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Chapter 11: Problem 42

The high melting points of ionic solids indicate that a lot of energy must be supplied to separate the ions from one another. How is it possible that the ions can separate from one another when soluble ionic compounds are dissolved in water, often with essentially no temperature change?

Short Answer

Expert verified
Soluble ionic compounds can dissolve in water without a significant temperature change because the energy released during the hydration process, where water molecules surround and interact with the ions, compensates for the energy needed to break the strong ionic bonds within the lattice structure. The water molecules play a crucial role in weakening the electrostatic forces between the ions, allowing them to separate from each other.

Step by step solution

01

Understand the melting process of ionic solids

Ionic solids have a lattice structure with positive and negative ions held together by ionic bonds. The high melting points of these solids are an indication of the strong electrostatic forces between the ions. In order to melt an ionic compound, a significant amount of energy is required to overcome these forces and separate the ions from the lattice structure.
02

Analyze the dissolving process of ionic compounds in water

When ionic compounds dissolve in water, the ions separate from each other and are surrounded by water molecules. This process is called hydration. The water molecules form a shell around each ion, with the oxygen atom of the water molecule (which has a partial negative charge) oriented towards the positively charged ion, and the hydrogen atoms (with partial positive charges) oriented towards the negatively charged ion. This interaction between water molecules and ions weakens the electrostatic forces between the ions and allows them to separate from each other.
03

Compare the energy changes in the dissolving process with the melting process

During the dissolving process, energy is required to break the ionic bonds between the ions in the solid lattice, but energy is also released when the ions interact with the water molecules and become hydrated. If the energy released during the hydration process is sufficient to compensate for the energy required to break the ionic bonds, the dissolving process can occur without a significant temperature change.
04

Explain why some ionic compounds dissolve without a temperature change

In the case of soluble ionic compounds, the energy released during the hydration process is able to balance the energy required to break the ionic bonds. This is why these compounds can dissolve in water without essentially any temperature change. In conclusion, soluble ionic compounds are able to dissolve in water without a significant temperature change because the energy released during the hydration process compensates for the energy needed to break the ionic bonds within the lattice structure. The water molecules play a crucial role in weakening the electrostatic forces between the ions and allowing them to separate from each other.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Melting Points of Ionic Solids
Ionic solids are composed of a structured lattice of alternating positive and negative ions. This arrangement is maintained by strong ionic bonds, which are essentially electrostatic forces acting between the oppositely charged ions. These forces are incredibly strong, which is why ionic solids typically have high melting points.
To melt these solids, you need a significant amount of energy to disrupt these forces and break the ions apart. This energy must overcome the electrostatic attractions that hold the lattice together, allowing the ions to move freely and transition into a liquid state.
  • High melting points indicate strong ionic bonds.
  • Energy is required to disrupt the crystalline structure.
  • This energy need is why ionic solids remain stable at lower temperatures.
Hydration Process
The hydration process is an interesting phenomenon that occurs when ionic compounds dissolve in water. When these compounds enter water, they dissociate into ions that get surrounded by water molecules. This surrounding process is known as hydration and plays a critical role in weakening the electrostatic forces between the ions.
Water molecules are polar, meaning they have a slight charge distribution鈥攐xygen carries a partial negative charge, while hydrogen carries a partial positive charge. This polarity allows water molecules to interact with the ions effectively.
  • Hydration involves water molecules forming a shell around each ion.
  • Oxygen end faces positive ions, hydrogen end faces negative ions.
  • This action helps ions to separate from each other more easily, facilitating dissolution.
Electrostatic Forces
Electrostatic forces are the backbone of ionic bonding, acting between the positive and negative ions. These forces are very strong, which contributes to the high melting points and stability of ionic solids. To separate the ions, either through melting or dissolving, these forces must be overcome in some way.
In the case of dissolving, the interaction with water molecules weakens these forces sufficiently for the ions to separate. During the hydration process, the energy released often compensates for the energy needed to separate the ions, allowing the process to occur smoothly, often without a noticeable temperature change.
  • Electrostatic forces ensure solid ionic bonds.
  • Both melting and dissolving need these bonds broken.
  • Hydration energy can offset the energy required to break these bonds.

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Most popular questions from this chapter

In a coffee-cup calorimeter, \(1.60 \mathrm{~g} \mathrm{NH}_{4} \mathrm{NO}_{3}\) was mixed with \(75.0 \mathrm{~g}\) water at an initial temperature \(25.00^{\circ} \mathrm{C}\). After dissolution of the salt, the final temperature of the calorimeter contents was \(23.34^{\circ} \mathrm{C}\). a. Assuming the solution has a heat capacity of \(4.18 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\), and assuming no heat loss to the calorimeter, calculate the enthalpy of solution \(\left(\Delta H_{\text {soln }}\right)\) for the dissolution of \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) in units of \(\mathrm{kJ} / \mathrm{mol}\). b. If the enthalpy of hydration for \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) is \(-630 . \mathrm{kJ} / \mathrm{mol}\), calculate the lattice energy of \(\mathrm{NH}_{4} \mathrm{NO}_{3} .\)

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