Reaction Rates: When Surface Area Matters!

Background Information for Teachers

A chemical reaction usually involves liquids, gases or solids. Some reactions happen very fast, whereas others seem to take ages. The speed of a chemical reaction is determined by its reaction rate. For many industrial applications, it is essential to be able to control reaction rates to ensure that processes happen fast enough to be economically viable, but at the same time not too quick, so as to prevent the risk of explosions. Studying chemical reaction rates allows students to investigate the factors that influence the speed of a reaction and explore reaction mechanisms in more detail.

How molecules or the reactants of a chemical reaction interact or react with each other is explained in the Collision Theory. This theory states that all reaction molecules are in constant motion and for a chemical reaction to occur they have to collide in order to form a product. A collision only leads to successful product formation if the molecules collide with sufficient energy as well as in the correct orientation. The collision frequency and the number of effective collisions determine how fast all the reactants are converted to the end product.

Any factor that affects the number of successful collisions will also change the speed of a reaction. This includes changing the number of reactant molecules (the reactant concentration) or the kinetic energy of the reactant molecules (the temperature), adding a catalyst or inhibitor to the reaction, and varying the nature of the reactants. In heterogeneous chemical reactions (whenever reactants are present in different phases), the reaction rate can also be changed by varying the surface area of the solid that reacts with the liquid or the gas. This is because molecule collisions can only happen at the surface of the solid, as all the other molecules are trapped within its body. If the same material is broken into smaller pieces, there is much more surface area exposed that is available for molecule collisions to occur (Figure 1).

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 1. Large particles (left) have less surface area available for reactions than smaller particles (right) of equal total volume. The total surface area increases with smaller particle sizes.

This surface area effect can be easily visualized by a simple experiment using effervescent tablets such as Alka-Seltzer® tablets. When these tablets are dropped into water, a chemical reaction happens that makes them dissolve and generate a lot of bubbles. These bubbles are carbon dioxide gas (CO₂), which is produced when the Alka-Seltzer ingredients react. The main ingredients of Alka-Seltzer tablets are aspirin, citric acid, and sodium bicarbonate (NaHCO₃). When sodium bicarbonate dissolves in water, it dissociates (splits apart) into sodium (Na⁺) and bicarbonate (HCO₃^-) ions. The bicarbonate reacts with hydrogen ions (H⁺) from the citric acid to form carbon dioxide and water as described by the following chemical equation:

Equation 1:

In this activity, students will measure how long it takes for an Alka-Seltzer tablet to completely dissolve depending on how big the pieces of the tablet are. As reaction rates are expressed as concentration change over time (Δ quantity/Δ t), and given that the Alka-Seltzer tablet dissolves throughout the reaction, students can calculate the average reaction rate (in grams of tablet dissolved over time) for each reaction according to Equation 2:

Equation 2:

Additional Background Links

• Introduction to kinetics, Khan Academy
• Rates of reaction, Teach It Science

Prep Work (5 minutes)

1. If you use ungraduated glasses or cups, then mark one cup on the outside to the same volume level for each group. As students will use the same glass for repeated trials, it is convenient to mark the desired water level beforehand.
   1. Use the measuring cup to add 8 ounces (236 mL) of water to the glass. (If you're using metric volume units, rounding up to 250 mL is fine.)
   2. Use a piece of masking tape on the outside of the glass to mark the water level. Place the tape with its top edge even with the water level in the glass.
   3. Now students can use the masking tape to fill the glass to the right level for each trial.
2. Take one whole effervescent tablet out of its wrapper and put it on the scale. Record its mass, as your students will need this information to calculate the reaction rates of the dissolution reaction. You will assume that each tablet will have the same initial mass for each experiment independent of the tablet's particle sizes.

Engage (30 minutes)

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

   What does the reaction rate tell you about a chemical reaction? Do all reactions happen at the same rate?

   The reaction rate gives information about how quickly or slowly a chemical reaction occurs, meaning how fast all the reactants are converted into product. Each chemical reaction has its own reaction rate.

   Why are chemical reaction rates relevant? Can anyone think of a chemical reaction that we want to happen really fast or very slowly? [Possible prompts for students include questions such as "What if you wanted to manufacture something using chemical reactions?", or "Do you think of chemical reactions could get 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?"]

   Reaction rates matter for many processes in industry and nature. Chemical reactions that we use to make products such as plastics, medicine of fuel should happen fast to produce more product in less time. 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 reactants at the right time.

   How can we measure chemical reaction rates?

   Reaction rates are a measure of concentration changes over time. That means we can measure and calculate chemical reaction rates by monitoring the concentration of the chemicals that react (reactants) or the chemicals that are produced (products) over time.

   How can we control the speed of a chemical reaction?

   There are several factors that can change the rate of a chemical reaction (temperature, reactant concentration, nature and surface area of reactants or the presence of catalysts or inhibitors). To understand why these factors affect the speed of a reaction, we need to know what happens during a chemical reaction on a molecular level.

2. Introduce the Collision Theory and link it to chemical reaction rates.

   What happens during a chemical reaction on a molecular level? [Possible prompts for students include questions such as "What needs to happen so that the reactants interact and combine with each other?" or "How can new bonds be formed to make a new product?"]

   In a chemical reaction, the reactants combine or interact to form new products. During that process, molecular bonds of the reactants are broken and new molecular bonds are formed. How reactants react with each other is explained in the Collision Theory. This theory states that the reactants' molecules are in constant motion and have to collide with each other in order to form a product.

   How does the collision frequency of the reactants affect the chemical reaction?

   Not every collision results in the formation of a product. Only when the reactant molecules collide in the right orientation and with enough energy will a reaction happen. A higher collision frequency increases the number of effective collisions which determines how fast all the reactants are converted to the end product.

   Based on the Collision Theory, can you explain which factors affect chemical reaction rates and why?

   Everything that increases the frequency of successful collisions of the reactants affects the reaction rate. Factors that affect reaction rates include the concentrations of the reactants, the nature of the reactants, temperature, and the presence or absence of a catalyst or inhibitor. In chemical reactions, in which a solid reactant is present, the surface area of this reactant also affects the reaction rates.

3. Explain to your students that in the following activity, they will explore the effect of surface area more closely. They will use a simple experiment that demonstrates how fast an Alka-Seltzer tablet dissolves in water dependent on its surface area to determine the correlation between the two.

   What happens to the surface area of an Alka-Seltzer tablet when you decrease its particle size by breaking it into smaller pieces?

   The surface area is the outermost layer of the tablet that faces the outside. If you break the tablet in smaller pieces more parts of the tablet are exposed which means that the surface area increases. In general, for the same mass, many small particles have a higher total surface area than one large particle.

   Optional: you can use 8 cubes to demonstrate how the surface area increases with smaller particles size according to Figure 1 in the Background section. Let the students count the exposed faces for each scenario (single large cube versus eight individual cubes).

   You can also show your students this introductory video to this lesson plan experiment:

   

   https://www.youtube.com/watch?v=fg_GFgGam34

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4. Introduce the experiment to your students (you can set it up yourself for a quick demonstration).

   Alka-Seltzer tablets are used as pain reliever and antacid. They contain aspirin, citric acid and sodium bicarbonate. Can anyone explain what happens when you drop an Alka-Seltzer tablet into water? Why does it fizz and bubble?

   The main ingredients of Alka-Seltzer tablets are aspirin, citric acid, and sodium bicarbonate (NaHCO3). When sodium bicarbonate dissolves in water, it dissociates (splits apart) into sodium (Na+) and bicarbonate (HCO2-) ions. The bicarbonate reacts with hydrogen ions (H+) from the citric acid to form carbon dioxide and water (shown in Equation 1 of the Background section).

5. Do not reveal how the surface area affects reaction rates to your students yet, but tell them that they will conduct a series of experiments with Alka-Seltzer tablets in different forms to find out about the correlation between surface area and reaction rates. Changing the surface area of the tablet can be achieved by dividing it up into different amounts of pieces, including grinding it into a powder.

   What happens if you convert the tablet into powder? How does that change the tablet's particle size and total surface area? Do you think the powdered tablet dissolves at a faster, slower or the same rate as the whole tablet?

   The particle size of the powdered tablet is much smaller compared to the whole tablet. This means that the combined surface area of the powdered tablet increases significantly.

6. On their worksheet, let the students write down their hypothesis of how the total surface area of the Alka-Seltzer tablet affects the rate of its dissolution in water.

   What would be the best way to measure the reaction rate of the dissolution reaction for different tablet surface areas?

   The dissolution of the tablet throughout the reaction allows us to see when the reaction is completed. You know that the reaction is done when all the solid material has disappeared and the bubbles stop forming. That means you can time the chemical reaction with a stopwatch by observing the dissolution process. Once you know how long it took for the tablet to dissolve, you can calculate the chemical reaction rate (grams of Alka-Seltzer tablet dissolved divided by reaction time).

Explore (20 minutes)

1. Based on the previous discussions, let each student formulate his/her hypothesis on how the surface area of the tablet affects the rate of the dissolution reaction.
2. Divide the class into groups of 2–4 students and inform them that each group will conduct experiments with effervescent tablets of different particle sizes to test their hypothesis, doing three trials for each, depending on the time available. Within each group, students should divide tasks. For example, two students could prepare the tablet and the solution, another could drop the tablet into the water, and a fourth student could measure the reaction time with a 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: Whole tablet

   1. Fill the beaker or cup with water up to the top of the masking tape.
   2. Take one whole Alka-Seltzer tablet out of its wrapper. Be careful—they break easily!
   3. Drop the whole tablet into the water and start the stopwatch immediately.
   4. Stop the stopwatch when the solid material has completely disappeared and the bubbles have stopped forming.
   5. Record the reaction time in the data table on your worksheet.

   Experiment 2: Tablet broken into 16 pieces

   6. Repeat steps a.–e. using an Alka-Seltzer tablet broken into 16 pieces. Take one whole effervescent tablet and start by halving it with your hands. Then with your fingers, halve each piece again until you have 16 pieces of approximately the same size as shown in Figure 2. Drop all pieces into the water at the same time.
   
   
   Image Credit: Svenja Lohner, Science Buddies / Science Buddies
   Figure 2. Take one whole Alka-Seltzer tablet and break it into 16 pieces of approximately the same size.

   Experiment 3: Tablet ground to powder

   7. Repeat steps a.–e. using an Alka-Seltzer tablet ground into powder. To do this, fold one whole tablet inside a clean piece of paper. Use the back of a metal spoon to carefully crush the tablet into a powder once it is wrapped. Stop immediately if the paper shows signs of tearing: you do not want to lose any of the powder. Drop all of the powder into the water at the same time.

Reflect (20 minutes)

1. Let students analyze their results. Combine the data for all three experiments from all groups into one table so the students can see and compare their results. Encourage the students to question and discuss their data. Possible prompts for students include questions such as

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

   If the surface areas were similar for each trial, the reaction times should be reproducible within and between groups. Depending on the experimenter or actual surface area of the tablet pieces, you might see a variance of up to 10% in your data.

   What are possible sources of variance in your data? For example, did you see a difference depending on which student conducted the experiment? Were all the tablet pieces really the same size?

   One possible source of variance is the water temperature. As the reaction rate is also dependent on temperature, all trials should be performed with the same temperature water. Variations might also result from different people conducting the same experiment or the tablet pieces broken into different sized pieces.

2. Ask your students to calculate the average reaction times from their combined class data for each experiment. Let them record the results in their worksheet.
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 with the powdered tablet compared to the whole tablet?

   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 (faster dissolution with smaller particle size or larger surface area) in their experiments.

   Which particle size resulted in the slowest reaction time? Why? Did you observe big differences in reaction time between the different surface areas?

   The slowest reaction time should have been observed with the whole tablet as it has the smallest surface area. Differences in reaction times depend on the actual surface areas of the tablet pieces, but probably are probably in the range of 5–30 seconds.

4. If you have time, you can combine all the groups' data into a scatter plot on the board to reflect the entire class' data. This way they can also see the trend of the data clearly, showing that higher surface areas (or smaller particle sizes) 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 (or mass) changes (of reactants or products) over time. As the mass of the tablet was monitored in this experiment, it can be calculated using Equation 3: Equation 3: $$Reaction \: rate = -\frac{\Delta[Alka-Seltzer \: tablet]}{\Delta t}$$ where Δ [dye] is the difference between the Alka-Seltzer mass at the beginning and end of the reaction ([Alka-Seltzer]start - [Alka-Seltzer]end), and Δ t is the reaction time determined above. As at the end of the dissolution reaction, the mass of the Alka-Seltzer tablet is zero as it all dissolved ([Alka-Seltzer]end = 0), Δ [Alka-Seltzer] = [Alka-Seltzer]start.

6. Tell your students the mass of one effervescent tablet (which you should have determined in your preparations before), so they can use it for their calculations. Students should assume that in each experiment and trial they started out with the same Alka-Seltzer tablet mass (independent of particle size). Given the Alka-Seltzer mass and time values, let students calculate and record each average reaction rate in the data table provided in their worksheet.
7. Let the students make a bar graph showing the average reaction rate, in grams/second, (y-axis) vs. surface area on the x-axis. As they cannot calculate the actual surface area of the tablet, they can use following x-axis labels instead: "whole tablet: low, 16 pieces: medium, powder: high") This way they can finish the lesson by visualizing the correlation between reaction rates and surface area.

   Do your results confirm or disprove your original hypothesis on how surface area affects the reaction rate of the dissolution reaction?

   The results of the experiments should confirm that the average reaction rate of a reaction increases with higher surface area (powder>16 pieces>whole tablet).

   Can you explain your observations? Why does the average reaction rate increase with an increase of surface area? What happens on a molecular level to the reactant molecules if the surface area is increased?

   A higher surface area increases the area of the solid reactant at which collisions with the other reactants are possible. According to the collision theory this makes it more likely that the reactant molecules collide and react. A higher collision frequency increases the number of successful collisions which results in a faster conversion of reactants into products or a faster reaction rate.

Assess

• Online quiz, assignable in any LMS
• Quiz (pdf) and answer key (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 particle sizes of solid reactants, 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

Materials Scientist and Engineer

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Career Profile

Material Scientists apply chemistry, as well as physics, to study all aspects of materials. This includes investigating how the properties of a material changes with its particle size or surface area. Nanoparticles of carbon for example have profoundly different characteristics than a piece of bulk carbon. Materials that are used for industrial processes need to be carefully developed and studied by Material Scientists and Engineers so they can be used safely and efficiently for the specified purpose. Read more

Lesson Plan Variations

• You can add an additional statistical component to this lesson by letting the students repeat each experiment several times and then let them calculate the standard deviation of the reaction times for each tablet's particles size. They can then add error bars to their graphs which reflects the variance in their data. This allows students to learn about the variability of data, how to assess the robustness of data sets, and how to represent errors or variance in their results.
• In a more advanced version of this experiment, let the students measure the volume of carbon dioxide gas produced by the dissolution reaction over time. To do this, they need to build a simple apparatus as shown in this Science Buddies' project. Students can collect data at multiple time points using this setup, so they will be able to get information about how the reaction rate changes over time.
• Use the same experiment to explore other factors that affect reaction rates, such as the temperature of the water. Let students dissolve a whole effervescent tablet in water of different temperatures and measure the reaction time for each. Here is another lesson plan, Reaction Rates: Speed It Up with Temperature!, that uses the bleaching reaction to demonstrate the effect of temperature on chemical reaction rates.
• To add a special twist to the experiment, you can let your students record the sounds the Alka-Seltzer tablet makes when disintegrating, using Google's Science Journal app. They can use their recorded data to derive the reaction times and rates for each reaction as explained in this this Science Buddies' project.