Strengths of acid and bases

In Chapter 4, we introduced the concept of acids and bases. Acids and bases may be strong or weak. Strong acids completely dissociate in water and weak acids only partially dissociate. For example, consider two acids HC1 (strong) and CH3COOH (weak). If we add each to water to form aqueous solutions, the following reactions take place  

The first reaction goes to completion—there is no HC1 left in solution. The second reaction is an equilibrium reaction—there are appreciable amounts of both reactants and products left in solution. 

There are generally only two strong bases to consider—the hydroxide ion and those species that produce hydroxide ion in aqueous solution. All other bases are weak. Weak bases also establish an equilibrium system, as in aqueous solutions of ammonia  

Chloric acid, HC103 Hydrobromic acid, HBr Hydrochloric acid, HC1 Hydroiodic acid, HI Nitric acid, HN03 Perchloric acid, HCIO4 Sulfuric acid, H2S04 

Alkali metal (Group IA) hydroxides (LiOH, NaOH, KOH, RbOH and CsOH) Calcium, strontium, and barium hydroxides 

For the complex acids Van Arkel and Carriere have drawn a number of conclusions from their model arguments regarding the order in acid strength of corresponding acids. 

Oxygen acids are weaker than fluorine acids in view of the fact that in the first case the proton of a divalent ion, in the second case of a monovalent ion, must be set free. 

Examples HB3+F4 is a very strong, H3B3+03 an extremely weak acid. 

Examples H2S04 stronger than H2S03, HNOs H2C03 

Of much less importance than the two points of difference mentioned above, the difference in the relative numbers of coordinated oxygen ions may be put forward, whereby a larger number by their combined effect leads to a stronger bonding and thus to smaller acid strength. 

The strong acid is assumed to be comptetety ionized in soiution. The proton exists in soiution as the hydronium ion (HsO ). 

Note that H2SO4 is a diprotic acid we show only the first stage of ionization here. At equilibrium, solutions of strong acids will not contain any nonionized acid molecules. 

Most acids are weak acids, which ionize only to a limited extent in water. At equilibrium, aqueous solutions of weak acids contain a mixture of nonionized acid molecules, ions, and the conjugate base. Examples of weak acids are hydrofluoric acid (HF), acetic acid (CH3COOH), and the ammonium ion (NH4). The limited ionization of weak acids is related to the equilibrium constant for ionization, which we will study in the next section. 

Like strong acids, strong bases are strong electrolytes that ionize completely in water. Hydroxides of alkali metals and certain alkaline earth metals are strong bases. [All alkali metal hydroxides are soluble. Of the alkaline earth hydroxides, Be(OH)2 and Mg(OH)2 are insoluble Ca(OH)2 and Sr(OH)2 are slightly soluble and Ba(OH)2 is soluble.] Some examples of strong bases are 

Strictly speaking, these metal hydroxides are not Brpnsted bases because they cannot accept a proton. However, the hydroxide ion (OH ) formed when they ionize is a Brpnsted base because it can accept a proton  

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In this chapter, we see what acids and bases are and why they vary in strength. We shall use thermodynamics, particularly equilibrium constants, to discuss the strengths of acids and bases quantitatively and thereby develop our insight into their behavior. We then use our knowledge of equilibria involving acids and bases to examine systems in which more than one equilibrium is taking place simultaneously. 

This is always the case for any two acids, and by measuring the positions of the equilibrium the relative strengths of acids and bases can be determined. Of course, if the two acids involved are close to each other in strength, a measurable reaction will occur from both sides, though the position of equilibrium will still be over to the... 

The strength of acids and bases is measured on a pH (potential of hydrogen) scale pH = -logio [H+]... 

It has already been mentioned several times that the strength of acids and bases is expressed by their dissociation constants KA and KB. This is... 

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