Question

Does the \(\mathrm{pH}\) increase, decrease, or remain the same on the addition of each of the following? (a) LiF to an HF solution (b) KI to an HI solution (c) \(\mathrm{NH}_{4} \mathrm{Cl}\) to an \(\mathrm{NH}_{3}\) solution

Short answer

(a) pH increases; (b) pH remains the same; (c) pH decreases.

Step-by-step solution

Understanding the Problem

We need to determine how the addition of different salts to acidic or basic solutions affects their pH. Each salt can possibly act as a source of conjugate acids or bases, altering the equilibrium.

Analyzing LiF and HF Solution

LiF in an HF solution provides fluoride ions (F^-). HF is a weak acid that partially dissociates into H^+ and F^-. Adding F^- from LiF will suppress HF dissociation due to the common ion effect, thereby reducing H^+ concentration and increasing the pH.

Analyzing KI and HI Solution

Adding KI to an HI solution introduces iodide ions (I^-), but HI is a strong acid that fully dissociates into H^+ and I^-. The presence of additional I^- ions from KI does not alter the pH significantly because HI's dissociation is already complete, keeping the pH constant.

Analyzing NH4Cl and NH3 Solution

NH4Cl is a salt that provides NH4^+ ions in solution. NH3 is a weak base, and NH4^+ acts as its conjugate acid. Adding NH4^+ ions will suppress NH3's ability to accept protons (H^+) leading to decreased OH^- concentration and thus decreasing the pH.

Key concepts

Common Ion Effect

The common ion effect is a vital concept in chemistry that occurs when a compound containing an ion already present in the solution is added. This phenomenon significantly influences the pH of the solution by shifting the equilibrium. When you add a salt like LiF to an HF solution, LiF dissociates to add extra fluoride ions (F^-) in the mix.

This addition of fluoride ions influences the HF to dissociate less because the system aims to maintain equilibrium and limit any change in ion concentration. The reduction of HF dissociation means that there are fewer hydrogen ions ( H⁺) in the solution, which directly impacts the pH by making it more basic, hence increasing the pH value.

Understanding the common ion effect helps predict and manipulate the behavior of solutions in chemical reactions. Lower dissociation leads to fewer free hydrogen ions, moving the pH upwards to a more basic value. Using this knowledge, chemists can carefully control reactions and solution properties in various fields such as pharmaceuticals and environmental science.

Equilibrium Shifts in Acid-Base Reactions

Equilibrium shifts in acid-base reactions are central to understanding how pH is influenced by different substances. Acids and bases in aqueous solutions establish an equilibrium between the dissociated and undissociated states. This equilibrium can be disrupted by the addition of salts that contain ions involved in the equilibrium.

For instance, when KI is added to an HI solution, the additional iodide ions (I^-) do not substantially alter the solution's pH due to HI being a strong acid. Strong acids like HI are fully dissociated in solution, meaning equilibrium is already shifted towards the formation of hydrogen ions and iodide ions. As a result, adding more iodide ions does not further shift this equilibrium, and the pH remains constant.

Recognizing shifts in equilibrium is crucial for predicting the outcomes of mixing various compounds in solution, which plays a substantial role in chemical manufacturing, drug formulation, and even biological processes within living organisms.

Impact of Salts on Solution pH

Salts can considerably impact the pH of a solution, depending on the components the salt breaks into upon dissolution. These ions can either increase or decrease the solution's pH. When NH₄Cl is added to an NH₃ solution, the salt dissociates into NH₄⁺ and Cl^- ions. The NH₄⁺ ion is particularly influential as it acts as a conjugate acid to NH₃, which is a weak base.

This introduction of NH₄⁺ leads to a suppression of NH₃'s ability to accept protons (H⁺), decreasing the amount of OH^- ions in the solution. The reduction of hydroxide ions results in a lower pH, making the solution more acidic.

Through careful chemical experimentation and analysis, chemists can adjust the acidic or basic nature of solutions by manipulating the type and concentration of salts introduced, allowing for precise control over pH in various chemical and industrial applications.