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Pchem lab report sucrose

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jessica reedy

on 10 December 2012

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Transcript of Pchem lab report sucrose

Experimental Objectives Use Guggenheim method to determine graphically the rate constant for the first- order reaction. To Determine:
The half-life at each temperature
The Arrhenius activation energy
The Arrhenius Constant A Follow the Optical Activity for the acid-catalyzed inversion of sucrose at two different temperatures;
35 C and 25 C Is the ability of an asymmetric compound to rotate plane polarized light.
Compounds that are optically active contain a chiral center.
Chirality is a property of the molecules structure. Optical Activity Is the ability of an asymmetric compound to rotate plane polarized light.
Compounds that are optically active contain a chiral center.
A chiral molecule, an enantiomer, is a molecule containing at least one carbon with four different sub groups.
Enantiomer have a non-superimposable mirror image and are asymmetric, lacking an internal plane of symmetry A enantiomer with a positive (+) rotation angle rotates the plane of plane polarized light clockwise is called dextrorotation
A enantiomer with a negative (-) rotation angle rotates the plane of linear polarized light counter clockwise is called levorotation. Rotation Angle Sucrose Glucose Fructose
+66.5° +52.7° -92.4° Rotation Angle Polarimeter In a polarimeter plane-polarized light passes through a sample cell containing a solution with a compound to be measured.
If the compound is optically active the plane of light will be rotated through the sample cell.
The rotation of light is due to the chiral centers of the compound interacting with parts of the plane light.
In this experiment the inversion of sucrose to fructose and glucose takes place, all 3 sugar molecules are chiral.
A polarimeter can be used to determine various characteristics, such as the identity of a specific compound. Optical activity can be used to determine the purity of molecule
Many sugars are enantiomers containing a chiral carbon, making them common models for optical activity.
Sucrose, a disaccharide can be broken down, in this experiment with the help of an acid catalyst, into two monosaccharide sugars which are glucose and fructose
All 3 sugarmolecules are chiral.
The Guggenheim method does not depend on the initial concentration
Using this method one only needs to know the time dependent on some quantity that is a linear function of the reactant concentration.
In this experiment the time depends on the decrease in the rotation angle as the reaction of inversion of sucrose by HCl proceeds.
By plotting a graph using the Guggenheim method, one can determine the rate constant the slope is equal to the negative rate constant. Guggenheim
Method Experimental Sucrose solution HCl Solution Guggenheim Plot Arrhenius Equation Results Rotation Angle at 25 C Rotation Angle at 35 C Rotation Angle In order to determine the value of the rate constant for each temperature, a Guggenheim plot is used. The Guggenheim method uses the data acquired from the polarimeter printed report and Equation below to create a linear plot of the optical rotation over time:
ln(ϴ - ϴ∞)
where ϴ is the rotation angle at a specific time, t and ϴ∞ is the rotation angle at a designated time, t∞ later in the data.The slope of the best fit line is equal to the negative rate constant, kexp. Guggenheim Plot at 35 C Guggenheim Plot 25 C Kexp= Ae
lnKexp=lnA-Ea/RT
ln(K /K )=-Ea/(1/T -1/T )
Arrhenius Equation is used to show the qualitative relationship between the rate a reaction occurs and the temperature.
As the temperature increases, the collisions between molecules increases.
When collisions increases this increase the kinetic energy, this has an effect on the activation energy.
The activation energy is the amount of energy need to allow the reaction to proceed. (-Ea/RT) From the equation of the graph we determined our activation energy to be 105.741 kJ
The Arrhenius constant A was determined to be A = 1.1072989x10 s
The literature value for activation energy is 102 kJ -15 o o o o o o o Kinetics of the Acid-Catalyzed Inversion of Sucrose Sucrose Fructose Rate Constant Rate = Ktrue[H2O] [H3O+] [Sucrose]Rate = kexp[Sucrose]The equations above become a pseudo-first-order reaction:
kexp = ktrue[H2O] [H3O+] x Y x Y Used 24.8mL of 12.1M HCl
This was diluted to 100mL with deionized water
Final concentration was 3.008M 5.005g of 99.895% pure solid sucrose was dissolved in 20mL of deionized water for the reaction done at 35 C
For the reaction done at 25 C 5.004g of sucrose was used. The prepared HCl solution and Sucrose solution are combined and stirred vigorously Perkin-Elmer Polarimeter Model 343 Sample Cell The sample cell is carefully filled with the Sucrose/HCl solution, making sure there are no air bubbles present.
The cell is resealed and then placed back into the polarimeter. Is equipped with a Na/NAL lamp and a constant flowing water bath, that can be set to a specific temperature.
In this experiment it is set to 25 C for the first run, and 35 C for the second run. From the polarimeter, a print out is given that shows 50 second interval measurements of the rotation angle.
This was then plotted using a 6 polynomial equation. Arrhenius Plot
By plotting each temperature at their respective rate, gives an equation of the line where the slope=-Ea/R and the y-intercept= lnA
Ea is the activation energy, and A is Arrhenius constant Residuals Residuals Rate constant for the reaction at 35oC is (1.32±0.004125)x10-3 s
Rate constant for the reaction at 25 C is (3.30682±0.01197)x10-4 s
This is consistent with what should happen. At a higher temperature, there will be more collisions, creating an increase in kinetics and increasing the rate constant.
The half-life was determined to be 2096±8 seconds at 25oC and 525±2 seconds at 35oC.
The Arrhenius constant A was determined to be 1.107298876 x 10-15.
Ea was found to be 105.74 kJ/mol Conclusion In this experiment the inversion of sucrose in to glucose and fructose is done with the help of an HCl (acid) catalyst.
The reaction of inversion of sucrose happens naturally in the stomach by the enzyme invertase. Half life From the graphs obtained using the Guggenheim method the rate constant at 35 C was determined to be (3.30682±0.01197)x10-4 s and at 25 C was determined to be (1.32±0.004125)x10-3 s -1 -1 The half-life, t1/2 of the sucrose using the equation:
t_(1/2)=ln⁡(2/k)
The half-life value shows how long it will take for a substance to be eliminated completely.
The t1/2 is the amount of time, in seconds, that it will take for sucrose to be completely broken down into glucose and fructose.
This term can be affected by a change in temperature.
At 25 C, t 1/2 was 2096 ±8seconds
At 35 C, t 1/2 was 525 ±2 seconds
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