Title: Preparation And Studies of Potassium Diaquabis(Oxalato) Chromate (II I) Dihydrate !"Cr (C#O$)# (%#O)#&'#%#O Obe*ti+e: To prepare potassium diaquabis(oxalate) *hromate (III) dihydrate *omplexes and study its rea*tions' Introdu*tion: Chro Chromi mium um (Cr) (Cr) is foun found d in ,rou ,roup p - (.I/ (.I/)) of the the Perio Periodi di** Table ble alon alon, , 0ith 0ith molybdenum (1o) and tun,sten (2)' It is lo*ated at Period $ and has atomi* number of #$' These metals3 therefore3 ha+e - +alen*e ele*trons' In the *hromium atom3 the 4d5orbital is half5filled 0ith 6 ele*trons and -th ele*tron is present in 4s orbital' It is in solid state at room temperature and density of 789$ ,m 54' ;le*tron *onfi,uration of *hromium is "Ar& 4d 6$s8' /oilin, point and meltin, point of *hromium are #-78 oC and 89<7 oC respe*ti+ely' /esides3 it has atomi* mass of 68'99-8 amu' This transition metal is a lustrous steel5,ray *olor 0hi*h taes a hi,h polish' It is hard and resistant to *orrosion' Chromium has a hi,h meltin, point3 stable *rystalline stru*ture3 and moderate moderate thermal thermal expansion' expansion' All *hromium *ompounds *ompounds are *olo *olored red'' Chro Chromi mium um *ompo *ompoun unds ds are are toxi toxi*' *' Chro Chromi mium um is used used to hard harden en steel steel'' It is a *omponent of stainless steel and many other alloys' The metal is *ommonly used for platin, to produ*e a shiny3 hard surfa*e that is resistant to *orrosion' Chromium is used as a *atalyst' It is added to ,lass to produ*e an emerald ,reen *olor' Chromium *ompounds are important as pi,ments3 mordants3 and oxidi=in, a,ents'The prin*ipal ore of *hromium is *hromite (>eCr #O$)' The metal may be produ*ed by redu*in, redu*in, its oxide oxide 0ith aluminum' aluminum' It *an be used dire*tly for the manufa*ture of steel and other *hromium *ompounds' An alloy of *hromium and ni*el3 ni*hrome is used for ele*tri*al heatin, elements' Chromium (III) *ompounds are sparin,ly soluble in 0ater' Chromium (III)3 the most stable oxidation state of *hromium3 forms a number of *omplexes3 in 0hi*h six monodentate monodentate li,ands surround the the *ation o*tahedrally' The *olors of these these *omplexe *omplexess are determin determined ed by the li,ands li,ands atta*hed atta*hed to the Cr *entre' *entre' In the abo+e abo+e *ompound3 t0o oxalate anions (bidentate li,ands) and t0o 0ater mole*ules *oordinate the *hromium *ation to pro+ide oxy,en atoms at the *orners of the o*tahedron' T0o ,eometri*al isomers exist for the *omplex ion3 "Cr(C #O$)#(%#O)#&53 t0o (C #O$)#5 ions "or t0o % #O
mole*ules& *an be on the same side of o*tahedron stru*ture of the *omplex ion ,i+in, the isomer or on the opposite sides ,i+in,
cis
trans isomer'
Preparation
(8)
A pestle and mortar 0ere used to po0der <'<666 mole oxali* a*id dihydrate and separately <'<<-? mole ! #Cr #O7' The po0ders 0ere mixed thorou,hly3 re,rindin, them ,ently'
(#)
The mixture 0as transferred into a 8<< ml beaer' The beaer 0as *o+ered 0ith a small 0at*h ,lass and heated ,ently the beaer on the hot5plate' A +i,orous rea*tion *ommen*ed 0ith the e+olution of steam and *arbon dioxide and 0ith the formation of a dar semi5solid *ompound'
(4)
The beaer 0as remo+ed from the hot5plate and immediately added 86 ml absolute ethanol' The mixture 0as stirred 0ith a ,lass rod and the mixture 0as transferred into a small mortar (or stir 0ith a beaer and ,lass rod)' Stirrin, 0as *ontinued for further 6 minutes breain, up the produ*t and de*ant the ethanol' A fresh 86 ml absolute ethanol 0as added' The produ*t 0as ,rinded usin, a beaer and ,lass rod until a *rystalline +iolet po0der is obtained'
($)
The *rystals 0ere filtered by +a*uum su*tion usin, a sintered ,lass *ru*ible' The *rystals 0ere 0ashed t0o times 0ith 6 ml ethanol and dried in the air by +a*uum su*tion' The produ*t 0as 0ei,hed on a top loadin, balan*e and transferred into a properly labelled +ial'
(6)
The follo0in, tests 0ere *arried out on the produ*t' @ualitati+e Analysis
(i)
Test for potassium
The flame5test for potassium 0as performed in a fume *upboard usin, a platinum 0ire' The *olour of the flame 0as noted'
(ii)
Test for cis and trans isomers
The follo0in, test 0as *arried out to determine the nature of the isomer prepared in this experiment:
A fe0 *rystals of the produ*t 0as moisten and pla*ed on a small filter paper restin, on a 0at*h ,lass3 first 0ith a fe0 drops of dilute ammonia solution and then 0ith dilute hydro*hlori* a*id' Obser+ations 0ere re*orded' 2ith the
cis5
form3 a deep5
,reen solution rapidly forms and spreads o+er the filter paper3 no solid remains' 2ith the
trans5
form there is no ,reen *olour3 but li,ht5bro0n solid forms and remains
undissol+ed' In ea*h *ase3 rea*tion 0ith dilute %Cl restores the ori,inal *olour of the isomeri* form'
esults:
1ass of oxali* a*id dehydrate
-'9?79,
1ass of ! #Cr #O7
#'<<6?,
1ass of 0at*h ,lass
89'?#<8,
1ass of 0at*h ,lass and produ*t
#4'97<8,
1ass of produ*t
#4'97<8,589'?#<8, B $'86,
Obser+ations:
i)
Test for potassium The *olour of the flame is pinish purple'
ii)
Test for *is and trans isomer A deep ,reen solution rapidly forms and no solid remains'
Dis*ussion:
The purpose of doin, this experiment is to prepare potassium diaquabis (oxalato) *hromate (III) dihydrate *omplexes and study its rea*tions' Chromium forms o*tahedral *omplex ion lie many transition metals do' In this experiment3 potassium diaquabis (oxalato) *hromate (III) dihydrate 0as formed' Oxali* a*id dihydrate and potassium di*hromate 0ere as the startin, material in this experiment to prepare the *omplex' Potassium di*hromate (! #Cr #O7) is a *rystalline ioni* solid in red oran,e *olour and it has mole*ular 0ei,ht of #9$'86 ,mol' It is also an inor,ani* *hemi*al rea,ent 0here mostly use as an oxidi=in, a,ent in the rea*tion' Its mole*ular ,eometry is tetrahedral' Oxali* a*id dihydrate (% #C#O$'#%#O) is a *olourless *rystalline solid' It is soluble in 0ater and its solubility in*rease 0ith the temperature' It is a stron, or,ani* a*id and simplest di*arboxyli* a*id' The oxalate ion from oxali* a*id dehydrate3 C#O$#5 is the *onu,ate base of oxali* a*id3 %#C#O$3 a*ts as a *helatin, a,ent for potassium ion be*ause the oxali* a*id is the redu*in, a,ent for this oxidation5redu*tion rea*tion' As the startin, pro*edures in this experiment3 both solution 0ere mixed and heated on a hot plate until the formation of a dar semi5solid *ompound 0ith e+olution of *arbon dioxide and steam 0as released' The formation of this *ompound indi*ates that *omplex 0as started to form after the potassium di*hromate rea*t 0ith oxali* a*id dihydrate' ext3 the produ*t 0as ,rinded until a +iolet po0der is obtained' At the end3 the produ*t 0as bein, filtered and thus the final produ*t formed in this rea*tion is potassium diaquabis(oxalate) *hromate (III) dihydrate !"Cr (C #O$)# (%#O)#&'#%#O' This produ*t formed is in solid state 0hi*h is in bla* *olour under sunli,ht but has a deep purple *olour under artifi*ial li,ht' The produ*t is in o*tahedral shape 0hi*h has - *oordination number' %alf ioni* equation for ea*h rea*tion in+ol+ed in the preparation of the *omplex usin, oxali* a*id dihydrate is % #C#O$
#CO# E #% E E #e 5' ext3 the half ioni* equation for potassium di*hromate for the preparation *omplex is Cr #O7#F(aq) E 8$%E E -e F G #Cr 4E(aq) E 7%#O' The balan*ed equation for o+erall rea*tion is ! #"Cr#<7& E 7%#C#<$H#%#<
#!"Cr(%#<)#(C#<$)#&'#%#< E -CO# E 84%#O'
In this experiment3 distilled 0ater 0as not allo0ed to rinse the produ*t be*ause it may *ause the produ*t to dissol+e and form another produ*t 0hi*h is hexaaqua*hromium (III)3 "Cr(%#O)-& 4E 0hi*h is soluble in 0ater' Therefore3 absolute ethanol 0as used to 0ash the *rystals durin, the filtration pro*ess' Absolute ethanol is ethanol 0ith lo0 0ater *ontent (6) and hi,h per*enta,e of ethanol (99)' /y usin, absolute ethanol3 the produ*t formed 0as less soluble in ethanol than in 0ater so it 0ill not dissol+ed mu*h of the produ*t in 0ater' In other 0ords3 absolute ethanol helped to redu*e the amount of *rystals from dissol+ed
further' >urthermore3 by dryin, the produ*t in the air by +a*uum su*tion as in the experiment3 the produ*t formed 0as far easier to dry 0ith absolute ethanol in it than 0ith 0ater as ethanol is +olatile' That is 0hy absolute ethanol 0as used to 0ash the produ*t for fe0 times' After had *olle*ted the produ*t3 follo0in, test 0ere *arried out on the produ*t' >lame test 0as used to test for the presen*e of potassium usin, a platinum 0ire' Colour of the produ*t formed 0as determined under the flame of /unsen burner' A flame test is to dete*t the presen*e of *ertain element3 basi*ally metal ions 0hi*h based on ea*h elementJs *hara*teristi* emission spe*trum' >rom the differen*e in *olour for ea*h of the element3 0e *an differentiate bet0een one element to another element' In this experiment3 the *olour of flame *han,es from oran,e to pinish purple' This obser+ation that obtained had indi*ates that potassium ion 0as present in this *ompound' ext3 test for *is and trans isomer by addin, ammonia solution and dilute hydro*hlori* a*id' /oth isomers are in equilibrium 0hen solid *omplex is dissol+ed in 0ater' Cis form is more soluble and exists in hi,h proportion solution but it is slo0ly *on+erted into trans from 0hen the solution is e+aporated slo0ly' In solid state only one isomer *an exits dependin, on the method of preparation of the *omplex' >irstly3 fe0 drops of dilute ammonia solution and then dilute hydro*hlori* a*id 0ere added' /ased on obser+ations obtained3 a deep ,reen solution 0as rapidly formed and spread o+er the filter paper but there 0ere no solid remains' This had sho0ed for the *is5form on the produ*t' >urthermore3 as the solution e+aporated slo0ly3 li,ht bro0n solid 0as formed and remains undissol+ed and there is no lon,er ,reen *olour solution' In this experiment3 per*enta,e yield 0as *al*ulated' /y tain, the theoreti*al mass and a*tual mass of the produ*t3 per*enta,e yield *an be obtained' Thus3 the formula is
Per*enta,e yield () B 7%#C#O$'#%#O
Actual mass Theoretical mass
×
8<<' >rom the equation3 ! #"Cr #O7& E
#!"Cr(%#O)#(C#O$)#&'#%#O E -CO# E 84%#O3 8 mole of ! #Cr #O7 produ*e
# moles of !"Cr(% #O)#(C#O$)#&'#%#O' >rom this equation3 0e *an find for the theoreti*al mass of the produ*t' On the other hand3 a*tual mass 0ill be obtained throu,h *ondu*tin, this experiment' The theoreti*al mass of this produ*t is $'-4, and a*tual mass that obtained is $'86,' Therefore3 per*enta,e yield that ,et from this experiment is ?9'-4,'
There 0ere some pre*autionary steps taen in this experiment' >irstly3 before started the experiment3 all the apparatus must 0ashed 0ith a*etone then put into the o+en to remo+e a*etone' 2e ha+e to use a*etone instead of usin, 0ater be*ause the produ*t formed later 0ill dissol+ed in 0ater and this 0ill affe*t the per*enta,e yield of the produ*t' /esides3 before the *rystalline +iolet po0der 0as bein, filtered by +a*uum su*tion3 the po0der must be spread in order to add in absolute ethanol' This is to ensure that the produ*t *an dry up faster and the unrea*ted impurities are able to 0ash a0ay' 1oreo+er3 absolute ethanol 0as *hosen to be used in this experiment be*ause it has a hi,h proportion of ethanol than 0ater and thus3 produ*t formed 0ould not easily dissol+ed' >urthermore3 ,lo+es and safety ,oo,les must be 0orn throu,hout the experiment to pre+ent any inuries and to a+oid any dire*t *onta*t 0ith *hemi*als' @uestions and Ans0ers: (8)
2hat is the oxidation state of *hromium in ! #Cr #O7 and in the produ*tK 2rite t0o half5equations for ea*h rea*tion in+ol+ed in the preparation of the *omplex usin, oxali* a*id dihydrate (%#C#O$'#%#O) and potassium di*hromate (! #Cr #O7) and a balan*ed equation for the o+erall rea*tion' Oxidation state of *hromium in !#Cr#O7 : E8(#) E #x 5#(7) B< #x B8# LB EOxidation state of *hromium in produ*t: x E <(#) E <(#) B 58 xB 58 T0o half equations: Oxali* a*id dehydrate: % #C#O$
#CO# E #%E E #e5 ( Oxidation rea*tion)
Potassium di*hromate: Cr #O7#F(aq) E 8$%E E -eF G #Cr 4E(aq) E 7%#O (edu*tion rea*tion) /alan*ed
equation
for
o+erall
equation:
#!"Cr(%#<)#(C#<$)#&'#%#< E -CO# E 84%#O
! #"Cr#<7&
E
7%#C#<$H#%#<
(#)
Cal*ulate the yield of the produ*t'
! #"Cr #O7& E 7%#C#O$'#%#O
#!"Cr(%#O)#(C#O$)#&'#%#O E -CO# E 84%#O
>rom the equation: 8 mole of ! #Cr #O7 produ*e # moles of !"Cr(%#O)#(C#O$)#&'#%#O 1olar mass of ! #Cr #O7 B #9$'8?6 ,mol 1olar mass of !"Cr(%#O)#(C#O$)#&'#%#O B 449'89$4 ,mol Mass ( g ) umber of moles of ! #"Cr #O7& B Molar Mass
2.0058
B
294.185
B -'?#
×
( ) g mol
g
( ) g mol
8<54 mol
umber of moles of !"Cr(%#O)#(C#O$)#&'#%#O B -'?#
×
8<54 mol
×
#
B <'<84-$ mol Theoreti*al mass of !"Cr(%#O)#(C#O$)#&'#%#O B no' of moles
B <'<84-$ B $'-4 , A*tual mass of !"Cr(%#O)#(C#O$)#&'#%#O B $'86 , Actual mass Theoretical mass
Per*enta,e yield () B
4.15 g
B
4.63 g
× 100
B ?9'-4
× 8<<
×
molar mass
× 449'89$4
(4)
2rite the *hemi*al equations for the rea*tions in+ol+ed in the tests for the dete*tion of *hromium (III) and oxalate ions' Indi*ate in ea*h *ase 0hether it is an a*id5base or oxidation5redu*tion rea*tion' The test for *hromium (III) is an a*id5 base rea*tion' Produ*t 0as dissol+ed in 0ater and form hexaaqua*hromium(III) ions "Cr(%#O)-&4E' 2hen a base 0as added into the solution3 neutral *omplex and pre*ipitate 0ere formed' >or example3 rea*tions 0ith ammonia solution' The ammonia a*ts as base and a li,and' /y addin, small amount of ammonia solution3 hydroxide ions (O%5) are pulled off the hexaaqua ionto ,i+e the neutral *omplex' ;quation: "Cr(%#O)-&4E E 4% 4
"Cr(%#O)4(O%)4& E 4%$E
/y addin, ex*ess ammonia solution3 ammonia 0ill repla*e 0ater as a li,and to ,i+e hexaammine*hromium(III) ions' ;quation: "Cr(%#O)-& 4E E-%4
"Cr(%4)-&4E E-%#O
ext3 the test for oxalate ion is oxidation5redu*tion rea*tion'
Purple *olour of
potassium perman,anate 0ill de*olourise immediately if rea*t 0ith a*id lie dilute sulphuri* a*id 0hile purple *olour of potassium perman,anate de*olourise slo0ly 0ithout dilute %#SO$' In this *ase3 a*id a*ts as a *atalyst in this redox rea*tion' The oxalate ion is oxidised by the perman,anate ion in the a*id solution' Potassium perman,anate (!1nO$) is a*ts as oxidisin, a,ent' In this oxidation5redu*tion rea*tion3 the C#O$#5 is oxidised to CO #' ;quation: C#O$#5
#CO# E #e5
The perman,anate ion (1nO$5) 0ill be redu*ed to 1n #E' ;quation: 1nO$5 E 6e5 ($)
1n#E
Dra0 the stru*tures of the t0o possible ,eometri*al isomers of the *omplex' 2hat is your *on*lusion re,ardin, the nature of the isomer in the test (i+)K 2hen dilute ammonia is added to the solid *omplex3 one mole*ule of the li,and % #O is repla*ed by O% 5 ion' 2rite equations for both the isomers to represent this rea*tion as 0ell as for the rea*tion of the hydroxo produ*t and %Cl to restore the ori,inal *omplex
The ,eometri*al isomer that exists in !"Cr(%#<)#(C#<$)#&'#%#< is *is isomer be*ause it formed a deep ,reen solution and spreads o+er the filter paper and there 0as no solid remains' The ammonia a*ts as both a base and a li,and' 2hen dilute ammonia is added to the solid *omplex3 one mole*ule of the li,and % #O is repla*ed by O% 5 ion' ;quation for both isomers: !"Cr(C#<$)# (%#<)#&'#%#< E %4
!" Cr(C#<$)# (%#<)(O%)&'#%#O E %$E
!"Cr(C#<$)# (%#<)(O%)&'#%#< E %Cl
!"Cr(C#<$)# (%#<)#&'#%#< ECl5
Con*lusion: The produ*t formed in this experiment is potassium diaquabis (oxalato) *hromate (III) dihydrate *omplexes3 !"Cr (C#O$)# (%#O)#&'#%#O 0hi*h is in o*tahedral shape' This *omplex has t0o oxalate anions bidendate li,and and t0o 0ater mole*ules *oordinate the *hromium *ation to pro+ide oxy,en atoms' This produ*t 0as prepared form potassium di*hromate and oxali* a*id dihydrate' The per*enta,e yield of the produ*t is ?9'-4' Pinish purple flame *an be seen 0hen the produ*t 0as tested for flame test sho0in, the presen*e of potassium ion' ext3 a deep ,reen solution 0as formed and no solid remains sho0in, the produ*t 0ith *is isomer' eferen*e: 8') Chromium' A+ailable from: http:*hemistry'about'*omodelementfa*tsa*hromium'html' "A**ess on 8 1ar*h #<8$&
#') ea*tions of *hromium (III) ions in solution' A+ailable from: http:000'*hem,uide'*o'uinor,ani*transition*hromium'html' "A**ess on 8 1ar*h #<8$& 4') Inor,ani* Synthesis' A+ailable from: http:files'rushim'ruboospolytominor,ani*5 synthesisinor,ani*5synthesis87'pdf ' "A**ess on 8 1ar*h #<8$&
