Physical Science Institute

Summer 2013

The Positives and Negatives of Gas

MATERIALS

(1) Arbor Scientific electrolysis kit: 1 plastic container w/ electrodes, 2 graduated test tubes, 1 plastic test tube holder

(2) connector cords w/ alligator clips, black & red 12”

(1) bottle, 8oz w/ flipcap, boston round with 5% sodium hydroxide solution

(1) battery, 9-volt

(~5) wood splints

(1) book of matches

PROCEDURE

  1. GOGGLES AND APRONS MUST BE WORN AT ALL TIMES DURING THIS ACTIVITY.
  2. Pour the sodium hydroxide solution into the electrolysis water bath up to about 1 cm above the tips of the electrodes.
  3. Grip a test tube with the tweezers and immerse the tube in the solution horizontally and fill it completely with solution.
  4. Place the tube over an electrode without lifting the mouth above the level of liquid. The mouth should be down and the tube filled with solution.
  5. Repeat for the other tube and the other electrode.
  6. After both tubes are placed over the electrodes, connect each of the metal electrodes protruding beneath the bath to a terminal of a battery. Connect one electrode to the positive (+) terminal and the other to the negative (-) terminal.
  7. Keep the battery connected to the electrodes until you have collected 10 mL of gas in one of the test tubes.
  8. Record the volume of gas—to the nearest 0.1-mL—in each test tube after you have disconnected the battery

- electrode = ______mL+ electrode = ______mL

Questions

1) Describe what is happening inside the test tube connected to the positive electrode?

2) Describe what is happening inside the test tube connected to the negative electrode?

3) Predict what gas is being produced at the positive electrode? The negative electrode?

  1. Test the gas in the test tube with the negative electrode. Carefully lift the test tube off the electrode without letting the open-end of the test tube come out of the water. Slip your finger over the opening and lift the test tube out of the water, keeping the open-end pointing down.
  2. Have your partner light a wooden splint; quickly invert the test tube and place the flame at the opening of the test tube. You should hear a “pop” sound. This is the test for hydrogen gas.
  3. Use the same procedure for testing the gas in the test tube with the positive electrode, except instead of a flaming splint, have your partner blow out the flame on the splint after it is lit, and put the glowing end of the splint in the test tube. You should see the glowing splint relight due to the presence of oxygen gas.

Questions

4) How did the results compare to your earlier prediction in Question 3 about what gas was being produced at each electrode?

5) Predict what will happen if you switch the positive and negative leads to the electrolysis device. Set up your electrolysis device as before and try it!

6) Calculate the ratio of the volume of hydrogen gas produced at the negative electrode to the volume of oxygen gas at the positive electrode. Express this ratio as a whole number:

______mL hydrogen/ ______mL oxygen = ______mL hydrogen/1 mL of oxygen

7) In this experiment, you used electricity to produce hydrogen and oxygen gas from the conducting solution. Where did the hydrogen and oxygen gas come from?

8) a) What do you think the ratio of hydrogen molecules produced to oxygen molecules produced is when the conducting solution has electricity passed through it?

b) How does your answer to 8a) compare to the volume ratio you calculated in question 6?

9) If you had 1 liter of hydrogen gas and 1 liter of oxygen gas at the same temperature and pressure, would you have more hydrogen molecules, oxygen molecules, or the same number of oxygen and hydrogen molecules? Explain.

10) If liquid hydrogen bromide—which has the chemical formula HBr—was used instead of water in the electrolysis apparatus, which diagram below best predicts the results you would expect if hydrogen and bromine gas are produced?

11) Chemists have measured the amount of energy needed to decompose 1 gram of water into hydrogen and oxygen gas. The energy required is 3800 calories.

a) Where did the energy needed to decompose the water in your electrolysis apparatus come from?

b) Suppose you mix, and react, the oxygen and hydrogen gas produced by electrolysis to produce 1 gram of water; what amount of energy, and in what forms, would be produced from this explosive reaction to produce water? What amount of energy would be produced if 2 grams of water were formed by the reaction of hydrogen with oxygen?

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