Types of bases and theories behind each PDF

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Types of bases and theories behind each...

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Two Types of Bases For bases, the concentration of OH – must exceed the concentration of H 3O + in the solution. This imbalance can be created in two different ways. First, the base can be a hydroxide, which merely dissociates to yield hydroxide ions:

where M represents the cation, usually a metal. The most familiar bases are such hydroxides. (See Table 1.)

The second type of base acts by extracting a hydrogen ion from a water molecule, leaving a hydroxide ion:

An example of this second type of base that is not a hydroxide can be an ammonia molecule in water (aqueous ammonia):

Ammonia acts as a base by stripping a proton from a water molecule, leaving an increased OH – concentration. Notice in the equilibrium reaction that

and NH 3 are

a conjugate acid‐base pair, related by transferring a single proton. Similarly, water acts as an acid by donating a proton to ammonia. H 2O and OH – are a conjugate acid‐base pair, related by the loss of a single proton. Alternatively, the base may be a particular kind of negative ion with a high attraction for a hydrogen ion:

In 1923, the English chemist Thomas Lowry and the Danish chemist Johannes Bronsted defined an acid and base in another way. An acid is a substance that can donate a proton, and a base is a substance that can accept a proton.

Polyprotic Acids Many acids contain two or more ionizable hydrogens. There are two in carbonic acid, H 2CO 3, and three in phosphoric acid, H 3PO 4. For any such multiple hydrogen acid, the first hydrogen is most easily removed, and the last hydrogen is removed with the greatest difficulty. These acids are called polyprotic (many protons) acids. The multiple acid ionization constants for each acid measure the degree of dissociation of the successive hydrogens. Table 1 gives ionization data for four series of polyprotic acids. The integer in parentheses after the name denotes which hydrogen is being ionized, where (1) is the first and most easily ionized hydrogen.

Remember: The strongest acids dissociate most readily. Of the nine acids listed in Table , the strongest is sulfuric (1), with the highest acid ionization constant, and the weakest is phosphoric (3). Here are the chemical equations for the three successive ionizations of phosphoric acid:

Consequently, an aqueous solution of phosphoric acid contains all the following molecules and ions in various concentrations:

Consulting the table of the dissociation constants K a's for phosphoric acid shows that the first dissociation is much greater than the second, about 100,000 times greater. This means nearly all the H 3O + ( aq) in the solution comes from the first step of dissociation. The second and third steps add very little H 3O + ( aq) to the solution. So a solution of phosphoric acid will contain H 3PO 4 molecules in highest concentration with smaller, and nearly equal, concentrations of H 3O + and present in very small concentrations.

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