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l16a - CH 203 O R G A N I C C H E M I S T R Y I Acids and...

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Acids and bases © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Acids and bases Identifying acids and bases An acid is a proton (H + ) donor. A base is a proton acceptor. An acid–base re- action is generically represented as follows: HA + B i A + HB + HA (an acid) loses H + to B (a base), which gains H + . An acid–base reaction is an equilibrium process, that is, it takes place in the forward direction and in the reverse direction. In the reverse direction, A picks up H + (i.e., A is a base) whereas HB + gives up H + (i.e., HB + is an acid). HA and A are conjugates: HA is the conjugate acid of A and A is the conju- gate base of HA. In like fashion, HB + and B are conjugates: HB + is the conju- gate acid of B and B is the conjugate base of HB + . The conjugate acid has one more H + than its conjugate base. Problem Identify the acids and the bases in the equilibrium CH 3 OH + CH 3 i CH 3 O + CH 4 Answer CH 3 OH + CH 3 i CH 3 O + CH 4 acid base base acid Problem Indicate which species on the reactant side is an acid and which is a base. (a) HCO 2 H + CH 3 O i HCO 2 + CH 3 OH (b) C 6 H 5 OH + H 2 SO 4 i C 6 H 5 OH 2 + + HSO 4 (c) CH 3 CH 2 OH 2 + + C 6 H 5 i CH 3 CH 2 OH + C 6 H 6 Answer (a) acid: HCO 2 H; base: CH 3 O (b) acid: H 2 SO 4 ; base: C 6 H 5 OH (c) acid: CH 3 CH 2 OH 2 + ; base: C 6 H 5
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2 Problem Write the formula of the conjugate acid of (a) CH 3 OH (b) CH 2 O (c) CH 3 NH . Write the formula of the conjugate base of (d) CH 3 OH (e) CH 3 NH 3 + (f) HCl. Answer (a) CH 3 OH 2 + (b) CH 2 OH + (c) CH 3 NH 2 (d) CH 3 O (e) CH 3 NH 2 (f) Cl The thermodynamics of acid–base reactions The acid-dissociation constant K a When an acid HA dissolves in water, the acid donates H + to water, which acts as a base: HA(aq) + H 2 O(l) i A (aq) + H 3 O + (aq) The equilibrium constant of this reaction is called the acid-dissociation constant and gets the special symbol K a : K a = [A - (aq)][H 3 O + (aq)] [HA(aq)] K a values vary over 60 powers of ten, an enormous range! Because an equilib- rium constant whose value is greater than unity implies that product forma- tion is favored, we say that an acid whose K a 1 is a “strong” acid, that is, such an acid donates H + readily. An acid whose K a < 1 is considered a “weak” acid, that is, such an acid donates H + only reluctantly. We might represent the reaction of a “strong” acid with water as HA(aq) + H 2 O(l) K a > 1 ! " !!!!! # ! !!! A (aq) + H 3 O + (aq) where, instead of using our customary equilibrium double arrows “ i ”, we draw a long arrow pointing in the direction of products to emphasize the fact that most of the HA molecules donate H + to water. Note that the relative size of the arrows suggests that a strong acid has a weak conjugate base, that is, A is not very good at picking up H + from the hydronium ion H 3 O + . We might represent the reaction of a “weak” acid with water as
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l16a - CH 203 O R G A N I C C H E M I S T R Y I Acids and...

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