Consider the following equilibrium for nitrous acid, HNO2, a weak acid:
HNO2 (aq) + H2O (l) <————–> H3O+ (aq) + NO2-
In which direction will the equilibrium shift if
NaOH is added?
Sol18:
When NaOH is added to the solution, it will react with HNO2 (the acid) to form NaNO2 (the salt) and water: HNO2 (aq) + NaOH (aq) ? NaNO2 (aq) + H2O (l)
This reaction will remove HNO2 from the solution, which will shift the equilibrium to the left to try to restore the concentration of HNO2. As a result, more HNO2 will dissociate to form H3O+ and NO2- until a new equilibrium is reached.
Therefore, the equilibrium will shift to the left if NaOH is added to the solution.
The Correct Answer and Explanation is :
The explanation in your provided response contains an error regarding the direction of the equilibrium shift. When NaOH, a strong base, is added to a solution of HNO2, a weak acid, the reaction that occurs can be described as follows:
[ \text{HNO}_2 (aq) + \text{NaOH} (aq) \rightarrow \text{NaNO}_2 (aq) + \text{H}_2\text{O} (l) ]
In this reaction, NaOH completely dissociates into Na(^+) and OH(^-) ions. The OH(^-) ions from NaOH will react with the H(^+) ions produced by the dissociation of HNO2, forming water. This effectively reduces the concentration of H(^+) ions in the solution, which is represented by the decrease in H3O(^+).
According to Le Châtelier’s Principle, if the concentration of a product in a reversible reaction is decreased, the equilibrium will shift to the right to produce more of that product. In the case of the nitrous acid equilibrium:
[ \text{HNO}_2 (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{H}_3\text{O}^+ (aq) + \text{NO}_2^- (aq) ]
Adding NaOH decreases the concentration of H3O(^+) because the OH(^-) ions neutralize some of the H3O(^+). Consequently, to compensate for the loss of H3O(^+), the equilibrium will shift to the right, not to the left. This will lead to more HNO2 dissociating to form additional H3O(^+) and NO2(^-) until a new equilibrium is established.
Therefore, the correct answer is that the equilibrium will shift to the right when NaOH is added to the solution, as this addition leads to the consumption of H3O(^+), and the system responds by producing more H3O(^+) and NO2(^-) to restore balance.
To summarize:
- Adding NaOH reduces H3O(^+) concentration by neutralization.
- This shifts the equilibrium to the right, increasing the dissociation of HNO2 to compensate for the decrease in H3O(^+).
- The result is increased production of H3O(^+) and NO2(^-) ions.
This explanation corrects the direction of the equilibrium shift and clarifies the chemical processes involved when NaOH is introduced to a solution containing HNO2.