0.4 Acid-base equilibrium

Foundation

We have developed an understanding of equilibrium involving phase transitions and involving reactions entirely in the gas phase. We will assume anunderstanding of the principles of dynamic equilibrium, reaction equilibrium constants, and Le Châtelier's Principle . To understand application of these principles to reactions in solution, we willnow assume a definition of certain classes of substances as being either acids or bases. An acid is a substance whose moleculesdonate positive hydrogen ions (protons) to other molecules or ions. When dissolved in pure water, acid molecules will transfer ahydrogen ion to a water molecule or to a cluster of several water molecules. This increases the concentration of $H^{+}$ ions in the solution. A base is a substance whose molecules accept hydrogen ions from other molecules. When dissolved in pure water,base molecules will accept a hydrogen ion from a water molecule, leaving behind an increased concentration of $O{H}^{-}$ ions in the solution. To understand what determines acid-base behavior, we will assume an understanding of the bonding,structure, and properties of individual molecules.

Goals

Acids and bases are very common substances whose properties vary greatly. Many acids are known to be quitecorrosive, with the ability to dissolve solid metals or burn flesh. Many other acids, however, are not only benign but vital to theprocesses of life. Far from destroying biological molecules, they carry out reactions critical for organisms. Similarly, many basesare caustic cleansers while many others are medications to calm indigestion pains.

In this concept study, we will develop an understanding of the characteristics of molecules which make themeither acids or bases. We will examine measurements about the relative strengths of acids and bases, and we will use these todevelop a quantitative understanding of the relative strengths of acids and bases. From this, we can develop a qualitativeunderstanding of the properties of molecules which determine whether a molecule is a strong acid or a weak acid, a strong baseor a weak base. This understanding is valuable in predicting the outcomes of reactions, based on the relative quantitative strengthsof acids and bases. These reactions are commonly referred to as neutralization reactions. A surprisingly large number of reactions,particularly in organic chemistry, can be understood as transfer of hydrogen ions from acid molecules to base molecules.

Observation 1: strong acids and weak acids

From the definition of an acid given in the Foundation, a typical acid can be written as $HA$ , representing the hydrogen ion which will be donated and the rest ofthe molecule which will remain as a negative ion after the donation. The typical reaction of an acid in aqueous solutionreacting with water can be written as

In this reaction, $HA\left(\mathrm{aq}\right)$ represents an acid molecule dissolved in aqueous solution. $H_3{O}^{+}\left(\mathrm{aq}\right)$ is a notation to indicate that the donated proton has been dissolved in solution. Observations indicate that the proton isassociated with several water molecules in a cluster, rather than attached to a single molecule. $H_3{O}^{+}$ is a simplified notation to represent this result. Similarly, the $A^{-}\left(\mathrm{aq}\right)$ ion is solvated by several water molecules. [link] is referred to as acid ionization .