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Chemistry LibreTexts

11.9: Predicting Equilibrium in Acid-Base Reactions

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    215764
  • Equilibrium in acid-base reactions always favors the weaker side. In the following example the pKa values for the substances acting as acids are shown under their structures. Equilibrium favors the left side because the substances on the left are the weaker acid and the weaker base.

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    We can arrive at the same conclusion looking at the bases. The strength of bases is measured by the pKa of their conjugate acids. To understand how this works, we must remember that the relative strengths of the acid and the base in a conjugate pair hold an inverse relationship. The stronger the acid, the weaker its conjugate base, and viceversa. Therefore, the relationship between the pKa of an acid and the strength of its conjugate base is direct: the stronger the base, the higher the pKa value of its conjugate acid. We can look at the same reaction again, except that now we’re focusing on the bases, and arrive at the same conclusion that equilibrium favors the left side.

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    The weaker acid and the weaker base are always on the same side. If you arrive at a different conclusion, something is not right.

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