Name_____________GSI______________ Page 1 of 8 Chemistry 1A, Fall 2008 KEY Midterm Exam #2 October 14, 2008 (90 min, closed book) Name:____________________ _____ SID:___________________________________ GSI Name:________________ • The test consists of 4 short answer questions and 11 multiple choice questions. • Put your written answers in the boxes provided. Answers outside the boxes may not be considered in grading. • Show your work to receive the maximum credit possible. • Write your name on every page of the exam. Page Points Score Question A 2 15 Question B(i-vii) 3 17 Question B (viii-ix) Question C (bonus) 4 6 Question D 5 27 Question E 6 13 Question F and G (multiple choice) 7-8 24 Total 100 Useful Equations and Constants: pH = - log[H3O+] pX = - log X ][][logHAApKpHa−+= N0 = 6.02×1023 1/mol Strong acids and bases: HCl LiOH HNO3 NaOH H2SO4 KOH HClO4 HBr HIName_____________GSI______________ Page 2 of 8 012345Time (sec)Number (moles)A) Gas Phase Equilibria Consider the reaction between ethylene, C2H4, and hydrogen, H2 to form ethane, C2H6. C2H4 (g) + H2 (g) ' C2H6 (g) The progress of a reaction between 5.0 moles of C2H4 (g) and 5.0 moles of H2 (g) is shown in the graph below. i. What is the equilibrium constant for the reaction? Assume the reaction container is 1.0 liter. Show your work. Imagine that the same reaction starts with the ethylene, C2H4 (g), and hydrogen, H2 (g) reactants only. This reaction mixture results in the moles listed below at equilibrium. moles ethylene, C2H4 (g) at equilibrium = 1.0 moles moles hydrogen, H2 (g) at equilibrium = 0.50 moles moles ethane, C2H6 (g) at equilibrium = 2.0 moles ii. How many moles of ethylene, C2H4 (g), and hydrogen, H2 (g) were present at the start of the reaction? C2H4 (g) + H2 (g) ' C2H6 (g) Initial X Y 0 Change -2.0 -2.0 +2.0 Equilibrium 1.0 0.50 2.0 X – 2.0 = 1.0, X = moles of C2H4 (g) = 3.0 moles Y – 2.0 = 0.50, Y = moles of H2 (g) = 2.5 moles Imagine that the same reaction begins with 5.0 moles of ethane, C2H6 (g) and no reactants. iii. moles C2H6 + moles C2H4 = ____5.0 moles _____ iv. moles C2H6 + moles H2 = ______5.0 moles ____ v. Draw a graph showing the progress of the reaction. Label the curves showing all 3 molecules. may have reasoned it out or done ICE table, if ICE then 4x2 – x – 5 = 0 and x = 1 = moles C2H4 and H2 moles of C2H6 (g) moles of C2H4 (g) or H2 012345Time (sec)Number (moles)initial equilibrium ()()()0.40.10.10.4]][[][24262===MMMHHCHCKeqmoles of C2H4 (g) or H2 moles of C2H6 (g)Name_____________GSI______________ Page 3 of 8 B) Heterogeneous Equilibria The common light bulb has a thin wire made of tungsten, W, in a glass bulb with all the air removed. The tungsten wire glows when it is heated with an electric current. If the tungsten wire breaks, the light bulb no longer lights. i. Write the chemical equation for the sublimation of tungsten. W (s) ' W (g) ii. Write the equilibrium expression for the sublimation of tungsten. KP = PW(g) or Keq = [W(g)] iii. For the sublimation reaction, K (300°C) < K (3500°C). Explain why. Heat is required to convert tungsten solid to gas (endothermic). Increasing the heat will favor more products (tungsten in gas form). Since the K expression is only dependent on the products formed, increasing the pressure or concentration of the products increases the value of K. After the light bulb is in use for a long time, you can observe a black substance coating the glass. Eventually, the light bulb no longer lights. iv. What is the black substance?____W (s)____ v. Why does it deposit on the inside of the glass bulb? The glass bulb is cooler than the tungsten filament. As the tungsten gas comes in contact with the cooler surface, some of the tungsten gas deposits as solid. Another way to explain this could be, since K decreases with lower temperatures, less product is favored so some of the tungsten gas deposits on the glass. Light bulbs with a small amount of iodine gas, I2, are called tungsten-halogen light bulbs. Iodine reacts with tungsten according to the chemical equation given on the left below. W (s) + 3 I2 (g) ' WI6 (g) For this reaction, K (300°C) > K (3500°C) vi. What happens when you raise the temperature? favors reactants more favors products more no preference vii. Explain why the addition of iodine makes the light bulbs last longer. The sublimation of tungsten is favored at higher temperatures and the solid tungsten gets “used up” as it is converted to a gas. Since the reaction with Iodine favors the reactants, I2 (g) and W (s), more at higher temperatures, more W(s) will deposit back onto the filament rather than on the cooler outside of the glass. Twire = 3500°C Tglass = 300°CName_____________GSI______________ Page 4 of 8 B) Heterogeneous Equilibria (continued) All of the air has been removed from inside the glass bulb, but there is always a small amount of water. The water reacts with the tungsten according to the chemical equation given below. W (s) + 4 H2O (g) ' WO2(OH)2 (g) + 3 H2 (g) viii. Balance the reaction. ix. How would you classify this reaction? single exchange or single displacement or redox C) Bonus Point What is the chemical analogy to this cartoon? Equilibrium reactions vs. product-favored reactions Customers (exiting) ' Customers (entering) VS. Customers (exiting) Æ Customers (entering) The left “reaction” is at equilibrium with K≈1 and the right “reaction” favors products (customers entering) with a K>>1.Name_____________GSI______________ Page 5 of 8 D) Lakes and Ksp A lake is surrounded by rocks containing gypsum, a mineral with the chemical formula CaSO4(s) (Ksp=1.9×10-4) and magnesite, a mineral with the chemical formula MgCO3(s) (Ksp=6.8×10-6). i. List at least three ions you would expect to find in the lake as a result of the dissolution of the minerals near the lake. Be sure to include the correct charge on each ion. i.___Mg2+ ii. ___Ca2+ iii. SO42- _ ___CO32-___maybe even HCO3- , HSO4- ii. If gypsum, CaSO4 (s), is dissolved in pure water, what would the equilibrium concentration of calcium ions be? Show your work. CaSO4 (s) ' Ca2+ (aq) + SO42- (aq) I ----- 0 0 C ----- + s + s E ----- s s concentration of calcium ions = _[Ca2+]= 0.014 M_ The water in this lake is found to have a