Materials Included In Kit

Additional Materials Required

Safety Precautions

Hydrochloric acid is a corrosive liquid. Avoid contact with eyes and skin and clean up all spills immediately. Magnesium metal is a flammable solid. Do not heat the hydrochloric acid directly on a hot plate! Heat the hydrochloric acid in a hot water bath. Keep the temperature of the hydrochloric acid between 0 and 60 °C. Do not react magnesium metal with hydrochloric acid in a closed system—do not stopper or cover the test tubes in which the reaction is taking place. Wear chemical splash goggles and chemical-resistant gloves and apron. Remind students to wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. The waste solutions may be neutralized with a base and rinsed down the drain with excess water according to Flinn Suggested Disposal Method 24b.

Teacher Tips

• Enough materials are provided in this lab for 30 students working in pairs or for 15 groups of students.
• The laboratory work for this experiment can reasonably be completed in one 50-minute lab period. The most important element for success in an inquiry-based activity is student preparation. Sufficient class time should be allotted before lab to think through the measurements that must be made and how the experiment should be conducted. The Prelab Preparation section contains leading questions to stimulate class discussion.
• To ensure a safe lab environment, it is essential that the teacher provide a mechanism for checking the students’ proposed procedures and their understanding of the necessary safety precautions, as recommended in the Procedure.
• See the Supplementary Information section (in Further Extensions) for a sample procedure and data table. These may be used as an alternative student handout, if desired.
• The magnesium metal ribbon will tend to float and surface area will play a much larger role in the reaction rate. Suspending the magnesium ribbon in the solution using a copper wire basket removes this source of error. Show students how to make a copper wire basket (see Figure 1).
  {13911_Tips_Figure_1_Experiment setup with Cu wire basket}
• Encourage student discussion of the optimum number of temperatures and trials for reliable results. The reaction must be carried out at a minimum of three different temperatures—two temperatures will always give a straight line relationship between reaction time and temperature. The reactions are relatively quick—once students feel comfortable with their procedure, additional trials will require only an extra 2–3 minutes. Additional trials may be done either at different temperatures or at the same temperatures to average the results.
• Reaction times are best measured based on the disappearance of the magnesium metal, especially at higher temperatures. Above 50 °C, the evolution of gas bubbles was observed even after all the metal had reacted. This may be due to “outgassing” of dissolved oxygen or hydrogen at higher temperatures. Indeed, it was found that the product mixture obtained from a room temperature run produced bubbles when placed in a 50 °C bath.
• At first glance, the experimental design seems very simple and straightforward. Upon closer examination, however, several factors emerge that have a bearing on the results. In order to isolate the effect of temperature on the reaction rate, it is desirable to carry out the reactions under conditions where the concentration of hydrochloric acid will not change significantly over the course of the reaction. Using 4-cm (0.03-g) strips of magnesium ribbon corresponds to 0.0012 moles of magnesium metal reacting. The amount of hydrochloric acid consumed in the reaction is twice the number of moles of magnesium, or 0.0024 moles. If the volume of 1.0 M hydrochloric acid used is 18 mL, the initial number of moles of HCl present is 0.018 moles, and the amount of HCl consumed is (0.0024/0.018) x 100, or 13% of the total. This is greater than the 5–10% “extent-of-reaction” generally advised for the method of initial rates—the reactant concentration will not be a controlled variable. The surface area of the magnesium metal also changes during the course of the reaction.
• The reaction rates reported in the data and results table are proportional rates obtained by taking the inverse of the reaction time in seconds. Actual reaction rates can be calculated from the number of moles of magnesium that have reacted divided by the reaction time.
• Some teachers tell us that for an inquiry-based lab, they require the students to submit their proposed procedures the day before the lab. The teachers then check the procedures and return the proofed copies to the students before lab. This ensures that students are prepared and that teachers have time to supervise the actual lab activity, not proof the procedures, during lab time.

Further Extensions

Supplementary Information: Sample Procedure

1. Prepare a hot water bath by filling a 400-mL beaker half-full with water and heating it on a hot plate at a low setting. The temperature of the bath should not exceed 50 °C.
2. Prepare an ice-water bath by filling a 400-mL beaker with water and ice. The temperature of the bath should be between 0 and 5 °C.
3. Using a graduated cylinder, add 18 mL of 1 M hydrochloric acid to each of three large test tubes (approx. 20 x 150 mm). All of the test tubes should be the same size.
4. Place one test tube in a beaker of room temperature water, one test tube in the ice water bath, and one test tube in the hot water bath.
5. Allow the test tubes to sit in their respective water baths for at least 5 minutes to reach thermal equilibrium.
6. Obtain a 24-cm strip of magnesium ribbon.
7. Using scissors, cut the magnesium ribbon into three, 4.0-cm long pieces. Note: Be as precise as possible. The reaction time will depend on the amount of magnesium reacting in each trial.
8. Twist and fold one end of the copper wire around a pencil to make a small “cage” into which the magnesium ribbon may be inserted. The other end of the wire must be long enough so that the wire will hang over the side of the test tube and the cage will be below the liquid level marked on the test tube.
9. Measure and record the temperature of the room temperature water bath.
10. Fit one piece of magnesium ribbon loosely through a copper wire cage so the magnesium will be held in place but not wrapped around too tightly.
11. Suspend the copper wire cage and the piece of magnesium in the hydrochloric acid solution in one of the room temperature test tubes and immediately start timing.
12. Measure and record the time until the metal has disappeared and the solution stops bubbling.
13. Repeat steps 9–12 with each test tube in the ice-water and hot-water baths.
14. Repeat steps 3–13 to collect a second set of data.
15. Average the reaction times at each temperature.

Answers to Prelab Questions

1. Write the balanced equation for the reaction of magnesium metal with hydrochloric acid.

   Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

2. What visible signs of reaction should be observed as the reaction proceeds? How will you be able to determine when the reaction has ended?

   The magnesium metal will slowly “disappear” as it reacts with hydrochloric acid to form a soluble magnesium compound, and hydrogen gas bubbles will be observed as the reaction takes place. The reaction is over when all of the metal has reacted and no more gas bubbles are observed. Note to teachers: Students will need additional experience or guidance to determine the best way for measuring the reaction time. Ideally, students will have enough time in the lab to conduct some trial runs and decide whether it is better to look for the metal to disappear or the bubbling to stop. See the Lab Hints section for a discussion of this problem.

3. What measurements must be made to determine the effect of temperature on the rate of the reaction?

   Measure the reaction temperature and the time needed for the magnesium metal to disappear and/or the gas bubbling to stop.

4. The independent variable in an experiment is the variable that is changed by the experimenter, while the dependent variable responds to (depends on) changes in the independent variable. Choose the dependent and independent variables for this experiment.

   Temperature is the independent variable, the time of reaction the dependent variable.

5. What other variables will affect the reaction times in this experiment? How can these variables be controlled?

   Other factors that will affect the reaction times include the concentration of the acid, the volume of solution, the amount (mass) of magnesium, and the particle size or surface area of the metal. The experiment should be carried out using only one concentration of hydrochloric acid (1 M) and keeping the volume of acid the same in each trial. The magnesium ribbon should be cut into equal-size strips so that the amount of magnesium reacting is the same in each trial. Using magnesium ribbon ensures that the surface area of the metal should be the same in each trial and should not affect the results. Because the magnesium ribbon will be encased in a wire cage to suspend it in solution, it is important that the entire surface of the metal be exposed to the acid. The magnesium ribbon should be loosely inserted into the wire cages and should not be twisted into a knot.

6. If a 4-cm strip of magnesium ribbon is used in one reaction trial, and if 1 meter of magnesium ribbon weighs 0.75 g, how many moles of magnesium are used in one trial?

   4 cm/100 cm x 0.75 g Mg = 0.030 g Mg

   0.030 g Mg x 1 mol/24.31 g Mg = 0.0012 mol

7. How many trials should be run to get a reliable straight line?

   Two trials will always give a straight line relationship, but more trials are required to determine if the data is indeed a straight line. A minimum of three (3) temperatures should be run and preferably four runs.

8. Read the Materials section and the recommended Safety Precautions. Write a step-by-step procedure for the experiment, including the specific safety precautions that must be followed.

   See the Supplementary Information (in the Further Extensions section) for a sample procedure.

Sample Data

Answers to Questions

1. According to the kinetic theory, the average kinetic energy of molecules is proportional to their absolute temperature in kelvins. What is the mathematical relationship between the reaction rate and the temperature in kelvins?

   The reaction rate appears to be proportional to the temperature, in kelvins, within the temperature range studied. Note to teachers: In general, a linear relationship is not expected.

2. The collision theory of reaction rates states that the rate of a reaction depends on the number of collisions between molecules, the average energy of the collisions, and the effectiveness of the collisions. Does the effect of temperature on the reaction rate support the collision theory of reaction rates? Explain.

   Increasing the temperature increases the reaction rate, decreasing the temperature decreases the rate, in support of the collision theory of reaction rates. The effect of temperature can be explained in terms of both the number of collisions between molecules and their average energy. As noted in Question 1, increasing the temperature increases the average kinetic energy of molecules—they move faster. As molecules move faster, the rate of collisions between molecules will increase, thus increasing the reaction rate. More importantly, as the average energy of the colliding molecules increases, more of the colliding molecules have sufficient energy to surpass the activation energy barrier and be converted to products.

References

This activity is from Flinn ChemTopic™ Labs, Volume 14, Kinetics; Cesa, I., Ed; Flinn Scientific: Batavia, L, 2003.

Introduction

The rate of a chemical reaction describes how fast the reaction occurs. How can the rate of a reaction be measured? What effect does temperature have on the rate of a chemical reaction?

Concepts

• Reaction rate
• Temperature
• Kinetic theory
• Collision theory

Background

The greater the rate of a chemical reaction, the less time is needed for a specific amount of reactants to be converted to products. This is analogous to “the greater the rate or speed of a car, the less time is needed to get someplace.” The rate of a reaction can be determined therefore by observing either the disappearance of reactants or the appearance of products as a function of time. Some of the factors that may affect the rates of chemical reactions include the nature of the reactant, the concentration of the reactant, the reaction temperature, the surface area of a solid reactant, and the presence of a catalyst. In this experiment, the effect of temperature on the rate of a chemical reaction will be investigated.

Experiment Overview

The purpose of this inquiry-based experiment is to design and carry out a procedure to determine the effect of temperature on the rate of reaction of magnesium with hydrochloric acid.

Materials

Prelab Questions

1. Write the balanced equation for the reaction of magnesium metal with hydrochloric acid.
2. What visible signs of reaction should be observed as the reaction proceeds? How will you be able to determine when the reaction has ended?
3. What measurements must be made to determine the effect of temperature on the rate of the reaction?
4. The independent variable in an experiment is the variable that is changed by the experimenter, while the dependent variable responds to (depends on) changes in the independent variable. Choose the dependent and independent variables for this experiment.
5. What other variables will affect the reaction times in this experiment? How can these variables be controlled?
6. If a 4-cm strip of magnesium ribbon is used in one reaction trial, and if 1 meter of magnesium ribbon weighs 7.5 g, how many moles of magnesium are used in one trial?
7. How many trials should be run to get a reliable straight line of temperature versus rate?
8. Read the Materials section and the recommended Safety Precautions. Write a step-by-step procedure for the experiment, including the specific safety precautions that must be followed.

Safety Precautions

Hydrochloric acid is a corrosive liquid. Avoid contact with eyes and skin and clean up all spills immediately. Magnesium metal is a flammable solid. Do not heat the hydrochloric acid directly on a hot plate! Heat the hydrochloric acid in a hot water bath. Keep the temperature of the hydrochloric acid between 0 and 60 °C. Do not react magnesium metal with hydrochloric acid in a closed system—do not stopper or cover the test tubes in which the reaction is taking place. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

1. Verify the procedure (see the Prelab Questions) with your instructor and review all safety precautions.
2. Carry out the procedure and record all data in a suitable data table.
3. Calculate the average reaction rate for each temperature and graph the data appropriately to determine the mathematical relationship, if any, between the rate of the reaction and the temperature.
4. Write a paragraph describing how temperature affects the rate of a chemical reaction. Include in this paragraph a discussion of the possible errors involved in the experiment and their effect on the results.
5. Answer the following Post-Lab Questions.

1. According to the kinetic theory, the average kinetic energy of molecules is proportional to their absolute temperature in kelvins. What is the mathematical relationship between the reaction rate and the temperature in kelvins?
2. The collision theory of reaction rates states that the rate of a reaction depends on the number of collisions between molecules, the average energy of the collisions and the effectiveness of the collisions. Does the effect of temperature on the reaction rate support the collision theory of reaction rates? Explain.