Reaction of Crystal violet with NaOH

The crystal violet was represented by the symbol CV in the experiment. The reaction rate rule for (CV) (dye) versus NaOH solution was determined in the laboratory. Aside from setting the rate law for the reaction, the experiment sought to determine the reaction order when taking CV into account.

The rate of the reaction between CV& and NaOH was calculated by monitoring the concentration of CV ([CV]) over time. The absorbance of the CV was used to calculate the concentration difference over time. The absorbance of CV declined as time passed. The inverse proportion between the absorbance and the time indicated that concentration of the CV was inversely proportional to time. Since from the reaction law, the reactant’s concentration decreases over time, the concentration of CV was used as a parameter for determining the rate of the reaction. The order of the reaction was determined through plotting the graphs for [CV] against time (t), 1/[CV] / time (t), and ln[CV] against time (t). A straight line graph was obtained for ln[CV] versus time.

Principles

Concept

Every chemical reaction has its individual rate, studied under chemical kinetics. The experiment involved the kinetics of the reaction between CV and NaOH. The LabWorks interface colorimeter was used in monitoring the concentration concerning time.

Hypotheses

The alternative hypothesis was that the reaction between CV and NaOH is a second order reaction. The hypothesis was based on the nature of the chemical reaction whose structure is summarized by the equation: A+B→ C. Where A, and B are the reagents and C is the product. The null hypothesis was stated as the reaction between CV & NaOH was hypothesized as a first-order reaction. Another null hypothesis was that the reaction is a 0th order reaction.

Central idea

In the experimental determination of a reactant order, the concentration of the reactant can be monitored over time. The rate of a reaction varies as the reactants become consumed over time. The experiment involved closely monitoring the concentration of the CV solution using the colorimeter. By using the calculus integrations, there are three orders 0th order, 1st order, and the 2nd order that can be assigned to a reactant. Following the rate law for the reaction depends on the concentration of CV solution over time, three equations can be developed (Illinois Central College).

In the above equations, the symbol represents the initial concentration of the CV (at t=0). The concentration after time t has passed during the reaction is represented by. Taking a closer look at the equations indicates that all of them are linear equations of the basic structure;

Where the gradient/ slope for each equation gives the constant of or.

The equations (1,2 , and 3) above can be reordered to obtain the linear form indicating the positive gradient as indicated below (Illinois Central College);

If one plots versus t and obtains a straight line, it would indicate that the reaction is of 0th order with relation to the Crystal Violet, and the y exponent is therefore zero as in equation 9. In the similar manner, if the plot of against t gives a straight line graph the the reaction is of 1st order with respect to CV. A second order reaction is represented by a liner graph of versus time (Miertschin).

Equation for the reaction

The following figure (Figure 1) indicates the equation for the chemical reaction that occurred between the CV and NaOH (Illinois Central College).

Figure 1: The chemical reaction between CV solution and NaOH

In this reaction, NaOH acted as the OH- donor. The Na+ ions served as spectator ions; they did not participate in the reaction. The OH- ions reacted with the CV to bring about a colorless solution as indicated in the simplified equating below;

Where CV stands for (Crystal Violet).

Parameters

The reaction rate was determined by monitoring the absorbance of the CV as a function of concentration over time. The general rate law is represented in the following equation (Miertschin);

The rate of the reaction equals the negative change in CV’s concentration over time. In the determination of the rate of the reaction represents the change in the concentration of CV. Moreover, indicates time change. Since the concentration was dynamic with time, the concentration variations with time was appropriate approximation of the reaction rate.

The colorimeter functions on the principles of Beer’s Law illustrated by the equation below;

Both ℇ and b values are constants. Therefore, the solution’s absorbance and concentration of the absorbing species are directly proportional.

Procedure

From the computer a new file for the experiment was opened by going to the menu bar, selecting ‘File’ and clicking ‘Open.’ The LabWorks toolbar was accessed, and the ‘Acquire’ option was selected. The ‘Start’ button was clicked, and an on-screen instruction for connecting the colorimeter interface was obtained. The experiment proceeded as illustrated below (Illinois Central College).

The initial reading of the distilled water was obtained. A clean cuvette was rinsed with approximately ¾ full distilled water. The outer surface of the cuvette was dried thoroughly using a towel to eliminate the fingerprint marks. The cuvette was inserted into the colorimeter. The cuvette was covered using a film canister to prevent the stray light. The onscreen observation was sued to obtain the absorbance value of the pure water.

The solution cell was emptied and dried out thoroughly on both inner and outer surfaces. By using a burette, 9.00mL of 0.000015M CV (CV) solution was delivered to a dry and clean 50mL beaker. The onscreen directions were observed to ensure that the timer started immediately on clicking the Enter key.

1.0mL of the 0.05 M NaOH was dispensed into the CV solution using a calibrated plastic dropper. The Enter key was pressed instantly so that the timer would start. The solution containing CV and sodium chloride was mixed thoroughly using stirring rod. A cuvette was ¾ filled with the CV/ NaOH solution. The cuvette was positioned exactly as for the pure water inside the colorimeter. The Enter key was pressed immediately, and the first current measurement for the sample was made after 10 seconds. The program took the current measurements at intervals of 2 minutes for 10 minutes and stopped automatically. After the program stopped, the button ‘Save Data’ was clicked, and the file was assigned a name. The experiment was conducted 2 times to produce two sets of data as ‘run 1’, and ‘run 2’.

Results

Problems addressed

In both run 1 and run 2, the resultant slopes for the three equations (equation 5, 6, and 7) were determined with a pseudo value of k^’=30134. All the plots were done to determine the one producing a linear graph and the corresponding reaction order with regards to the concentration of the CV. The gradient of the slope indicated the relationship between the concentration of the CV and time and hence the value of k’. The data showing the changes in the concentration of the CV over time was obtained, and the respective values of 1/[CV], and -ln[CV] were calculated using Microsoft Excel 2013 (Illinois Central College).

The table below indicates the data collected on the concentration of the CV and the absorbance.

Conc. (mol/L)

Absorbance

0

0

0.000001

0

0.000002

0.035

0.000004

0.117

0.000006

0.173

0.000008

0.222

0.00001

0.283

0.00002

0.589

From the table, above, a graph of the best fit was obtained as shown in figure 2 below.

Figure 2: Graph of Absorbance versus concentration

Data Analysis

The plots for [CV] versus t, 1/[CV] against t and ln[CV] versus t were made using the data from the experiment. Each plot had two sets of graphs from the first and the second run. Before the graphs were obtained, the values for 1/[CV] and ln[CV] were obtained using the excel formula. The 1/[CV] values were obtained by calculating 1/ the corresponding concentration of CV. The values of ln[CV] was obtained by taking natural logarithms of the corresponding concentrations (Illinois Central College).

Figure 3: Concentration of CV versus Time (Run 1)

The figure indicated a curve showing that the concentration decreased with increase in time. A line of best fit indicated an R-Square value of 0.9913. Since the plot is not a straight curve, the reaction is not a 0th order.

Figure 4: Concentration of CV versus Time (Run 2)

The figure indicated a curve showing that the concentration decreased with increase in time. A line of best fit indicated an R-Square value of 0.9707. The R2 value is approximately equal to the one obtained in the first run.

Figure 5: ln [CV] versus Time (Run 1)

The plot of ln[CV] versus time indicated a straight line. The graph suggests an inverse relationship between the concentration of the CV and time. The R2 value was 0.9998.

Figure 6: ln [CV] versus Time (Run 2)

When the second run was conducted, a plot of ln[CV] versus time indicated a straight line. The graph indicates an inverse relationship between the concentration of the CV and time. The R2 value was 0.9986. The R-square value was close to the one obtained in the first run for ln[CV] versus t.

Figure 7: 1/[CV] against Time (t) (Run 1)

Figure 8: 1/[CV] versus Time (Run 2)

In both runs, a straight line was not observed. Therefore, the reaction in the experiment was not a 1st order reaction.

Discussion

Qualitative meaning of the results. (Agreement with the principles)

When the plots of [CV] versus time and 1/[CV] versus time did not indicate a linear graph. The best of fit line graphs were plotted to determine the value of k’ and the equation for the reaction. The straight line graph was obtained when the graph of ln[CV] v. time was drawn. The linear graph of ln[CV] / time indicated that the reaction of CV and NaOH was a second order reaction.

Agreement of results with the expected value

The result indicated that a linear graph was obtained for the plot of ln[CV] versus time. Therefore, the experimental results indicated that the reaction between the CV and the NaOH solution at room temperature is a second order reaction. The experimental results supported the alternative hypothesis and rejected the null hypotheses.

Error Analysis

Inherent difficulties and sources of errors

Although the study indicated a high precision in the data, there were challenges in the experiments that could have contributed to errors in the experiment. For instance, the duration for the time lost between the start of the reaction and the click of the Start button to initiate the timer could contribute to errors. Such errors may be eliminated by having two people working together, while the first person concentrates on the addition of the reactant (NaOH), the other one clicks the start button instantly. Since the experimental results depend on the software, the problems such as connection failures between the computer and the software could have caused errors. Another possible source of errors is contamination of the reagents and apparatus. Care should be paramount to ensure that the cuvettes are not contaminated since the absorbance would be affected. Moreover, the contamination of the reagents would result in incorrect results.

Works Cited

Illinois Central College. “Reaction of Crystal Violet with Sodium Hydroxide: A Kinetic Study.” Illinois Central College (2017): 1-10. Web. 22 November 2017. .

Miertschin. A Kinetic Study: Reaction of Crystal Violet with NaOH. 2007. Web. 22 November 2017..