Demonstrations
Six drops of glycerin are placed on top of a mound of potassium permanganate. The manganese is in a high oxidation state (oxidation state will be discussed in Chapter 18) and therefore can easily be reduced. When the glycerine is first added no reaction is observed. The reaction begins slowly and heat is generated as the reaction proceeds. After about 20 seconds "smoke" (that is, water vapor) is generated, the heat builds up, and the rate of the reaction increases. When the rate becomes fast enough, the reaction proceeds very violently and essentially all of the glycerine is converted to carbon dioxide and water vapor. The permanganate ion is reduced to manganese oxide.
A strip of copper is placed in a freezer bag with a zip-lock closure. Concentrated nitric acid is added to the opposite corner of the bag and the bag is closed. The bag is then tipped so that the nitric acid and copper mix. The reaction produces the red-brown gas, nitrogen dioxide, which is in equilibrium with the colorless gas, dinitrogen tetraoxide. A sample of the mixture is withdrawn from the bag by syringe, the end of the syringe is then sealed, and the mixture is placed alternately in cold and hot water baths. That the equilibrium is shifted to the tetraoxide when the syringe is placed in ice-water can be seen in the change in color of the gas (notice that the volume also decreases as expected). When the mixture is heated more nitrogen dioxide is produced and the gas becomes darker. That the process is reversible can be demonstrated by repeated heating and cooling.
When cobalt (II) chloride is dissolved in water the cobalt ion is hydrated; that is, forms an adduct with water which we will discuss in Chapter 17. The hydrated (hexaaqua) ion in solution is pink. When chloride ion is added (from a hydrochloric acid solution), the chloride ion displaces the water molecules from the cobalt and the tetrachloro complex is formed. This complex is blue. The equilibrium between the hexaaqua complex and the tetrachloro complex can be shifted from one side to another by adding either chloride ion or water. The equilibrium can also be shifted by heating or cooling the solution.