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CHMB41H3 Lecture 2: CHMB41 - Chapter 2

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13 Feb 2017

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Chapter 2 acids and bases: central to understanding organic chemistry. 2. 1 an introduction to acids and bases. Bronsted-lowry: acid loses a proton, base gains a proton. Any species that has a hydrogen can potentially act as an acid. Any species that has a lone pair can potentially act as a base. The rxn of an acid w/ a base is called an acid-base rxn or a proton transfer rxn. A conjugate base is formed by removing a proton from an acid. A conjugate acid is formed by adding a proton to a base. Acidity is a measure of the tendency of a compound to lose a proton. Basicity is a measure of a compound(cid:495)s affinity for a proton. The stronger the acid, the weaker its conjugate base. The degree to which an acid (ha) dissociates in an aqueous solution is indicated by the equilibrium constant of the rxn, keq.

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Related Questions

Experiment 18: Buffers Purpose In this experiment you will prepare several buffers and investigate their properties. Discussion A solution of a weak acid and one of its salts, in approximately equal concentrations, resists changes in pH when acid or base is added to it. (Note: The pH of a solution is a measure of the concentration of free H3Ot ion; it does not indicate concentration of combined acid such as HA.) The same is true for a solution of a weak base and one of its salts. A solution with this property is called a buffer (see illustration next page). Bufter systems are extremely important in many chemical and biological systems. For example, your blood contains a buffer system utilizing carbonic acid and bicarbonate ion. The important equilibrium in an acidic buffer is HA (aq) H20 ) A (a)0 () acid conjugate base in which the concentrations of acid and conjugate base are approximately equal. If a small amount of free acid (H30) is added to this solution, it reacts with the conjugate base A (reaction 1 in reverse). The equilibrium constant for this reaction is very large, so the reaction proceeds to near completion, absorbing the added acid. Similarly, added base (OH-) reacts with the acid HA as shown below: 2. HA (aq) + OH-(aq) ATaq) + H2O (1) This reaction also proceeds to completion, effectively absorbing any added base. Since the H30 and OH- concentrations are kept constant, the solution thus resists pH changes when an acid or base is added to it. A similar argument can be made for a basic buffer The actual pH of a buffer can be determined from the value of the Ka and the concentrations of the acid and conjugate base obtained from an initial-change- equilibrium table. However, a shortcut for buffers is available, and is shown below. We begin by writing and rearranging the equilibrium constant expression for the acid dissociation. If the -log of both sides of equation (3) is taken, a relationship involving pH is then obtained. H30IA] HA] 3. HA HAI -loglH30'1- -log Ka A- 141