First-Degree Reaction—the Transform of Sucros

Content

Purposes and Demands Principle Apparatus and Reagent Procedure Data Records and Processing Questions Attentions

Purposes and Demands

Measure the reaction rate constant and half-life of the transformation of sucrose.

Master the method of polarimetry to determine optical activity.

Principle

A reaction in which rate would increase in proportion to the concentration of reactant is defined to be the first-degree reaction. With the catalysis of H+ ion, the equation of transformation of sucrose is:

As the sucrose solution is very dilute, we can consider the concentration of water is invariable and rate is only due to the concentration (Cs) of sucrose. So this reaction is considered to be a first-degree reaction.

It can be expressed as the following formula:

(A)

V: reaction rate T: reaction time K: reaction constant C: concentration of sucrose (Time=t)

实验原理

The integral of formula A ln(C0/C)=-kt+B （ B ）

The half-life of the reaction:

（ C ）

实验原理

optical activity

t=0 C0 0 0 α0 （ positive ）

t C C0 － C C0 － C αt

t∞ 0 C0 C （ negative ）

+66.6º +52.5º -91.9º

direction dextrogyrous dextrogyrous levorotatory

实验原理

Given by α ＝ KBCB (D)

α ： optical activity KB ： proportion constant

CB ： concentration of substance B

α0 ＝ KS C0 (D1)

αt ＝ KS C + (KD + KF )(C0 –C) (D2)

α∞ ＝ (KD + KF )C0 (D3)

We can obtain

实验原理

Combining Eqs. (D1), (D2) and (D3), we obtain Eqs.(E1)and .(E2):

C0=(α0–α∞){KS–(KD +KF )}–1 （ E1 ）

C=(αt–α∞) {KS–(KD +KF )}–1 （ E2 ）

实验原理

Adding Eq. (E2) to Eq.(B),we obtain another equation: ：

Where αt stands for optical activity of an ongoing

reaction,α∞ stands for optical activity when reaction

terminates, K stands for reaction rate constant, T stands for time when reaction is ongoing and B is a constant.

实验原理

Draw a beeline of

against time, from which we can figure out the constant K that is negative to the slope.

Apparatus and Reagent

Polarimeter 1Super thermostatic trough 1Stopwatch 1Volumetric flask (50ml) 2Pipette (25ml) 1Spheric aspirator 1Y-pipe 1Washing bottle (250ml) 1Conical flask 2Sucrose (analytical pure) HCl solution （ c=2.0mol·dm-3 ）

Procedure

1.Adjust temperature of the thermostatic bath at 30℃±0.1℃.

2. Preparation and thermostatic process Dissolve 10g of sucrose and make up the soluti

on in 50ml of volumetric flask. Shaking up, Pipette 25ml solution into a conical flask.

Pipette 25ml of HCl solution into another conical flask then take them into thermostatic trough.

3. Regulate the zero point 1) Electrify the polarimetry. After 2-3, regulate the focus to make the trisection visual field clear 　　

or

实验步骤

2) Filled the optical pile full of distilled water and set it into the polarimeter. Rotate the microscopy, when trisection visual field disappears, mark down the angle that is just the zero point.

Repeat this procedure three times. Subsequently, calculate the average value, then pour out water and get ready for next operation.

4. Determine optical activity of sucrose in transfer process.

1) Determination ofαt

After solution being kept temperature constant for 10min, add HCl solution into sucrose, meanwhile record the time with a stopwatch.

Wash the optical pile with mixture for 2-3 times, fill it with mixture, without any air bladder and leak, then put it into polarimeter again.

Rotate the microscopy, mark down the angle(Xt) as soon as trisection visual field disappear. In the first hour, take an angle per 5min, then per 10min after one hour until a negative value appears.

2) Determination ofα∞

Heat the mixture remained in a water bath at 50-60℃. After half an hour, cooled it till room temperature. Then take it into thermostatic trough staying for 10 min. Repeat the complete procedure and determine X∞.

Data Records and Processing

Room temperature: Concentration of sucrose solutionConcentration of HCl solution:Xt: X∞:αt ＝ Xt － X0 α∞ ＝ X∞ － X0

t/min 5 10 15 20 25 30 35 40 45

Xt/℃                  αt /(°)                  

(αt-α∞)/(°)                  ln{(αt-α∞)/(°)}                  

t/min                  Xt/℃                  αt/(°)                  

(αt-α∞)/(°)                  ln{(αt-α∞)/(°)}                  

实验数据记录和处理

Draw a graph of against time. Take two points A and B on the beeline to calculate the slope by the formula as following:

Half-life ：

Questions

Why sucrose should be confected freshly at the time you start the experiment?

Attentions

1. Optical pipe must be filled full of liquid without any air bladder.

2. Rotate the cover moderately. If it is too tight, the knob will be damaged, and the glass patch pressed may produce the fault optical activity.

3. For the mixture is rather acid and corrosive, the apparatus must be washed cleanly and wiped dry after the expriment is finished.

Outline for using the polarimeter

1. Switch on the electrical source(220V) and turn on the Na-light. As the lamp-house is stable, regulate focus of the ocular until trisection visual field is clear, just like the following pictures.

or

2. Regulate the zero point. Fill the optical pipe full of distilled water. When rotate the microscopy, mark down the point on which trisection visual field disappears.

Reading on the dial after “0”.

6

0

A

0

X0 ＝＋ 1.6° 、

B 20

0X0 ＝﹣ 2.8°

(177.2°﹣180°)

3. Repeat the same procedure and determine the optical activity of sample.

Finally, the apparatus should be cleaned and put away trimly.