Effect Of Concentration Of Glucose On Rate Of Blue Bottle Reaction

How does the rate of reduction (in s^-1) of methylene blue by alkaline glucose solution depends on the concentration of the glucose solution used, determined using a stop-watch?

Blue bottle reaction was perhaps one of the first Chemistry activity done in my middle school Science classes during the Science Exhibition that stimulated my interest in the subject. Though at that time, what the reaction is, how it happens and why there is a color change was not known to me properly. However, an observation that triggered me at that point was though all students in the classroom was doing the activity yet some for us the color change was faster while for others it took quite a much time for the color to change. While going through the Topic-6 in DP Chemistry course, I came to know about how concentration affects the rate of the reaction. This intrigued me and I wondered if the difference in the time taken for color change was due to the difference in the mass of glucose that we used or not. To answer this question, I have zeroed down to the research question stated above.

Methylene blue is an indicator. The chemical name of methylene blue is methylthionium chloride. This is also used as a blue colored dye. It is a heterocyclic aromatic compound where two substituted benzene rings are fused through C chains that connects through N and S atoms. It exists as an ion pair where the organic compound with a positive charge on the S atom of the ring combines with a chloride ion. In the oxidized form, the dye is blue in color whereas in the reduced form, the dye is colorless. The reduction occurs due to abstraction of a proton by the N atom present in the ring.

Figure-1: Structural formula of oxidized and reduced methylene blue¹

Glucose is a polyhydroxy aldehyde with five OH groups and an aldehyde (CHO) group. It falls under the category of carbohydrates. It can exist in both open chain and closed chain form. This is the main product of photosynthesis by which the plants can make food for itself as well as for others.

Alkaline glucose is glucose dissolved in a solution with pH of more than 7. As glucose contains OH groups which are acidic in nature and can easily reacts with alkalis to liberate a H+ ion and thus generate the alkoxide forms (RO-), glucose is readily soluble in alkaline medium. The alkaline solutions of glucose are made by dissolving glucose in a solution of either NaOH or KOH.

Alkaline glucose solution can act as a reducing agent. It can reduce the dye- methylene blue to the colorless form. The reduced form-Leucomethylene blue (colorless) can be oxidized back to the blue colored form in presence of dissolved oxygen in the medium. Apparently, it is like that blue color disappears and then again reappear on shaking. This gives the reaction the name of the blue bottle reaction.

The current investigation aims to study the effect of temperature on the first step of the reaction- the reduction of the blue colored dye to the colorless form using alkaline glucose solution.

As temperature increases, the average kinetic energy of the glucose molecules would increase too. This means that higher the temperature to which the alkaline glucose solution is heated, more will be the fraction of glucose molecules with sufficient energy to overcome the activation barrier and that will eventually result into higher number of successful collisions between the glucose molecules and the dye molecules resulting in a faster rate of reaction, lesser time for blue color to become colorless.

Thus, it can be expected that with the rise of temperature, the time taken for the blue color to change into colorless should decrease.

The rate of a reaction can be measured in multiple ways. In general, it is expressed as change of concentration of the reactant or product per unit time. Here, in this investigation the rate will be calculated using the following formula:

Rate = \(\frac{1}{Time\ taken \ for\ the \ blue \ color \ to \ become \ colorless \ }s^{-1}\)

As discussed in the background information, the rate of the reaction must increase with the increase in temperature. Thus, it is expected that as temperature rises, it should take less time for the blue color to become colorless and the rate of the reaction must increase.

Temperature

The reaction will be carried out at various temperatures – 30.0℃, 40.0 ℃, 50.0 ℃, 60.0 ℃, and 70.0 ℃.  As the reaction can be performed at room temperature, high values of temperature has not been chosen. A water bath will be used to vary the temperature. A laboratory thermometer will be used to check the temperature of the solution.

Rate of reduction of methylene blue by alkaline glucose in s-1

The time taken for the solution to change color from blue to colorless will be recorded using a stop-watch and then the rate of the reaction will be calculated using the formula given below:

Rate = \(\frac{1}{Time\ taken \ for\ the\ blue \ color \ to \ become \ colorless }s^{-1}\)

• Wear a protective laboratory coat.
• Use safety gloves and mask.
• Do not expose any of the chemicals to your skin.
• Dispose all used chemicals into the disposal bin and dilute them using tap water before disposal.
• Return all unused materials to the laboratory technician for re-use.
• Wash and clean all used apparatus before returning them.
• Keep your workstation clean and dry.

• Take a clean and dry 100 cc glass beaker.
• Take a watch glass and place it on the digital mass balance.
• Adjust the reading of the mass balance to 0.00 ± 0.01 g using the Tare button.
• Transfer solid NaOH from the reagent bottle to the watch glass until it reads 0.40 ± 0.01 g.
• Transfer the weighed NaOH from the watch glass to the empty glass beaker.
• Add 100 cc distilled water using a graduated measuring cylinder to the glass beaker.
• Use a glass rod to stir the solution and dissolve the NaOH.
• Using a watch glass and a spatula, weigh 10.00 ± 0.01 g of glucose.
• Place the beaker on the water bath and set the temperature at 30.0 ℃. Use a thermometer to check the temperature.
• Add the weighed glucose to the beaker as soon as the temperature is reached.
• Using a spatula, a watch glass and the digital mass balance weigh 3.19 ± 0.01 g of methylene blue indicator solid.
• Transfer the weighed solid into the beaker and start the stop-watch.
• Record the time taken by the solution to change color from blue to colorless using the stop-watch.
• Repeat same steps for 4 more times.
• Repeat all of the above steps at 40.0 ℃, 50.0 ℃, 60.0 ℃ and 70.0 ℃.

• The initial color of the solution was colorless which immediately became deep blue on adding the methylene blue indicator.
• The beaker appeared hot when touched on dissolving NaOH in water.
• The time taken to change the color from blue to colorless reduced on increasing the temperature.

Sample calculation

For Row-1

Mean time taken =\(\frac{Trial-1+Trial-2+Trial-3+Trial-4+Trial-5 }{5}\)

=\(\frac{343.06+344.06+343.08+343.09+341.06}{5}\)=342.87

Standard deviation (SD)

\(=\frac{\sum (trial \ value-mean \ value )2}{5}\)

\(\frac{ = (343.06-342.87)^2 + (344.06-342.87)^2 + (343.08-342.87)^2 + (343.09-342.87)^2 + (341.06-342.87)^2 }{5}\)

= 1.10

Sample calculation

Rate of the reaction=\(\frac{1}{Time \ taken\ for\ the \ color\ to \ change}\)=\(\frac{1}{342.87}\)=2.92 × 10^-3 s^-1

Absolute error in rate = absolute error in time recorded = ± 0.01 s

Thus, percentage error in rate

\(=\frac{absolute\ error \ in \ rate }{rate}× 100\)=\(\frac{± 0.01}{ 2.92 × 10^{-3}}\)× 100=342.87 %

The magnitude of percentage error is abnormally high as the value of rate is extremely low.

Graph-1: Rate of reduction of methylene blue in × 10^-3 s^-1   versus temperature of the reaction in ± 0.5 ℃

Graph-1 displays how the rate of the reaction changes with temperature. As indicated in the graph above, the rate of the reaction increases from 1.00 × 10^-3 s^-1  to 2.92 × 10^-3 s^-1 as the temperature increases from 30.0 ± 0.5 ℃  to 70.0 ℃.

This indicates that as the temperature at which the reaction is performed is higher, the reactants collide more and thus the reaction is faster and it takes lesser time for the color to change from blue to colorless.

The change in rate is quite gradual as the distance between the successive data points is mostly same.

A linear trend line has been displayed in the graph above using MS-Excel. The trend line represents a quantitative relationship between rate and temperature; y= 0.4494 x – 0.515. The intercept of the trend line is 0.515. It means that at a temperature of 0.0 ℃  the rate of the reaction will be 0.515 × 10^-3 s^-1. Considering that the value obtained mathematically is really low and thus insignificant, it can be accepted.

The value of the gradient is 0.4494. It indicates that as the temperature increases by 1.00 ℃, the rate must increase by 0.4494 × 10^-3 s^-1.

Evaluation of hypotheses

The trend line displayed in Graph-1 shows a value of gradient as 0.4494 which is a positive value. Moreover, the trend line is moving upwards. This confirms that there is a positively linear relationship between the rate and temperature. The magnitude of correlation coefficient has been obtained using MS-Excel and is displayed in the graph. The value is 0.9002. This again confirms that there is a 90.02 % of correlation between the rate and the temperature. Thus, the null hypotheses is rejected and the alternate hypotheses have been accepted.

How does the rate of reduction (in s^-1) of methylene blue by alkaline glucose solution depends on the concentration of the glucose solution used, determined using a stop-watch?

• The rate of the reaction increases from 1.00 × 10^-3 s^-1 to 2.92 × 10^-3 s^-1 as the temperature increases from 30.0 ± 0.5 ℃  to 70.0 ℃.
• The change in rate is quite gradual as the distance between the successive data points is mostly same.
• A linear trend line has been displayed in the graph above using MS-Excel. The trend line represents a quantitative relationship between rate and temperature; y= 0.4494 x – 0.515. The intercept of the trend line is 0.515. It means that at a temperature of 0.0 ℃  the rate of the reaction will be 0.515 × 10^-3 s^-1. Considering that the value obtained mathematically is really low and thus insignificant, it can be accepted.
• The value of the gradient is 0.4494. It indicates that as the temperature increases by 1.00 ℃, the rate must increase by 0.4494 × 10^-3 s^-1.
• The trend line displayed in Graph-1 shows a value of gradient as 0.4494 which is a positive value. Moreover, the trend line is moving upwards. This confirms that there is a positively linear relationship between the rate and temperature. The magnitude of correlation coefficient has been obtained using MS-Excel and is displayed in the graph. The value is 0.9002. This again confirms that there is a 90.02 % of correlation between the rate and the temperature. Thus, the null hypotheses is rejected and the alternate hypotheses have been accepted.
• As the temperature increases, there are higher number of glucose and methylene blue molecules that surpasses the energy barrier and thus there are higher number of useful collisions within the
  given time frame. This leads to the formation of higher number of reduced methylene blue and thus it takes lesser time for the color to change from blue to colorless.

• A range of continuous independent variable has been used to make the data processing mathematically accurate.
• The design of the investigation is simple and reliable.
• The values of standard deviation is low confirming high values of precision in the raw data collected.
• The experimentally collected data displays a trend that can be scientifically justified.

As the rate of the reaction depends on temperature, it depends on the pH of the reaction medium too. Thus, by varying the amount of NaOH added, the concentration of the base added can be changed. This will eventually change the pH at which the reaction is carried out. Following this, the same method can be used to calculate the rate of the reaction. This will allow us to investigate how the rate of the reaction depends on the pH of the medium.

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