Introduction

Have you ever wondered how chemists slow down reactions that are potentially explosive or speed up reactions to synthesize a product in a shorter period of time? In this laboratory activity, we will use a familiar reaction:

Mg(s) + 2 HCl(aq) --> MgCl2(aq) + H2(g)

to investigate this problem. The rate may be measured in several different ways. For example, it may be expressed as the volume of H2 gas produced per second or as the mass of magnesium metal used per second.

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Purpose

To design a procedure to measure the rate or speed of the Mg/HCl reaction. You will then identify two factors other than catalysis to alter the speed of this reaction and examine each factor quantitatively.

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Safety Considerations

Wear protective glasses and an apron at all times. Avoid skin contact with solids and solutions. Hydrochloric acid is a very corrosive reagent. Wash any spills copiously with water. If you need to dilute the hydrochloric acid, remember AAA--"Always Add Acid" to water. No Bunsen burners should used during this activity due to the production of potentially explosive hydrogen gas. Wash your hands before leaving the laboratory.

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Procedure

  1. During the first laboratory period, your group of four students will investigate the Mg - HCl reaction. Based on these observations, devise a way to measure the rate of this reaction using one of the two approaches mentioned in the Introduction. Then propose two different factors which will alter this rate. The group should agree on a procedure in preparation for the second laboratory period. Students within the group should assume the following roles:
  2. Before the second laboratory period, you and your partners individually should:
    1. Sketch a diagram of the apparatus.
    2. List the steps involved in your procedure for investigating the effect of the two factors.
    3. Create suitable data tables for each factor. You will be expected to complete four different trials for each factor.
    Your four-person team will be given approximately 50 cm of magnesium ribbon, 250 mL of 3 M HCl, steel wool, a balance, a centimeter scale, meter stick, scissors, common glassware, a thermometer, and a hot plate. Other equipment may be requested if you provide a list the previous day.
  3. Your teacher must approve your procedure before you begin the experiment.
  4. Obtain a piece of magnesium ribbon and polish it with steel wool. Because magnesium has such a low density, it is not practical to weigh small pieces. Therefore, determine the mass of your uncut length of magnesium ribbon. Determine its length to the nearest 0.1 cm. Then calculate the mass of individually-measured lengths used in various steps of the experiment. Record the ribbon lengths and calculated masses.
  5. Each group of four students should form two pairs. One pair will collect data for one factor, while the second pair collects data for the other factor. At the completion of the experiment, data for both factors should be shared.
  6. Wash hands thoroughly before leaving the laboratory.

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Data Analysis, Concept

  1. Calculate the reaction rate for each trial performed. Be sure to include units.
  2. Prepare a graph of your data for each factor, putting the independent variable on the x axis vs. the reaction rate on the y axis.
  3. For each graph, predict the reaction rate at another data point not measured in the experiment.
  4. What generalizations are you able to make concerning the effect of each factor you varied on the rate of the reaction?
  5. How is the rate of a reaction related to the time of the reaction?

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Imply, Apply

  1. It is found that a 10oC increase in temperature roughly doubles the rate of many chemical reactions. If a reaction takes 20 seconds at 40oC, how long would it take at 60oC?
  2. Look up the collision theory in your text. Use it to explain why each factor tested in this experiment altered the rate of the reaction.
  3. Using a dashed line, sketch what would happen to the appearance of one of the graphs if a catalyst were used in this reaction. Label the graphs.
  4. What errors were inherent in the design of your experiment? Explain how each error would affect your measurement of the rate (high, low, or no effect).
  5. It has been found that rates are more rapid at the beginning of a reaction than toward the end, assuming the temperature is constant. Design an experiment using the reaction between magnesium and hydrochloric acid to verify this statement quantitatively.
  6. List several instances in every-day life where you face the need to alter the rates of chemical reactions.
  7. Did any individuals in your group make unique contributions during this investigation? Explain.
  8. Which approach enables you to learn more about a chemical concept -- designing your own procedure or following a predetermined set of laboratory directions? Why?
  9. Do you prefer developing your own laboratory procedures or following directions that are given to you? Why?
  10. List several unanswered questions you have that arose from this experiment.

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Teachers Guide

Preparing for the Laboratory Activity

Conducting the Laboratory Activity

Assessing the Laboratory Learning


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Preparing for the Laboratory Activity

Major Chemical Concept

Various methods can be used to measure the rate of a chemical reaction. Such procedures allow a quantitative study of the effect that two factors which will alter this rate.

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Level

General and advanced chemistry

This lab is designed to be done before any discussion of reaction rates occurs in class other than a very brief introduction on how to measure rates.

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Expected Student Background

Students should be able to:

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Time

Two class periods

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Safety

Remind students to

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Materials

Non-consumables (per lab team of four)

Consumables (per lab team of four)

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Advance Preparation

  1. During the first laboratory period, have small quantities of magnesium ribbon and 3 M HCl available at the lab stations.
  2. To prepare 250 mL of 3 M HCl: Slowly add 62 mL of 12 M HCl to sufficient distilled water with stirring to produce 250 mL of solution.

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Conducting the Laboratory Activity

Pre-Lab Discussion

Discuss the term "rate" and mention its meaning in terms of the speed of a car. Demonstrate the reaction between magnesium and hydrochloric acid or a related one; ask students how they might measure the rate. Remind students to control variables by changing only one factor at a time. Also remind students that since they will be limited to 50 cm Mg ribbon and 250 mL of 3 M HCl they should be judicious in their approach. As part of the assessment process, each student will be asked to evaluate the participation of group members. Pass out the evaluation sheet (see the Check-List in the Assessment section of this guide) so students can see which behaviors will be noted during the laboratory activity. Students may be given the option of selecting only one of the behaviors listed, in keeping with cooperative learning procedures, if you so choose.

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Teacher-Student Interaction

Remind students not to heat the hydrochloric acid above 45oC and to watch for splattering. While students are involved in experimentation, watch for proper lab techniques and safety concerns; also check for proper recording of data and use of significant figures. See Check-List in the Assessment section.

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Anticipated Student Results

Factor 1: Effect of decreasing the HCl concentration on the rate

Typical data obtained using a 5.0-cm length of Mg ribbon, 20.0 mL HCl in a small beaker, measuring the total time for all the magnesium to be consumed:

Concentration of HCl (M)

3.0 2.0 1.5 .8

Time (s)

8 11 18 36

Typical data obtained using a 5.0-cm length of Mg ribbon encased in a copper wire cage, 15 mL HCl added to a eudiometer tube filling the remainder of the tube with water, measuring the total time to generate 16.0 mL of hydrogen gas:

Concentration of HCl (M)

3.0 2.0 1.5 .8

Time (s)

127 210 230 355

Factor 2: Effect of increasing the temperature on the reaction rate:

Typical data using a 5.0-cm length of Mg ribbon, 1.0 M HCl in a small beaker, measuring the total time for all the magnesium to be consumed:

Temperature (&Mac251;C)

5.0 15.4 23.5 32.8 43.5

Time (s)

168 107 66 45 32

Factor 3: Effect of increasing the surface area on the reaction rate:

Typical data using 10.0 cm length of Mg ribbon, 1.0 M HCl in a small beaker, measuring the total time for all the magnesium to be consumed when the strip is rolled into a tightly-coiled piece, a medium-size coil and a loosely-coiled piece.

Circumference of Mg ball (cm)

2.0 2.9 4.8

Time (s)

148 111 95

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Answers to Data Analysis, Concept

  1. Rate is determined by dividing volume of hydrogen gas produced by time increment or mass of Mg used by time increment. For example, using the data above: 5.0 cm Mg used x 0.41 g Mg/(50.0 cm Mg)/ 8 s = 0.005 g Mg/s or 16.5 mL H2 produced/127 s = 0.130 mL H2 /s
  2. Graphs: A computer program for curve fitting of data such as Graphical Analysis III (Vernier Software) could be used to analyze the data.
  3. Students use their graphs (see above) to estimate two other data points.
  4. As the concentration of a reactant increases, the rate of the reaction increases. As the temperature of the reaction increases, the rate increases. As the surface area increases, the rate increases.
  5. The rate of a reaction is inversely proportional to the time of the reaction.

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Answers to Imply, Apply

  1. 5 s
  2. As the concentration of a reactant decreases, the number of effective collisions per unit time decreases; hence the reaction rate decreases. As the temperature increases, more molecules have sufficient energy of activation, causing the reaction rate to increase. As the surface area increases, there is an increase in the total number of effective collisions; hence the reaction rate increases.
  3. Using a catalyst, the graph would resemble this:
  4. Possible errors:
    1. Reaction itself is exothermic. The rate would increase from this factor alone.
    2. The temperature is difficult to control using the eudiometer tube, producing rates too high or too low.
    3. Student errors such as nonuniform strips of Mg, spillage, improper dilution, etc.
  5. Sample answer: Measure the volume of hydrogen gas produced from the reaction in a eudiometer tube over a period of several minutes, taking volume readings at 10 s intervals. Record both the volume and the time. The volume produced initially is greater than the volume produced later in the reaction for a given time interval.
    1. Use of a platinum catalyst in contact lens sterilization vials to decompose hydrogen peroxide solutions.
    2. Use of kindling in campfires.
    3. c) Preserving perishable foods in a freezer or refrigerator.
  6. - 10. Use teacher discretion.

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Post-Lab Discussion

Separate the students according to the factor they chose to investigate; have them share their data and reach a collective conclusion to be shared with the entire class.

Discuss various unique experimental designs and the features offered by each. Discuss the importance of reactions rates in real-world applications.

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Possible Extensions

This experiment provides an excellent lead-in to rate law expressions.

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Assessing the Laboratory Learning

Quiz

  1. What is meant by the term "rate of reaction"?
  2. What experimental data must be collected to determine the rate of a reaction?
  3. You have three samples of calcium carbonate, CaCO3, of equal mass cut in the following pieces: a) 1 perfect cube, 2.0 cm on a side; b) 8 perfect cubes, 1.0 cm on a side; c) 64 perfect cubes, 0.50 cm on a side. Assume you are interested in determining the effect of the total surface area on the rate of the reaction:

    CaCO3(s)

    +

    2 HCl(aq)

    -->

    CaCl2(aq)

    +

    H2O(l)

    +

    CO2(g)

    You decide to use this apparatus:
    1. What data would you need to collect?
    2. Explain how you would use these data to calculate the rate of this reaction.
    3. Predict how the rate would be affected as the surface area of CaCO3 is increased in this reaction.
    4. Explain your prediction in (c) using collision theory.

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Laboratory Report

Formal Laboratory Report should include:

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Pencil/Paper Items

  1. This assessment item could be done as a teacher demonstration, possibly video-taped for ease of repetition. Hydrogen peroxide decomposes at a reasonable rate in the presence of a catalyst according to the equation: 2 H2O2 --> 2 H2O + O2
    In the diagram above, the catalyst is placed into a tissue paper bag. When it drops into the solution of hydrogen peroxide in the flask, the released oxygen gas is collected in the eudiometer tube.
    If you were interested in studying the effect of changes in the concentration of H2O2 on the reaction rate,
    1. What data would be convenient to collect to determine the rate of this reaction?
    2. How would you calculate the rate of this reaction from the data described in Part (a)?
    3. What variables must be controlled in order to carry out this investigation?
  2. From data obtained in this activity, the student found that the rate of reaction increased as the concentration of hydrogen peroxide increased.
    1. Sketch the general shape of the graph which might be obtained from this data, with the rate of reaction on the y axis vs. the concentration of hydrogen peroxide on the x axis. Label the axes.
    2. Redraw the graph from Part II (a) and, by use of a dotted line, indicate the graph that would be expected if the reaction were run without a catalyst.
      Label the two relationships.
    3. Sketch the graph which might be obtained plotting the time of the reaction on the y axis vs. concentration of H2O2 on the x axis. Label axes.
    4. Explain the effect of increasing the concentration of H2O2 on the reaction rate in terms of the collision theory.
  3. Rates are observed to be more rapid at the beginning of a reaction than toward the end if the temperature is held constant.
    1. Design an experiment using the reaction between magnesium and hydrochloric acid to gather data to verify this observation.
    2. Indicate what data need to be collected in this experiment.
    3. Explain this observation in terms of the collision theory.
  4. State three ways in which the rate of a reaction can be increased.

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Answers to Paper/Pencil Items

    1. For a particular concentration of hydrogen peroxide, measure the volume of oxygen produced in a given time interval.
    2. Divide the volume of oxygen produced by the time interval.
    3. Temperature, air pressure, volume of hydrogen peroxide used.
    1. As the concentration of hydrogen peroxide increases, there are more chances for effective collisions per unit time. Hence the rate increases.
    1. See answer to Question 6 in Implications/Applications section.
    2. Data: volume of hydrogen gas produced over a fixed time period (such as 10 s intervals). Record both volume and time.
    3. There are more collisions among reactant molecules per unit time at the beginning of a reaction due to their higher concentration. Toward the end of a reaction, fewer reactant molecules remain; hence there are fewer collisions and the rate decreases.
  1. Increase temperature, concentration of reactants, surface area; add a catalyst.

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Check-List

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Teacher Evaluation

  1. Procedure written out prior to class
  2. Data sheet done prior to class
  3. Use of proper lab technique, recording of data (paying attention to proper use of significant digits)
  4. Use of proper safety precautions (goggles, dilution procedures, heating, etc.).
  5. Clean up.

Student evaluation of group interaction: To be done by each member of the group anonymously

  1. Listening to others
  2. Communicating ideas
  3. Contributions to overall design of experiment
  4. Helping others where possible
  5. Overall participation

Possible scoring scheme:

3 points: Excellent
2 points: Satisfactory
1 point: Poor

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