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BRONSTED ACIDS AND BASES Conjugate Acid Base Pairs A Bronsted Acid is any substance capable of donating a proton. A Bronsted Base is any substance capable of accepting a proton. An extension of these definitions is the concept of the conjugate acid-base pair , which is defined as an acid and its conjugate base or a base and its conjugate acid. The conjugate base of a Bronsted acid is the species that remains when one proton has been removed from the acid. Conversely, the conjugate acid of a Bronsted base is the species that results from the addition of a proton to a Bronsted base. Every Bronsted acid has a conjugate base, and every Bronsted base has a conjugate acid. ACID-BASE PROPERTIES OF WATER Water can act as either an acid or as a base. Water functions as a base in reactions with acids, and water functions as an acid in reactions with bases. Water is a very weak electrolyte, and therefore a poor conductor of electricity, but it does undergo ionization to a small extent. H 2 O(l) H + (aq) + OH - (aq) This reaction is referred to as the autoionization of water . This can also be represented by the following equation: H 2 O(l) H 3 O + (aq) + OH - (aq) The conjugate acid-base pairs for this equation are H 2 O (acid) and OH - (base) H 3 O + (acid) and H 2 O (base) The Ion Product of Water In the study of acid-base reactions, the concentration of hydrogen ion (hydronium) is the key: it’s value indicates the acidity or basicity of the solution. Only a very small fraction of water molecules are ionized, and therefore [H 2 O] remains virtually unchanged.
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Therefore, the equilibrium constant for the autoionization of water is: K c = [H 3 O + ][OH - ] Since the H + (aq) and H 3 O + (aq) are used interchangeably to represent the hydrated proton, the equilibrium constant can also be expressed as K c = [H + ][OH - ] To indicate that the equilibrium constant refers to the autoionization of water, we replace K c with the symbol K w : K w = [H 3 O + ][OH - ] = [H + ][OH - ] In this case K w is referred to as the ion-product constant , which is product of the molar concentrations of H + and OH - ions at a particular temperature. In pure water at 25°C, the concentrations of H + and OH - are equal and found to be [H + ] = 1.0 x 10 -7 M [OH - ] = 1.0 x 10 -7 M Therefore, we can attest from the previously given information that whether we have pure water or an aqueous solution of dissolved species, the following relation always holds true at 25°C: K w = [H + ][OH - ]= (1.0 x 10 -7 ) (1.0 x 10 -7 ) = 1.0 x 10 -14 Whenever [OH - ] = [H + ], the aqueous solution is said to be neutral . In an acidic solution, there is an excess of H
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This note was uploaded on 02/02/2011 for the course CHEM 122 taught by Professor Bellew during the Fall '07 term at New Mexico.

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