Reaction Rates: Speed It Up with Temperature!

Background Information for Teachers

Chemical reactions can happen either very fast, as in an explosion, or they can be really slow, such as the rusting of iron. Being able to control the speed of a reaction is crucial to many industrial processes. Studying chemical reaction rates allows students to investigate the factors that influence the speed of a reaction and explore reaction mechanisms in more detail. In this activity, students will study a chemical reaction that we use every time we apply bleach to our clothes: the bleaching of colored stains. A bleaching reaction, in which a colored compound becomes colorless, is ideal for studying reaction rates because the color change between the reactants and products can be easily monitored. The active ingredient in bleach, hypochlorite (OCl^-), acts as an oxidant, which transforms the dye into a colorless molecule. The bleaching reaction can be summarized as:

Food colors, such as FD&C Blue #1 or Yellow #5 are large aromatic molecules that contain an extended conjugated system of alternating single and double bonds. This allows the absorption of light in the visible spectrum. This is why dyes appear colored to us. Once the molecular structure is altered, for example, through oxidation by bleach, the oxidized dye loses the ability to absorb light in the visible spectrum, rendering it colorless.

The reaction rate is a measure of how fast a chemical reaction proceeds and is expressed as a concentration change (of reactants or products) over time. Given that the food color disappears throughout the reaction, the reaction rate can be defined by the concentration change of the colored reactant over time, as stated in following rate law:

where Δ indicates "delta", or "change in", t is time [s], k is the rate constant [unit is dependent on the reaction order] for the reaction at room temperature, m and n indicate the reaction orders for the food dye and hypochlorite [unitless], respectively, and the squared brackets [ ] express the concentration of the reactant [mol/L or M].

Determining the reaction rate of the bleaching reaction requires measuring the dye concentration over time. This can be done visually by using the ability of the dye to absorb light. With more dye present in the solution, it gets darker and consequently absorbs more light. Once all dye has been oxidized, the solution will become colorless and the dye concentration can be assumed to be zero.

The collision theory states that for a chemical reaction to happen, the reactant molecules have to collide in the right orientation and with enough energy to react and form a product. This means that more collisions lead to a faster reaction. Many factors can influence the probability of reactant collisions, and therefore, determine the rate of a chemical reaction, including; 1) the temperature, 2) the concentration of the reactants, 3) the nature (and surface area) of the reactants, or 4) the presence/absence of a catalyst or an inhibitor. In this lesson plan, you will explore the effects of temperature only. A change in reaction temperature influences the reaction rate because a higher reaction temperature increases the average kinetic energy of the reacting molecules, which a) makes it more likely that they collide and react, and b) increases the proportion of reactant molecules that exceed the required activation energy of the reaction. In the rate law (Equation 2), this temperature dependency is reflected by the rate constant k which is temperature-dependent (as stated in the Arrhenius equation). As a rule of thumb, a reaction temperature increase of about 10 Kelvin (K) approximately doubles the reaction rate.

In this activity, students will conduct the bleaching reaction at three different temperatures in order to directly observe the effect of temperature on the reaction rate. They will use a stopwatch to time how long it takes for the food color to fade in each reaction. The results will allow students to determine the average reaction rate for each temperature.

Additional Background Links

• Introduction to kinetics
• Kinetics Studies of the Bleaching of Food Dyes
• Rates of reaction

Prep Work (10 minutes)

1. Prepare a cup or beaker of diluted bleach solution (1:20) for each student group. To do this, add 2 mL of concentrated bleach (8.25%) into a labeled 2 oz mini cup or beaker and add 38 mL of room temperature water.
2. Practice going through the experimental procedure first so you can explain it to the students.
3. Have the student worksheet ready and pass it out before you begin the activity.

Engage (15 minutes)

1. Define and discuss the concept of chemical reaction rates.

   Do all chemical reactions work at the same rate?

   No, each chemical reaction happens at a different rate.

   Can you think of examples of chemical reactions in everyday life that happen really fast or very slow?

   Possible answers for slow reactions could be the rusting of iron, photosynthesis, chemical weathering of rocks, alcohol fermentation, or rotting of food. Examples for fast reactions could be combustions or explosions such as fireworks, acid-base neutralizations, or precipitation reactions.

   What does the reaction rate tell you about a chemical reaction?

   The reaction rate gives information about how quickly or slowly a chemical reaction occurs. It is a measure of concentration changes (of reactants or products) over time.

2. Explore why the speed of a chemical reaction matters and explain that it is important to be able to control reaction rates. Ask students about factors that influence the reaction rate of a chemical reaction.

   Why do you think it is important to know and be able to control the speed of a chemical reaction? [Possible prompts for students include questions such as "What if you wanted to manufacture something using chemical reactions?", or "Can you think of chemical reactions that could be dangerous if they happened too fast or too slow?" or "What if a process consists of a series of chemical reactions and one reaction is dependent on another?" et cetera.]

   Possible answers might include that reaction rates matter in industries that make products from chemical reactions. Speeding up reactions in industry makes processes faster, cheaper and more profitable. Another example could be the careful control of reaction rates in our body chemistry. All reactions are balanced out to provide products and use up substrates at the right time.

   What factors affect how fast chemical reactions happen?

   Factors that affect reaction rates include the concentrations of the reactants, the nature and surface area of the reactants, temperature, and the presence or absence of a catalyst or inhibitor.

3. Explain to the students that in the following activity, they will focus on the effect of temperature only. They will use a specific chemical reaction, the bleaching reaction, to assess reaction rates at different temperatures to determine the correlation between the two.

   Optionally, you can show your students this introductory video to this lesson plan experiment:

   

   https://www.youtube.com/watch?v=wG3OVpaHGkc

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4. Introduce the bleaching reaction to your students (you can set it up yourself for a quick demonstration) and explain why it is ideal for studying reaction rates.

   Can anyone explain what happens during the bleaching reaction? Why does the color in the solution disappear?

   The food color molecule has a chemical structure that allows the absorption of light in the visible spectrum range, which makes it appear colored. The bleach, which includes hypochlorite (OCl-) as an oxidant, reacts with the dye to change its structure in a way that no longer allows absorption of light in the visible spectrum, which is why it becomes colorless.

   Why would we be interested in the temperature dependence of the bleaching reaction?

   Clothes can be washed or cleaned in hot or cold water. For the bleach to be most effective, we have to know at what temperature the bleaching reactions happens fast enough to get the clothes clean within a wash cycle.

   How do you think temperature will affect the bleaching reaction? Do you think your stained clothes would get clean faster when washed in hot or cold water?

   There are three possible answers: reaction rates increase with temperature, decrease with temperature, or are not affected by temperature.

5. Do not reveal how temperature affects reaction rates to your students yet, but tell them that they will conduct a series of bleaching experiments at different temperatures to find out the correlation between temperature and reaction rates.

   What would be the best way to determine the reaction rate of the bleaching reaction at different temperatures?

   The color change of the solution once the dye is fully oxidized allows us to see when the reaction is completed. This makes it easy to assess the reaction time for each reaction when using different solution temperatures.

Explore (20 minutes)

1. Based on the previous discussions, let each student formulate his/her hypothesis on how temperature affects the rate of the bleaching reaction.
2. Divide the class into groups of 2-4 students and inform them that each group will conduct the bleaching reaction at three different temperatures to test their hypothesis, doing three trials for each, depending on the time available. Within each group students should divide tasks. For example, one student could prepare the dye solution, another could add the bleach to the reaction mixture, and a third student could measure the reaction time with the stopwatch. Students can rotate through each task for each of the different experiments.
3. Walk the students through the experimental procedure described below. (A slide show is available that you can use to guide your students through the experiments.)

   Experiment 1: Ice Water

   1. Prepare the dye solution for the first experiment using 500 mL of ice water and eight drops of yellow food dye.
   2. Fill the beaker with 150 mL of dye solution. Measure and record the temperature of the solution.
   3. Suck up 2 mL of the pre-prepared 1:20 diluted bleach solution with the pipette and hold it above the dye solution.
   4. Then, start the stopwatch and immediately add all of the bleach to the cup. Use the pipette to stir the solution and keep stirring throughout the experiment. You will observe the food color fade in the reaction mixture.
   5. Stop the stopwatch as soon as the color has fully disappeared from the solution. Record the time on your worksheet.
   6. Repeat the ice water experiment for two more trials with the same temperature, following steps b–e again.

   Experiment 2: Room Temperature Water

   7. Once all trials for the first temperature have been completed, repeat steps a–f using room temperature water for the new dye solution. Make sure the difference between each temperature is at least 10°C.

   Experiment 3: Hot Water

   8. Once all trials for the room temperature experiment have been completed, repeat steps a–f using hot water for the new dye solution. Make sure the difference between each temperature is at least 10°C.

Lesson Completion

Let the students clean up their experimental supplies. All the solutions, including the diluted bleach, can be disposed of in the sink.

If time is short, break here and have students analyze their data the next day of class.

Reflect (15 minutes)

1. Throughout the data analysis process, encourage the students to question their data. Possible prompts for students include questions such as:

   How reproducible was your data? Did all your trials for the same temperature result in a similar outcome? Did all groups observe the same trend in their results?

   If the temperatures were similar for each trial, the reaction times should be reproducible within a group and between groups.

   What are possible sources of error in your data? For example, did you see a difference depending on which student conducted the experiment?

2. Let each group review the data they have recorded in their student worksheets.
3. Discuss the results with the students and ask them to interpret the meaning of their data.

   How many of you observed a faster reaction in the hot dye solution?

   All students should raise their hands. If there is a group that has opposite results, do some joint troubleshooting to find out what led to their results. Everyone should have seen the same trend (shorter reaction times with higher temperature) in their experiments.

   Which temperature resulted in the slowest reaction time? How big were the differences in reaction time?

   The slowest reaction time should have been observed in the ice-cold dye solutions. Differences in reaction times depend on the actual reaction temperatures, but probably range in between a couple of seconds to about one minute.

4. If you have time, you can combine all the groups' average reaction times into a scatter plot on the board to reflect the entire class's data. This way they can also see the trend of the data clearly, showing that higher temperatures lead to shorter reaction times.
5. Make the connection between the reaction time and the reaction rate.

   Can we determine the average reaction rate from the measured reaction time? How are both related?

   The reaction rate is a measure of concentration changes (of reactants or products) over time. It can be calculated using equation 3: Equation 3: $$Reaction \: rate = - \frac{\Delta[dye]}{\Delta t}$$ where Δ[dye] is the difference between the dye concentration at the end and beginning of the reaction ([dye]final - [dye]initial), and Δt is the reaction time determined above. As at the end of the bleaching reaction, the dye concentration is zero ([dye]final = 0), Δ[dye] = [dye]initial. Note: The actual concentration of Yellow #5 in the food dye is proprietary information by the company and therefore not known. For your students to complete this step, you have to assign an arbitrary initial dye concentration (here we will us an arbitrary concentration of 0.0005 M). Instructions for calculations are also given in the student worksheet.

6. Given the concentration and time values, let students calculate and record each average reaction rate in the data table provided in their worksheet.

   Do your results confirm or disprove your original hypothesis on how temperature affects the reaction rate of the bleaching reaction?

   The results of the experiments should confirm that the average reaction rate of a reaction increases with higher temperatures.

   Can you explain your observations? Why does the average reaction rate increase with an increase of temperature? What happens on a molecular level to the dye and bleach molecules if the reaction solution is cold/warm?

   According to the collision theory, a higher reaction temperature increases the average kinetic energy of the reacting molecules, which makes them move much faster and with more energy. This makes it more likely that the reactant molecules collide and react, and also increases the proportion of reactant molecules that exceed the required activation energy of the reaction.

Assess

You can use this quiz to assess student learning after the activity; quiz is available in online and pdf formats:

• Online quiz, assignable in any LMS
• Quiz (pdf) and answer sheet (pdf)

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Chemical Engineer

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Career Profile

Chemical engineers design and manage large chemical processes such as the manufacturing of products like food additives, polymers or cosmetic chemicals. As you did in this project, they have to find out which conditions, including reaction temperatures or reactant concentrations, result in the optimal outcome for the process. Controlling chemical reaction conditions allows a Chemical Engineer to manipulate the cost, reliability and safety of the products they make. Read more

Biochemical Engineer

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Biochemical engineers design equipment and processes that involve living cells to manufacture products such as medicine, foods or chemicals. They conduct studies to determine the optimal reaction temperatures and optimal conditions for growing their cells or expressing their target proteins. This allows to maximize their product yields and lower their manufacturing costs. Read more

Food Scientist or Technologist

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Career Profile

Food scientists help produce the food we are eating and make sure that it is safe. Finding ways to manipulate chemical reaction rates is an everyday business for them, such as finding ways to slow down food degradation and spoilage during processing or while making foods through chemical reactions such as cheese from milk curdling or alcohol via sugar fermentation. Read more

Environmental Engineer

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Career Profile

Environmental engineers develop solutions to environmental problems such as water and air pollution. To design processes that can reduce contaminants in the environment, they have to find out how fast or slow the pollutants can be degraded through chemical or microbiological reactions. By controlling process parameters such as reaction temperature or reactant concentration, they are able to speed up the cleanup of the environment. Read more

Lesson Plan Variations

• For advanced students, take this activity beyond determining the average reaction rate and let them calculate the reaction constants using the overall rate law of the reaction. You will need to provide the hypochlorite concentrations (0.001 M) and the reaction order for both reactants (m = 1 and n = 1) given in Equation 4.

  Equation 4:

  $$Reaction \: rate = k \times [Food \: dye]^1 \times [OCl^-]^1$$

  Using their results for k, they can also calculate the activation energy for the bleaching reaction from the Arrhenius equation shown in Equation 5.

  Equation 5:

  $$ln(k) = -\frac{E_a}{RT} + ln(A)$$

  You have to create an Arrhenius plot of ln(k) vs. 1/T using a spreadsheet program. Fitting a linear trendline through the data then results in a trendline equation in from of y = ax + b. Linking this equation to the natural log of the Arrhenius formula makes y = ln(k), a = -Ea/R, x = 1/T, and b = ln(A). That means that the slope a, can be used to solve for the activation energy.

• This lesson plan focused on the effect of temperature on the reaction rate. You can also use the bleaching reaction to investigate how the reactant concentration (dye or bleach) influence the speed of the reaction. Results from these experiments will allow students to derive the complete rate law by determining the reaction order with respect to the bleach and dye based on the information given in Table 1.

• How does the nature of the reactant affect the reaction rate? You can find out by comparing the reaction rates of the bleaching reaction using different dye colors (yellow, red, and blue).