Question

Would you expect the mobility of an electrolytic ion to increase or decrease for each of the following? (a) increasing the size of the ion;(b) increasing charge on the ion; (c) increasing temperature and (d) placing it in a solvent of lower viscosity.

Short answer

In summary, for an electrolytic ion, mobility will (a) decrease with an increase in ion size, (b) increase with an increase in ion charge, (c) increase with an increase in temperature, and (d) increase in a solvent of lower viscosity.

Step-by-step solution

Factor (a): Increasing the size of the ion

When the size of the ion increases, the resistance it faces while moving through the medium also increases due to stronger interactions with solvent molecules. These increased interactions and larger surface area lead to less mobility. Therefore, as the size of the ion increases, its mobility will decrease.

Factor (b): Increasing charge on the ion

When the charge on an ion increases, the force applied to the ion by the external electric field also increases. This increased force helps the ion overcome resistance in the medium and move more quickly. Therefore, as the charge on the ion increases, its mobility will also increase.

Factor (c): Increasing temperature

The temperature of the medium has a direct effect on the movement of ions. As the temperature increases, the kinetic energy of the individual molecules and ions in the medium increases. This increased energy leads to more frequent collisions and faster movement of the ions. Also, with higher temperature, the viscosity of the solvent tends to decrease which facilitates the movement of ions. Therefore, as the temperature of the medium increases, the mobility of the electrolytic ion will also increase.

Factor (d): Placing it in a solvent of lower viscosity

Ion mobility is affected by the viscosity of the solvent; higher viscosity results in the slower movement of ions, whereas lower viscosity allows ions to move more freely. When an ion is placed in a solvent of lower viscosity, there will be less resistance to the ion's movement. This reduced resistance will lead to increased mobility. Therefore, when the electrolytic ion is placed in a solvent with lower viscosity, its mobility will increase.

Key concepts

Ion Size and Mobility

The size of an ion plays a crucial role in determining its mobility. When an ion increases in size, it faces more resistance when traveling through a medium. This is because larger ions have a greater surface area that interacts with the molecules in the solvent. These interactions slow down the ion's movement, reducing its overall mobility. Imagine trying to push a large, heavy box across a surface compared to a small one; the larger box is more difficult to move due to greater friction. Similarly, in the world of ions, bigger ions have a "sluggish" mobility compared to their smaller counterparts.
Thus, as the size of the ion increases, its mobility through the medium decreases.

Charge Effect on Ion Mobility

Ion mobility is heavily influenced by the charge of the ion. The charge is essentially how much force the ion can generate within an electric field. When the charge of an ion is increased, the force acting on the ion also increases. This stronger force enables the ion to overcome the resistance encountered in the medium more effectively. Think of charge as a personal boost of energy that allows an ion to zip through its surroundings more efficiently. Consequently, as the charge on an ion increases, its mobility tends to rise as well, allowing it to move faster through the electrolyte solution.

Temperature Effect on Ions

Temperature can dramatically influence ion movement. As the temperature rises, the kinetic energy of the molecules and ions in the medium increases. This results in more energetic particles moving faster and colliding more frequently. The increase in movement creates a dynamic environment where ions can travel swiftly. In addition, higher temperatures typically reduce the viscosity of the solvent, which further aids in the movement of ions. Think of a warm soup with less resistance for stirring compared to a chilled one. Therefore, increasing the temperature not only provides energy for faster movement but also lowers resistance, resulting in increased ion mobility.

Viscosity and Ion Movement

The viscosity of the solvent in which the ion resides is a key determinant of how easily ions can cruise through. Viscosity is essentially the "thickness" of the liquid; higher viscosity means thicker liquid, which translates to more resistance against ion movement. When ions are placed in a solvent with lower viscosity, there's less "thickness" to push through, allowing the ions to flow more freely. The effect is akin to moving your hand through water compared to honey; water provides less resistance, allowing for quicker and smoother movement. Therefore, lower viscosity in the solvent results in enhanced ion mobility, as ions can glide with greater ease and speed.