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Chemical History of the Candle

Lecture IV: Analyzing the Products of Combustion

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Web Lecture
Lab: Hydrogen

Lecture IV: Products: Hydrogen in the Candle

Contents of this webpage

This lecture looks at the interesting chemical properties of hydrogen. Faraday starts with the ability of electrical current passing through water to separate hydrogen and oxygen into individual gases with properties very different from those they shared when bound together in water molecules. Together with his mentor , Sir Humphrey Davy, Faraday pioneered the use of electrolysis to separate molecules into their constituent atoms, thereby discovering a number of new elements.

If it seems like we are getting far from the candle--remember that Faraday is using it as a jumping off point to discuss many aspects of chemistry and physics!

For more information on Faraday himself...

You may want to check your library for Harry Sootin's Michael Faraday: From Errand Boy to Master Physicist (Julian Messner 1954). This book was written for young adult readers, and concentrates on Faraday's early career.

The video series The Mechanical Universe spends episode #29 discussing the concept of the electric field, which Faraday first described using lines of electrical force. The video, which is about 30 minutes long, has scenes set at the Royal Institution where Faraday worked and shows some of his original apparatus.

Lecture outline

  1. Review of previous observations on hydrogen
    1. Burning hydrogen gas to make water
    2. Capturing gas from steam passing over hot iron fragments (see Lecture III)
  2. Experiments with the voltaic cell (zinc/acid battery--similar to our car batteries)
    1. Preparation of copper nitrate
    2. Current through lead acetate (the salt mentioned on p. 76)
      1. Testing the battery
      2. Connecting the battery in a complete circuit to the tin foil
      3. Extracting lead oxide (brown powder) from lead acetate with current
    3. Current through copper nitrate
      1. One platinum rod in solution at a time: incomplete circuit, nothing happens
      2. Both rods into the solution--the liquid acts as a conductor, and one of the rods is plated with copper.
      3. Reversing the connections of the rods cleans the copper plate off the one and puts it on the other.
    4. Current through water: first experiment (p. 79)
      1. Description of apparatus: ends of wires from battery in water, gas given off by water is captured in jar.
      2. Identifying the gas
        1. It isn't steam, because it doesn't condense back to water when it cools
        2. It isn't hydrogen, which burns explosively -- it "pops" when tested with a small flame
        3. It does burn, without access to the air
        4. The gas is isolated by exhausting everything above the water in the collector jar into a vacuum chamber (p. 83) and a spark is passed through the gas.
          1. There is an explosion
          2. Water now coats the inside of the container
          3. Refastening the container to the collector shows that the vacuum chamber has been partially drained of gas by the formation of water on its sides.
        5. Water has apparently been split into components and reassembled
    5. Current through water: second experiment (p. 85)
      1. Apparatus now has two collector cylinders, each with one of the wires from the battery inside.
      2. Gases formed by each terminus are collected separately.
      3. Identification of gas in first collector
        1. Burns with pale flame
        2. Remains in jar when open jar is inverted (is lighter than atmosphere)
        3. Gas is therefore hydrogen
      4. Identification of gas in second collector
        1. Gas itself does not burn!
        2. Lighted and doused splint reignites and burns more brightly than in air alone when placed in gas
        3. Gas it therefore oxygen!
      5. Summary of results
        1. Twice as much hydrogen as oxygen gas obtained from the electrolysis of water--BY VOLUME, oxygen:hydrogen from water is 1:2
        2. Mass of oxygen is (88.9 ) compared to mass of hydrogen (11.1)--BY MASS, oxygen:hydrogen from water is 8:1.
    6. Oxygen from manganese oxide: third experiment (p. 88)
      1. Manganese oxide heated with potassium chloride produces gas in the collector cylinder
      2. Identification of gas
        1. Does not dissolve in water
        2. Transparent
        3. Lighted taper burns more brightly in gas than in air alone
        4. Gas is heavy: it is not hydrogen but oxygen
          1. Comparison of weights of hydrogen and oxygen
        5. Burning pure oxygen still only produces water, just like candle in air
    7. Ability of oxygen to sustain and increase combustion
      1. Brightness of lamp in oxygen compared to air
      2. Burning wood and iron in oxygen compared to air
      3. Burning sulfur in oxygen
      4. Burning phosphorus in oxygen
    8. Oxygen and hydrogen recombining: fourth major experiment (p. 95)
      1. Use mixture of both gases, 2 parts hydrogen to 1 part water
      2. Release mixture in soap bubble "containers"
      3. Ignite bubble, which pops and forms water
  3. Summary
    1. Potassium (chemical symbol K) burns readily in oxygen, and will extract oxygen from water (and even ice) in order to burn, releasing hydrogen:

      K + 2 H2O --> KO2 + 2H2

      li>Understanding the laws of combustion and the function of oxygen allows us to burn fuels safely.

Reading notes

voltaic battery:
Faraday is using the type of battery invented by Allesandro Volta. If you want to know more about Volta's work, check out Duracell's website on the history of the battery , which also has some information about Davy's experiments.
"the wire became red-hot" (p. 77):
Faraday has shorted the battery through his wire. You can do the same thing using one of the little battery-tester strips to connect one end of an AA battery to the other. A heat-sensitive chemical will change color (just like the flat forehead thermometers) as current flows through and heats the strip. Don't leave the tester connected too long, though, oryou'll drain the battery!
2 parts hydrogen to one part oxygenFaraday's experiment here demonstrates the work of many of his fellow chemists to determine the proportions of elements in different substances. It demonstrates Dalton's law that all substances have a constant composition in which the elements combine in whole number ratios--a phenomenon we now explain by saying that whole atoms of an element combine with whole atoms of other elements to form molecular compounds.

Discussion points

  • What tests does Faraday use to identify the different gases he produces?
  • What conclusions does he draw from his experiments about the role of oxygen and hydrogen in combustion?