Acetone Iodine kinetics

Introduction

In this lab, we will apply the concepts of reaction rate, order of reaction and catalyst. Our example reaction is the iodination of acetone catalyzed by hydrochloric acid.

\[\ce{CH3C(=O)CH3 + I2 ->[\ce{HCl}] CH3C(=O)CH2I + I- + H+}\]

The reaction is zero order in iodine. You will figure out the order in acetone and the order in HCl, and the rate constant. Then, you will predict which initial concentrations are necessary to make the reaction run for exactly 2 minutes (120 s).

We will figure out the reaction rate by visual observation of the iodine. Iodine in water is brownish-yellow, and all other species are colorless. In the experiments, you will start a timer once iodine is added to the other substances in the reaction mixture, and measure the time until the solution appear colorless. We will run the reaction at room temperature. Try not to warm up any of the reactants or the reaction mixture with your hands or other sources of heat (e.g. sunlight near window).

Experimental design

Finding the rate

Usually, the reaction rate of a reaction changes over time as reactants are depleted. Also, the concentrations change over time. To be able to "plug in" the rate and the concentrations into the rate law and determine the order of reactions, the typical strategy would be to measure initial rate.

Today, however, we are measuring an average rate. We can get away with that because it turns out the rate stays constant over the course of the experiment, which is unusual. There are three reasons for this:

1. The HCl concentration does not change (catalyst)
2. The acetone concentration does not change significantly (in large excess over iodine)
3. The iodine concentration changes a lot, but it does not affect the rate (zero order in iodine)

To calculate the rate, we consider the time it took for the iodine to "fade", and the initial concentration of the iodine. For example, if the initial iodine concentration is 0.001 mol /L, and it takes 180 s for the reaction mixture to become clear, the rate would be:

\[rate = \frac{\Delta [\ce{I2}]}{\Delta t} = \frac{1.0\ \times 10^{-2} \mathrm{M}}{\ 180 \mathrm{s}} = 5.5\ \times 10^{-5} \mathrm{M}^{-1} \mathrm{\ s}^{-1}\]

Varying concentrations to find the order

You will set up multiple reactions, varying the concentrations in the reaction mix. To quickly figure out the concentrations, it makes sense to always keep the final volume the same (25 mL), and use the dilution law to figure out concentrations (or volumes you need to add to get the desired concentration). Here is the dilution law:

\[c_\mathrm{mix} V_\mathrm{mix} = c_\mathrm{stock} V_\mathrm{stock}\]

If you want to know the (initial) concentration of a component in the reaction mixture, plug in the volume and concentration of the stock solution. For example, if our 25 mL reaction mixture contains 5 mL iodine stock with an original concentration of 0.005 M, we get:

\[c_\mathrm{mix} = c_\mathrm{stock} \frac{V_\mathrm{stock}}{V_\mathrm{mix}} = \mathrm{0.005 M}\ \frac{\mathrm{5 mL}}{\mathrm{25 mL}}\]

If you want to know how much of the 4-M acetone stock solution you need to get 0.8 M in the reaction mixture, you solve for the volume of stock solution:

\[V_\mathrm{stock} = V_\mathrm{mix} \frac{c_\mathrm{mix}}{c_\mathrm{stock}} = \mathrm{25 mL}\ \frac{\mathrm{0.8 M}}{\mathrm{4 M}}\]

Procedure

This is the procedure for measuring a single rate. You will run this procedure twice for each different reaction mixture. You can figure out the concentrations of the mixture from the recipe (the volumes mixed) using equation 4 or this spreadsheet. Make sure the volumes in your recipe always add up to 25 mL.

1. Mix water, acetone stock and hydrochloric acid stock in a 125 mL Erlenmeyer. The reaction can not proceed at this stage (no iodine).
2. At your bench, add the correct amount of iodine stock (5 mL) to the mixture and start the timer.
3. For better visibility of the color change, pour the reaction mixture into a medium test tube, and observe from the top, with a white paper under the test tubes. Prepare a large test tube with 25 mL water and one with 0.5 mL iodine solution in 24.5 mL of water for comparison (see picture below: the iodine solution labeled 1:10 looks like water in the side view, but has a faint brownish tint in the top view).
4. Stop the timer once the solution is colorless. Record the time in your lab notebook and calculate the rate for your experiment (like in equation 2).
5. Keep the test tube with water and the 1:10 mix for the remainder of the runs. Discard the reaction mix, rinse the 125 mL Erlenmeyer and the large test tube and invert to dry so you can use them again for the next run

 

Determining the orders of reaction

The easiest way to find an order of reaction is to halve the concentration of the species you are interested in while keeping all other concentrations the same. If the reaction time stays the same, it is zero order in the species whose concentration you changed. If the reaction time doubles (the rate halves), it is first order. If the change is more extreme, it is a higher order than one. If you learned a mathematical way of determining the reaction order in your lecture section, feel free to use that method. Because of experimental errors, we don't expect the reaction order to come out exactly as 0, 1, or 2, and you should feel free to round your result as you formulate the rate law.

To find out two orders of reaction (with respect to acetone and with respect to hydrochloric acid), you need three sets of concentrations (see data sheet). You will run each set in duplicate, so you end up running a total of six experiments. 

We have aqueous stock solutions containing acetone (c = 4 M), hydrochloric acid (c = 1 M), and iodine (c = 0.005 M). For the first set of concentrations, you will use 10 mL acetone solution, 5 mL water, and 5 mL hydrochloric acid (which is corrosive and should be handled under the hood until diluted), giving a subtotal of 20 mL. Measure out 5 mL iodine solution in a graduated cylinder, and start the reaction at your bench by adding the iodine to the other components in the Erlenmeyer. Record the time and repeat the experiment once to check for consistency.

For the second set of concentrations, we are using half the volume of acetone, and for the third set of concentrations, we are using half the volume of hydrochloric acid (compared to the first set). The volumes are given in the data sheet. Calculate the volume of water you need. You should check your recipe before you make the reaction mixture. Make sure the volumes in your recipe always add up to 25 mL.

Predicting the rate of reaction

Once you know the form of the rate law (i.e. all the orders of reaction), you can plug in all the concentrations and the measured rate to calculate the rate constant. It should be the same for all of your trials (if the temperature is constant), so you can average it to average out random errors. Once you have the rate constant, you can predict the rate (and the reaction time) for any set of concentrations. We will have a contest to see who can get their reaction to end after exactly 2 minutes (120 s), or closest to that time.

The contest will be done bench by bench. Once everyone has calculated the concentrations they want to use, and has prepared their reaction mixture (minus the iodine), you can ask for the contest to start. Your instructor starts a countdown, and you will add the iodine to the flask when the timer starts. Then, you will transfer your reaction mixture to a large test tube and place it in a test tube rack in the front. A doc camera will project the image of the tubes on the screen, so everyone in the room can see. Whenever a test tube turns colorless, we will record the time. The reaction completing closest to 120 s wins our appreciation for good calculations and good experimental technique (with a bit of luck).