Kinetics Review


In one of my last blog posts I wrote of how I sometimes enjoy ending a unit with a series of demonstrations and using them to elicit a dialog between the students and myself to check for understanding. It is always a fascinating experience to hear the misconceptions that many students have the day before the test.

My kinetics unit has a very good review lecture where I take the kids through the six factors that affect reaction rate (concentration, temperature, agitation, catalysts, surface area, and nature of the reactants) by showing them demonstrations that illustrate each of the factors.  Most importantly I am trying to get my students to engage in a detailed discussion of what is happening at the molecular level. My whole approach is based on the idea of “Effective Molecular Collisions”. What is it that we can do to produce more collisions and speed up a chemical reaction? I have a phrase that I keep using over and over again: “the species must hit, they must hit hard enough, and hit in the right place”. How can these six factors help with that?

I use light sticks to illustrate the effect temperature has and ask them why light sticks in cold water glow dimmer and ones in hot water glow brighter to try to elicit a discussion of the speed at which the molecules are moving. My main point is the faster molecules at the higher temperature hit harder, more often, and because of the increased number of collisions there is an increase chance of it being in the right orientation to produce a reaction.

To discuss agitation I have a demo that is sometimes called “The Blue Bottle” or the “Traffic Light Reaction". When it is shaken in a closed flask oxygen gas in the flask collides with a mixture of dextrose and base to cause color changes in different acid base indicators. I have even seen some instructions that allow you to try and tailor it to your schools colors in one demo book but I can’t seem to recall which one. I like to point out to the students that most young kids learn these first two factors while watching people cook. Raise the temperature and mix the pot to help cook things faster. My main point here is that if we were not shaking the bottle the molecules would not be hitting as often. Especially since the oxygen is in the gas phase and not exposed to the entire volume of the solution.

Chemistry Comes Alive - Blue Bottle

To discuss surface area I have often shown a candle making contact with a block of wood, then a paper towel, and then Lycopodium powder. This is a little dangerous and maybe one I should rethink. I am not sure I want to show the old classic empty paint can version of this demo and teach students how to make an impromptu bomb. Showing a metal like magnesium in ribbon versus turnings reacting with HCl can illustrate the point nicely. My main point here is that the greater surface area provides more likely points of collision.

Chemistry Comes Alive - Lycopodium powder

For catalysts I have a great demo that shows the decomposition of tartrate ion in hydrogen peroxide. The catalyst is cobalt chloride and it changes to a deep green color while reacting (illustrating a visible activated complex) and then returns to its red color. Ironically the AP Biology teacher in the next room watched me do this one day and is now incorporating it into his lessons on enzymes. My main point here is that the cobalt provides a catalytic intermediate and surface for the reaction to happen on.

Chemistry Comes Alive - Decomposition of tartrate ion in hydrogen peroxide

For the kind of eclectic idea of the nature of reactants I show several things that do react and several that don’t. I like to put copper in water (no reaction), show a precipitation reaction, and even the classic freeze a beaker to a board reaction. In a very large lecture hall I have even done this by exploding balloons of (1) helium, (2) hydrogen, and (3) hydrogen and oxygen in a 2:1 mix. This is not for the inexperienced teacher. My main point being that some things will not react fast (or at all) no matter what you do.

Chemistry Comes Alive - Precipitation

To illustrate concentrations effect on a reaction I like to use the iodine clock reaction. I have a student help me run several mixtures in a row where I start with 50 mL of the first solution (A) and 50 mL of the second solution (B) and time the appearance of blue color. We then do the same thing with only 40 mL of solution A that has been diluted with 10 mL of distilled water to maintain a constant volume. We continue with 30, 20, and 10 mL samples each diluted with enough distilled water to make a constant volume of 50 mL. Each one takes longer than the previous one and we plot the data (mL A versus time) to get a nice curve. I ask my students to predict how many mLs of A would be needed for a reaction of a specific time that I choose and have them predict from their graphs. I like to tell them that it is a “group quiz” and if they are correct they will get 50 points. For every second they are off I will reduce their score by ten points. It can get some heated debate started in the room. My main point here being that increasing the number of molecules present increases the chance of a reaction.

Chemistry Comes Alive - Iodine Clock

Depending on the time I have available for this lecture I have several other kinetics demos I like. I show “Hooberman Balls” to illustrate activation energy, the Old Nassau Clock reaction to show intermediates, and even an Oscillator to show the same thing. For rate determining steps I use a stack of funnels on a ring stand and pour water through them into a bucket. I learned that one from a Flinn workshop.

I have been doing these demonstrations for many years. My personality has allowed me to come up with many anecdotes about them that are high school appropriate. I am sure that all of you have your own ways of tailoring these demos to your stories. 

I am very grateful to Don Showalter of the University of Wisconsin at Stevens Point for showing me how to bring these specific reactions together. Also Larry Quimby and Lanny Larsen of my school (Francisco Bravo Medical Magnet High School) spent many years dong them with me and helping me develop the presentations. In fact we often brought all three of our classes together in one room to team-teach these days. It has proved to be a very effective method of reviewing having three teachers all together in the same room at the same time. I have a great love now for team teaching.

I have attached a file to this blog post that I use to set the lecture up. It has the recipes and equipment lists for the way I do the lecture. It also contains a copy of the graph we pass out for the iodine clock experiment. As always with any experiment or lab be sure to follow the necessary safety practices and remember the five P’s. Prior practice prevents poor presentations. I have provided a basic outline of the activities and materials below. You can find more detailed directions online, in a demo book or in some of the videos linked here. 

As always you can follow all my NErDy adventures on Twitter @morganchem

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