JOHN 8. DAVIS American International College, Springfield, Massachusetts
A
N U ~ E Rof years have elapsed since there has been any significant change in the formulation and the teaching of the basic concepts of chemistry. Moreover, what seems to be more serious, several of the textbooks that have passed through this laboratory during the past year seem to represent a tendency on the part of some textbook authors to revert to ancient and wornout expedients which have long since outlived their usefulness. Many new data have been obtained from many sources, and these new facts demand a decided revision of (1) certain chemical theories, and (2) equations representing certain reactions. One of the several reactions that needs reevaluation in the light of the modern theory of valence is the reaction between ammonia and water. Ammonia is commonly represented as reacting with water to form ammonium hydroxide, NH,OH. Now if a molecule is defined as a group of atoms held together by covalent bonds, it is immediately obvious that no such molecule as that represented by the formula, NH,OH, actually exists. The water molecule and the ammonia molecule are indeed attracted to each other, and this attraction is the result of the so-called hydrogen bond. The name, hydrogen bond, is misleading, since the force acting between the two molecules is not of the nature of a true chexical bond but can be described more accurately as a dipole-dipole attraction. It appears that when the hydrogen atom is bonded to a small, electronegative atom, the strong pull which the atom exerts on the bonding electrons leaves an effective positive charge on the hydrogen atom which is sufficient, because of the absence of shielding electrons, t o cause attraction for a second electronegative atom.' Hence the hydrogen atom bonded to the electronegative oxygen atom of the water molecule will attract the electronegative nitrogen atom of the ammonia molecule. Using a dotted line to represent the dipole-dipole, the combination of the two molerules is represented as follows:
H H:P*T: H
H
+ H:O: .. =H:N: .. . . . H:O: .. H
H
H
This bond is longer than the normal covalent bond and this fact seems t o preclude the possibility of resonance. The hydrogen bond is comparatively weak, having roughly 5 per cent the energy of a covalent bond. It follows that the energy of activation of such a bond is small and, consequently, the bond forms and breaks with ease a t ordinary temperatures. 1 MOELLER, T.,''Inorpmic Chemistry," John Wiley & Sons, h e . , New York, 1952, p. 180.
In a 0.1 molar solution of ammonia in water, the water molecules outnumber the ammonia molecules 550 to 1; and this would appear to be an ideal environment for the practically complete reaction of ammonia with water if any valence bonds of significant strength existed between these molecules. This reaction is far from complete, however, and, according to S i d g ~ i c k about ,~ 46 per cent of the ammonia remains free and unhydrated in such a solution. This unfavorable equilibrium shows beyond a doubt that the force of attraction between these two molecules is very weak, and the combination of these two molecules is unstable and tenuous. The well-known fact that ammonium hydroxide cannot be isolated by carefully eva~oratinpan aqueous solution is further proof for thi; conclusion. If any additional clarity were gained by the use of the fiction of the ammonium hvdroxide molecule. Derhaps its continued use might bk justified; But &is is not the case. The teacher can present the chemistry of ammonia more clearly, and at. the same time find himself in a much sounder position from the point of view of the theory of chemical valence, by simply representing the reaction between water and ammonia thus: H H:N: H
H
+ H : 6..: = H : N : H + + :o:H.. H
H
and t,he equilibrium constant for the reaction by the following equation:
To summarize: (1) There is no true chemical bond acting between ammonia molecules and water molecules. (2) These molecules are attracted to each other by a dipole-dipole force which is roughly 5 per cent of the strength of a covalent bond. (3) Even under favorable conditions only about -half of the ammonia molecules are hydrated in aqueous solution. (4) The force between the water molecules and the ammonia molecules is so weak that the hydrate cannot he isolated by the most careful evaporation of the aqueous solution. (5) Nothing is gained in clarity or understanding by continuing the fiction of the reality of the ammonium hydroxide molecule. 2 SII~WICK, N. V., "Chemicd Elements and Their Compounds," Oxford University Press, London, 1950, Val. 1, 65%-60.
