Which of the following accounts for the observation that the pH of pure water at 37°C is 6.8?

Question

Which of the following accounts for the observation that the pH of pure water at 37°C is 6.8?

1. At 37°C water is naturally acidic.
2. At 37°C water has a lower density than it does at 25°C; therefore, [H⁺] is greater.
3. At 37°C the autoionization constant for water, Kw, is larger than it is at 25°C.
4. At 37°C water ionizes to a lesser extent than it does at 25°C

(Is the highlighted answer correct?)

Answer 1

Hello, Sdasd,

Excellent! Thanks for proposing the (corrct answer). Yes, the equilibrium constant does depend on the temperature of the water. The forward reaction of water dissociating into OH^- and H₃O⁺ ions is slightly endothermic - the dissociation absorbs a small amount of energy When more energy is available, as when the temperature is increased, the reaction will move more to the right - the equilibrium "constant" changes slightly. (Le Chatelier's principle - the system will move to relieve the added energy, so the concentrations of H⁺ and OH^- will increase slightly and their product will be reflected in the new equilirium constant, for that temperature.

At 25C the constant is 1.008x10^-14

At 40C it becomes 2.916x10^-14

Neutral pH at 25C is pH 7

Neutral pH at 40C is pH 6.77

Good job,

Bob