Question

The failure of Ostwald's dilution law in case of strong electrolytes is due to (a) strong electrolytes are almost completely dissociated at all dilutions and \(\lambda_{v} / \lambda_{\infty}\) do not give accurate value of \(a\) (b) the law of mass action in its simple form cannot be applied when the concentration of the ions is very high (c) the ions get hydrolysed and affect the concentration terms (d) all of the above

Short answer

(d) all of the above

Step-by-step solution

Understanding Ostwald's Dilution Law

Ostwald's dilution law describes the relationship between the degree of dissociation (α) of a weak electrolyte and the molar concentration of the electrolyte (C) in an ideally dilute solution. It's important to recognise that this law applies mainly to weak electrolytes, indicating their incomplete dissociation.

Analyzing Each Option

The following option analysis is done: (a) Strong electrolytes are almost completely dissociated at all dilutions, hence do not follow the Ostwald's Dilution law because it is applicable for weak electrolytes having partial dissociation.(b) The law of mass action in its simple form operates within the premise of low concentration of ions. Therefore, for high ion concentrations, the prediction of the law of mass action becomes inaccurate.(c) The hydrolysis of ions can indeed influence concentration terms, thus affecting the efficacy of Ostwald's law.

Final Answer Determination

With the above points, it is clear that all the options (a, b, and c) are correct reasons for the failure of Ostwald's dilution law in the context of strong electrolytes.

Key concepts

Strong Electrolytes

Strong electrolytes are substances that completely dissociate into ions when dissolved in water, regardless of the concentration. Unlike weak electrolytes, which only partially dissociate, strong electrolytes break apart almost entirely into their constituent ions.

Because of this, their electrical conductivity is quite high, and their dissociation doesn't depend on the concentration.

• Examples include salts like sodium chloride (NaCl), strong acids like hydrochloric acid (HCl), and strong bases like sodium hydroxide (NaOH).
• The complete dissociation makes it difficult for Ostwald's Dilution Law to be applicable to strong electrolytes.

This is why strong electrolytes behave differently and do not fit the predictions made by the Ostwald's Dilution Law, which is primarily for weak electrolytes.

Degree of Dissociation

The degree of dissociation, represented by the Greek letter \( \, \alpha \), indicates the fraction of an electrolyte that dissociates into ions in solution. For weak electrolytes, this measure is particularly significant because they don't fully dissociate.

The degree of dissociation can vary with concentration.

• In dilute solutions, weak electrolytes tend to dissociate more, increasing \( \, \alpha \).
• With strong electrolytes, \( \, \alpha \) is typically approximated to 1, meaning almost complete dissociation.

Ostwald's Dilution Law uses \( \, \alpha \) to relate the concentration of a weak electrolyte to its dissociation, and this is where the discrepancy arises when we apply it to strong electrolytes, as they do not follow this pattern.

Law of Mass Action

The law of mass action is a key principle in chemistry that describes how the rate of a reaction is related to the concentration of the reactants. For electrolytes in solution, it explains the dynamics of dissociation and association of ions.

According to this law, for a reaction \( aA + bB \rightleftharpoons cC + dD \), the equilibrium constant \( K \) is given by:\[K = \frac{{[C]^c [D]^d}}{{[A]^a [B]^b}}\]In the context of electrolytes:

• We apply this to understand how ions associate or dissociate in solution.
• For strong electrolytes, the law becomes less applicable because the concentrations of ions are so high that assumptions break down.

High ion concentrations mean that the interactions are not as straightforward as the law assumes, leading to deviations from predicted behavior.

Hydrolysis of Ions

Hydrolysis of ions occurs when ions in a solution interact with water, leading to partial decomposition and the formation of other compounds, often affecting the pH of the solution. This is especially relevant for salts that form from strong acids and weak bases, or vice versa.

During hydrolysis:

• Water molecules react with the dissolved ion species, which can change the concentration of reactants or products.
• This interaction can further complicate how electrolyte solutions behave, deviating from simple laws like Ostwald's.
• Hydrolysis can lead to the formation of acidic or basic solutions, depending on the ions involved.

The effect of hydrolysis is another reason why Ostwald's Dilution Law struggles with accurately predicting behavior for certain solutions. Understanding hydrolysis is crucial in chemistry, as it helps explain pH changes and other dynamic reactions in solutions.