Student Performance Q&A:

Transcription

1 Student Perfrmance Q&A: 2011 AP Chemistry Free-Respnse Questins The fllwing cmments n the 2011 free-respnse questins fr AP Chemistry were written by the Chief Reader, Larry Funck f Wheatn Cllege in Wheatn, Ill. They give an verview f each freerespnse questin and f hw students perfrmed n the questin, including typical student errrs. General cmments regarding the skills and cntent that students frequently have the mst prblems with are included. Sme suggestins fr imprving student perfrmance in these areas are als prvided. Teachers are encuraged t attend a Cllege Bard wrkshp t learn strategies fr imprving student perfrmance in specific areas. Questin 1 What was the intent f this questin? This questin assessed students understanding f and ability t slve prblems and explain cncepts that pertain t acid/base equilibrium and buffers. Parts f the questin required calculatin. Students were presented with three samples f equal vlume and cncentratin a strng acid, a weak base and the cnjugate acid f the weak base. Part (a) asked students t determine the ph f the slutin f the strng acid and assessed their understanding f the ph scale. Part (b) required students t write the equilibrium expressin fr the hydrlysis reactin f the weak base and t determine the hydrxide in cncentratin in the slutin, given the cncentratin and the value f K b fr the weak base. Part (c) required students t determine the value f the K a f the cnjugate acid. Students then were tld that the slutins cntaining the weak acid and weak base were cmbined and that they were t determine the ph f the resulting slutin. T d s they needed t apply any f the equilibrium equatins fr K b, K a r the Hendersn- Hasselbalch equatin apprpriately. All three slutins were then cmbined, and part (d) required students t state with justificatin whether this slutin wuld be an effective buffer and t determine the cncentratin f the weak acid in this resulting slutin. Hw well did students perfrm n this questin? Students did reasnably well n this questin, earning a mean scre f 3.65 ut f a pssible 10 pints and shwing a brad distributin, with many students earning 4 r 5 pints. Apprximately 12 percent f students failed t address the questin r earned n pints, while 1.4 percent earned full credit. Mst frequently, students earned all their pints early in the questin, and it was cmparatively rare that a respnse wuld fail t earn pints n a given part and then earn pints n subsequent parts f the questin. The large percentage f scres f 4 typically earned all 4 pints n parts (a), (b)(i) and (b)(ii) and failed t earn pints n subsequent parts f the questin. Thse respnses that earned 5 pints usually earned them all n parts (a), (b) and (c)(i). Students were generally successful n part (a), earning 1 pint fr the crrect ph f a M HCl slutin, and in part (b), writing a crrect expressin fr K b f NH 3, substituting apprpriate values int the

2 equatin, and slving t determine the crrect [OH - ]. A small but significant percentage f respnses set up a crrect equatin but did nt slve crrectly. Mst papers made the simplifying assumptin that the [NH 3 ] equilibrium [NH 3 ] initial and did nt need t slve a quadratic equatin. A range f appraches were taken n part (c). Many students recgnized that K a = K w /K b in part (c)(i). Others were able t determine the value f K a frm the expressin and the cncentratins f species in beaker 2, previusly determined. Regardless f the answer t part (i), part (ii) was much mre challenging fr the students. Mst incrrect respnses failed t recgnize that the resulting slutin was a buffer. Thse respnses that crrectly indicated that [NH 3 ] equilibrium = [NH 4 + ] equilibrium generally went n t earn credit. Part (d)(i) was the least ften earned pint; few students referred t the stichimetry f the reactin. Credit in part (d)(ii) was als rarely earned. Students attempted t use equilibrium equatins t answer a questin that was based n stichimetry. What were cmmn student errrs r missins? Part (a): Nt recgnizing HCl as a strng acid Cnfusin between cncentratin and mles Part (b): Incrrect chemical frmulas, missin f charges n ins in equilibrium expressin Including [H 2 O] in equilibrium cnstant expressin fr K b Nt recgnizing that [OH - ] equilibrium = [NH 4 + ] equilibrium Arithmetic errrs, particularly invlving scientific ntatin Part (c): Nt recgnizing the relatinship between K a and K b attempting t determine value f K a frm given cncentratins Nt recgnizing the resulting slutin as a buffer and treating it as a weak acid r weak base Adding (r averaging) the ph values fr beaker 2 and beaker 3 Part (d): Making general assertins withut accunting fr stichimetry (e.g., The slutin is nt a buffer because HCl is a strng acid ) Neglecting the reactin f HCl and NH 3 as a surce f NH 4 + Neglecting the effect f dilutin n cncentratins 2

3 Based n yur experience f student respnses at the AP Reading, what message wuld yu like t send t teachers that might help them t imprve the perfrmance f their students n the exam? Suggestins fr teachers: Distinguish between amunt (mles) and cncentratin (mlarity = ml/l). Reinfrce ntatin that [ ] refers t the mlar cncentratin f a species. Demnstrate r prvide a labratry exercise t familiarize students with the ph scale. Emphasize the distinctin between an equilibrium cnstant expressin and its value. Practice identifying buffers have students make up buffers in different ph ranges. Demnstrate that a buffer can be treated as a weak acid in equilibrium with its cnjugate base OR as a weak base in equilibrium with its cnjugate acid, and that the appraches are equivalent. Require clear wrk shwn n calculated answers stress prcess rather than memrizatin f frmulas. Suggestins fr students: Read and answer the questins that are asked. Be sure that numerical answers are reasnable. Differentiate between mles and cncentratin (mles/liter). Practice writing clear, expsitry statements regarding chemical systems. Use vcabulary crrectly. Questin 2 What was the intent f this questin? This questin assessed students knwledge and skills pertaining t a labratry experience invlving the determinatin f the mass percent f silver in a cpper-silver ally. The questin cnsisted f bth mathematical and cnceptual applicatins f chemistry. Part (a) was primarily cncerned with the labratry prcess itself: diluting an acid (a mathematical determinatin and an experimental prcess in which students are prvided with a specific list f equipment), cmparing and cntrasting apprpriate glassware based n the level f precisin f the dilutin, and cleaning up an acid spill. Parts (b) and (c) fcused n the manipulatin f experimental data: students determined the number f mles f AgCl precipitate and the mass percent f Ag in the Cu-Ag ally. Hw well did students perfrm n this questin? The mean scre was 3.51 ut f a pssible 9 pints. Apprximately 26 percent f students earned a scre f 3 r 4, 14 percent earned n pints r did nt address the questin, and nly 1 percent earned the maximum scre. Students generally answered all parts f the questin, with mst f the pints being earned in part (a)(i) fr crrectly calculating the vlume f cncentrated HNO 3 needed t prepare 50 ml f a 6 M HNO 3 slutin and in part (c) fr recgnizing that the mass percent f Ag in the ally was based n the mass f the ally. Students had a difficult time expressing themselves in parts (a)(ii) thrugh (a)(iv) and determining the crrect mass fr the dried AgCl precipitate in part (b). 3

4 What were the cmmn student errrs r missins? A large majrity f students crrectly answered part (a)(i). The mst cmmn errr was the incrrect algebraic manipulatin f the M 1 V 1 = M 2 V 2 equatin in rder t slve fr the vlume f HNO 3 needed fr the dilutin. This incrrect expressin resulted in the cmmn answer f 133 ml f 16 M HNO 3 needed fr the dilutin. Acceptable values reprted fr the answer in part (a)(i) include 20, 19, 18.8 and ml. In part (a)(ii) students culd earn 1 pint based n the prcedure used t carry ut the dilutin. Many students seemed t experience a discnnect between the mathematical and the experimental prcesses assciated with the cncept f dilutin. Fr example, students wh calculated a vlume f 133 ml HNO 3 were required t prepare the 50 ml slutin in part (a)(i) did nt realize (r chse t ignre) that this vlume f cncentrated HNO 3 was t large fr the 100 ml graduated cylinders (r beaker) and als t large fr a ttal slutin vlume f 50 ml. Students als incrrectly perfrmed the dilutin prcess by starting with 50 ml f water and then adding the acid. They incrrectly used the 100 ml beaker t measure the vlume f water r acid used in the dilutin. Whether their result in part (a)(i) was crrect r nt, many students did nt specify the vlume requirements f either the acid r the water when cnducting the dilutin. Althugh n penalty was given, many students used the eyedrpper t deliver their calculated vlume f acid r water t the graduated cylinder. The secnd pint students culd earn was based n addressing the safety precautins needed t carry ut the dilutin. Mst students listed put n safety gggles and rubber glves n the first line f their respnse but ften incrrectly pured water int acid. A variatin f this errr ccurred when students neglected t specify the rder f mixing, such as pur the acid and water int the beaker fr mixing. In part (a)(iii) the mst cmmn errr was nt recgnizing that the relative precisin f the glassware shuld be cmpared t the level f precisin needed fr the diluted acid. Students als indicated unfamiliarity with the vlumetric flask by suggesting that diluting the acid in the flask may result in an verflw (spillage). Additinally, students ften preferred the graduated cylinder ver the flask because the cylinder has gradatins, whereas the flask has nly a single calibratin line. Sme students thught that the graduated cylinder was a mre precise instrument with which t measure vlume (i.e., the miscnceptin that mre gradatins yield better precisin). Anther cmmn errr was assciating the vlumetric flask with a specific experiment, such as titratin, r a prcess, such as the disslutin f a slid. Sme students argued that the calculated vlumes f acid and water (19 and 31 ml, respectively) were accurate t nly ±1 ml and thus that the graduated cylinders were mre apprpriate t use. In part (a)(iv) there were tw primary surces f errr. First, students chse the incrrect slutin (water r NaCl) t clean the acid spill; secnd, students tried t justify why NaHCO 3 wuld be the best ptin by stating that the NaHCO 3 wuld lwer the ph (crrelating a decrease in H + with a decrease in ph), indicating that the NaHCO 3 culd be used t clean the acid spill because it is a buffer, r stating that the NaHCO 3 diluted r absrbed the acid. In part (b) the mst cmmn mistake fr the first pint was taking an average f the masses after each drying and then subtracting the mass f the dry crucible frm the average. Overall students did a very gd jb f crrectly cnverting frm grams f AgCl t mles f AgCl. The mst cmmn mistake fr the secnd pint was reprting the mles f AgCl with the imprper number f significant figures. Since students were manipulating experimental data, they were held stringently t the prper number f significant figures. Many students did nt use the mlar masses (r all f the significant figures in the mlar masses) prvided t them in the peridic table (143 g/ml was cmmn). 4

5 In part (c) the mst cmmn mistake fr the first pint was incrrectly using the mass f AgCl frm part (b) in the mass percent calculatin. Fr the secnd pint sme students did nt recgnize that the mass percent was based n the initial mass f the ally. A general miscnceptin was that the Cu-Ag ally was a cmpund ( CuAg ) with a 1:1 mle rati and therefre that the mass percent f Ag is determined based atmic mass Ag n atmic masses, Ê ˆ 100% Ëatmic mass Cu + atmic mass Ag. Anther cmmn mistake was calculating the mass percent f Ag in AgCl. Based n yur experience f student respnses at the AP Reading, what message wuld yu like t send t teachers that might help them t imprve the perfrmance f their students n the exam? Suggestins fr teachers: D multiple labs thrughut the academic year. Based n student respnses, it seems as thugh students may nt be ding enugh labratry exercises. Have students becme familiar with cmmn glassware (uses, applicatins, and precisins) and review basic labratry prcedures. Emphasize the cncept f significant figures prviding infrmatin pertaining t the quality (precisin) f the measurement. Emphasize all safety prcedures fr safe lab practices. Encurage students t review their lab ntebks prir t taking the exam. Require wrk with units (dimensinal analysis). Review the rules fr determining the significant figures expressed in measured (experimental) and calculated (+, -,, ) values. Review a prir year s exam with yur students. Shw students where t find the infrmatin that is prvided t them in the exam. Suggestins fr students: Never measure vlumes with a reactin vessel, like a beaker r an Erlenmeyer flask. All vlume measurements must take place in a calibrated instrument (graduated cylinder, vlumetric flask, vlumetric pipet, etc.). If yu are asked t select the best answer, make a single selectin and justify the reasning fr making that chice. If yu are asked t make a cmparisn, mentin bth pssibilities and then make a single chice with accmpanying justificatin. Be cgnizant f significant figure rules and hw t apply them crrectly. Shw all yur mathematical wrk and make sure t include units. Analyzing the units can help yu determine if yu have dne the crrect calculatin. Familiarize yurself with the resurces prvided t yu in the exam bklet. DO NOT write yur answers in the bdy f the questin; write them n the lined pages. Skip a line between subsectins fr clarity and ease f evaluatin Yur written, nnmathematical, answers shuld be neat and cncise respnses. 5

6 Remember, AP Readers can best grade yur exam if yu write clearly and use prper grammar. Write in pencil r blue-ink pens and avid using pens that smear easily. Questin 3 What was the intent f this questin? This questin assessed students understanding f selected cncepts f thermchemistry, electrchemistry, and envirnmental chemistry. Thermchemistry cncepts included the determinatin f a standard enthalpy change, applicatins f Hess s law, and stichimetric determinatin f heats f reactin given mle r mass amunts. Electrchemistry cncepts included the determinatin f cell reactins frm half-cell reactins, the determinatin f standard cell ptentials frm half-cell ptentials, and applicatins f Faraday s law. Finally, students were asked abut the ptential impact f carbn dixide emissins n glbal climate. Hw well did students perfrm n this questin? The mean scre was 3.18 ut f a pssible 10 pints. The mst cmmn scre was zer. Part (a) was the mst accessible part f the questin. The pint was lst fr mechanics (ff by a factr f tw, wrng sign, r n units) mre than fr cnceptual misunderstandings. Students had few cnceptual prblems with part (b) as well. This is prbably because heat is mre tangible than enthalpy. Still, abut 10 percent f students, upn seeing mass and heat in the questin, used the heat transfer equatin, q = mcδt instead f q = nδh. Part (c) was the least accessible thermchemistry pint. The mst cmmn wrng answer was kj, which indicated a lack f understanding f thermchemical principles (applicatin f Hess s law). Students were slightly less successful with the electrchemistry parts f the questin than they were with the thermchemistry parts. On the ther hand, a number f students wh were unable t d the thermchemistry parts were able t d the electrchemistry parts successfully. Sme students had difficulty prperly cmbining half-cell reactins in part (d). Abut 30 percent f the respnses left water mlecules, hydrxide ins, r electrns n bth sides f the equatin. Abut half the students crrectly cmputed the cell ptential in part (e). Many f thse wh erred added a minus sign, drpped the units, r multiplied the ande half-cell ptential by tw. Over half the students struggled with the stichimetric cnversin required t find mles f electrns frm mles f hydrgen in part (f)(i). In part (f)(ii), ver half were insufficiently familiar with Faraday s law calculatins: (mles f substance mles f electrns charge in culmbs current in amperes). Finally, part (g) was the least accessible part. It required a familiarity with the cncept f glbal warming r an enhanced greenhuse effect thught t be caused by carbn dixide emissins. What were cmmn student errrs r missins? Part (a): Nt multiplying the standard mlar enthalpy f frmatin f water, DH f, by tw Drpping units (nly kj, kj ml -1, r kj/ml rxn were accepted) Drpping the negative sign Nt using zeres fr the standard enthalpies f frmatin f elements 6

7 Part (b): Nt cnverting grams t mles Nt recgnizing that the standard frmatin reactin f liquid water is the same as the standard cmbustin reactin f gaseus hydrgen Using g ml -1 as the mlar mass f H 2 Using the answer t part (a) fr the standard enthalpy f cmbustin f ne mle f H 2 (instead f tw mles f H 2 ) Nt shwing the calculatin setup Using the heat transfer equatin (q = mcδt) t determine the heat f cmbustin Part (c): Giving the enthalpy f vaprizatin f tw mles f water (+88.0 kj) instead f the enthalpy f cmbustin f tw mles f hydrgen t give tw mles f water vapr ( kj kj = kj) fr the final answer H vap Nt multiplying the standard mlar enthalpy f vaprizatin f water, D, by tw Part (d): Nt crrectly balancing the equatin Nt canceling tw mles f water (i.e., 2 H 2 O(l) + 2 H 2 (g) + O 2 (g) = 4 H 2 O(l)) Nt canceling electrns, hydrxide ins r bth Reversing the reactin Part (e): Nt reversing the sign f the xidatin half-cell ptential (giving E = V) Reversing the sign f the reductin half-cell ptential (giving E = V) Reversing but dubling the xidatin half-cell ptential (giving E = V) Reversing bth and dubling the xidatin half-cell ptential (giving E = V) Drpping the units Nt shwing the calculatin setup Part (f)(i): Using the wrng mle rati f mles e - t mles H 2 Nt shwing the calculatin setup Nt using 0.93 ml H 2 in the calculatin Wrngly attempting t use the cell ptential frm part (e) (ΔG = -nfe ) Multiplying the given 0.93 ml H 2 by the mlar mass f H 2 Finding the ttal number f e - (instead f mles f e - ) 7

8 Part (f)(ii): Nt indicating the cnversin f mles f e - t charge in culmbs using the Faraday cnstant Nt dividing the charge in culmbs by the time in secnds t btain current in amperes Wrngly dividing by 96,500 C/ml e -, multiplying by 600. s, r bth Nt shwing the calculatin setup Drpping the units Part (g): Nt specifically identifying carbn dixide r CO 2 as the substance mst likely t cause an envirnmental disadvantage in the butane fuel cell Nt identifying the specific envirnmental phenmenn ptentially affected by carbn dixide (glbal warming, the greenhuse effect, climate change, and acid rain were all accepted) Wrngly identifying a harmful effect f carbn dixide n the zne layer as the envirnmental disadvantage Based n yur experience f student respnses at the AP Reading, what message wuld yu like t send t teachers that might help them t imprve the perfrmance f their students n the exam? Suggestins fr teachers: Have students practice mre prblems using thermchemical equatins. Have students manipulate thermchemical equatins by adding, subtracting, reversing, r multiplying them by stichimetric factrs. Have students practice applying Hess s law f cnstant heat summatin. Have students practice mre prblems using electrchemical cells. Make sure students understand that vlts are intensive units (jules per culmb) whereas jules and kiljules are extensive (i.e., dubling a chemical reactin dubles its energy r enthalpy in kiljules but des nt duble its cell ptential in vlts). Have students practice writing cell reactins frm half-cell reactins and decmpsing cell reactins int ande and cathde half-cell reactins with the apprpriate numbers f electrns. Make sure students understand the accepted practice f canceling nnparticipating species frm bth sides f a chemical equatin, and f reducing cefficients t the lwest set f whle numbers. Have students practice sme simple Faraday s law calculatins (e.g., they shuld be able t cnvert frm mles f reacting species t mles f electrns t charge in culmbs t current in amperes, and back again). Suggestins fr students: Shw yur wrk explicitly n all calculatin prblems. Remember that units f quantities are imprtant and may be manipulated algebraically thrughut a calculatin. On calculatr questins a numerical answer is required; shwing a setup is nt sufficient. 8

9 Use the lined space allwed fr answers instead f cramping a few numbers between the lines f a questin. Questin 4 What was the intent f this questin? This questin assessed students ability t cmmunicate their knwledge f chemical prcesses. Imprtant skills tested included writing chemical frmulas fr substances and cmmunicating hw these substances react with ne anther in the frm f a balanced net-inic equatin. Additinal aspects f the prblem evaluated general understanding f chemical cncepts presented t students in the classrm and the labratry. Hw well did students perfrm n this questin? The mean scre was 6.62 ut f a pssible 15 pints. Students displayed a wide range f knwledge and skills in their respnses t this questin. The scres cvered the entire scale and had a relatively nrmal distributin. The mst cmmn scres were in the 5 7 range. Apprximately 1.4 percent f the respnses earned a scre f 15, and there were very few blank respnses. What were cmmn student errrs r missins? Part (a): Writing slid species in inic frm Nt determining the xidatin state f magnesium frm the peridic table. Nearly 10 percent f students used Mg 1+. Incrrect prducts f an acid base (neutralizatin) reactin Nt recgnizing that strng acids are cmpletely inized in slutin Nt recgnizing that sluble inic species are cmpletely dissciated in slutin Inability t crrectly recall slubility rules Nt reducing cefficients t lwest terms Inability t recgnize that the vlume calculatin depends n the 2:1 stichimetry f OH - : Mg(OH) 2 in additin t the 1:1 stichimetry f H + :OH - Incrrect applicatin f the 1:1 stichimetry f H + :OH - taken directly frm the net-inic equatin Applicatin f the mlar vlume at STP (22.4 L/ml) rather than the mlarity f the hydrbrmic acid in an attempt t calculate the vlume Cnverting frm liters t milliliters incrrectly Part (b): Lack f familiarity with crdinatin chemistry. The mst cmmn accepted respnse was C Cl - CCl 2. Readers were certain that mst students actually intended this as a precipitatin reactin rather than an aqueus uncharged cmplex. Several students cnfirmed this by indicating the state f CCl 2 as (s). 9

10 Nt recgnizing that strng acids are cmpletely inized in slutin Nt recgnizing that sluble inic species are cmpletely dissciated in slutin Inability t crrectly recall slubility rules Inability t crrectly recall the frmula and charge fr the nitrate in Nt calculating the crrect verall charge n a cmplex in Nt recgnizing Cl - as the preferred ligand fr the cmplex Attempting t prduce a crdinatin cmplex by simply cmbining a variety f incrrect reacting species tgether Nt understanding the term Lewis base Cnfusing Lewis acid-base thery with that f Brønsted and Lwry r Arrhenius. Part (c): Nt using the table f standard reductin ptentials t determine the mst likely redx reactin t ccur. Apprximately 15 percent f students xidized Cu t Cu 1+. Nt recgnizing that sluble inic species are cmpletely dissciated in slutin Inability t crrectly recall slubility rules Inability t crrectly recall the frmula and charge fr the nitrate in Inability t crrectly recall the symbl fr silver and, less frequently, cpper Nt reading the questin carefully and nt answering what was asked. Apprximately 10 percent f students gave a descriptin f electrn transfer prcesses r single replacement reactins rather than giving an bservatin. Giving incmplete bservatins such as clr change r precipitate frms All parts: Writing cmplete balanced frmula (mlecular) equatins instead f net-inic equatins Writing cmplete inic equatins rather than net-inic equatins Writing miscellaneus charges ver frmulas in the bx Writing several equatins in the bx (nly the last equatin gets scred) Based n yur experience f student respnses at the AP Reading, what message wuld yu like t send t teachers that might help them t imprve the perfrmance f their students n the exam? Suggestins fr teachers: Give students practice balancing equatins nt nly fr mass, but als fr charge. A rutine f checking fr the balance f bth shuld be established. Offer students practice writing chemical frmulas fr mlecules, atms and ins in slutin. Stress that all sluble inic cmpunds shuld be dissciated and any f the six strng acids shuld be inized. All ther species, such as elements r cvalent (mlecular) cmpunds, shuld nt be written as ins. 10

11 Encurage students t use the psitin f an element in the peridic table t determine its inic charge. Remind students frequently that all nitrate cmpunds are sluble and hence dissciate in slutin. Remind students that neutralizatin reactins and crdinatin cmplex frmatin reactins d nt invlve xidatin r reductin f any species. Expse students t sme crdinatin chemistry. They need t recgnize cmmn ligands and practice writing acceptable crdinatin cmpunds with apprpriate charges. Encurage students t use the table f reductin ptentials prvided t help them write a crrect redx reactin. Remind students that the answer bx shuld cntain nly the final, balanced chemical equatin. Emphasize that the bx shuld nt cntain xidatin numbers, scratch wrk, any ther extraneus charges r numbers, r extra equatins. Practice the applicatin f stichimetry frequently thrughut yur curse. Emphasize the imprtance f cnsidering the surce f the reacting species rather than applying the stichimetry f the net-inic equatin. Use reactins that invlve plyprtic and plybasic species ften. Students remember what they see and d better than what they hear and read. D labratry wrk and frequent demnstratins t help students learn cmmn reactins and practice equatin writing. Stress the recrding f qualitative bservatins during labratry wrk. Suggestins fr students: Cancel ut cmmn species that are present in bth the reactants and the prducts. Practice writing net-inic equatins in bxes fr this questin. Reduce the stichimetric cefficients t lwest whle numbers. Multiply fractinal cefficients fr lwest whle numbers as well. Learn the names and symbls f the cmmn elements. Silver is nt Au r Si. Knw the frmulas and charges fr cmmn plyatmic ins. An errr in the frmula f nitrate culd have led t pints lst in parts (b) and (c). Becme very familiar with the list f the six cmmn strng acids. Many students presented HBr, HCl, and HNO 3 as weak acids. Others indicated that hydrbrmic acid was HBrO 3 r hydrchlric acid was HClO 4. These miscnceptins led t pints lst in parts (a) and (b). Questin 5 What was the intent f this questin? This questin asked students t cmplete the Lewis diagram fr the N 2 H 4 mlecule in part (a). Part (b) asked students t determine, based n their diagram, whether all six atms were n the same plane and t justify their answer. Part (c) gave students the biling pints fr N 2 H 4 and C 2 H 6 and asked them t explain the difference in biling pints in terms f the intermlecular frces in each liquid. In part (d) students wrte a balanced chemical equatin fr the reactin between N 2 H 4 and water t explain a ph greater than 7. Part (e) gave students a balanced chemical equatin fr hydrazine reacting in air and asked them t identify the type f reactin and justify their answer. In part (f) students were asked t refer t the 11

12 equatin prvided in part (e) and predict the sign f the entrpy change, with justificatin. In part (g) students were given a statement regarding energy and the breaking f bnds and were asked t justify whether the statement was true r false. Hw well did students perfrm n this questin? The mean scre was 3.04 ut f a pssible 8 pints. Students generally attempted all parts f the questin, with mst f the pints earned in part (a) by cmpleting the Lewis diagram, in part (b) by identifying the arrangement f atms r by citing electrn-pair repulsin, in part (e) by identifying and justifying an xidatin-reductin reactin, and in part (f) by assigning an apprpriate sign t entrpy with a valid justificatin. What were cmmn student errrs r missins? In part (a) sme students expanded the ctet f nitrgen and drew a structure with a duble r triple bnd and ccasinally with additinal lne electrn pairs. A few students cmpleted an ctet fr each atm, including hydrgen, and seemed t shw all electrns, nt just valence electrns. Occasinally structures included lne electrns in a variety f lcatins. In part (b) students ften gave an incmplete justificatin fr why all six atms were nt n the same plane. A statement that lne electrn pairs are present is a fact and des nt prvide an explanatin fr the gemetry f the mlecule. Sme students described the tw-dimensinal image n the paper, citing that tw hydrgen atms were belw the nitrgen atms and tw hydrgen atms were beside the nitrgen atms, fr an incrrect justificatin. Sme did nt seem t tie the structure drawn in part (a) t the explanatin fr part (b). Part (c) was difficult fr many students. One cmmn missin was discussing the intermlecular frces f nly ne f the tw mlecules. Anther cmmn errr was the cnfusin between intramlecular and intermlecular frces. Students ften referred t the breaking f cvalent bnds when the substance bils. Students with an incrrect structure in part (a) shwing a multiple bnd ften referred t multiple-bnd strength versus single-bnd strength, clearly indicating a misunderstanding f biling as a phase change. Mre ften, hwever, students referred t the hydrgen bnds between the N and H atms in the mlecule and did nt make a clear distinctin that IMFs are frces between mlecules. The cvalent bnds between the N and H were errneusly identified as hydrgen bnds, and many students stated that the hydrgen bnds in N 2 H 4 were much strnger than the cvalent bnds in C 2 H 6. Anther cmmn missin was identifying the intermlecular frces in each mlecule but making n meaningful cmparisn between them. In part (d) a variety f prducts were written fr the reactin between hydrazine and water. Sme f thse included NH 3 and OH 2-, N 2 H 6 and OH -, H 3 O + and N 2 H 4 -, and a cmplete decmpsitin int elements. One cmmn missin was leaving ut charges n ins in slutin. In part (e) a cmmn missin was identifying nly ne species as xidized r reduced. A cmmn errr was assigning an incrrect xidatin number t nitrgen withut referring t the apprpriate cmpanin prcess r identifying a net xidatin number change. Sme students identified xidatin number changes fr three species H, N, and O in the reactin. There were sme vague answers that simply stated that the reactin was xidatin-reductin since atms change charge, with n specific identificatin. The mst cmmn incrrect answer was citing a decmpsitin reactin because N 2 H 4 decmpsed int N 2. Anther cmmn errr was fr students t prvide a ratinale by eliminating the chices f acid-base r decmpsitin but never explaining why the reactin is an xidatin-reductin reactin. Occasinally students reversed which species were xidized and reduced. Anther cmmn errr was identifying N 2, a prduct, as a species xidized. 12

13 One cmmn missin in part (f) was failure t indicate the type f reactants and prducts; a statement f an increase in mles f prduct withut mentining the state did nt receive credit. Als, a cmmn reference t a phase change f the liquid hydrazine t a gaseus frm, rather than the chemical prcess, did nt earn the pint. Anther cmmn errr was fr students t explain that because ΔH was negative, the entrpy must be psitive, even thugh n reference was made t ΔG in the questin, r simply substituting sign values int ΔG = ΔH - TΔS. Many students indicated that the statement in part (g) was true. The miscnceptin that energy is released when bnds are brken still exists. A cmmn errr was a discussin f energy required t break bnds leading t a cnclusin that the given ΔH shuld have been reprted as psitive instead f negative, thereby failing t recgnize that the value f ΔH is the cmbinatin f the endthermic breaking f bnds and the exthermic frmatin f bnds. Sme students cited the negative ΔH value as prf that the reactin was actually endthermic. Based n yur experience f student respnses at the AP Reading, what message wuld yu like t send t teachers that might help them t imprve the perfrmance f their students n the exam? It is impssible t veremphasize the imprtance f writing precisely. Vague and unclear answers ften made it impssible fr AP Readers t determine whether the student understd the chemistry. Clear, precise, and succinct prse des nt cme naturally; we all must wrk n this and hld ur students accuntable fr use f crrect terminlgy. Students ften referred t intermlecular frces as bnds, making it difficult t distinguish between intramlecular and intermlecular. When justifying answers, statements f fact are nt enugh; an explanatin f why must be prvided. Suggestins fr teachers: D nt accept incrrect terminlgy frm students. Give students practice balancing a variety f reactins stichimetrically as well as balancing fr charge. Encurage students t explain and justify answers fr all types f questins. Thugh time is required t scre student respnses, explanatin, and justificatin are very imprtant cmpnents in assessing understanding and misunderstanding. Allw students the pprtunity t explre miscnceptins in chemistry. Intramlecular and intermlecular interactins are always difficult fr students t distinguish r describe. Part (c) prvided evidence that the cnfusin cntinues. Wrk with students t help them understand these cncepts. It may be helpful t have students rganize thughts int categries when addressing this type f questin: Identify the majr binding frce first (inic, cvalent, metallic). Identify types f intermlecular attractive frces if apprpriate. Emphasize that the term van der Waals frces is a general term that includes all IMFs, and have students identify the types f van der Waals frces. Allw students t explain differences in data using the principles f bnding, IMFs, r bth. Have students illustrate attractins between mlecules and clearly identify the binding frces and the intermlecular frces. 13

14 Suggestins fr students: Read the questin carefully and answer the questin asked. Begin by writing the answer and then prvide supprting evidence. Often infrmatin is used fr multiple parts f a questin; be sure t refer t infrmatin that is given. When respnding t the questin f whether a statement is true r false, begin by writing dwn ne f these chices and then prvide supprting evidence. D nt begin the answer with the wrd yes. Be sure t address each substance within a given questin stem. When asked t cmpare tw substances, always talk abut bth. Simply discussing ne and expecting the reader t assume that the ther is implied will nt receive credit. Use apprpriate language when answering questins. It is nt acceptable t refer t an atm as a mlecule r t refer t an intermlecular frce as a bnd. When drawing Lewis structures, take time t calculate the ttal number f electrns befre beginning. When writing balanced equatins, pay attentin t details such as balancing the charges. Ins written withut charges receive n credit. Avid vague generalizatins when answering questins. Give details as ften as pssible. Write legibly and be sure t write all answers in the lined spaces prvided instead f squeezing wrds between questin parts. Questin 6 What was the intent f this questin? There were tw parts t this questin. The first part explred the cncept f dynamic equilibrium with respect t vapr pressure. In part (a) students had t recgnize that at the given temperature, when the vlume f liquid was n lnger changing, the system was at equilibrium. In part (b) students needed t invke tw f three arguments that used kinetic mlecular thery t explain why an increase in temperature wuld increase the pressure in a fixed vlume. The secnd part f the questin explred a decmpsitin reactin with graphical data. In part (c)(i) students used the graphical data t determine that the reactin was zer rder. In part (c)(ii) students wrte the crrect rate law fr the reactin. In part (c)(iii) students used graphical data frm the first graph fr a crrect setup with units fr the value f k, the rate cnstant. In part (d) students were asked t cmpare the pressure in the flask befre and after the reactin tk place. Hw well did students perfrm n this questin? The mean scre was 2.98 ut f a pssible 8 pints. Students generally attempted all parts f the questin. If parts were mitted it was generally part (a) r part (c)(iii). Many students earned just 1 pint in part (b). Of the pints missed, they cmmnly were missed in all parts f (c). Apprximately 13 percent f students either earned n pints with their respnse r failed t address the questin, while 2 percent achieved a scre f 8. 14

15 What were cmmn student errrs r missins? In part (a) students needed t make a cnnectin between bservatin (the cnstant vlume f the ethanl) and the cnclusin (the system is at equilibrium). Cmmn errrs included the fllwing: Nt making a chice f greater than, less than, r equal t 100 trr Trying t use gas law arguments fr pressure and temperature Stating that the amunt f ethanl remained cnstant s the pressure remained the same Stating that because everything was cnstant, the pressure f 100 trr wuld als be cnstant, with n mentin f equilibrium Stating that at equilibrium, the lack f change f pressure was due t the lack f change n the mlecular level (i.e., n evapratin r cndensatin ccurred) In part (b) answers had t be in terms f kinetic-mlecular thery t earn full credit. Cmmn errrs included the fllwing: Talking abut the electrns r atms instead f mlecules (i.e., blurring the distinctins between electrns, atms, and mlecules). Nt clearly explaining that an increase in kinetic energy results in mre gas mlecules mving faster, which results in mre mlecules clliding with the walls f the flask and clliding with a greater frce Using language such as mre intense cllisins rather than harder cllisins, r excited atms (r electrns) rather than faster-mving mlecules Recgnizing that kinetic energy increased with increasing temperature and that the particles mve faster and cllide with the walls f the flask mre frequently, but just restating the phrase and giving it as a secnd reasn Listing speed, energy, and frequency as separate reasns fr increased pressure Invking cllisins between mlecules, rather than cllisins with the wall f the flask, as a reasn fr increased pressure Stating that the vlume decreased s the pressure had t increase Applying strictly gas law arguments, ignring KMT Writing incrrect statements, such as: The gas expanded thus increasing the pressure. The increase in temperature caused the mlecules t expand (sunding like the mlecules swell). The mlecules cllide with each ther t cause an increase in pressure (rather than cllide with the walls f the cntainer). As temperature increases, vlume increases, but since it is in a rigid cntainer, pressure ges up instead. Treating the phase change as a reactin and using activatin energy and heats f reactin fr reasns 15

16 Part (c)(i): Trying t identify the reactin rder that each graph represented rather than picking the first graph, which clearly represented a 0 rder reactin (because f the straight line plt f [ethanl] versus time) Writing that the slpe is either +1 r -1 (rather than linear) t describe the [ethanl] versus time graph Identifying the [ethanl] versus time graph as indicating a first-rder reactin Part (c)(ii): Crrectly identifying the rder f the reactin as 0 but nt writing the crrespnding rate law Including the prducts in the rate law equatin Writing an equilibrium expressin fr the rate law Writing the integrated rate equatins rather than the rate law equatin Writing the first-rder rate equatin after identifying the reactin as zer rder Part (c)(iii): Using cncentratin brackets t represent units r mles rather than mles/liter Omitting units even thugh the questin specifically asked fr units Incrrectly substituting values fr rate int the rate equatin when slving fr k, the rate cnstant: Using a single data pint fr the rate Using secnds fr the rate Using mlarity fr the rate Sme students crrectly stated that the reactin was zer rder in part (c)(i), wrte the rate law incrrectly in part (c)(ii), but then interpreted the graph crrectly and were able t calculate the value f k with crrect units and earned 2 pints in part (c)(iii). Part (d): Trying t make this a PV = nrt prblem r a Byle s law prblem by pulling 2.0 L, 1.0 L, and 100 trr frm ther parts f the questin Stating that because the ethanl cmpletely decmpsed, the pressure drpped t zer, r that the flask is empty s the pressure = 0. Stating that the pressure is cnstant s the final pressure is 0.40 atm Based n yur experience f student respnses at the AP Reading, what message wuld yu like t send t teachers that might help them t imprve the perfrmance f their students n the exam? Help students understand the cnnectin between cnstant bservable prperties and a system at equilibrium dynamic equilibrium. Make sure that students can interpret graphical data. 16

17 Remember t teach zer rder reactins. Make sure that students give a reasn fr their chice. Remind students t carefully read the questins. Teach students the difference between a rate law, an integrated rate law, and an equilibrium expressin, and t distinguish between rate and time. Teach students what [ ] represents. It was clear that many students did nt knw hw t write the rate law equatin. Teach what cncentratin brackets represent. Help students understand that [ ] (cncentratin brackets) are nt units. Teach students t set up the prblem, substitute the numbers in, and then d the calculatin. Often in a nncalculatr prblem, if the wrk was clearly shwn, a pint wuld be earned even when a cmputatinal errr was made. 17