GSC101 GDB Solution 2023

 

GSC 101

GDB Solution

Spring 2023

 

Topic:

The formation of hydrogen ions (hydroxonium ions) and hydroxide ions from pure water is an endothermic process at room temperature.

Using the simpler version of equilibrium (reversible reaction) H₂O ⇌ H⁺ (aq) + OH^- (aq)

 

If the temperature increases, the rate of forward reaction increases more than the rate of reverse reaction, which causes it to shift to the right. The concentrations of both hydronium cations and hydroxide anions increase when the temperature increases. From the above discussion, can we say that the pH level of pure water increases with increases in temperature because the concentration of hydronium cations also increases? Either yes or no, explain the scientific reason behind this.​

 

Solution:

 

No, the pH level of pure water does not increase with an increase in temperature. The reason for this lies in the autoionization of water and the self-ionization constant (Kw) of water.

The autoionization of water is the process in which water molecules can spontaneously dissociate into hydronium ions (H+) and hydroxide ions (OH-). This is represented by the equilibrium reaction: H2O ⇌ H+ (aq) + OH- (aq).

At room temperature, the concentration of hydronium ions (H+) and hydroxide ions (OH-) in pure water is very low, but they are always present due to the autoionization process. The concentration of hydronium ions is equal to the concentration of hydroxide ions in pure water, resulting in a neutral pH of 7.

When the temperature increases, according to Le Chatelier's principle, the equilibrium of the autoionization reaction will shift to the right, favoring the formation of more hydronium ions and hydroxide ions. This means that the concentrations of both H+ and OH- will increase.

The pH level of a solution is determined by the concentration of hydronium ions (H+). Since the concentration of both H+ and OH- increases with temperature, the increase in Kw will result in an equal increase in both H+ and OH-. Therefore, the concentration of hydronium ions does increase with temperature, but the concentration of hydroxide ions also increases in the same proportion. As a result, the pH of pure water remains neutral at 7, indicating that the concentration of hydronium ions (H+) does not change, and thus, the pH level does not increase with an increase in temperature.