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Reactions With Water

Sodium metal reacts so strongly with water that it reacts with the water vapor in the air. The metal is stored submerged in kerosine, where it has no contact with moisture.

I take a piece of sodium metal the size of a large grain of sand, handled with tongs or tweezers to prevent burns on the fingers, and drop it on the surface of a cup of water. The sodium skims about on the surface of the water, chased by a wisp of blue flame.

Sodium has 11 protons, 12 neutrons, and 11 electrons. The second layer of electrons is crowded with 8 electrons. The eleventh electron can't fit in the second layer. It is loosely bound to the atom. In a piece of sodium metal, the outer electron of each atom seems to belong to no atom in particular. The weakness of the attraction between the electron and the rest of the atom is what makes sodium so active chemically.

When the sodium metal is in contact with water, a sodium atom without its outer electron is pulled into the water. It is attracted to the negative side of a water molecule. The attraction is not terribly strong, but it is somewhat stronger than the attraction between the sodium atom and its outermost electron. The deciding factor is the weakness of the weaker attraction, rather than the strength of the stronger attraction. The net result is that the atom of sodium, lacking one electron, gains very much kinetic energy on the way to the water molecule.

When an atom is short one or two electrons, it is a positive ion. In water, the sodium ion is surrounded by four water molecules. Each water molecule has its negative side toward the sodium ion. The attraction of water for sodium ion is partly due to the fact that it is the attraction of four molecules at a time. The group of four water molecules and one sodium ion migrate together through the body of water.

The sodium metal, having lost a positive charge, has an excess electron. The metal retains a negative charge. After the departure of several positve ions, the metal has many excess electrons. The negative charge of the metal attracts positive charges that are in the water.

A water molecule that is in contact with the metal loses a proton. The proton attaches itself to the surface of the metal until an electron joins the proton. The proton-electron combination leaves the metal surface as a hydrogen atom. A second hydrogen atom soon combines with the first as a hydrogen molecule.

In the meantime there is the remainder of a water molecule, H-O, which was deserted by a proton. The reaction is:

H-O-H -------> H-O - + H +

Most of the hydroxide ions and the positive sodium ions remain in the ionized condition in the water. The attraction of water molecules for H-O and Na is stronger than the attraction that Na and H-O have for each other. Even though the H-O and Na are not combined, the solution is called sodium hydroxide solution. Solid sodim hydroxide remains if the water is boiled away.

I return to the hydrogen atoms. The attraction between hydrogen atoms is very strong. All along, the sodium metal floats on the water because it is less dense than water. The escapng hydrogen is immediately in contact with the oxygen of the air, All it takes to start the reaction between the hydrogen and the oxygen is one free hydrogen atom. The heat that the reaction yields and the presence of free atoms ensures the further reaction between hydrogen and oxygen, That accounts for the flame. The last reaction is:

H-H + H-H + O=O -------> H-O-H + H-O-H

Sodium is not the only metal that reacts with water. Lithium is similar to sodium. Lithium reacts less violently and produces no flame. Potassium, rubidium, and cesium also react with water, but the reactions are explosive. Each of these three elements has a single electron in its outer layer. These atoms are larger than sodium atoms. The electron is so far from the nucleus that it is weakly held. Therefore the atom is extremely active chemically.

Calcium, strontium, barium, and radium, each have two outermost electrons. They are held more strongly than the single outermost electron, because the nuclei have an additional proton. Calcium and the others react with water, but not violently.

Ca + 2H2O -----> Ca(OH)2 + H2

Potassium has 19 protons and one outermost electron. Calcium has 20 protons and two outermost electrons. The extra proton in the calcium nucleus pulls all of the electrons in closer. The outermost two electrons of calcium are closer to the nucleus than the single outermost electron is to the nucleus of potassium.

I perform an experiment with crystals of potassium hydroxide, a beaker of water, a thermometer, and a heater. I add potassium hydroxide to the beaker of water. The temperature goes down slightly. In order to maintain the solution at 298K, I turn on the heater briefly. The reaction is:

KOH + H2O -----> K+ HO- + H2O

All that happens is that the ions that are in the crystal separate and become surrounded by water molecules. I could not melt the potassium hydroxide at 298K. Potassium ions and hydroxide ions attract each other very well, The presence of water is necessary for the separation of the ions. The addition of heat is necessary because the attraction of the ions for each other in the crystal state is stronger than the attraction of water for the ions. The most enegetic of the surface ions leave the crystal. That lowers the temperature of the crystal. The departing ions carry potential energy in the crystal-ion system. Their energy in the ion-water system is slightly less. Some of the dissolved ions return to the crystal, but most of the ions get lost in the body of water, and don't return readily. When the ratio of the number of potassium ions to water molecules is 210/555, any additional crystals do not dissolve.

I turn on the heater to raise the temperature to 363K. The extra crystals dissolve. I find that the limit of the solubility at 363K is 301 potassium ions to 555 molecules of water. The higher temperature raises the rate at which ions break away from the crystal.



The crystals stop dissolving when there are so many ions in the water, that ions return to the crystal as fast as ions leave the crystal.

I can let the solution cool from 363K to 298K. As the temperature falls, the crystals grow larger until there are 210 potassium ions per 555 molecules of water.

I try the same experiment, this time with calcium hydroxide and water. At 298K, the water holds only 15 calcium ions per 55500 molecules of water. This time the temperature rises slightly as the crystals dissolve. I turn the heater up to 363K. Instead of dissolving, the crystals grow. The solution holds only 8 calcium ions per 55500 molecules of water at 363K.

The crystal of calcium hydroxide is an array of molecules, not ions. The water attracts Ca(OH)2 molecules, but not as strongly as the crystal attracts additional Ca(OH)2 molecules. The molecules enter the water at a slow rate. Once the Ca(OH)2 molecule is in the water, the attraction of the water for ions breaks up the molecule into two HO- ions and one Ca+ ion. The rush of the ions toward water molecules increases the kinetic energy of the ions, raising the temperature. The association of the ion with a group of water molecules becomes stable when the heat is transferred to the surroundings.

When the heater raises the temperature to 363K, the ions vibrate and separate from their positions among the water molecules. This gives the ions a chance to reassemble as Ca(OH)2 molecules and to crystalize.


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