Chemical Kinetics with a Smartphone

Chemical Kinetics with a Smartphone

An article recently published in the Journal of Chemical Education by Ruth E. Nalliah1 describes a great experiment for studying chemical kinetics that is extremely simple to set up and carry out. The experiment involves monitoring the reaction between blue food dye and hydrogen peroxide in the presence of base:

Blue dye #1 + H2O2    à  colorless products                 Equation 1

This article piqued my interest so I decided to try it out for myself. When doing so, I made a few modifications. I found that it was quite simple to monitor the kinetics of this reaction using an iPhone outfitted with the RGB analyzer app called “Color Name”. You can see the overall process of how I carried out the experiment as well as collected and analyzed the data in the video below.

 

 

The goal of the experiment is to determine the order of the reaction with respect to the blue dye:

Rate = k[dye]x[H2O2]y                        Equation 2

Where “Rate” is the rate of the reaction, k is the rate constant for the reaction, x is the order of the reaction with respect to the dye, and y is the order of the reaction with respect to H2O2. The reaction is run with a very large excess of H2O2, therefore the concentration of H2O2 essentially remains constant throughout the reaction. As a result, we can treat [H2O2]y as a constant, and Equation 2 may be written as:

Rate = k’[dye]x                                   Equation 3

Where k’ =  [H2O2]y. If the reaction is first order in the dye, the following equation can be used to describe the concentration of the dye at various time intervals:

ln[dye]t = [dye]0 - k't                         Equation 4

Where [dye]t is the concentration of the dye at the beginning of the experiment and [dye]0 is the concentration of the dye at some time, t, during the experiment. Equation 4 takes on the form of a line if we let y = ln[dye], b = ln[dye]0, m = k’ and x = t. Because of this, if we plot the natural log of the dye concentration vs. time, a straight line will result if the reaction is first order in the dye. Furthermore, the slope of the resulting line is equal to the opposite of k’ (Figure 1).

Figure 1: First order plot of ln[dye] vs. time. The straight line indicates the reaction is first order with respect to the dye concentration. The slope of the line yields the rate constant for the reaction (k’ = 0.47 s-1)

 

On the other hand, if the reaction is second order in the dye, then the equation below should be used to describe the concentration of the dye at various time intervals:

1/[dye]t = 1/[dye]0 +k't                   Equation 5

In this case, we get a line if we let y = 1/[dye]t, b = 1/[dye]0, k’ = m and x = t. Thus, if the reaction is second order in dye concentration, a plot of 1/[dye] vs. time results in a straight line with a slope = k' (Figure 2). Thus collecting [dye] vs. time data and constructing the accompanying first and second order plots can provide insight into whether the reaction described in Equation 1 is first or second order with respect to the dye. 

 

Figure 2: Second order plot of 1/[dye] vs. time. The straight line indicates the reaction is second order with respect to the dye concentration. The slope of the line yields k' in Equation 3 (k’ = 1.38 M-1 s-1)

 

Because the dye is colored blue, it can be monitored by measuring the amount of red light that passes through the dye in the reaction mixture (recall that blue dye absorbs red light quite well).2 Thus, the RGB app on the smartphone can be used to measure the red light that passes through the solution. Recall that the absorbance, A, of a dye is directly proportional to its concentration:

 A a [dye]                     Equation 6

Because of this direct relationship we can use absorbance as a measure of dye concentration. Note that in the context of this experiment, the RGB app on an iPhone is used to monitor the amount of red light that passes through the blue dye. The absorbance at each time interval is calculated as:

A = -log(R/R0)                                    Equation 7

Where R is the amount of red light transmitted through the solution and R0 is the amount of red light that passes through the solution when it is colorless (which is essentially the “blank”).

I had the opportunity to try this experiment out with students and it worked very well. If you use this experiment, let me know how it works for you. Also let me know if you monitored the reaction using the method described in the article or by using the RGB app. I’m curious to hear about any other variations you might try!

References: 

  1. Ruth Nalliah, , Journal of Chemical Education, 2019, 96, 7, 1453 -1457.  (accessed 11/26/19)
  2. Kuntzleman, Tom,, ChemEd X, 3/30/16. (accessed 11/26/19)

 

Collection: 

Safety

General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

Other Safety resources

: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

 

NGSS

Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data.

Summary:

Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution.

Assessment Boundary:
Clarification:

Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.

Summary:

Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.
Evaluate the claims, evidence, and reasoning behind currently accepted explanations or solutions to determine the merits of arguments.

Assessment Boundary:
Clarification:

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.

Summary:

Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.

Assessment Boundary:
Clarification:

Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.

Summary:

Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.

Assessment Boundary:
Clarification:

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

*More information about all DCI for HS-PS1 can be found at  and further resources at .

Summary:

Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Assessment Boundary:

Assessment is limited to chemical reactions involving main group elements and combustion reactions.

Clarification:

Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.

Students who demonstrate understanding can apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.

*More information about all DCI for HS-PS1 can be found at  and further resources at .

 

Summary:

Students who demonstrate understanding can apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.

Assessment Boundary:

Assessment is limited to simple reactions in which there are only two reactants; evidence from temperature, concentration, and rate data; and qualitative relationships between rate and temperature.

Clarification:

Emphasis is on student reasoning that focuses on the number and energy of collisions between molecules.