Chemistry 122, Spring, 1996


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Chemical Reactions Involving Gases

Perform and discuss these activities with your group but write your responses here in your own words. You must wear your goggles for this experiment.

1. Burning the candle at one end. One of the most common chemical reactions, and probably the first chemical reaction intentionally managed by early humans, is combustion (burning). A candle flame is a familiar example.

a. What do you suppose happens chemically when a candle burns? What happens to the wax when a candle burns? If you weighed a candle, then burned it and collected all the wax drippings and weighed the remains candle and its drippings, do you suppose the total weight afterwards would be greater than, the same as, or less that the initial weight? Why?

b. Light one of the small candles. Light a match, then blow out the candle, keeping the match lit. Then immediately bring the burning match close to the smoking candle wick and observe closely. At exactly what point in time did the candle flame re-ignite? Repeat this experiment with a cold candle that has not been recently burning. Notice the difference? How can you explain the behavior of the hot candle? What could exist in the apparently "empty" space between the hot wick and the match flame that could be capable of burning? (Hint:

c. A old demonstration in science classes, chemistry sets, etc, is the burning of a candle in an enclosed air space, typically under an inverted jar submerged in water, so that changes in the volume of trapped gas can be observed. What would you predict would happen to the volume of trapped gas during such an experiment?

d. Here is how to perform this experiment: light one of the small floating candles provided and float it carefully it in a beaker half-filled with water. Invert a large graduated cylinder over it and carefully observe the lever of water inside the graduated cylinder as the candle burns, until the candle burns out. Repeat once or twice to insure that your observations are reproducible.

What gas do you suppose are in the trapped volume of gas after the candle burns?

e. To help explain your observations, you need to know something about the chemistry of candle burning. Candle wax, or paraffin, is a mixture of large (high molecular weight) saturated hydrocarbons. Butane and octane, which you saw before, are examples of saturated hydrocarbons, but paraffin molecules is much longer, consisting mostly of long chains of -CH2- units. Almost all hydrocarbons are good fuels. When hydrocarbons are completely burned in air, in the presence of excess oxygen (O2), the products are carbon dioxide (CO2) and water (H2O). For example, methane (CH4) is the simplest hydrocarbon:

CH4 + 2O2 -> CO2 + 2H2O

This is called a balanced reaction. It means that, for every one molecule of CH4 , two molecules of O2 are required to burn it completely, producing a molecule of CO2 and two molecules of H2O.

Why is oxygen written as O2 and not just O?

Why would it not be just a satisfactory to write the reaction as

CH4 + O2 -> CO2 + H2O

which is even simpler?

f. If we approximate paraffin wax as just a collection of CH2 units, the balanced reaction for the complete combustion of one single -CH2- unit would be:

2CH2 + 3O2 -> 2CO2 + 2H2O

Assuming that each molecule of gas take of the same volume, what does this predict would happen to the total gas volume when solid paraffin wax reacts with gaseous oxygen in the air to produce carbon dioxide and water?

g. What gas volume change would you have predicted if you had used the unbalanced reaction

CH2 + O2 -> CO2 + H2O

as a model for the burning of wax?

h. Why do you suppose the trapped gas volume increases at first, then decreases at the end? Hint: water is a gas at temperatures above 100deg. C at atmospheric pressure but condenses to liquid water at lower temperatures.

2. Plop, plop, fizz, fizz. Alka Seltzer tablets are interesting from a chemical point of view because the package label not only tells what the tablets contain (the reactants) but what they become when added to water (the products). What product of the reaction is not mentioned by the package but is visible when Alka Seltzer tablets are added to water? Can you guess what the evolved gas bubbles might be?

Using words rather than the actual chemical formulae, write the reaction for Alka Seltzer tablets in water.

a. Place a small amount of water in a plastic cup and place on the pan of the electronic scale. Cover the top with a piece of paper or metal foil. Place two Alka Seltzer tablets on top of the cover, taking care that they don't drop in prematurely.

Record the initial mass in grams. _______________.

b. What do you predict will happen to the mass when the Alka Seltzer tablets are dropped into the water?
Prediction: Mass will increase, mass will decrease, or no significant change?

c. Tilt the cover so that the tablets drop into the water and immediately replace the cover so water droplets will not escape. Observe the mass reading every few seconds until it stops changing.

Record the final mass in grams. _______________.

d. Explain your observations in terms of the masses of the products and reactant of the reaction.

e. Repeat this same experiment with the cup enclosed in a sealed container. Place a small amount of water in a large 2-liter plastic "Coke" bottle. Break two Alka Seltzer tablets in pieces that will fit in the bottle. Weigh the bottle. bottle cap, and Alka Seltzer tablets together, then dump the Alka Seltzer tablet pieces into the bottle and quickly replace the bottle cap tightly and place back on the scale. Follow the displayed mass as the reaction proceeds.

Prediction: Mass will increase, mass will decrease, or no significant change?


Initial mass in grams. _______________.

Final mass in grams. _______________.

Why do these results differ from the experiment carried out in an open container?

f. What do your predict will happen if you loosen the bottle cap?

Try it. Remove the bottle for the scale, loosen the bottle cap (what happens when you do that?), and re-measure the bottle and its cap.

Mass in grams after loosening the bottle cap. _______________.

g. How does this experiment relate to the Law of Conservation of Mass?

h. Do gases have mass, or are they "massless"?

The course is sponsored by the Maryland Collaborative for Teacher Preparation