e– Cd (s) Cd(OH)2 (s) + 2 e– Cd (s) + !° = – 0.400 V 2 HO– (aq) !° = – 0.821 V (a) Demonstrate that these can be combined to give the overall reaction above, and calculate ε° for the overall reaction. (b) Label the half-reactions you wrote above to indicate which one occurs in the anode compartment of the cell, and which occurs in the cathode compartment. page 2 (c) At 80°C, when [Cd2+] = 0.0630 M and [OH–] = 0.0100 M, what is the measured cell voltage? (d) What is ΔG° at 25°C for the reaction at the top of this problem? (e) What is the equilibrium constant for this reaction at 25°C? (f) What is the value of Ksp for Cd(OH)2 at 25°C? (g) When water is saturated with Cd(OH)2, what is the concentration of Cd2+? 4. (a) List two species that can be reduced by metallic aluminum under standard conditions. (b) Under standard conditions, is Cu2+ capable of oxidizing NaI? (c) In addition to being a strong acid, nitric acid, HNO3, is an oxidizer. Under standard conditions, which of the following species can be oxidized by nitric acid? (circle one or more) Ag Au KCl CdSO4 H2 O 5. 78.1 g (1.00 mol) of benzene is cooled from 60.0°C to its freezing point, 5.5 °C, where the entire mass becomes solid. Liquid benzene's heat capacity, Cp = 135.7 J mol–1 K–1, and benzene's ΔHfusion = 9.866 kJ mol–1. Calculate ΔS and ΔH for the conversion of liquid at 60.0°C to solid at 5.5°C. 6. 1.00 mol of an ideal gas having an initial volume of 1.00 L and a pressure of 2.00 atm is allowed to expand isothermally into an evacuated vessel so that the total volume is 5.75 L. (a) How much heat flows during the expansion, and does it flow into or out of the system? (b) What would the heat flow be if the expansion had been carried out reversibly? Would heat flow into the system or out of the system during such a reversible expansion? (c) Calculate ΔS for the expansion. (d) If the same expansion were carried out with a higher initial pressure, corresponding to a higher initial temperature, how would ΔS and the reversible heat flow differ from the expansion above. No numbers are necessary; just state whether ΔS and qrev would be larger, smaller, or the same. page 3 7. Understanding the conformations (the three-dimensional structures) of long chains of atoms is important in polymer chemistry. Chain conformation can affect physical properties like elasticity, durability, as well as the chemical properties of polymers. We can understand some of the thermodynamic CH2 CH2 CH2 CH3 principles involved in polymer chain CH2 CH2 CH2 conformations by looking at smaller molecules like CH3 octane. The conformation with the lowest enthalpy of formation is the zig-zag conformation shown at right. We'll call this "conformation A". There is also a group of conformations having a single "kink" in the chain. Each is 3.8 kJ/mol higher in enthalpy than the zig-zag. As shown below, a single –CH2—CH2– unit can be "kinked, in two mirror-image ways. This is true for each of the 5 C-C bonds indicated with arrows above. It's not necessary for us to analyze these conformations in detail; we will simply treat them as a group, called "conformations B". CH3 CH2 CH2 CH2 CH2 CH2 CH2 or CH2 CH2 CH3 CH2 CH3 (a) How many conformations are there in the group we refer to as B? (b) What is ΔS° for the conversion of A to B? (Again, "B" always refers to the entire ensemble of conformations with one "kink" in the chain.) (c) Use the ΔH° given above and the ΔS° you calculated to determine ΔG° for the equilibrium at 25°C. (If you couldn't determine ΔS° in part b, then use ΔS° = 25.0 J mol–1 K–1.) A B (d) Calculate the equilibrium constant, K, at 25°C, and calculate the % of A and B present at equilibrium. (e) What is the temperature (in °C) at which 50% of the molecules have the zig-zag conformation (A) and 50% have one kink in the chain (B)? 8. For the reaction below, (a) use the enthalpies of formation and 3rd-law entropies in the tables provided to calculate ΔH° and ΔS° (per mol). Note that for the reaction below, everything is in the gas phase, but the table only has values for the product, ethanol, in its liquid state. Drat! But according to NIST, ΔH°vaporization of ethanol is 42.0 kJ/mol, and its boiling point is 78°C. O H2C gas CH2 ethylene oxide + H2 phase OH CH3 CH2 ethanol (b) Calculate ΔG° at 100°C. page 4