Problem 58 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

The pH of \(0.1\) M solution of the following compounds increases in the order (a) \(\mathrm{NaCl}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCN}<\mathrm{HCl}\) (b) \(\mathrm{HCl}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCl}<\mathrm{NaCN}\) (c) \(\mathrm{NaCN}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCl}<\mathrm{HCl}\) (d) \(\mathrm{HCl}<\mathrm{NaCl}<\mathrm{NaCN}<\mathrm{NH}_{4} \mathrm{Cl}\)

Step-by-Step Solution

Answer

The correct order for the increasing pH of the compound's solution is (b) \(\mathrm{HCl}<\mathrm{NH}_{4} \mathrm{Cl}<\mathrm{NaCl}<\mathrm{NaCN}\).

Step 1: Identify the Effect of Each Compound on pH

Determine how each compound affects the solution's pH. NaCl is a neutral salt and does not affect pH. NH4Cl is an acidic salt because it comes from a strong acid (HCl) and a weak base (NH3). NaCN is a basic salt from a strong base (NaOH) and a weak acid (HCN). HCl is a strong acid and significantly lowers pH.

Step 2: Compare the Acidity and Basicity

Rank the compounds based on their influence on acidity and basicity: HCl is the most acidic so it has the lowest pH, followed by NH4Cl due to its acidic nature. NaCl is neutral and does not affect the pH, and NaCN is basic raising the pH the most.

Step 3: Arrange in Order of Increasing pH

Based on their expected effects on pH, arrange the compounds from lowest to highest pH to be: HCl, NH4Cl, NaCl, and NaCN.

Key Concepts

Understanding Acidic and Basic Salts

A crucial aspect of chemistry is understanding how various salts can affect the acidity or basicity of a solution. Acidic and basic salts are the products of reactions between acids and bases. When a strong acid reacts with a weak base, the result is an acidic salt. Conversely, a reaction between a strong base and a weak base yields a basic salt.

For instance, NH₄Cl (ammonium chloride) is formed from the reaction of hydrochloric acid (HCl), a strong acid, with ammonia (NH₃), a weak base. This makes NH₄Cl an acidic salt. In contrast, NaCN (sodium cyanide) arises from the reaction of sodium hydroxide (NaOH), a strong base, with hydrocyanic acid (HCN), a weak acid, thus classifying it as a basic salt.

In terms of pH, an acidic salt tends to release hydrogen ions (H⁺) into the solution, causing the pH to drop. However, a basic salt dissociates to release hydroxide ions (OH−) into the solution, which causes the pH to rise. The terms 'acidic' and 'basic' are indicative of the resulting pH shift when the salt dissolves in water, not the pH of the salt itself.

Strong and Weak Acids and Bases

When we talk about the strength of an acid or base, we're referring to its ability to dissociate in water. Strong acids and bases virtually completely dissociate, while weak acids and bases only partially dissociate. This level of dissociation has a direct impact on the pH of a solution.

Identifying Strong vs. Weak

Strong acids, such as HCl (hydrochloric acid), ionize completely to yield a large number of hydrogen ions, which makes the pH drop significantly. Weak acids, like HCN (hydrocyanic acid), only partially ionize, producing fewer hydrogen ions and thus affecting the pH less.

Similarly, strong bases like NaOH (sodium hydroxide) fully dissociate to release a lot of hydroxide ions (OH⁻), which drastically increase the pH. Weak bases, such as NH₃ (ammonia), slightly dissociate and do not raise pH as much as strong bases.

Understanding the strengths of acids and bases is essential when predicting the pH level of their solutions and knowing your solute's properties greatly assists in anticipating its impact on pH.

pH Ranking of Compounds

The pH scale, ranging from 0 to 14, is a tool to measure how acidic or basic a solution is. Substances with a pH less than 7 are considered acidic, while those with a pH greater than 7 are deemed basic. To properly rank compounds by pH, it is essential to consider the nature of the solutes involved.

Compounds formed from strong acids yield solutions with a low pH because of the high concentration of hydrogen ions in the solution. Conversely, compounds from strong bases result in higher pH values owing to the abundant presence of hydroxide ions.

Ranking Compounds by Their pH

Like the exercise explained, HCl, a strong acid, will have the lowest pH among the given compounds. Ammonium chloride, NH₄Cl, while less acidic than HCl, will still have a lower pH than neutral salts like NaCl (sodium chloride) due to its acidic properties. In contrast, NaCN, being a basic salt, makes the solution more basic and hence has the highest pH of the compounds listed. Understanding the theory behind the acidity and basicity of salts, along with the strengths of acids and bases, allows for accurate pH ranking of compounds.