ABSTRACT
In this experiment, two clear solutions are added together in one container. At frst, no reaction can be visibly seen ater mixing. But, ater a short delay, the clear solution turned into blue-black shade solution. his is due to the variations that we did in our set-up. he frst set-up was in a room-temperature having a mixture o !"."m# o ".$""% &I, !"."m# o ".$""% &'l, !"."m# o ".""("% )a$*$ )a$*$+, +, ".( m# o *tarch *tarch *olution *olution,, and and $"." $"." m# o ".!""% ".!""% &$*$+. &$*$+. he second set-up was also in a room-temperature having a mixture o $"."m# o ".$""% &I, !"."m# !"."m# o ".""("% )a$*$+, )a$*$+, ".( m# o *tarch *olution and $"m# o ".!"" ".!""% % &$*$+ &$*$+. . he he third third set-up set-up was was also also in a room room-te -temp mpera eratur ture e having having a mixture o $"."m# ".$""% &I, !"."m# ".""("% )a$*$+, ".( m# *tarch *olution, !"m# o ".!""% &$*$+ and !"m# o )a$*+. #astly, the ourth set-up was under under an ice-ba ice-bath th having having a mixtur mixture e o $"."m $"."m# # ".$" ".$""% "% &I, !"."m !"."m# # "."" ".""("% ("% )a$*$+, ".( m# *tarch *olution, and $"m# o ".!""% &$*$+. hrough these, we were able to determine the order o reaction, computed or the rate constan constantt o the reaction reaction at a given temperatur temperature e and ionic strength strength.. And lastly, we proved proved that the temperature temperature a/ects on the rate o the reaction. reacti on.
INTRODUCTION
he purpose o this experiment experiment is to determine determine the rate law and the activation energy or the reaction between persulate ion, * $+$-, and iodide ion, I-0 *$+$-1a23 4 $ I -1a23
$ *+$-1a23 4 I$1a23
he rate law can be written written as0
5eaction rate 6 7here m and n are the orders with respect to * $+$- and I-, respectively, and k is the rate constant. 8etermining 8etermining the rate law involves determining the values o m and n. he temperature temperature dependence o the rate constant constant is given given by0 by0
he Arrhenius Arrhenius 92uation, where A is the pre-exponential pre-exponential actor, 9 is the activation energy with units o :;mol, is the absolute temperature, and 5 is the gas consta constant nt 156 156 .! .!( ( : & -!mol-!3 expla plains ins that that the the acti activa vati tion on ener energy gy can can be obtained obtained by measuring measuring the rate constant constant at several several temperatur temperatures, es, and then plotting ln k versus !;.
*el-
indicating reactions o this type, in which nothing seems to happen or a while and then a change suddenly becomes visible, are known as clock reactions. 'lock reactions oten appear in chemistry shows, and many involve iodine, since the colour change resulting rom the generation o iodine is particularly marked. he stoichiometric e2uation tells us that the rate o consumption o persulate e2uals the rate o iodine production. In each experiment, time measurement, =t, is taken to consume a known amount o thiosulate 1at which point the solution turns blue3. In this experiment, the e/ect o reactant concentration on the rate o reaction will be investigated, and thereby obtain the rate e2uation. he e/ect o temperature is also later studied, and the results used to fnd the activation energy or the reaction. >inally, the e/ect o the ionic strength o the solution on the rate is measured, rom which a possible structure o the reaction intermediate may be deduced.
METHODOLOGY
PROCEDURE •
•
>our experiments had to be perormed, the three had been done under room temperature whilst the th experiment was under an ice bath. All the solutions except &'l and )a$*+ had to be prepared. hese two solutions were used to maintain the ionic strength o the solutions.
!. o prepare the needed solutions, computed amount o each solutions was poured into the volumetric ?ask and distilled water had been added up to the mark on the neck o the ?ask. hese diluted solutions would be used along the experiment. $. >or each experiment there were two reagents@ A and B. All the experiments had ".( m# o resh starch solution in reagent A. And in the frst experiment, the other solutions in 5eagent A, prepared in the e-?ask were0 !"m# o &I, &'l and )a$*$+ %eanwhile or reagent B which was prepared in a beaker contained0 $" m# o & $*$+ and there was no )a $*+ present in the reagent. . +nce the two reagents had been prepared, the contents o the beaker could now then be added to the contents o the ?ask, the timer should be started once the transerring started. he contents o the ?ask was poured back into the beaker then returned it back to the ?ask to mix the reagents well. . he ?ask had been allowed to sit undisturbed and had been observed. he timer was then stopped once the blue-black color appeared. he time then was recorded and the experiment was duplicated. (. his procedure was repeated or experiments $, , and . >or experiment $, 5eagent A was composed o $" m# &I, no &'l, !" m# )a $*$+. >or experiments and , there were $" m# &I, also no &'l, !( m# o )a $*$+ in the reagent A or the compositions in 5eagent B, experiment $ and had $" m# o & $*$+ and or experiment , there was !" m# each o & $*$+ and )a$*+. . In order to get the temperature o the solutions, the temperature o one o the reaction solutions could be recorder or all. 1since experiments had been done under room temperature3 . >or experiment , the reagent A0 $" m# &I and !" m# )a $*$+ and reagent B0 $" m# o & $*$+ were put into two beakers with ice and they had been allowed to remain in the ice or atleast ( minutes. . he temperature o the ice bath was then recorded. C. he reagents had been mixed and was put back to the ice back. he timer then was started when the reaction ?ask started to show a color change. !".'alculations had been done ater perorming the experiment0 dilution calculations, calculation o the average elapsed time or each experiment and the rate or each reaction, the order o reaction with respect to the Iodide ion and the
RESULTS AND DISCUSSIONS
able -! Initial 'oncentrations o the Ions
9xperiment
DI-E
D*$+E$-
D*$+E$-
!
"."C %
"."C
C.C"! x !"-
$
"."C$ %
"."C
C.C"! x !"-(
"."C$ %
"."!C
C.C"! x !"-
emperature o *olution 1o'3
"
"
"
"."C$ %
"."C
C.C"! x !"-
emperature o *olution 1o'3
!"
!"
!"
able -$ 8etermination o the rate o reaction with respect to iodide and persulate ions 9xperiment !
9lapsed ime 1s3 rial 1A3
rial 1B3
(.$(
$
(.!!
$$.(
$$.
.!
1ice bath3
.$$
C.($
CC.
Average ime 1s3
5ate 1%;s3 !."C( !"-( $.!CC !"-( !."$( !"-( (."$C( !"-
(.! $$.(( .! C.
able - 8etermination o rate constant,k, and activation energy 5eaction +rder 5ounded to 'alculated numbers 7ith respect to Iodide Ion ! ! 7ith respect to
!."
+verall Falue o k rom 9xpt ! Falue o k rom 9xpt $ Falue o k rom 9xpt Average value or & 1including units3 room temp value o k rom experiment 1ice bath3 including units activation energy, 9a 1k:;mol3
x x x x
whole
! $ ".""C 1!;%-s3 ".""CC( 1!;%-s3 ".""(($ 1!;%-s3 ".""( 1!;%-s3 ".""!" 1!;%-s3 (!. k:;mol
1-!3
I$1a23 4 $ *$+$-1a23 G $ I - 4 *+$-1a23 I$1a23 4 starch G blue complex
>ast *low
1-$3 1-3
5eaction 1-!3 is much slower than reaction 1-$3, and, as a result, the I $ in ormed in reaction 1-!3 is immediately consumed by reaction 1-$3, and the concentration o I$ remains at a very low value as long as thiosulate ion is present. 7hen all o the thiosulate ion was used up, the concentration o I $ rom reaction 1-!3 increased. he presence o I $ was detected by the ormation o a blue complex resulting rom the reaction o I $ with starch, reaction 1-3. he rate o reaction 1-3 did not become signifcant until the concentration o I$ became appreciable. A characteristic o this reaction is that the reaction mixture remains colorless or several minutes ater the reactants are mixed. 8uring this time both reactions 1!3 and 1-$3 were occurring. he solution remained colorless because the I $ rom reaction 1-!3 was being consumed by reaction 1-$3, and canHt react with the starch. As soon as the thiosulate ion was used up, the I$ reacted with the starch, and an abrupt and dramatic color change, rom colorless to blue, occured. he rate o reaction 1-!3 is the rate o consumption o * $+$- ion. >or every *$+$- ion used up two * $+$- ions are consumed. hus,0 1-3 5ate o reaction 1-!3 6 I 1*$+$-3o is the initial concentration o * $+$- ion, and =t is the time interval rom the start o the reaction until the solution changed color, then in e2uation 1-3, =1*$+$-3 6 " - 1* $+$-3o , and i the initial concentration o *$+$- is constant then e2uation 1-3 becomes 1-(3 5ate o reaction 1-!3 6 As the name implies, rate constant, k must be constant at a specifc. In this experiment, the values o rate constant obtained at room temperature using experiments were similar to each other. o be able to fnd the value o k, experiments were carried out in which concentration o iodide ions were held constant or experiments $ and while concentation o persulate ions were held constant or ! and $. his was done to be able to fnd the corresponding exponents,m and n, to be used or the ollowing rate e2uation0 rate 6 kDI -Em D1*$+$-En. he values o m and n do not correspond to the coecient o the ions in the chemical e2uation. It can only be determined experimentally. It has been observed that the rate, reported in molar;s, increased when the molarity o one o the ion increased provided the other was held constant as in the case o experiment ! and $ where D1*$+3 $-E was held constant and exerment $ has a higher concentration o DI -E. 7hen experiment $ and was compared in which DI -E was constant while D1*$+3$-E was higher in experiment $, the rate o reaction had a higher value in $. his could be due to the act that as ionic streghth increases, the aster the reaction proceeds. Ionic strength is a actor o concentration and charge o the ion.
It is important to stress out that it is the rate o reaction which varies depending on the concentration and values o m and n exponents while the rate constant, k, is the same or a specifc temperature no matter what the concentration is. At lower temperature, & became lower as well which is expected already as implied by the Arrhenius e2uation.
his is because at low temperatures, molecules have lower kinetic energy to meet the re2uired activation energy. In act, had here been more values, the graph o ln & vs !; would show a linear correlation which would support the said observation. 8etermination o activation energy was done using the ollowing e2uation derived rom Arrhenius e2uation,
As the name implies, rate constant must be constant at a specifc. In this experiment, the values o rate constant obtained at room temperature are similar to each other. At lower temperature though, & became lower as well which is expected already as implied by the Arrhenius e2uation. his is because at low temperatures, molecules have lower kinetic energy to meet the re2uired activation energy. In act, had here been more values, the graph o ln & vs !; would show a linear correlation which would support the said observation. SAMPLE CALCULATIONS
9xperiment !0 '!F!6'$F$ DIodideE0 1".$%3 1"."!"#3 6 1' $3 1"."("(#3@ ' $6"."C"% D
[ thiosulfate] 5ate0
2 t
−4 6 !."C(x!" -( %;s 2 ( 45.18)
9.901 x 10
6
9xperiment $0 DIodideE0 1".$%3 1"."$"#3 6 1' $3 1"."("(#3@ ' $6"."C$% D
[ thiosulfate] 5ate0
2 t
−4 6 $.!CCx!" -( %;s 2 ( 22.565 )
9.901 x 10
6
9xperiment 0 DIodideE0 1".$%3 1"."$"#3 6 1' $3 1"."("(#3@ ' $6"."C$% D
[ thiosulfate] 5ate0
2 t
−4 6 !."$(x!" -( %;s 2 ( 48.18)
9.901 x 10
6
9xperiment 0 DIodideE0 1".$%3 1"."$"#3 6 1' $3 1"."("(#3@ ' $6"."C$% D
[ thiosulfate] 5ate0
2 t
−4 6 (."$C(x!" - %;s 2 ( 98.43)
9.901 x 10
6
5eaction order0 rate 6 kDiodideE mDpersulateE n i3
w; respect to Iodide ion 1m3
!0 !."C(x!"-( 6 k 1"."C"3 m 1"."C"3 n $0 $.!CCx!" -( 6 k 1"."C$3 m 1"."C3 n !J$0 m6! ii3
w;respect to
$0 $.!CCx!" -( 6 k 1"."C$3 m 1"."C3 n
0 !."$( x!" -( 6 k 1"."C$3 m 1"."!C3 n !J$0 n6! +verall0 m4n 6 $ 9xperiment !0 !."C(x!" -( 6 k 1"."C"3 ! 1"."C"3 ! & !6 ".""C ;%.s 9xperiment $0 $.!CCx!" -( 6 k 1"."C$3 ! 1"."C3 ! & $6 ""CC( ;%.s 9xperiment 0 !."$( x!" -( 6 k 1"."C$3 ! 1"."!C3 ! & 6 ".""(($ ;%.s 9xperiment 0 (."$C( x!" - 6 k 1"."C$3 ! 1"."C3 ! & 6 ".""!" ;%.s 1
Ea
ln& $ K ln& ! 6
R
1
T 2
1
-
T 1
3 1
Ea
ln1".""(3 K ln1"."""3 6
8.314
1
10
+ 273.15 -
1 30
+ 273.15 3
9a6 (!(.$ :;mol 9a 6 (!. k:;mol
CONCLUSION
he process mainly involved proving the validity o rate law e2uations and e2uilibrium constants so that the activation energy can be determined. he time recorded, wherein the solution turned to blue-black symboliLing that the all o the thiosulate has converted to tetrathionate, was used or calculating the rate. he overall order o reaction which was determined experimentally, is second order. #iterature value also shows that the peroxydisulate-iodide reaction is a second order reaction. A considerably high activation energy was obtained which implied that the reaction was slow, this was also observed during the ourth experiment in the procedure. he value o activation energy obtained is dependent o the e2uilibrium constants and temperature at which the e2uilibrium constants were obtained.
1n.d.3. 5etrieved on %arch $"!(, rom http0;;wwwchem.csustan.edu;chem!!!$;
Arrhenius Equation. 1n.d.3. 5etrieved on %arch $"!(, mula sa M'8avis 'N9%7I&I0 http0;;chemwiki.ucdavis.edu;
#iu, P., *hea/er, 5. #., Q Barker, :. 5. 1$""3. 9/ects o emperature and Ionic *trength on the 5ate and 92uilibrium 'onstants or the 5eaction IRa2 4 Ia2 S I$R-a2. The Journal of Physical Chemistry .
