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Rate Law for the Crystal Violet Reaction
- 1. What is the Rate Law for the Crystal Violet Reaction? 3/27-4/3/2014 Section 10th TA: Jinwei Zhang Group 6 Daylin Morgan Ahmed Alsharif Shili Tong Chang Chuan Introduction The main goal of this experiment was about observing the reaction between crystal violet and sodium hydroxide, CV+ + OH- -> CVOH. The objectives of this experiment are to monitor the absorbance of the crystal violet solution as a function of time, and determining the order of the reaction with respect to CV+. The pseudo rate constant k’ in this reaction is derived from the knowledge that OH- and the rate constant can be combined to a single constant. The Beer’s Law relationship between absorbance and concentration for CV+ is used to calculate the concentration in a solution given the Absorbance and a calculated constant. The half-life for this reaction with respect to CV+ and the rate law for this reactionare derived from total data. Materials & Procedure Materials 2.5*10-5M CV+(aq) 2.5*10-5 M OH-(aq) Volumetric measuring equipment Beaker Cuvette Stirring Rod
- 2. Spectrophotometer Each student in a group will measure kinetics of the reaction at different concentration of sodium hydroxide. Before starting kinetic measurements, you should measure the spectrum of the crystal violet in the aqueous solution and establish the wavelength suitable for absorption measurements. Of course, this wavelength is the same for the whole class, so there is no reason to establish it once and once again. Created 4 different solution of H2O and Crystal Violet at specific concentration. Measured the absorbance of each of these concentration using spectrophotometer and noted absorbance. Graphed the absorbance vs. concentration, and found line of best-fit with an intercept of 0 to find value of m= εb. Prepared spectrophotometer for kinetics measurements (separate instructions will be provided for each group). Pour 10 mL of NaOH solution into 10 mL beaker. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. Pour 10 mL of 2.0 X 10-5 M crystal violet solution. CAUTION: Crystal violet is a biological stain. Avoid spilling it on your skin or clothing. Initiated the reaction, simultaneously poured the 5-mL portions of crystal violet and sodium hydroxide into a 25-mL beaker and stirred the reaction mixture. Rinsed the cuvette with ~1-mL amounts of the reaction mixture and then filled it 3/4 full. Placed the cuvette in the cuvette slot of the spectrophotometer, and clicked "Collect" button. The program collected the absorbance data. Analyzed the data graphically to decide if the reaction is zero, first, or second order with respect to crystal violet. Results
- 3. Conc.(M) A 0.000125 1.609 0.0001 1.244 0.000075 1.019 0.00005 0.53 *Measured with λmax= 600nm Determining Beer-Lambert’s Law A=εbC Using the linear-fit line above εb was determined to equal 12695. Determining the order wrt [CV+]: To find molarity of CV+ used Beer-Lambert’s Law and value for εb found above. A=εbC 1.331=12695C C=1.331/12695 C=1.05*10-4 Below is the graph comparing the concentration vs. time. Below is the graph comparing ln(conc.) vs. time. Below is the graph comparing 1/conc. vs. time. Given the graphs and linear regressions above it was determined that the Crystal Violet undergoes a first order reaction. The psuedo rate constant or k’=|slope| is k’=.0066. At this psuedo constant the temperature is room temperature,
- 4. and the OH- in excess and considered to be constant. Half-life: Using the equation t1/2 = t * ln(2)/ln(N0/Nt). The half-life was estimated to be t1/2 = 200 * ln(2)/ln(1.05*10- 4/5.7*10-5) t1/2=226.923s The rate law expression for the reaction is Rate=.0066[CV+]. Discussion For this reaction the λmax was 600nm. This was calculated using the spectrophotometer and an aqueous solution of CV+.In regards to this experiment it was necessary to use the Beer- Lambert’s law to determine the concentration of the solutions as it reacted. Since there is no way to measure the concentration accurately every 2 seconds over a long interval the absorbance rate can be used to calculate what the concentration is at any given point in time. Absorbance can’t be used to calculate the rate constant because the values would not be correct with respect to OH- concentrations. The rate constant and pseudo rate constant are affected by things like the temperature of a given system. This system was performed under the condition of room temperature. The rate of a reaction will increase with the concentrations of either CV+ or OH-. Since crystal violet is a first order reaction doubling it for instance should double the rate. A common error for any experiment involving the spectrophotometer could be a cuvette that is not clean on the smooth sides, or even inserted the wrong way. These errors would substantially affect the absorbance measurements. Another error might be spilling solution in the spectrophotometer itself. The errors can be minimized by executing caution and consideration for the equipment during when in lab. Conclusion The goal of this experiment was to observe the reaction kinetics
- 5. between crystal violet and an excess of sodium hydroxide to determine the order of the reaction with respect to crystal violet. Once the proper wavelength has been chosen for the crystal violet and the spectrometer has been calibrated, the absorbance is measured. The rate law for this reaction as listed above is .0066[CV+]. Beer-Lamberts law states that absorbance is proportional to concentration, which is significant in this experiment because it allows one to plot absorbance vs concentration to find the constant needed. With this in mind, the order with respect to crystal violet can be determined by the absorbance versus time plot. Which was found to be a first order reaction with respect to CV+. Appendix T(s) A A/eb=C ln[CV] 1/CV T(s) A A/eb=C ln[CV] 1/CV 0 1.331 1.05E-04 -9.16303 9.54E+03 102 0.712 5.61E-05 -9.78864 1.78E+04 2 1.311