Within Chemistry

  1. The mole concept was devised to explain (predict) the mass relationships involved in chemical reactions.
  2. Whole fields of study in chemistry are based on a single class of reaction. Acid-base and redox are two prime examples (see these modules as well as the Solubility and Precipitation module).

Between Chemistry and Other Disciplines

  1. The precipitate Co 3 (PO 4 ) 2 prepared in Laboratory Activity 1 is used as a ceramics coloring agent.
  2. Photosynthesis (a process students may have encountered in biology) is an example of a combination or synthesis reaction. It is considerably more complex than reactions studied in this module, but the result of a series of reactions is that carbon dioxide and water combine to form glucose, a sugar.
  3. Geology (see Rocks, Minerals, and Gems module)
  4. Biochemistry (see Enzymes module)

To the Contemporary World

  1. The remnants of the space shuttle Challenger were under the sea for several weeks before being recovered. This caused the tapes carrying data and conversations of the crew to become covered with a foam-like cement of Mg(OH)2 . The tapes had been stored in a magnesium casing that reacted with seawater essentially as

    Mg(s) + 2H 2 O(l) ---> H 2 (g) + Mg(OH)2 (s)

    catalyzed by salts in the water. The tapes were restored to a readable form by a painstaking removal of the cement layer using the double replacement reaction:

    Mg(OH)2 (s) + 2HNO 3 (aq) ---> 2H 2 O(l) + Mg(NO 3 ) 2 (aq)

  2. Baking powder is added to doughs to generate the gas CO 2 inside the pastry to help it "fluff up." The gas comes from the reaction of sodium bicarbonate with an acid salt such as potassium acid tartrate.

    NaHCO 3 (s) + KHC 4 H 4 O 6 (s) ---> NaKC 4 H 4 O 6 (s) + H 2 O(l) + CO 2 (g)

  3. Cooking an egg can also be viewed as a decomposition reaction, because the primary event is the breaking of chemical bonds. The white part of the egg is the protein, albumin. In its natural state, this very long chain of linked amino acids wraps around itself in a prescribed way. The prescribed wrapping is held in place by chemical bonds between adjacent parts of the chain. Heating breaks these "cross-linking" bonds, the molecular chain unwraps, stretches out, and then rebonds randomly to other unwrapped protein molecules. This whole process is seen as the clear, slimy liquid turning into a white pliable solid. Biochemists call it denaturation.

  4. Rusting of iron and steel is a chemical reaction readily recognized by everyone. Overall, it can be classified as a synthesis reaction.

    4Fe(s) + 3O 2 (g) + 6H 2 O(l)---> 2Fe 2 O 3 . 3H 2 O(s)

    The product, rust, is a hydrated form of iron(III) oxide. The actual sequence of reactions that yields this overall change is quite complex, and not yet completely understood. An interesting characteristic is that the process is catalyzed by salt. One possible explanation of most of the data is as follows. The metallic iron in contact with water is oxidized

    Fe---> Fe 2+ + 2 e -

    These electrons then reduce O 2 dissolved in the water

    1 / 2 O 2 + 2H + + 2 e - ---> H 2 O

    The acid needed for this comes from CO 2 dissolved in the water, forming an acidic solution. Concurrently, dissolved O 2 combines with the Fe 2+ ions as

    1 / 2 O 2 + 2Fe 2+ + 5H 2 O ---> Fe 2 O 3 . 3H 2 O + 4H +

  5. This last compound is rust, a hydrated form of iron(III) oxide. Essentially the oxygen atom oxidized Fe 2+ ions to Fe 3+ ions. An interesting feature of the process is that these three reactions (two of them redox half-reactions) can each occur at a different spot on the metal surface. Thus corrosion (the first reaction) can be seen in one spot while rust formation (the third reaction) can be seen at another. It is believed that salt water catalyzes the process by helping to transport Fe 2+ ions and electrons to different points along the metal surface.

  6. Formation of stalagmites and stalactites in Solubility and Precipitation module.

  7. Space scientists suggest the removal of carbon dioxide from the cabin atmosphere of a space mission by absorption in sodium hydroxide. Since the average human body breathes out about 925 g of carbon dioxide per day, for a six-passenger, 10-day expedition into space, 101 kg of sodium hydroxide would be required.

    CO 2 (g) + 2NaOH(s) ---> Na 2 CO 3 (aq) + H 2 O(l)

  8. If astronauts drank an average volume of 2 L of water a day, they would eliminate 2.4 L of water, with the additional amount coming from food metabolism. Scientists suggest that astronauts use lithium oxide (because of its low molar mass) to absorb the water. For the expedition hypothesized above, 240 kg of Li 2 O would be needed.

    Li 2 O(s) + H 2 O ---> 2LiOH(aq)

    You might try carrying through the stoichiometric calculations.

  9. Fermentation, a not so simple chemical reaction, is the basis of the brewing and baking industries.