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 = \(\frac{1}{Time\ taken \ for\ the\ blue \ color \ to \ become \ colorless }s^{-1}\)
Variable | Why do we control it? | How do we control it? |
---|---|---|
pH | As alkaline glucose is used in the reaction, changes in pH of the medium may cause changes in the rate of the reaction. | 0.10 moldm-3 NaOH solution will be used in all trials. A glass beaker and a digital mass balance will be used to prepare the solution. |
Surface area | The rate of reaction depends on surface area. Larger the surface area, there are more frequent collisions and thus the rate of the reaction is higher. | All the reactions will be performed in a 100 cm3 glass beaker. |
Concentration of glucose and methylene blue | More the concentration or amount of the glucose and methylene blue used as reactants, faster the reaction and lesser the time taken for the color to change. | In all trials, 10.00 g of glucose (0.05 moles) and 3.19 g of methylene blue (0.01 moles) has been used. The glucose is taken in excess so as to make methylene blue the limiting reactant and consume that completely. |
Apparatus | Quantity | Least count | Absolute uncertainty |
---|---|---|---|
Water bath | 1 | NA | NA |
Digital mass balance | 1 | 0.01 g | ± 0.01 g |
Graduated cylinder-100 cm3 measuring | 1 | 1.00 cm3 | ± 0.05 cm3 |
Graduated pipette-20 <p>cm3</p> | 1 | 0.10 cm3 | ± 0.05 cm3 |
Spatula | 1 | NA | NA |
Watch glass | 1 | NA | NA |
Glass rod | 1 | NA | NA |
Thermometer | 1 | 1.0 ℃ | ± 0.5 ℃ |
Digital stop-watch | 1 | 0.01 s | ± 0.01 s |
Glass beaker-100 cm3 | 1 | NA | NA |
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
Temperature ± 0.5 ℃ | Mean time taken for the color to change | Rate of the reaction in × 10-3 s-1 |
---|---|---|
70.0 | 342.87 | 2.92 |
60.0 | 515.32 | 1.94 |
50.0 | 667.85 | 1.50 |
40.0 | 773.25 | 1.29 |
30.0 | 998.15 | 1.00 |
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.
Figure - 8 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
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?
Type of error | Source of error | Impact of error | Improvements |
---|---|---|---|
Random | The digital mass balance has been used to weigh the mass of glucose and it has an absolute uncertainty of ± 0.01g. | Reduces the precision in the values of mass of glucose, NaOH and methylene blue recorded. | Repeat the experiment, collect the data in multiple trials and use average values. |
Random | The digital stop-watch has been used to record the time taken for color change and it has an uncertainty of ± 0.01 s. | Reduces the precision in the values of time recorded. | |
Systematic | There is a time lag between the instant the reactants are mixed and the stop-watch is started. | The values of time recorded is inaccurate. | Use a slow motion time lapse video recorder to record the video of the experiment and use that later to identify the time change. |
Methodological | The entire amount of methylene blue may not be used up and thus the color change would be from blue to lighter shade of blue instead of being colorless. | It would be difficult to monitor the color change. | Use excess glucose to make methylene blue the limiting reactant. |
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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