Name GSI 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 Question A Question B i vii 2 3 Points 15 17 4 Question D 5 27 Question E 6 13 7 8 24 Total Useful Equations and Constants pH log H3O pX log X pH pK a log A HA N0 6 02 1023 1 mol Question B viii ix Question C bonus Question F and G multiple choice Score 6 Strong acids and bases HCl LiOH NaOH HNO3 H2SO4 KOH HClO4 HBr HI 100 Page 1 of 8 Name GSI 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 Number moles 5 moles of C2H6 g 4 3 2 moles of C2H4 g or H2 1 0 initial Time sec equilibrium i What is the equilibrium constant for the reaction Assume the reaction container is 1 0 liter Show your work C2 H 6 4 0 M 4 0 K eq C2 H 4 H 2 1 0 M 1 0 M 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 H2 g C2H6 g C2H4 g Initial X Y 0 2 0 2 0 2 0 Change 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 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 5 Number moles Imagine that the same reaction begins with 5 0 moles of ethane C2H6 g and no reactants 4 moles of C2H6 g 3 2 moles of C2H4 g or H2 1 0 Time sec Page 2 of 8 Name GSI Twire 3500 C 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 Tglass 300 C 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 Page 3 of 8 Name GSI 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 viii Balance the reaction WO2 OH 2 g 3 H2 g 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 Customers entering The left reaction is at equilibrium with K 1 and the right reaction favors products customers entering with a K 1 Page 4 of 8 Name GSI 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 SO42 CO32 maybe even HCO3 HSO4 iii ii If gypsum CaSO4 s is dissolved in pure water what would the equilibrium concentration of calcium ions be Show your work I C E CaSO4 s Ca2 aq 0 s s SO42 aq 0 s s K Ca2 aq SO42 aq 1 9 x 10 4 K s s s2 1 9 x 10 4 s 0 014 M Ca2 concentration of calcium ions Ca2 0 014 M The water in this lake is found to have a Ni concentration of 1 0 10 5 M A river that feeds the lake brings the OH concentration to 1 0 10 6 M where the lake and river water meet The Ksp 6 5 10 18 for Ni OH 2 2 iii …