When a weak acid (HA) dissolves in water it partially dissociates (ionizes) into its conjugate base (A-) and hydronium ion (H3O+). At the same time as these ions form in solution a reverse reaction takes place. This reverse reaction leads to the formation of the acid (HA) from its dissociate ions. The rate of forward reaction increases as the reaction progresses while the rate of the reverse reaction increases as the reaction progresses. At some time after the dissolution of the acid an equilibrium is achieved. At equilibrium, the rate forward is equal to the rate backward. Thus, the concentration of all species (HA, A- , H3O+) in solution remain constant at equilibrium. The acid dissociation constant, Ka can then be expressed as the following equation:
Thus, Ka is a constant that is equal to the hydronium ion concentration, [H3O+], multiplied by the conjugate base concentration, [A-], and divided by the weak acid concentration, [HA], at equilibrium.
Ka values for weak acids can range from being very small (e.g. caffeine, Ka= 1×10-14 at 25 °C) to relatively large (e.g. picric acid, Ka= 4.2×10-1 at 25 °C ).
Ka gives an insight into the extent of ionization of the acid when it is dissolved in water. A larger Ka value means that the acid generates hydronium ion more readily in solution. For example, Assuming that we have two solutions with the same concentrations of caffeine and of picric acid (see above for Ka values), then it is expected that the hydronium ion concentration in the picric acid solution is larger than that in the caffeine solution. Because picric acid has a larger Ka value and would have more potential to generate hydronium ion in solution.
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Thus, Ka is a constant that is equal to the hydronium ion concentration, [H3O+], multiplied by the conjugate base concentration, [A-], and divided by the weak acid concentration, [HA], at equilibrium.
Ka values for weak acids can range from being very small (e.g. caffeine, Ka= 1×10-14 at 25 °C) to relatively large (e.g. picric acid, Ka= 4.2×10-1 at 25 °C ).
Ka gives an insight into the extent of ionization of the acid when it is dissolved in water. A larger Ka value means that the acid generates hydronium ion more readily in solution. For example, Assuming that we have two solutions with the same concentrations of caffeine and of picric acid (see above for Ka values), then it is expected that the hydronium ion concentration in the picric acid solution is larger than that in the caffeine solution. Because picric acid has a larger Ka value and would have more potential to generate hydronium ion in solution.
Feel free to write comments, or questions?
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