Rate Law and Activation Energy

Topics: Chemistry, Reaction rate constant, Rate equation Pages: 8 (2160 words) Published: February 28, 2013
Absorption to Activation Energy

Calculating the rate constants, half-lives, and activation of a reaction by monitoring the change in absorption

Department of Chemistry
Dusten Dussex

Lab partner: John Drury

Lab date: February 7th, 2013

Due date: February 21st, 2013

In this experiment we are analyzing the relationship between reaction rates at different concentrations and temperatures to determine the true rate constant, activation energy, reaction orders, and half-life of a reaction. The reaction of interest is the addition of a hydroxyl group to the nucleus of Crystal Violet. Crystal Violet, or hexamethylparaosaniline chloride for short, is a strongly colored purple dye with the chemical formula C25H30N3Cl and disassociates completely in solution. The relevant structure for this compound can be seen in figure 1

Figure 1
The base that is being used for the reaction is the strong base Sodium Hydroxide, or NaOH. This molecule also completely disassociates in water. Because measuring the concentrations of reactants is difficult in a simple lab setting, the reaction between Crystal Violet and Sodium Hydroxide will be measured through light absorbance. As the reaction between the chemicals takes place and the Crystal Violet receives the hydroxide the overall intensity of the purple color will decrease thus affecting the absorbance. The absorbance of the solution will be measured with a colorimeter as the reaction takes place and will be interpreted as a direct representation of concentration of Crystal Violet. After the reaction has taken place, through analysis of graphs plotting absorption vs. time, the natural log of absorption vs. time, and the inverse of absorption vs. time the reaction will be determined to be either zeroth, first, or second order with respect to crystal violet. From here the a pseudo rate constant can be determined, and using comparisons of different constants at different concentrations of NaOH solution and different temperatures, the reaction order with respect to hydroxide, the true rate constant for the reaction, and the activation energy for the reaction can all be determined with the following equations respectively.

equation 1
Where k2’ is the pseudo rate constant of the reaction using twice the initial OH- concentration as is used in the k1’ reaction and n is equal to the reaction order with respect to OH-. equation 2

Where k’ is a pseudo rate constant based off of absorption and n is the reaction order with respect to OH- determined by equation 1.
equation 3
Where k1 is the reaction constant at temperature T1, a is a constant that can be ignored due to the way the equation will be utilized, R is that gas constant, and Ea is the activation energy.

The following materials were needed for the experiment:
4 100mL beakers
250mL beaker
2.5×10-5M Crystal Violet Stock solution
0.10M NaOH Stock solution
Distilled Water
10 dry plastic cuvettes and caps
Stirring rod
Vernier Colorimeter
50mL volumetric pipet
100µL syringe
2 10mL vials
Logger Pro software
Vernier computer interface
Hot plate
Vernier temperature probe

1. First, 100mL of 0.10M NaOH solution was obtained using a 50mL volumetric pipet, and 0.05M was prepared using a the pipet, the stock 0.10M NaOH solution, and distilled water. 2. The Logger Pro software was engaged and both the Vernier colorimeter and temperature probe were plugged into the appropriate channels. The temperature of the room was measured and the colorimeter was calibrated by setting the 0% light and 100% light conditions. 3. The colorimeter was set to 565nm and 1mL of 2.5×10-5M Crystal Violet solution was mixed with 1mL of 0.05M NaOH solution and quickly added to the colorimeter. Data correlating time, temperature, transmittance, and absorbance was then recorded for seven minutes as the reaction between the two solutions took place, and this data was saved....

References: Alberty, A. A.; Silbey, R. J. Physical Chemistry, 2nd ed.; Wiley: New York, 1997.
Department of Chemistry. (2013, Spring). CHEMISTRY 441G Physical Chemistry Laboratory Manual. Lexington: University of Kentucky
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