Acid-Base Buffers

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CHEMICAL REACTIONS: ACID-BASE BUFFERS

Short Overview

Acids and bases represent two of the most common classes of compounds. Many studies have been done on these compounds, and their reactions are very important. Perhaps the most important reaction is the one in which an acid and base are combined, resulting in the formation of water (in aqueous solution) and a salt; this reaction is called neutralization.

A buffer solution is a solution that contains both an acid and a salt containing the conjugate base anion in sufficient concentrations so as to maintain a relatively constant pH when either acid or base is added. In this experiment you will prepare a buffer solution and observe its behavior when mixed both with an acid and a base. You will also compare the behavior with that of solutions containing only the acid.

Theory
In his theory of ionization in the 1880’s, Svante Arrhenius defined acids are substances which form H+ and bases as substances which form OH- in water. He further defined a salt as a substance other than an acid or base which forms ions in aqueous solution. Such substances are thus capable of producing an electric current and are called electrolytes. The amount of electricity produced is directly proportional to the concentration of ions in solution.

With regard to electrolytes we have learned previously that strong acids and strong bases ionize completely, and are therefore strong electrolytes because they produce a large electric current. Soluble salts are the other type of strong electrolytes. We also learned that weak acids and weak bases ionize only partially in solution, producing smaller quantities of current; these substances are called weak electrolytes. Materials which do not produce an electric current are called nonelectrolytes. To complete our understanding, we concluded that strong electrolytes exist primarily as ions in solution, while weak electrolytes exist as both ions and molecules in solution. Nonelectrolytes must exist as polar molecules only in solution.

While it is useful, the Arrhenius definition of acids and bases is limited to aqueous solutions. This may seem insignificant to a student in introductory chemistry or general chemistry, but it imposes restrictions for understanding more advanced topics. As such, we now introduce two additional definitions of acids and bases, which expand our understanding.

| | |Acid | |Base | | | | | | | |Arrhenius | |forms H+ in water | |forms OH- in water | |Brønsted-Lowry | |donates H+ (proton) to base | |accepts H+ (proton) from acid | |Lewis | |accepts electron pair from base | |donates electron pair to acid |

The Brønsted-Lowry concept of acids and bases was introduced by Johannes Brønsted and Thomas Lowry in 1923, and led to an understanding of many proton transfer reactions observed to occur in both non-aqueous and aqueous solutions. Gilbert Newton Lewis quickly recognized that a substance which is a proton acceptor must also be one which contains an unshared valence electron pair to accept the positive charge. He therefore proposed his own theory of acids and bases based upon electron transfer rather than proton transfer. The Lewis acid-base concept is the most general and allows us to understand reactions which may not involve proton transfer. However, the Brønsted-Lowry concept provides the simplest description of acid-base buffer solutions, and it is this one which we will utilize in further discussion....
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