Question

Which of the following postulatales of Debye-Huckel theory is/are true? (a) The strong electrolyte is completely ionised at all dilutions. (b) The oppositely changed ions are completely distributed in the solution but the cations tend to be found in the vicinity of anions and vice versa. (c) Decrease in equivalent conductance with increase in concentration is due to fall in mobilities of ions due to inter-ionic effect. (d) All of the above.

Short answer

(d) All of the above.

Step-by-step solution

Analyze Postulate (a)

The first postulate suggests that a strong electrolyte will be completely ionised at all dilutions. In Debye-Huckel theory this is a widely recognized assumption. When strong electrolytes are completely dissolved in water, they dissociate entirely into cations and anions.

Analyze Postulate (b)

The second postulate claims that all ions are completely distributed in the solution and that cations tend to be found in the vicinity of anions and vice versa. This is also a true principle in Debye-Huckel theory. It's essentially describing that in electrolyte solutions there is some level of ion association due to opposite charges attracting.

Analyze Postulate (c)

Postulate (c) suggests a decrease in equivalent conductance with an increase in concentration due to a decline in ion mobilities from the inter-ionic effect. This is also correct in accordance with Debye-Huckel theory. It is known that as concentration increases, the conductance decreases due to the increase in inter-ionic interactions which limits the mobility of the ions.

Determine if all Postulates are True

Having gone through each of the postulates, it can be seen that all are correct within the context of Debye-Huckel theory. Thus postulate (d) claiming all of the above are true is also correct.

Key concepts

Electrolyte Ionisation

Understanding the full ionization of strong electrolytes is crucial when studying solutions in chemistry. According to Debye-Huckel theory, a strong electrolyte refers to substances like table salt (sodium chloride) or hydrochloric acid (HCl), which completely dissociate into their respective ions when dissolved in a solvent, usually water. This postulate explains why such solutions conduct electricity well - it's due to the free-flowing ions that are formed.

Every student should grasp that a strong electrolyte doesn’t merely partially dissociate; it yields a solution where originally associated ions are now fully separated and dispersed throughout the solvent. This is significantly different from how weak electrolytes behave, which partially ionize resulting in a mixture of ions and un-ionized molecules. Appreciating this cornerstone of ionic solutions forms the bedrock for understanding subsequent properties such as electrical conductance.

Ionic Distribution in Solutions

The spatial arrangement of ions in a solution is far from random; they are influenced by their charges and interactions with solvent molecules. Debye-Huckel theory sheds light on this ionic distribution, where cations (positively charged ions) and anions (negatively charged ions), despite being uniformly distributed, reveal a tendency to be found near each other. This is because of the electrostatic attraction between opposing charges.

In a solution, you won't find isolated territories of cations and anions; instead, there's a delicate dance as they attract and repel, creating a balanced zone of influence around each other. This affects everything from the properties of the solution to how we can measure and predict their behavior.

Equivalent Conductance

A solution's equivalent conductance is a measure of its ability to conduct electricity. However, this property isn't static; it varies with concentration. Debye-Huckel theory highlights that as the concentration of an electrolyte solution increases, the equivalent conductance typically decreases. The reason for this pattern lies in the concept of ion mobility.

At lower concentrations, ions have more freedom to move, which means they can carry electric current more efficiently. As solutions become more concentrated, ions get closer together, and their mutual electric fields interfere with each other's movement, resulting in reduced conductance. Students should realize that this reduction is a direct manifestation of increased inter-ionic interactions.

Inter-ionic Effect

The inter-ionic effect describes how ions influence each other's movements in solution, a pivotal part of the Debye-Huckel theory. When ions are in close proximity, their electric fields interact, creating a force that affects an ion's mobility, and hence, the solution's overall electrical conductance.

For younger scholars or those new to electrochemistry, it may be helpful to think of this effect as a crowded room: When the room is less crowded (low concentration), it’s easier to move through it (higher mobility). But as more people (ions) enter the room (higher concentration), moving becomes more difficult (lower mobility), due to the increased interactions (inter-ionic effect). It's this fundamental concept that helps explain why equivalent conductance decreases with increasing concentration.