Problem 9 - Chemistry and Chemical Reactivity

Question

In each pair of ionic compounds, which is more likely to have the more negative enthalpy of hydration? Briefly explain your reasoning in each case. (a) LiCl or CsCl (b) \(\mathrm{NaNO}_{3}\) or \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) (c) RbCl or \(\mathrm{NiCl}_{2}\)

Step-by-Step Solution

Answer

LiCl, Mg(NO3)2, and NiCl2 have more negative enthalpies of hydration.

Step 1: Understanding Enthalpy of Hydration

Enthalpy of hydration refers to the energy change that occurs when one mole of gaseous ions dissolves in water to form a solution. A more negative enthalpy indicates stronger attractions between water molecules and the ions. This is influenced by the charge and size of the ion.

Step 2: Comparison of LiCl and CsCl

LiCl will have a more negative enthalpy of hydration than CsCl. Lithium ions (Li⁺) are smaller than cesium ions (Cs⁺), leading to a higher charge density in Li⁺. This results in stronger attractions to water molecules.

Step 3: Comparison of NaNO3 and Mg(NO3)2

Mg(NO3)2 will have a more negative enthalpy of hydration than NaNO3. Magnesium ions (Mg²⁺) have a higher charge than sodium ions (Na⁺), leading to stronger ionic attractions with water molecules despite having a similar ionic size.

Step 4: Comparison of RbCl and NiCl2

NiCl2 will have a more negative enthalpy of hydration than RbCl. Nickel ions (Ni²⁺) have a higher charge than rubidium ions (Rb⁺), resulting in stronger attractions to water molecules.

Key Concepts

Ionic Compounds

Ionic compounds are made up of positive and negative ions held together by electrostatic forces. These compounds are usually formed when metals bond with non-metals, transferring electrons from the metal to the non-metal. This transfer results in the formation of positively charged cations and negatively charged anions. Ionic compounds have unique properties, including:

• High melting and boiling points due to strong ionic bonds.
• Solubility in water as the water molecules can interact with the charged ions.
• Ability to conduct electricity when dissolved or molten due to free-moving ions.

Understanding these basic properties helps explain why ionic compounds behave the way they do in water, especially through the process of hydration.

Ion Charge and Size

The charge and size of ions significantly influence how they interact with water. Higher charge and smaller size result in a greater charge density. Imagine charge density as how concentrated an ion's charge is, like how much weight is packed into a single box. A higher charge density means stronger attraction to opposite charges, like water molecules.
For example, magnesium ions \(\text{Mg}^{2+}\) have a higher charge than sodium ions \(\text{Na}^{+}\), so they attract water more strongly due to the extra positive charge. Similarly, smaller ions like lithium \(\text{Li}^{+}\) occupy less space and bring water molecules closer, which increases the strength of attraction. By understanding how differences in charge and size affect ions, we determine which compound might have a more negative enthalpy of hydration.

Hydration Energy

Hydration energy is the energy released when ions dissolve in water. It signifies how favorable the dissolution process is. More negative hydration energies reflect stronger interactions between ions and water, meaning it requires more energy to separate them.

• Ions with high charge and small size, like \(\text{Mg}^{2+}\), often have very negative hydration energies because the water molecules can come really close and strongly interact.
• Ions with lower charge or larger size, like \(\text{Cs}^{+}\), have weaker interactions, resulting in less negative hydration energies.

This concept is crucial to understanding why different ionic compounds dissolve with differing ease and how they affect the solution's properties.

Water-Ion Interactions

Water-ion interactions play a key role in determining how ionic compounds dissolve. Water is a polar molecule, meaning it has a partially positive and a partially negative end. The positive and negative ends allow water to surround ions and stabilize them as they dissolve.
Hydration Process:

• Anions are surrounded by the partially positive hydrogen atoms of water molecules.
• Cations are surrounded by the partially negative oxygen atoms of water molecules.

These interactions are what release hydration energy. Stronger water-ion interactions lead to more negative hydration energies, making understanding this interaction fundamental for predicting the solubility of ionic compounds.