Reaction Kinetics

Unit Overview

Gasoline is a hydrocarbon. You want it to burn quickly and smoothly in the cylinder of your car. Vinyl is also a hydrocarbon, used as trim in many cars. You want it to oxidize very slowly. You hope that the steel components of your car also oxidize slowly, so that your car doesn't turn into a large pile of rust. But you hope the lead in the battery oxidizes quickly, to release electrons to run the starter.

This core unit provides students with an opportunity to investigate factors which influence rates of chemical reaction, and a basis for understanding the movement and rearrangement of particles during a reaction. If students see the connection between understanding the principles governing the rates of chemical reactions and understanding how a multitude of applications work, they will have achieved the goal of this unit.

Placing undue emphasis on the mathematical relationships involved in rate laws is unnecessary. Calculating rate laws, determining the order of reactions, or developing equilibrium expressions from rate laws may be of interest to some, but such topics place an unnecessary amount of pressure on students who lack mathematical ability. A descriptive treatment of the unit is preferable.

The use of models and audiovisual aids is practically indispensable in this unit. Many good resources are available. Teachers should attempt to use a variety of them in this unit.

There are several classic rate investigations that may be done. The use of laboratory activities will enhance this unit by making it less theoretical and abstract.

Factors of scientific literacy which should be emphasized

• A4 ..........replicable
• A6 ..........probabilistic
• A8 ..........tentative

• B10 ..........cause-effect
• B11 ..........predictability
• B13 ..........energy-matter
• B15 ..........model
• B20 ..........theory

• C3 ..........observing and describing
• C5 ..........measuring
• C8 ..........hypothesizing
• C9 ..........inferring
• C10 ..........predicting
• C12 ..........interpreting data
• C13 ..........formulating models
• C16 ..........designing experiments

• D3 ..........impact of science and technology
• D4 ..........science, technology, and the environment
• D7 ..........variable positions

• E3 ..........using equipment safely
• E4 ..........using audio visual aids
• E5 ..........computer interaction
• E8 ..........measuring time
• E9 ..........measuring volume
• E10 ..........measuring temperature
• E12 ..........using electronic instruments

• F1 ..........longing to know and understand
• F2 ..........questioning
• F7 ..........demand for verification
• F8 ..........consideration of premises

• G1 ..........interest
• G2 ..........confidence
• G5 ..........avocation
• G7 ..........vocation

Foundational Objectives for Chemistry and the Common Essential Learnings

Examine the factors which influence reaction rates in the context of the collision theory.

• Suggest some ways in which the rate of a chemical reaction could be measured.
• Identify some factors which affect the rate of chemical reactions.
• Apply collision theory to account for the factors which affect the rates of chemical reactions.
• Recognize that chemical reactions may occur in successive elementary steps.
• Understand how a series of simple reactions can constitute a reaction mechanism for a complex reaction.

Consider molecular level events in a chemical reaction.

• Discuss the concept of threshold energy.
• Interpret energy versus reaction pathway diagrams.
• Consider the existence of transition states in which activated complexes exist.
• Explain the role of catalysts in chemical reactions.
• Interpret energy versus reaction pathway diagrams for catalyzed and uncatalyzed reactions.
• Describe the use of catalysts in a variety of applications.

Strengthen knowledge and understanding of how to compute, measure, estimate and interpret quantitative data, when to apply these skills and techniques, and why these processses are important in studying chemical energetics. (NUM)

• Recognize whether computed answers are sensible.
• Understand the principles and difficulties of measuring rates of chemical reactions, and inventing suitable procedures for measurement.
• Understand that divergent thinking and reasoning often precede convergent thinking and solutions to problems.
• Distinguish between situations where quantitative precision is required and those where approximations are acceptable.
• Use quantitative problem solving tools such as tables of conversion or equivalence.

Promote both intuitive, imaginative thought and the ability to evaluate ideas, processes, and experiences in meaningful contexts. (CCT)

• Seek alternate ways of responding to activities, projects or assignments.
• Summarize information in a variety of ways.
• Understand that real life problems often have more than one solution.
• Discover relationships and patterns.
• Generate, classify and explore reasons or rules underlying categories.
• Propose generalizations which explain relationships.

Treat themselves and others with respect. (PSVS)

• Work toward improving self-esteem in themselves and others.
• Work cooperatively and contribute positively in group learning activities.
• Demonstrate capacity and ability to act with respect, empathy, sympathy, fairness, loyalty, cooperation and patience for others.

Develop their abilities to meet their own learning needs. (IL)

• Connect what is already known with new experiences.
• Focus on and complete learning tasks.
• Look for associations among items of knowledge and extend identified relationships through additional inquiries.
• Interpret and report results of learning experiences.
• Plan, manage and evaluate own learning tasks.

Suggested activities and ideas for research projects

• To help illustrate the influence that the nature of the reactants has on the progress of a reaction, place about 1 cm³ of potassium permanganate onto some paper towelling on a fire retardant surface, preferably in a fume hood. Dropwise, add glycerine until the mixture becomes moist. Quickly wrap the towelling around the mixture. After a few minutes the paper towelling will burst into flames. This is a good demonstration of spontaneous combustion.

• Support a paraffin candle in a 50 mL or 100 mL beaker. Measure and record the mass of the system. Light the candle and let it burn for exactly 60 seconds. Reweigh the mass of the system and calculate how much mass has been lost. Repeat three times.

  Use the molecular mass of paraffin to calculate the rate of burning of paraffin. Investigate what variables are important in the rate of burning. Is the length of the wick significant? the diameter of the candle? the ambient temperature? the length of time the candle has been burning? Devise experimental procedures to test these variables.

• Design a procedure to measure the rate at which a gas burner consumes gas.

• Support a sugar cube on a clay triangle. Use a match to ignite it. Describe the rate at which it burns and the characteristics of the flame.

  Remove the cube and extinguish the fire. Get another cube. Rub fine paper ashes or cigarette ashes over its surface. Place this cube on the clay triangle and ignite. Compare the rate and characteristics of the burning.

• Mix a very small (<1 g) sample of Pb(NO₃)_2(S) with an equal size sample of NaI_(S). Use a glass stirring rod to grind them together. Describe the rate and characteristics of the reaction.

  Get another <1 g sample of Pb(NO₃)_2(S) and dissolve in about 10 mL distilled water. Add 1 gram of NaI_(S) to this solution. Describe the reaction rate and characteristics.

  Get a third <1 g sample of Pb(NO₃)_2(S) and dissolve in about 10 mL distilled water. Dissolve 1 gram of NaI_(S) in 10 mL water, and add this solution to the Pb(NO₃)_2(aq). Describe the reaction rate and characteristics. Compare results of the three trials.

• Set up three beakers with 200 mL of 15^oC water in each. In beaker 1 dissolve 0.5 g Na₂S₂O_3(s) (sodium thiosulphate). In beakers 2 and 3 dissolve 1.0 g and 1.5 g Na₂S₂O_3(S). Add 200 mL of 15^oC 1.0M HCl(aq) to beaker 1 while stirring the mixture.

  Note the time from the addition to the first appearance of cloudiness. Alternatively, set the beaker over an X marked on a piece of paper. Looking down through the beaker at the X, record the time when the X disappears.

  Repeat the addition of HCl to each of the other beakers. Compare the time until cloudiness appears.

  Repeat the experiment using a range of solution temperatures, such as 25^oC, 35^oC, 45^oC, and 55^oC. Predict what would happen if 75^oC was the temperature of the solutions.

  The equation which represents the dominant reaction when these chemicals are mixed is

  S₂O₃^-2_(aq) + 2H⁺_(aq) S_(s) + SO_2(g) + H₂O_(l)

  (This activity was adapted from CHEM13 NEWS, #81, November 1976, page 3, based on an idea contributed by L. Sibley, St. Catharines, ON)

• The "clock reaction" is an interesting reaction which can be used to examine the effect of temperature and concentration on the rate of a reaction.

  There are several ways to prepare the chemicals used in this experiment. One method is to prepare three different solutions. Solution A is 0.2 M potassium iodide, solution B is 0.0050 M sodium thiosulphate and starch, and solution C is 0.1 M ammonium peroxydisulphate. Pour 10 mL of solution B into a beaker and vary the amounts of solutions A and C that are added. Add solutions A and C quickly, stirring while they are being added. Begin timing as soon as the solutions come into contact with one another. To keep the total volume constant in each trial, a small amount of water can be added to either solution A or C prior to mixing solutions. Samples can be placed in hot and cold water baths to compare the rates at different temperatures.

  A variation of the clock reaction uses two solutions, one of 0.02 M potassium iodate, and another of 0.002 M sodium hydrogen sulphite. Prepare several samples of the iodate solution that have been diluted, so that the total volume of the mixture is 20 mL (i.e., 15 mL KIO₃ and 5 mL of water, 10 mL of KIO₃ and 10 mL of water, etc.). Mix the iodate solution with the sodium hydrogen sulphite solution. Begin timing as soon as the two solutions come into contact. Use a hot and cold water bath for different trails to observe the effect of temperature on reaction rate. (For further details, refer to the "clock reactions" found in a variety of lab manuals.)

• Students can perform microscale explosions using tapered (not thin-stem) Beral pipets. Take two Beral pipets. Cut off the stem of one about 2 cm from the bulb. This is the projectile. Insert a piece of thin copper or iron wire (or straight pins) into each side of the bulb of the other pipet. Leave a suitable spark gap between the inner ends. This is the reaction chamber. For a multiple explosion connect several of these chambers in series.

  

  Draw only one drop of methanol or ethanol into the reaction chamber. Rotate the bulb to coat the walls with the alcohol. Stand the reaction chamber in a test tube rack, bulb down. Securely fit the cut-off pipet over the vertical, tapered end of the reaction chamber pipet. Touch one of the wires with a Tesla coil. (This activity was adapted from CHEM13 NEWS, #184, March 1989, page 3. It is based on an article in the September 1988 New Jersey Science Teachers Association Newsletter by George Gross. The experiment was developed by Rob Lewis and Jim Tarnowski at a Butler University workshop in 1988.)

• Use Co⁺²_(aq) as a catalyst to generate oxygen gas from ordinary laundry bleach. 0.5 mL of 0.1M CoCl_2(aq) will provide enough Co⁺²_(aq) to catalyze the decomposition of 2 mL to 5 mL of undiluted bleach.

  This system can be used to test the effect of temperature on the rate of a reaction. Some students may want to test the effect on the rate of the reaction of changing the concentration of the bleach used.

  This activity also suggests a research project in which the action of catalysts on the rates of reactions is investigated. How do catalysts lower the activation energy of the reaction? Why are metals and metal ions often used as catalysts? What reactions does the catalytic converter of a car promote? Are reaction inhibitors (such as antioxidants, for example) related to catalysts? Do they work by raising the activation energies in their reactions or is there a different mechanism?

• Dissolve 3 grams of sodium potassium tartrate in 50 mL of distilled water and warm to 70°C. Add 20 mL of 3% or 6% hydrogen peroxide. Observe the mixture carefully and record a description.

  Add a few crystals of cobalt(II) chloride so that the solution turns pink. Observe the reaction and record a description of what is seen. What evidence is there that the cobalt chloride catalyzed the reaction? Are there other salts which will catalyze the reaction in a similar manner? (This activity was adapted from CHEM13 NEWS, #81, November 1976, page 15, based on an idea contributed by J. Huxley, Simcoe, ON)

• Investigate the production of ozone in the upper atmosphere to form the ozone layer, the mechanisms by which ozone in that layer filters out ultraviolet light, and the issue of catalytic decomposition of ozone by free chlorine atoms.

• How does the Haber process use catalysts? Is the synthesis of ammonia from nitrogen gas and hydrogen gas economically feasible without catalytic assist? What process does the Saferco plant at Belle Plain use to synthesize ammonia? Do all ammonia synthesizing plants in Western Canada use the same process?

• Research the Fischer-Tropsch process. What are the raw materials? What are the products? What catalyst is used? What are the technical details of the process? When was the process first used on a large scale? Why was its use important then? Where is the process used now?

• How many industrial processes can be identified which rely on the use of catalysts to make the process economically viable?

• To demonstrate enzyme catalysis, put some H₂O_2(aq) (30% or less) in a petri dish on an overhead projector. Add a drop of blood to one area of the dish, a drop of juice squeezed from a piece of fresh liver in another area, and a drop of juice squeezed from a fresh slice of turnip in a third area. The slight foaming indicating decomposition should be visible on the screen. (This activity was adapted from CHEM13 NEWS, #81, November 1976, page 10, based on an idea contributed by S. Sharpe and D. Humphreys, Hamilton, ON.)

• Students who would be interested in doing a research project on the treatment of fabrics with flame retardants can be referred to an article by Peter Carty in CHEM13 NEWS "Fires and Flame Retardants for Polymers" (March 1989, #184, pages 7-10).

• Use Alka-Seltzer^TM tablets to investigate the influence on the rate of reaction of phase (solid and aqueous), amount of surface area (whole tablet versus powdered), and the temperature of the reaction medium (in hot water and cold water).

Sample ideas for evaluation and for encouraging thinking

Compare the curves which represent the rates of two reactions. What are the similarities in the rates? What are the differences? What factors might cause the differences?

• Zn_(s) + 2HCl_(aq) ZnCl_2(aq) + H_2(g)
  Outline three ways that you could measure the rate of this reaction.

• Zn_(s) + 2HCl_(aq) ZnCl_2(aq) + H_2(g)
  What would be the effect on the rate of the reaction if
  • a piece of zinc screen were substituted for a piece of solid zinc metal
  • powdered zinc was sprinkled into the acid instead of using apiece of solid metal
  • the strength of the HCl_(aq) was doubled
  • the reaction was done in a plastic container instead of a glass beaker

  Explain why the effect indicated for each change would occur.

• Gold is preferable to silver for jewellery because its rate of reaction with oxygen is slower. Why does this make it preferable? Why does it react more slowly with oxygen?

• How is the mechanism of a reaction like the recipe for a cake? Create and explain some other analogies for reaction mechanisms.

• A grade 12 chemistry class was given the task of measuring the rate at which the butane in a disposable lighter burned. Outline a procedure that could be used.

  One group reported the rate they measured was 3.0 grams of butane per minute. Another reported their results as 0.9 mmol·sec^-1. A third group reported the rate as 1.3 litres C₄H_10(g) at SATP/minute. Are these results equivalent?

• Catalysts are thought to lower the activation energy for a reaction. What are some of the ways which catalysts act to do this?

• An increase in temperature of 10°C rarely doubles the kinetic energy of particles and hence the number of collisions of particles is not doubled. Yet this temperature increase alone may be enough to double the rate of a slow reaction? Explain how this can happen.

• Discuss the proper adjustment of the gas supply and air regulator of a gas burner, in terms of the rate of reaction of the burning gas.

• Phosphorus, P₄, burns spontaneously when exposed to air. The product of the combustion is P₄O₁₀ and the H is -2.98 x 10⁺³ kJ·mol^-1 P₄. Draw an energy diagram (reaction pathway) for the net reaction.

  Calculate how much energy is produced when 20.0 grams of phosphorus burn.

• Mg_(s) + 2HCl_(aq) MgCl_2(aq) + H_2(g)

  

  If 10.0 grams of magnesium metal ribbon were put into the acid in this reaction, why would only 5.0 grams of it have reacted?

  If the volume of HCl_(aq) used was 200 mL, what was the approximate concentration of the HCl_(aq) used?

  What was the rate of the reaction during the first minute? during the time between minute three and minute four of the reaction?

  Sketch a possible shape of the curve if powdered magnesium was sprinkled into the acid instead of using magnesium ribbon.

• Which of the following affects the rate at which a candle burns?
  • temperature of the air
  • shape of the candle
  • % of O_2(g) in the air
  • % of CO_2(g) in the air
  • length of wick exposed
  • age of the candle
  • composition of the candle

  For each factor briefly discuss why you have made your decision.

• Name three industrial processes which use catalysts. Identify the catalyst and how each speeds the reaction.

• What are two ways one can slow down the chemical reactions which cause food to spoil? How does each method slow down the reactions?