Why do strong acids dissociate

On average, only about 1 percent of a weak acid solution dissociates in water in a 0. Vinegars : All vinegars contain acetic acid, a common weak acid. Weak acids ionize in a water solution only to a very moderate extent.

The generalized dissociation reaction is given by:. The strength of a weak acid is represented as either an equilibrium constant or a percent dissociation. The equilibrium concentrations of reactants and products are related by the acid dissociation constant expression, K a :. The K a of weak acids varies between 1. Acids with a K a less than 1. If acids are polyprotic, each proton will have a unique K a.

For example, H 2 CO 3 has two K a values because it has two acidic protons. The first K a refers to the first dissociation step:. This K a value is 4. The second K a is 4. The K a of acetic acid is [latex]1. What is the pH of a solution of 1 M acetic acid? This quadratic equation can be manipulated and solved. A common assumption is that x is small; we can justify assuming this for calculations involving weak acids and bases, because we know that these compounds only dissociate to a very small extent.

Therefore, our above equation simplifies to:. Although it is only a weak acid, a concentrated enough solution of acetic acid can still be quite acidic. Calculate percent dissociation for weak acids from their K a values and a given concentration. Calculate the percent dissociation of a weak acid in a [latex]0. Therefore, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows.

In addition, the smaller the pKa value, the stronger the acid. For example, the pKa value of lactic acid is about 3. Another important point is the relationship between pH and the pKa of an acid. This relationship is described by the following equation. If the pH changes by 1 near the pKa value, the dissociation status of the acid changes by an extremely large amount.

In the case of acetic acid, for example, if the solution's pH changes near 4. When the pH is 3. Except for their names and formulas, so far we have treated all acids as equals, especially in a chemical reaction. However, acids can be very different in a very important way. Consider HCl aq. HC 2 H 3 O 2 is an example of a weak acid:.