SPECTROPHOTOMETRIC DETERMINATION OF THE EQUILIBRIUM CONSTANT OF A REACTION
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Experiment 1 Spectrophotometric Spectrophotometric Determination of the Stability Constant of a Complex Ion
Student Name: Student Number: Partner Name: Practical Section Number: 012 Date the Experiment !as Performed: 1"#01#201$ Date the %eport !as Submitted: 2#201$ Submitted to: Clarissa Soo'lal
Introduction Stable complex ions can be formed from the reaction bet!een transition metal cations and li(ands) *he stability of these ions can be expressed by the e+uilibrium constant for the formation of the complex and these constants are referred to as stability constants) ,no!led(e of the -alue of stability constants has many applications such as (uidin( the synthesis of ne! inor(anic materials. complexometric titration. and colorimetric analysis) In this experiment. an a+ueous solution of ferric ions is made to react !ith the thiocyanate ion to form a complex ion as follo!s: /e 234"56&a+4 6 SCN7a+4 ⇌ /e234$SCN4562a+4 6 23 14 *he purpose of this experiment is to determine the -alue of the stability constant for the formation of the complex ion. eSCN4 62 !hich can be expressed as: , 8 /eSCN4625#/e6&5/SCN75 24 In Part 9 of this !or'. fi-e different solutions !ere prepared usin( constant amounts of excess SCN 7 and -aryin( amounts of e 6&. actin( as the limitin( rea(ent. thus ensurin( the reaction (oes to completion) *he absorbance of each solution !as measured usin( the Spectronic 200 calibrated to ; nm. then plotted -ersus the molar concentration of eSCN4 determined from the initial concentration of the limitin( reactant.
62
. !hich !as
in order to establish a
calibration cur-e) In Part <. fi-e different mixtures !ith the same amount of e
6&
but different
amounts of SCN 7 !ere prepared) In these mixtures an e+uilibrium bet!een products and reactants !as established and the absorbance of each solution !as measured) =sin( the calibration cur-e as !ell as dilution calculations. the e+uilibrium concentrations. and thus the stability
constant.
!ere
determined)
3ur
!or'
spectrophotometric determination of stability constants)
pro-ides
added
insi(hts
into
the
Experimental Method *he determination of the stability constant for the eSCN4 62 complex -ia spectrophotometry !as performed accordin( to the procedures (i-en in pa(es 2172" of the lab manual) *he calculations to determine the stability constant are presented in 9ppendix I) *he ori(inal data sheet from the experiment is presented in 9ppendix II)
Results In Part 9 of the experiment. )0 m> 0)2000 ? NaSCN and -aryin( -olumes of 0)000$ ? eN3&4& !ere added to solutions 07$. !ith 0 as the blan' solution) *he absorbance of each solution !as measured usin( the Spectronic 200 calibrated to ; nm and the results are sho!n in *able 1) belo!) Solution Number 0 1 2 & $
9bsorbance 0 0)0;2 0)2@; 0)$1& 0)"0 0);
*able 1) 9bsorbance of solutions 07$ =sin( this data and the calculated concentration of eSCN4 62 for each solution. a calibration cur-e of absorbance -s) concentration of eSCN4 62 !as created !hich can be seen in i(ure 1) belo!)
1 0)@ f(x) = 6424.88x - 0.02 R² = 1
0) 0); e c n a b r o s b A
0)" 0)$ 0) 0)& 0)2 0)1 0 0
0
0
0
0
0
0
0
0
Concentration (mol/L)
i(ure 1) Calibration cur-e of absorbance -s) concentration of eSCN4 62 for solutions 07$
In Part <. $)0 m> of 0)002 ? eN3 &4& and -aryin( amounts of 0)002 ? NaSCN !ere added to solutions "710) *he absorbance of each solution !as measured and the results are sho!n in *able 2) belo!) Solution Number " ; @ 10
9bsorbance 0)22 0)0 0);2 0)"1 0);$
*able 2) 9bsorbance of solutions "710
=sin( the calibration cur-e and the absorbance data. the stability constant for eSCN4
62
in
solutions "710 !as determined to be 2")$;. 2"&)$&&@. 1$@)&1. 1$)&"1 and 1$;)&;". respectfully. for an a-era(e -alue of 1)@; x 10 2)
Discussion *he eSCN462 complex is a stable ion !hich means that in the formation reaction of eSCN462 the e+uilibrium lies far to the ri(ht) *hus. the lar(er the stability constant. the more stable the complex) In this !or'. the stability constant !as determined to be a lar(e number of 1)@; x 102. !hich is in correspondence !ith theory) *he spectrophotometer used in this experiment !as the Spectronic 200 !hich is accurate to A2 nm and A0)01 9 at 0)& 9)
Summary In this experiment. the stability constant for eSCN4 62 !as determined -ia spectrophotometry) In Part 9. a constant amount of SCN 7 and -aryin( amounts of e 6& !ere added to solutions 07$. !ith the reaction (oin( to completion and e
6&
actin( as the limitin(
reactant) *he absorbance of each solution !as measured usin( the Spectronic 200 calibrated to ; nm) =sin( this data. a calibration cur-e of absorbance -s) concentration of eSCN4
62
!as
created) In Part <. -aryin( amounts of SCN 7 and a constant amount of e 6& !as added to solutions "710 such that the reaction !ould reach an e+uilibrium instead of (oin( to completion) Conse+uently. the absorbance of solutions "710 !ere measured) =sin( this data and the calibration cur-e. the e+uilibrium concentrations of e 6&. SCN7. and eSCN4 62 !ere calculated) *he stability constant !as determined to be 1)@; x 10 2)
Appendices Please find attached 9ppendix I and 9ppendix II) 9ppendix I sho!s the calculations for the stability constant. ,) 9ppendix II contains the ori(inal data sheet from the lab)
Reerences 1) 2)
Budith. P) CHM 120 Course Manual =ni-ersity of *oronto ?ississau(a 201$ p 2172" umdahl. S) and Decoste. D) Chemical Principles, 7th Edition
&)
Appendix I ! Calculations Part 9 In Part 9 of this experiment. the composition of solutions 17$ included -aryin( amounts of 0)1 ? N3& and 0)000$ ? eN3 &4& as !ell as a constant m> of excess 0)2 ? NaSCN) In these solutions. e 6& !as the limitin( reactant and so the concentration of eSCN4 62 formed in the reaction is e+ual to the initial concentration of e 6& put into the solution) *o find the initial concentration of e 6& in each solution. dilution calculations must be performed) *his can be done by multiplyin( the -olume and concentration of eN3 &4& added and then di-idin( by the total -olume of the solution) Solution 17$: /e 6&5initial 8 initial concentration4-olume eN3 &4& added4#total -olume Solution 1: /e 6&5initial 8 0)000$ ?40) m>4#10 m> 8 2)0 x 10 7$ ? Solution 2: /e 6&5initial 8 0)000$ ?41)0 m>4#10 m> 8 $)0 x 10 7$ ? Solution &: /e 6&5initial 8 0)000$ ?41)" m>4#10 m> 8 )0 x 10 7$ ? Solution : /e 6&5initial 8 0)000$ ?42)2 m>4#10 m> 8 1)1 x 10 7 ? Solution $: /e 6&5initial 8 0)000$ ?42) m>4#10 m> 8 1) x 10 7 ? *he absorbance of each solution !as then measured as indicated in the data sheet found in 9ppendix II4 usin( a spectrophotometer. and a calibration cur-e for eSCN4 62 !as created found in %esults section4) Part < In Part < of this experiment. the composition of solutions "710 included -aryin( amounts of 0)1 ? N3& and 0)002 ? NaSCN as !ell as a constant -olume of $)00 m> 0)002 ? eN3 &4&) Since the reaction does not (o to completion. the e+uilibrium concentrations of e 6&. SCN7. and eSCN462 must be calculated) *he e+uilibrium concentration of eSCN4 62 can be read off the calibration cur-e from the measured absorbance for each solution as indicated in the data sheet found in 9ppendix II4 usin( the linear line of best fit) *he e+uilibrium concentrations of e 6& and SCN7 can be determined by subtractin( the e+uilibrium concentration of the complex since the ions and the complex react in a one7to7one ratio4 from the initial concentrations determined by a dilution calculation as in Part 94) Solution "710: /e 6&5initial 8 initial concentration4-olume eN3 &4& added4#total -olume Solution "710: /e 6&5initial 8 0)002 ?4$)0 m>4#10 m> 8 0)001 ? mol e6& 8 0)002 ?40)00$ >4 8 1 x 10 7$ mol
Solution "710: /e 6&5e+ 8 /e6&5initial 7 /eSCN4 625e+ *he line of best fit of the calibration cur-e yields the follo!in( e+uation: y 8 "2)@x 7 0)022& *he absorbance of solution " !as measured to be 0)22 y7-alue4 and so the x7-alue is the concentration of eSCN4 62 at e+uilibrium) 0)22 6 0)022&4#"2)@ 8 x 8 eSCN4 62 8 &)&&$ x 10 7$ ? *he same calculations !ere done for solutions ;710 usin( their respectful absorbance data) Solution ": /e 6&5e+ 8 0)001 ?4 7 &)&&$ x 10 7$ ?4 8 @)"1""$ x 10 7 ? Solution ;: /e 6&5e+ 8 0)001 ?4 7 ;)1 x 10 7$ ?4 8 @)212 x 10 7 ? Solution : /e 6&5e+ 8 0)001 ?4 7 ;)"@&$ x 10 7$ ?4 8 @)2&0"$ x 10 7 ? Solution @: /e 6&5e+ 8 0)001 ?4 7 @)@@0&; x 10 7$ ?4 8 @)00@"& x 10 7 ? Solution 10: /e 6&5e+ 8 0)001 ?4 7 1)21@ x 10 7 ?4 8 );$$1 x 10 7 ? *he same calculations must be done in order to find the concentration of SCN 7 at e+uilibrium: Solution "710: /SCN 75initial 8 initial concentration4-olume NaSCN added4#total -olume Solution "710: /SCN 75e+ 8 /SCN75initial 7 /eSCN4 625e+ Solution ": /SCN 75initial 8 0)002 ?41)0 m>4#10 m> 8 2)0 x 10 7 ? mol SCN7 8 0)00240)001 >4 8 2 x 10 7" mol /SCN75e+ 8 2)0 x 107 ?4 7 &)&&$ x 10 7$ ?4 8 1)"1""$ x 10 7 ? Solution ;: /SCN 75initial 8 0)002 ?42)0 m>4#10 m> 8 )0 x 10 7 ? mol SCN7 8 0)00240)002 >4 8 x 10 7" mol /SCN75e+ 8 )0 x 107 ?4 7 ;)1 x 10 7$ ?4 8 &)212 x 107 ? Solution : /SCN 75initial 8 0)002 ?4&)0 m>4#10 m> 8 ")0 x 10 7 ? mol SCN7 8 0)00240)00& >4 8 " x 10 7" mol /SCN75e+ 8 ")0 x 107 ?4 7 ;)"@&$ x 10 7$ ?4 8 $)2&0"$ x 10 7 ? Solution @: /SCN 75initial 8 0)002 ?4)0 m>4#10 m> 8 )0 x 10 7 ? mol SCN7 8 0)00240)00 >4 8 x 10 7" mol /SCN75e+ 8 )0 x 107 ?4 7 @)@@0&; x 10 7$ ?4 8 ;)000@" x 10 7 ? Solution 10: /SCN 75initial 8 0)002 ?4$)0 m>4#10 m> 8 1)0 x 10 7& ?
mol SCN7 8 0)00240)00$ >4 8 1 x 10 7$ mol /SCN75e+ 8 1)0 x 107& ?4 7 1)21@ x 10 7 ?4 8 );$$1 x 10 7 ? =sin( the e+uilibrium concentrations of the ions calculated abo-e and the e+uilibrium concentration of the complex read off the calibration cur-e. the stability constant can be determined usin( the follo!in( e+uation: , 8 /eSCN4625#/e 6&5/SCN75 914 ") , 8 &)&&$ x 10 7$ ?4 # @)"1""$ x 10 7 ?41)"1""$ x 10 7 ?4 8 2")$; ;) , 8 ;)1 x 107$ ?4 # @)212 x 10 7 ?4&)212 x 10 7 ?4 8 2"&)$&&@ ) , 8 ;)"@&$ x 10 7$ ?4 # @)2&0"$ x 10 7 ?4$)2&0"$ x 10 7 ?4 8 1$@)&1 @) , 8 @)@@0&; x 10 7$ ?4 # @)00@"& x 10 7 ?4;)000@" x 10 7 ?4 8 1$)&"1 10) , 8 1)21@ x 10 7 ?4 # );$$1 x 10 7 ?4);$$1 x 10 7 ?4 8 1$;)&;" *hus. the a-era(e of the stability constants is: ,a-( 8 2")$; 6 2"&)$&&@ 6 1$@)&1 6 1$)&"1 6 1$;)&;"4 # $ 8 1@;)0&1 *he a-era(e , stability for eSCN4 62 in this experiment !as therefore determined to be 1)@; x 10 2)
Appendix II ! Data Sheets Please see the next pa(es for data sheets)
