Consider a room temperature 0.30M ammonia buffer at pH 9.5. As you raise the temperature of the buffer, would you expect the pH to change? Explain. (Pka of NH4+ is 9.26)
See below.
To determine whether the pH of a buffer will change with a change in temperature, we need to understand the relationship between temperature and the dissociation constant (pKa) of the acidic component in the buffer.
In this case, the primary acidic component of the ammonia buffer is NH4+. It can donate a proton (H+) to water, which forms NH3 and H3O+. The dissociation reaction is as follows:
NH4+ + H2O ⇌ NH3 + H3O+
The pKa value of NH4+ in this case is given as 9.26. The pKa represents the pH at which half of the acid is dissociated, and half is undissociated. Therefore, when the pH of the buffer is equal to the pKa, NH4+ and NH3 concentrations will be equal.
Now, let's consider the effect of temperature on the buffer system. When you raise the temperature, the equilibrium for the dissociation reaction will shift. In general, increasing temperature favors an endothermic reaction. In an endothermic reaction, the forward reaction absorbs heat, so increasing the temperature will shift the equilibrium towards the product side.
In the case of the ammonia buffer, the dissociation reaction is exothermic as it releases heat. Therefore, increasing the temperature will shift the equilibrium towards the reactant side.
NH4+ + H2O ⇌ NH3 + H3O+
Raising the temperature will result in the absorption of heat, which favors the reverse reaction (production of NH4+ and H2O). As a consequence, the concentration of NH4+ will increase relative to NH3, leading to a decrease in the concentration of H3O+. Consequently, the pH of the buffer will decrease.
To summarize, as the temperature of the ammonia buffer is raised, the pH of the buffer is expected to decrease due to the endothermic nature of the dissociation reaction, which causes a shift towards the reactant side.