Question

Given the chemical equation for the ionization of water $$ \mathrm{H}_{2} \mathrm{O}(l) \rightleftarrows \mathrm{H}^{+}(a q)+\mathrm{OH}^{-}(a q) $$ Predict the direction of equilibrium shift for each of the following stresses: (a) add gaseous \(\mathrm{HCl}\) (b) add solid \(\mathrm{NaOH}\) (c) add liquid \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (d) add solid NaF

Short answer

(a) Shift left, (b) Shift left, (c) Shift left, (d) No change.

Step-by-step solution

Understanding Le Châtelier's Principle

Le Châtelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. In this reaction, we need to predict the direction of shift when different substances are added.

Impact of Adding Gaseous HCl (a)

Adding gaseous HCl introduces additional \( H^+ \) ions to the system. According to Le Châtelier's Principle, the equilibrium will shift to the left to reduce the concentration of the added \( H^+ \) by converting it back to water.

Impact of Adding Solid NaOH (b)

Adding NaOH introduces \( OH^- \) ions to the system. The equilibrium will shift to the left, as the system attempts to remove the excess \( OH^- \) ions by forming more \( H_2O \).

Impact of Adding Liquid H2SO4 (c)

Like HCl, \( H_2SO_4 \) adds more \( H^+ \) ions to the system. The equilibrium will shift to the left to reduce the excess \( H^+ \) ions, favoring the formation of water.

Impact of Adding Solid NaF (d)

Adding NaF doesn't directly alter the concentration of \(OH^- \) or \(H^+ \) ions. NaF dissociates to give \(F^- \) and \(Na^+ \) ions, neither of which are part of the reaction in question. Therefore, the equilibrium position remains unchanged.

Key concepts

Chemical Equilibrium

Chemical equilibrium occurs in a reversible reaction when the rates of the forward and reverse reactions are equal. In this state, the concentrations of reactants and products remain constant over time. For instance, in the ionization of water: \[ \mathrm{H}_{2} \mathrm{O}(l) \rightleftarrows \mathrm{H}^{+}(aq) + \mathrm{OH}^{-}(aq) \] At equilibrium, the rate at which water splits into hydrogen and hydroxide ions is equal to the rate at which these ions recombine to form water. The concept of equilibrium is crucial because it helps us predict how a system responds to changes in concentration, pressure, or temperature through Le Châtelier's Principle. In practice, equilibrium is dynamic, meaning that the reaction doesn't stop but instead continues with no net change in concentrations.

Ionization of Water

The ionization of water is a fundamental process occurring in aqueous solutions, described by the chemical equation: \[ \mathrm{H}_{2} \mathrm{O} (l) \rightleftarrows \mathrm{H}^{+} (aq) + \mathrm{OH}^{-} (aq) \] Pure water is almost neutral, but it slightly dissociates into hydrogen ions (\(\mathrm{H}^{+}\)) and hydroxide ions (\(\mathrm{OH}^{-}\)). The concentration of these ions in pure water at 25°C is very low, \(1.0 \times 10^{-7}\) mol/L, which leads to a neutral pH of 7. In this process, water acts as both an acid and a base, demonstrating its amphoteric nature. This delicate balance of ionization forms the foundation for reactions in more complex solutions, influencing phytochemistry, biology, and industrial chemistry. Understanding this sets the stage for comprehending how external substances affect equilibrium through Le Châtelier's Principle.

Equilibrium Shift Prediction

Predicting the shift in chemical equilibrium involves applying Le Châtelier's Principle, which states that a system at equilibrium will adjust to counteract any imposed change. Let's consider the ionization of water and various stresses:

• Adding an acid like \( \mathrm{HCl} \) or \( \mathrm{H_{2}SO_{4}} \) introduces more \( \mathrm{H}^{+} \) ions, causing the equilibrium to shift left, converting \( \mathrm{H}^{+} \) back to water and reducing ion concentration.
• Introducing a base, such as \( \mathrm{NaOH} \), increases \( \mathrm{OH}^{-} \) ion concentration, prompting a leftward shift to form more water and balance the system.
• Substances like \( \mathrm{NaF} \) that do not participate in the ionization reaction of water won't affect the equilibrium position directly, as they do not change the \( \mathrm{H}^{+} \) or \( \mathrm{OH}^{-} \) concentrations.

These predictions are vital when manipulating chemical reactions in labs, supporting industries or understanding environmental chemistry. Recognizing how systems react to changes aids in controlling reaction conditions efficiently.