How does temperature affect the acid–base equilibria and pH of a buffer solution?

How does temperature affect the acid–base equilibria and pH of a buffer solution?

• A. Temperature changes Ka and Kb; as Ka or Kb shift, the pH of the buffer can drift; some buffers become stronger or weaker with temperature changes.
• B. Temperature has no effect on the pH of buffers.
• C. pH always increases with temperature for buffers.
• D. All buffers become weaker at high temperature.

Answer: (A)

The key idea is that temperature changes alter the acid–base equilibria themselves. The pH of a buffer is tied to the acid–base dissociation constants, Ka and Kb, through Henderson–Hasselbalch: pH = pKa + log([A−]/[HA]). When temperature changes, Ka and Kb shift because the ionization of the weak acid or weak base has an enthalpy of reaction. According to the van’t Hoff relation, if the ionization is endothermic, Ka increases with temperature (pKa decreases), causing the pH to drift in one direction; if the ionization is exothermic, Ka decreases with temperature (pKa increases), causing the pH to drift in the opposite direction. Water’s own autoionization also depends on temperature, which further influences pH in aqueous solutions. Because different buffers have different enthalpies of ionization, some become effectively stronger (better at resisting pH change) and others weaker as temperature shifts occur. That’s why temperature can cause the pH of a buffer to move and why buffers don’t respond the same way to temperature changes.

The key idea is that temperature changes alter the acid–base equilibria themselves. The pH of a buffer is tied to the acid–base dissociation constants, Ka and Kb, through Henderson–Hasselbalch: pH = pKa + log([A−]/[HA]). When temperature changes, Ka and Kb shift because the ionization of the weak acid or weak base has an enthalpy of reaction. According to the van’t Hoff relation, if the ionization is endothermic, Ka increases with temperature (pKa decreases), causing the pH to drift in one direction; if the ionization is exothermic, Ka decreases with temperature (pKa increases), causing the pH to drift in the opposite direction. Water’s own autoionization also depends on temperature, which further influences pH in aqueous solutions. Because different buffers have different enthalpies of ionization, some become effectively stronger (better at resisting pH change) and others weaker as temperature shifts occur. That’s why temperature can cause the pH of a buffer to move and why buffers don’t respond the same way to temperature changes.