Can You Change the Rate of a Chemical Reaction by Changing the Particle Size of the Reactants?

1. Wear safety goggles for this step.
2. The hole should be slightly smaller than the outer diameter of the aquarium tubing to assure an air-tight fit.
3. Mark the location for the hole in the center of the bottle cap. Use a center punch or tap a nail with a hammer to make a small depression at the center location so the drill bit won't slip.
4. Carefully drill the hole in the bottle cap. It is easiest to do this with the cap screwed in place on the empty bottle. Have your helper hold the bottle straight.

1. Fill your plastic dishpan (or bucket) about one-third full with water.
2. Fill the graduated cylinder with water.
   1. If your dishpan is deep enough, fill the graduated cylinder by tipping it on its side inside the dishpan. Allow any bubbles to escape by tilting the cylinder up slightly, while keeping it under water. Keeping the opening of the cylinder under water, turn it upside down and attach it to the side of the dishpan with masking tape (or have your helper hold it in place).
   2. If your dishpan is not deep enough, fill the graduated cylinder completely using the faucet and cover the top tightly with plastic wrap. Quickly invert the cylinder and place the opening in the dishpan, beneath the surface of the water. Remove the plastic wrap. Attach the cylinder to the side of the tub with packing tape (or have your helper hold it in place).
3. The graduated cylinder should now be upside down, full of water and with its opening under the surface of the water in the dishpan. Place the free end of the tubing from the plastic bottle inside the graduated cylinder. Your apparatus is now ready to trap carbon dioxide from the Alka-Seltzer® chemical reaction (see illustration below).

   Schematic diagram of inverted graduated cylinder gas collection apparatus.

4. You can test your gas collection apparatus by blowing gently into the tube. The bubbles you create should be captured inside the cylinder. (You'll need to re-fill the cylinder before starting your experiment.)

Running the Experiment

1. In this experiment, you will be measuring the reaction rate for the production of carbon dioxide gas from a single Alka-Seltzer® tablet.
   1. You will measure the volume of gas produced at 10-second time intervals after the reaction begins.
   2. You will investigate how the reaction rate changes as you vary the particle size of the reactants.
2. You'll use the same plastic bottle for repeated trials, so it is convenient to mark the desired water level.
   1. The actual volume of water used is not critical, as long as it is at least 240 ml (8 oz.).
   2. The smaller the air space in the bottle, the sooner you will be able to start collecting carbon dioxide gas.
   3. You do want to use the same amount of water for each trial. Use a piece of masking tape on the outside of the bottle to mark the water level. Place the tape with its top edge even with the water level in the bottle.
3. For measuring the reaction rate, you will use the same volume of water at the same starting temperature. You will use four different particle sizes for the Alka-Seltzer® tablets:
   1. A whole tablet
   2. A tablet broken in half
   3. A tablet broken in quarters
   4. A tablet ground into powder. To do this, fold a single tablet to be ground inside a clean piece of paper. Place the folded paper on a piece of scrap wood, and use the hammer to firmly pound the tablet about ten times. Stop immediately if the paper shows signs of tearing: you don't want to lose any of the powder. You can also use the back of a metal spoon or ice cream scoop to carefully crush your tablet into a powder once it is wrapped in paper if you do not want to use a hammer and piece of wood. Crushing the tablet this way may take a little more time.
4. Here is how to measure the reaction rate:
   1. Fill the glass bottle water to the level of the masking tape.
   2. Measure the temperature of the water, and record it in your lab notebook.
   3. Remove the thermometer.
   4. Have your helper get ready with the stop watch, while you get ready with an Alka-Seltzer® tablet. Hold the tablet in one hand and the bottle cap ((with tubing attached) in the other hand.
   5. Have your helper count one–two–three. On three, the helper starts timing and you drop the tablet (or tablet pieces) into the water.
   6. Quickly cap the bottle tightly. You'll immediately see bubbles of CO₂ streaming out from the tablet.
   7. Using the hand that you don't use for writing, swirl the bottle gently, keeping the bottom of the bottle flat on the table top.
   8. Every ten seconds, your helper should call out "Time!" You should immediately read the carbon dioxide volume (in the syringe or graduated cylinder) and write it down in your lab notebook. Prepare a table like the one below to keep your data organized.

      Particle Size (whole/halves/quarters/powder)	Trial #	Volume of CO2 after reaction begins (times in seconds)
      		10	20	30	40	50	60	70	80	90	100	110	120

   9. Continue recording the volume of gas at ten-second intervals until the volume is no longer changing. At this point, the reaction is complete.
   10. Tip: be careful when opening the packets and handling the Alka-Seltzer® tablets. The tablets are thin and brittle, so they break easily. Remember that you need to have three whole tablets for this experiment.
5. For each of the four particle sizes, you should repeat the experiment three times, for a total of 12 trials.

Analyzing Your Data

1. For each particle size, calculate the average volume of gas at each time point for the four trials (see the table below)

   Particle Size (whole/halves/quarters/powder)	Trial #	Volume of CO2 after reaction begins (times in seconds)
   		10	20	30	40	50	60	70	80	90	100	110	120
   whole tablet	1
   whole tablet	2
   whole tablet	3
   Average	—

2. Make a graph of the volume of CO₂, in mL, (y-axis) vs. time after the reaction begins, in seconds (x-axis).
   1. You can include the data from all four particle sizes on one graph.
   2. Use a different symbol and color for each particle size.
   3. Remember to include a legend that identifies the particle size associated with each symbol.
3. More advanced students should also calculate the standard deviation of the reaction times for each particle size.
   1. Use the standard deviation to add error bars to your graph.
   2. For example, say that the average volume for two half-tablets 30 seconds after the reaction began was 45 mL, and the standard deviation was 5.2 mL (these are made-up numbers). You would graph the symbol for the data point at 45 mL, and then draw short vertical bars above and below the symbol. Each vertical bar would have a length equivalent to 5.2 mL.
   3. Error bars give your audience a measure of the variance in your data.
4. How does reaction rate change with particle size?

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