Class Note for CHEM 242 at UMass(4)
Class Note for CHEM 242 at UMass(4)
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Date Created: 02/06/15
Experiment 7 Thermochromism and Ionic Conductivity in Solid State CqugI4 Introduction A great variety of structure types are derived from lling cations into the voids of a face centered cubic FCC anion lattice One simple example is the zinc blende structure of ZnS where one half of the tetrahedral FCC voids are lled Figure 1 far left The ordered tetragonal roomtemperature structure of CqugI4 Figure 1 far right involves a more complex arrangement of cations in the tetrahedral voids and closely resembles a doubled zinc blende unit cell Figure 1 center left Cu and Hg2 ions are arranged in separate layers both sandwiched between closepacked layers of 139 ions The stability of this complicated order of the cations is tenuous When solid CqugI4 is heated the cations begin to move freely and a degree of randomness will be incorporated into the manner in which the Cu and Hg2 ions f111 tetrahedral voids As a result CqugI4 undergoes a structural phase transition to a disordered cubic structure of onehalf the volume of the tetragonal phase Figure 1 center right At high temperatures the Cu and Hg2 tetrahedral sites are indistinguishable to xrays because Xray diffraction measures the cell contents averaged over all unit cells within a crystal and on average each of the four available sites has a 50 chance of being occupied by a Cu ion a 25 chance ofbeing occupied by a Hg ion and a 25 chance ofbeing vacant giving the high temperature phase the same high symmetry cubic unit cell as ZnS The phase change is accompanied by a color change thermochromism and a marked decrease in electrical resistance ie an marked increase in electrical conductivity The thermochromic color change of CqugI4 is due to a small decrease in the semiconducting band gap 21 to 19 eV with the temperatureinduced change in structure 9 O O Cqugl4 Figure 1 Left A single and doubled unit cell of zinc blende structure of ZnS Right The closely related disordered cubic hightemperature HT and ordered tetragonal lowtemperature LT structures of CqugI4 Despite the fact that four different tetrahedral voids in the cubic HT cell can host Cur and Hg2 ions only three of these sites actually contain an atom in a typical cell Ionic Conduction Unlike most electrical conductors in which electrons move in response to an applied voltage ionic conductors such as CqugI4 and the related solid AnggI4 can transport a current due to the ability of their ions to move in response to an applied voltage Ionic conductors are the solid analogues of electrolyte solutions which also conduct electricity via the motion of ions Ionic conductors are generally broken into two classes 7 those which conduct cations and those which conduct anions The latter class is of crucial importance to fuel cell applications which demand a material capable of conducting oxygen in the form of 0239 ions through a solid barrier Above its transition temperature CqugI4 exhibits ionic conductivity Fiveeighths of the tetrahedral holes and all of the octahedral holes formed by the iodide ions are vacant and these open sites provide a possible mechanism for the small copper cations to move through the crystal carrying charge It is easiest for a copper cation to jump between tetrahedral holes by moving to an octahedral hole and then to the new tetrahedral hole rather than jumping directly between tetrahedral holes Such substantial ionic conductivity is rare in ionic solids Determination of the Resistivity and Ionic Conductivity of CqugI4 An ohmmeter measures the resistance R of a material through the relation Voltage I X R where I the current in units of amperes A and where the units of resistance are Ohms S2 de ned as voltsampere However the resistance in Ohms is not independent of sample geometry The resistance increases linearly with the length of a sample and decreases proportionally to the cross sectional area of the sample You will therefore convert the measured resistance of your CqugI4 t0 the resistivity p in units of Ohmscm The resistivity of a sample is de ned to be independent of sample geometry through the formula below area pRx length The conductivity 0 of a sample is then defined as the inverse of its resistivity l a i p The units of a material39s conductivity are reported in Siemens Scm where S 182 You will therefore determine the ionic conductivity of the hightemperature structure of CqugI4 by l measuring its resistance with an ohmmeter while it is in the hightemperature phase 2 converting the hightemperature resistance to the geometryindependent resistivity and 3 calculating the hightemperature phase39s ionic conductivity Syntheses Despite having a solid state rather than molecular structure CqugI4 is readily prepared through a solution synthesis as are most nanoparticles CopperI tetraiodomercurateH CqugI4 is prepared by combining c0pperI iodide with mercuryII iodide CopperI iodide is formed by reacting copperII sulfate with potassium iodide in which the iodide ion reduces CuII to CuI thereby forming solid CuI 2Cu2 aq 4I39aq S 2CuIs I aq 1 Note that this reaction follows the HSAB rule that you studied in Experiment 1 the most stable oxidation state of the Cun ions in water is manipulated to be the softer Cu1 by the addition of the soft base I39 The E0 of this reaction halfreactions are derived from Table l in Experiment 1 is also favorable 2Cu2 21 2e39 3 2CuI 086V 2139 r 12 2e39 054v 2Cu2 4139 r 2CuI 12 aq 032v However in the presence of excess iodide ion the iodine undergoes further reaction forming triiodide ions 12 aq I39 S 1339 aq 2 Thus the true net ionic equation for the formation of copperI iodide is shown in Equation 3 2Cu2 aq 5139 aq S 2CuI s 1339 aq 3 Since the reaction is an equilibrium triiodide ions can act as oxidizing agents taking the Cu1 back to Cu2 the reverse of Equations 2 and 1 so the triiodide must be removed from solution Sodium sulfite is used to reduce the triiodide ion back to iodide Equation 4 1339aq 303239 aq 3H20 r 3139 aq SO4239aq 2H30aq 4 The solid copper I iodide can then be separated from the reaction mixture by carefully removing the excess solution of supernatant liquid A mercuryII iodide precipitate is synthesized by an anion metathesis meaning quotexchangequot reaction in which mercuric nitrate is combined with potassium iodide The nitrate anion is exchanged for the iodide The insolubility of HgIz helps to pull the equilibrium toward the right of Equation 5 Hg aq 2139 aq 3 Hglz S 5 Finally CqugI4 is prepared by adding the solid copperI iodide with heating to the mixture containing the mercuryII iodide precipitate 2 CuI s HgIz s gt CqugI4 s 6 Experimental Procedure SAFE T Y N 0T ES The mercurycontaining compounds are all quite toxic Avoid creating or breathing dust Use gloves when working with the mercury compounds and avoid eye contact Make extra efforts to ensure that all mercury waste ends up in the proper container Keep your work surfaces clean at all times so that mercury waste is not inadvertently transferred Synthesis of CqugI4 Synthesis of CuI Add 25 mL of 05 M CuSO4 solution 30 mL of 1M KI solution and five drops of 6M acetic acid to 25 mL of deionized water in a 100 mL beaker while stirring A precipitate of Cul will form Add to this precipitate with continuous very vigorous stirring a solution of0 10g Na2803 dissolved in 5 mL of water Allow the precipitate of Cul to stand for ve to ten minutes and then pour off as much as possible of the supernatant solution without losing the precipitate It may be better to slowly pipette off the supernatantit forms atiny layer on top of the CuI Synthesis of HgIz in situ In situ is Latin for in its original place In synthesis this means that an intermediate compound is not isolated before using it in a further reaction Combine 125 mL of 005 M HgN03z 15 mL of 1M KI and 50 mL of deionized water in a 200 mL beaker while stirring Note your observations Synthesis of CqugI4 Transfer the CuI precipitate into the suspension of Hglz using a stream of deionized water to wash all the CuI from the beaker Make sure to transfer all the CuIdon t lose too much precipitate Heat the mixed suspension of CuI and Hglz almost to boiling for about 20 minutes on a stirring hot plate A single brightred brickred or darkbrown solid should form Record your observations of initial colors etc If the CqugI4 appears green or light pink it should be resynthesized Filter the solution using a vacuum aspirator while it is still hot and wash the precipitate of CqugI4 with small portions of acetone Dry the solid as much as possible on the lter using the aspirator and then put on gloves collect the solid and spread it on a piece of filter paper to air dry for 10 minutes Determination of the Transition Temperature of CqugI4 The transition temperature of CqugI4 is the temperature at which it switches from the roomtemperature phase to the hightemperature conductive phase This temperature lies between 40 and 900 C for CqugI4 and can be determined using a meltingpoint apparatus mel temp by observing the thermochromism of CqugI4 Wearing gloves pack a small amount of dry CqugI4 into a piece of capillary tubing that has one end sealed and immerse the capillary in a meltemp apparatus Heat the capillary and record the temperature at which a color change takes place as the sample is heated This is the thermochromism the change in color upon heating to the phase transition that CqugI4 displays The color change is thus an indicator of the phasetransition temperature Record the color of the hightemperature phase and the phase transition temperature Allow the capillary to cool observe and record its roomtemperature color and then repeat the transition temperature measurement on the exact same sample Determine and record in your notebook whether the sample can be thermally cycled or whether the rst phase transition is irreversible Cnmpalixnnn lu Emmi Cundmtili u am Twn Plum 1wath Wem glwves cmyynursnhd cmiLgmp mg minim m wmchglaS mch prwmi Wimp pipu CmmbmakaglaS capillary le inhalf andmsen asmmpm nfl gauge mppr 3 in mm mm n 922 mg Figme mm The m Sl39lmlldbe ppm mandipr mg ppnmipm capillame mp sums CmHglmnhlat 12m m cm a mammals smka lube Thenmsen mimmgipm prepppuwm man up npnendm39 uz glaS capum mi gemlypllsh he cmiLgi dawnsn m n is packedbelween he m Emmi m 21mm The cud1g m be mgnnd suntan wiihbmh ms airshmdd mpg mppi beiween he ends imp wimsand he CmILgh Aim mg endsn he ms munnnlbe in mm m lube shnmbe packzd wiihasnhdcyhndememILgh wimp mcnnm wi h he ends nfbmhcnpprwues 1Q gauge Copper Wire m Glass Cap ary Tube Cqugl4 anwip up all mus prcmixgi andbmkznglas Emmiin imprpm mi msppsi imp gims mi mils mm mm rnzicurywnsle Enmmr Allmucurywasbzs must be segpgmimayrmm mherwasbzs 5n m heycanbe mated pmpiy wip in mud pr ynmcaplllarg wm apparmls wnhapipu m1 Alm wash ynurhands hnmughly m ynmberlch measm mi maid in 1mm in cm nfynur 53mph prcmixgi mg mg nlb pmvided Reunide mKIEId amEMni l39lz capl ary lbe whlchis wnmnnn he cap lanz plashl minim Take ynmcaplllarg wireCm gh sampb p in mm Wham m m 11m guns mi nhmmems Cannecnhz m ends imp Emmi wne m up nhmmemelenmdes mi pm in capillary at on the bench Your TA will adjust the ohmmeter to the correct scale Read and record the electrical resistance R of your CqugI4 sample at room temperature Your TA will now heat the capillary with the heat gun Read and record the electrical resistance R of your sample above the phase transition temperature The ohmmeter reading may be jumpy so record several R values and use either 1 the one that seems to be most stable or 2 an average of the values that occur in a reasonable range In the Data Analysis section of your lab report you will use this experimentallydetermined resistance value to calculate the dimensionindependent resistivity p and conductivity 0 of your sample Be sure to dispose of all the CqugI4 in the waste container provided Data Analysis 1 6 points Record your value of the transition temperature of CqugI4 and the colors of the lowtemperature and hightemperature phases 2 6 points Record the electrical resistance R of your CqugI4 sample at room temperature and the values of R that you got at high temperature If you got several values of R at high temperature state which one you will use in the remaining calculations judging by either 1 the one that seems to be most stable or 2 an average of the values that occur in a reasonable range 3 7 points Calculate the dimensionless resistivity p of CqugI4 in the hightemperature phase Show all your calculations including those for the crosssectional area of your sample 4 6 points Calculate the conductivity of CqugI4 in the hightemperature phase 10 Discussion 1 8 points Discuss whether the CqugI4 phase transition can be thermally cycled or whether the first phase transition is irreversible and how you determined this Why or why not can this phase transition be thermally cycled 2 7 points Compare the value of the resistance R of CqugI4 in the room temperature phase and the high temperature phase What are the likely resistivity and conductivity characteristics of CqugI4 in the low temperature phase 3 7 points Discuss the magnitude of the conductivity of CqugI4 in the hightemperature phase and the reason for its difference from that of the lowtemperature phase Questions 1 6 points When you made CqugI4 you filtered it while the suspension was still hot What impurity or impurities were you trying to avoid by ltering while hot 2 6 points CqugI4 is an ionic conductor which means that charge is carried by the net movement of ions rather than by the motion of electrons through a stationary lattice Give two reasons for why the cations are mobile in this compound but the anions are stationary 3 6 points Do you expect the Cu or Hg ions to be more mobile Explain your answer
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