Hi,
I am stuck on this question and would appreciate some guidance. The question states " The value of Kw decreases as the temperature decreases. Is the auto-ionization of water an exothermic or endothermic reaction in the forward direction?"
I feel like the answer should be exothermic as I know from studying Le Chatelier's principle that a decrease in temperature favours the reaction to occur in the forward direction. Is that a correct assumption?
ionization decreases as temp decreases so..
ionization increases as temp increases
water+heat>>ions
endothermic
ionization decreases as temp decreases so..
ionization increases as temp increases
water+heat>>ions
endothermic, so decrease in temp favors reverse.
check my logic
when temp decreases, water will decrease in solubility or in ionization and the reaction will favour the backward reaction;
H+ + OH- --> H2O
so we know that the backward reaction is endothermic and the forward is exothermic.
Yes, your assumption is correct. In general, a decrease in temperature favors exothermic reactions, which means that the forward reaction is exothermic and releases heat. Let's go through the reasoning step-by-step.
The auto-ionization of water is the process by which water molecules spontaneously dissociate into hydronium ions (H3O+) and hydroxide ions (OH-) through a proton transfer reaction:
H2O(l) ⇌ H3O+(aq) + OH-(aq)
The equilibrium constant for this reaction is called the ion product of water, Kw. It is defined as the product of the concentrations of hydronium and hydroxide ions:
Kw = [H3O+][OH-]
At a given temperature, Kw has a constant, specific value. However, the value of Kw changes as the temperature changes. As the question states, when the temperature decreases, the value of Kw also decreases.
According to Le Chatelier's principle, when a stress is applied to a system at equilibrium, the system responds by shifting its equilibrium position to counteract that stress. In this case, when the temperature decreases, the system tries to counteract that stress by shifting the equilibrium position to increase the temperature.
Since the forward reaction of auto-ionization of water releases heat (exothermic), decreasing the temperature will favor the forward reaction to produce more heat. This means that more hydronium and hydroxide ions will be formed, resulting in an increase in their concentrations and a decrease in the value of Kw.
Therefore, it is correct to conclude that the auto-ionization of water is an exothermic reaction in the forward direction.