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Reduction of H+ by Zn: 
Gas Experiment 1

Introduction

This experiment demonstrates a number of ideas in chemistry. First we see a single-replacement oxidation-reduction (redox) reaction. We witness the evacuation of a gas from a solution. Upon reaction of that gas, we witness an addition/combination reaction, also a redox reaction.

Zinc metal (Zn) and hydrochloric acid (HCl) are both easily obtainable in almost any laboratory and react vigorously when combined. Zinc is generally available in two forms: mossy zinc, which appears as chunks (it is this form that we use in our photos), and a sheet form, like the kind used for making most zinc materials. A powder is also found, but is somewhat harder to work with than the other forms.

WARNING AND DISCLAIMER

Zinc is reasonably harmless, but don't eat it or cut yourself on zinc chips. Hydrochloric acid usually comes from chemical companies in very high molarity solutions (i.e. 12M). We do not use stock HCl, and we recommend that inexperienced experimenters obtain the diluted solution from a lab technician or teacher, rather than preparing it by hand. The 6M solution called for in the procedure is less of a hazard than the fuming 12M solution, but will damage tissue if used carelessly. The 6M solution provides a rapid reaction; more dilute solutions may be used if required. When working with it, use goggles (and gloves, if desired). If a solution must be mixed, always add acid to water, never water to acid.

The International Order of Nitrogen, its officers, and/or its members are not responsible for your actions.  Carry this out at your own risk.

Read the entire procedure TWICE before starting the experiment!

Reactions In Summary

  1. 2HCl (aq) + Zn (s) --> ZnCl2 (aq) + H2 (g)
  2. 2H2 (g) + O2 (g) --> 2H2O (l)

Facilities

To do this experiment safely, you will need to have ready access to the following:

  • Water supply
  • Eye wash station
  • Emergency shower

Supplies and Equipment

The following are listed in the order they will be used.

  • Ring stand with test tube holder
  • One large glass culture/test tube (see photos below for example)
  • Zinc (about 10 g)
  • 10 mL 6M hydrochloric acid
  • One stopper with curved tube fitting (see photos below for example)
  • One small glass culture/test tube (see photos below for example)
  • Test tube "gripper"
  • Matches and thin wooden "tongue depressor"

Procedure

Before going any further, be sure to have read the warning and disclaimer.

Use goggles and gloves (if desired) during this experiment.

  1. Set up the ring stand and test tube holder on a lab bench near a water supply.
  2. Mount the large test tube in the holder.
  3. Add the zinc to the mounted test tube.
  4. Carefully add the hydrochloric acid to the test tube.
  5. Quickly but carefully insert the stopper assembly into the large test tube. 
  6. Angle the test tube with the HCl and Zn so that the tube fitting points somewhat upward (as shown in the photos below).
  7. Place the small test tube over the tube, so that the gas accumulates in the tube. Viewing the photos may be helpful in order to observe proper assembly.
  8. Record observations.
  9. Once the bubbling in the large tube has begun to subside, carefully remove the small test tube with the "gripper," being sure to hold the tube upside down.
  10. Hold an ignited "tongue depressor" to the bottom of the test tube (with the tube slightly angled).
  11. Record observations. If nothing is observed, repeat experiment.

Photos and Movies

The following are thumbnails of digital photos of this reaction being performed. Click on them to open a full size photo. Thanks to John Thompson of Crescent Valley High School for the use of his lab.

Adding HCl to a beaker in preparation for a transfer to a graduated cylinder
Adding HCl to a beaker in preparation for a transfer to a graduated cylinder		 Adding zinc metal to the test tube
Adding zinc metal to the test tube
The compounds reacting
The compounds reacting		 Collecting hydrogen gas
Collecting hydrogen gas
Igniting the hydrogen gas
Igniting the hydrogen gas		 No Photo Here
  • To view a slow-motion video of the acid being added to the zinc (about 692 KB), click here. The video is an animated GIF image.
  • To view a slow-motion video of the reaction occurring vigorously (about 641 KB), click here. The video is an animated GIF image.

What's Happening

The hydrochloric acid, by definition, consists mainly of dissolved H+ (actually H3O+) and Cl- ions. Zinc is a stronger reducing agent than hydrogen (it is above hydrogen in the activity series of the metals), and thus tends to reduce (give electrons to) the hydrogen and force it out of solution. Zinc chloride is thus left in solution in the test tube. The hydrogen gas is collected in the second test tube and, when heated sufficiently, reacts with oxygen to form water. In this second reaction, the oxygen is actually oxidizing (taking electrons from) the hydrogen; this is where the term "oxidation" comes from.

Questions

If you fully understand what's happening in this reaction, see if you can answer the following questions for yourself:

  1. What are the ionic and half-cell equations for these reactions?
  2. Why does a higher molarity acid speed the initial reaction?
  3. If, instead of air, pure oxygen was used for the second reaction, what would you predict?
  4. What should the pH of the zinc chloride solution be, approximately?
  5. How good a conductor should the zinc be? How about the HCl? And the ZnCl2?

Author: C. Shultz

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