AN INEXPENSIVE KINETICS EXPERIMENT
This experiment illustrates the effect of concentration on reaction rate.
This experiment is appropriate for a general or first-year college-prep course. The students study the effect of the concentrations of NaOH and HCl solutions on the rate of the reaction of each solution with aluminum.
One lab period.
Concentrated solutions of NaOH and HCl will burn skin and damage clothing. Fumes from acid may be caustic and/or irritating; carry out the reactions in a well-ventilated room (hood, if available). Do not mix the NaOH and HCl solutions; splattering may occur. Some splattering may occur near the end of the reaction of the NaOH or HCl with the aluminum; use care. Wipe up any splatters with wet paper towels. Goggles must be worn throughout the experiment.
- saturated NaOH solution (about 13 g NaOH/25 mL distilled or deionized water) in small dropper bottle*
- 6 M HCl solution (500 mL concentrated HCl solution diluted to 1.00 L with distilled or deionized water) in small dropper bottle*
- 2.5-cm square pieces of aluminum foil
- test tubes
- test tube support
- clock which can measure seconds
- NaOH is available in grocery stores lye.
- HCl solution is available from a hardware store as muriatic acid, 28% HCl. This solution is approximately 8 M and may be substituted for 6 M HCl solution.
Holding pieces of aluminum foil with forceps, rinse with water, wrap in paper towel and place in solid waste. Solutions of NaOH and HCl may be flushed down drain with water.
Drano and other solid drain cleaners usually consist of lye, NaOH, and a small amount of metallic aluminum. The aluminum reacts with the lye solution to form hydrogen gas. The bubbling of the gas provides a physical action that helps unclog a drain. The reaction is:
- Place a test tube in a firm support. Gently lay a small piece of foil across the test tube and with the bottom of a similar-sized test tube force the foil into the tube to form a basin-like depression.
- Have a timing device ready. Carefully add 10 drops of the concentrated NaOH solution to the depression in the foil. Begin timing when the first drop hits the foil.
- End the timing when there is evidence that a hole has formed in the foil. The solution may simply drop into the tube or a small hole may form with a drop of solution visible below the hole.
- With forceps, carefully pick up the foil and rinse with water. Wrap in paper towel and place in trash can.
- Repeat using a new piece of foil and test tube, but in this trial, place 2 drops of water on the foil and then add 8 drops of the NaOH solution. Start timing when the first drop of NaOH solution is added.
- Continue using new pieces of foil and new test tubes with various dilutions of NaOH as listed in the table below.
- Repeat the experiment using HCl solution in place of the NaOH solution.
- Prepare graphs of time (or rate which can be considered as 1/time) on the vertical axis and number of drops of NaOH or HCl solution on the horizontal axis.
- Describe the effect of the concentrations of NaOH and HCl on the rate of the reaction. Suggest a reason for why the two graphs might differ.
2 Al(s) + 2 OH-(aq) + 6 H2O(l) 2 Al(OH)4-(aq) + 3 H2(g)
Aluminum also reacts with HCl solution to form hydrogen gas according to the following equation.
2 Al(s) + 6 H+(aq) 2 Al3+(aq) + 3 H2(g)
The effects of concentration for NaOH and HCl are different because of differences in the activities of the two solutions.
This experiment might be a good student project. An advanced student might wish to calculate the activities of the two solutions or otherwise approach the experiment more quantitatively.
Submitted by Paul Hobe
Woodrow Wilson Leadership Program in Chemistry
The Woodrow Wilson National Fellowship Foundation
CN 5281, Princeton NJ 08543-5281