0.9 Bis2a 02.appendix ii detailed look at acid-base equilibrium  (Page 6/8)

We next consider mixing together a weak acid solution with a strong base solution, again with equal molarquantities of acid and base. As an example, we mix 100ml of 0.1M acetic acid(HA) solution with 100ml of 0.1M sodium hydroxide. In this discussion,we will abbreviate the acetic acid molecular formula CH ₃ COOH asHA and the acetate ionCH ₃ COO ^– as A ^– . The reaction ofHA andNaOH is:

$A^{-}\left(\mathrm{aq}\right)$ is the acetate ion in solution, formed when an acetic acid molecule donates the positive hydrogen ion. We have thus created a saltsolution again, in this case of sodium acetate in water. Note that the volume of the combined solution is 200ml, so the concentrationof sodium acetate (NaA)in solution is 0.050M.

Unlike our previous NaClsalt solution, a measurement in this case reveals that the pH of the product salt solution is 9.4, so the solution is basic. Thus,mixing equal molar quantities of strong base with weak acid produces a basic solution. In essence, the weak acid does not fullyneutralize the strong base. To understand this, we examine the behavior of sodium acetate in solution. Since the pH is greaterthan 7, then there is an excess of OH _– ions in solution relative to pure water. These ions must have come from the reaction of sodium acetate with the water. Therefore, thenegative acetate ions in solution must behave as a base, accepting positive hydrogen ions:

The reaction of an ion with water to form either an acid or a base solution is referred to as hydrolysis . From this example, the salt of a weak acid behaves as a base in water, resulting in a pH greater than7.

To understand the extent to which the hydrolysis of the negative ion occurs, we need to know theequilibrium constant for this reaction. This turns out to be determined by the acid ionization constant forHA. To see this, we write the equilibrium constant for the hydrolysisof A ^– as

Multiplying numerator and denominator by [H ₃ O ⁺ ],we find that

Therefore, for the hydrolysis of acetate ions in solution, K _h = 5.8 × 10 ^–10 . This is fairly small, so the acetate ion is a very weakbase.

Observation 5: acid strength and molecular properties

We now have a fairly complete quantitative description of acid-base equilibrium. To complete our understandingof acid-base equilibrium, we need a predictive model which relates acid strength or base strength to molecular properties. In general,we expect that the strength of an acid is related either to the relative ease by which it can donate a hydrogen ion or by therelative stability of the remaining negative ion formed after the departure of the hydrogen ion.

To begin, we note that there are three basic categories of acids which we have examined in this study. First,there are simple binary acids :

• HF
• HCl
• HBr
• HI