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Acid Base Buffers Essay

2801 words - 12 pages

Acis
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 ...view middle of the document...

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.

We will use the formula HA for an acid and B: for a base in our discussion. Accordingly, the reaction between an acid and base is described by [1].

HA + B: ( :A- + BH+ [1]
In the reaction above, the products which are produced are :A-. and BH+. :A- is called the conjugate base of HA because it has donated a proton (H+) to the base B: .Likewise, BH+ is the conjugate acid of B: since it has accepted the proton from HA. The substances HA and :A- are called a conjugate acid-base pair. Likewise, BH+ and B: are also a conjugate acid-base pair. Some common acid-base pairs are:

H3O1+ / H2O H2O / OH1- HCl / Cl1-

HNO3 / NO31- H2SO4 / HSO41- HSO41- / SO42-

CH3COOH / CH3COO1- (acetate) NH41+ / NH3 (ammonium)

H2CO3 / HCO31- (bicarbonate) H2PO41- / HPO42- (phosphate)

The first two pairs show that hydronium ion and hydroxide ion are the conjugate acid...

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