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 NiCl_

Step-by-Step Solution

Answer

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

Step 1: Understanding Enthalpy of Hydration

The enthalpy of hydration is the amount of energy released when one mole of ions dissolve in water to form an aqueous solution. Compounds with smaller cation size or higher charge typically have more negative enthalpies of hydration.

Step 2: Comparing LiCl and CsCl

Lithium (Li) has a smaller ionic radius than Cesium (Cs). This smaller size allows Li+ to interact more strongly with water molecules, resulting in a more negative enthalpy of hydration compared to CsCl.

Step 3: Comparing NaNO3 and Mg(NO3)2

Magnesium (Mg) has a +2 charge compared to Sodium's (Na) +1 charge. The higher charge density of Mg2+ allows it to interact more strongly with water molecules, leading to a more negative enthalpy of hydration compared to NaNO3.

Step 4: Comparing RbCl and NiCl2

Nickel (Ni) in NiCl2 has a +2 charge compared to Rubidium's (Rb) +1 charge. The higher charge density of Ni2+ makes NiCl2 have a stronger attraction with water molecules, therefore having a more negative enthalpy of hydration compared to RbCl.

Key Concepts

Ionic Compounds

Ionic compounds are chemical compounds composed of ions held together by electrostatic forces known as ionic bonding. These compounds consist of cations, which are positively charged ions, and anions, which are negatively charged ions. Together, they form a structure that is electrically neutral.

When ionic compounds dissolve in water, the ions disassociate and interact with water molecules. This process typically results in the release or absorption of energy, defined by the enthalpy of hydration. The formation of hydration shells around the ions is crucial as it influences various properties of solutions, such as electrical conductivity and solubility.

For instance, in the exercise, the dissolution of compounds like LiCl and CsCl showcases how ionic bonding promotes strong interactions with water, influenced predominantly by ionic characteristics like cation size and charge density.

Cation Size

The size of cations plays a vital role in determining the enthalpy of hydration. Cations are smaller than their parent atoms due to the loss of an electron shell, resulting in a higher positive charge density that can strongly attract water molecules.

When a cation interacts with a water molecule, the oxygen atom in water, which is slightly negative, is attracted to the positive ion. This results in the formation of hydration shells. Smaller cations like Li⁺ can interact more strongly with water than larger cations because their charge is concentrated over a smaller volume, leading to stronger electrostatic attraction.

In the exercise, Li⁺ from LiCl demonstrates a more negative enthalpy of hydration than Cs⁺ from CsCl, largely due to its smaller size, which increases its ability to attract water molecules effectively.

Charge Density

Charge density refers to the concentration of electrical charge in a given space and is a pivotal factor affecting the enthalpy of hydration. It is influenced by both the magnitude of the charge and the size of the ion. Higher charge density usually results in stronger attractions between ions and water molecules, leading to more negative enthalpy of hydration.

Cations with higher charges exert more substantial attractions on the surrounding water molecules. For example, a Mg²⁺ ion in Mg(NO₃)₂ has a higher charge density compared to a Na⁺ ion in NaNO₃ because it not only possesses a greater charge but is also smaller in size. Consequently, Mg²⁺ will result in stronger interactions with water molecules, releasing more energy during hydration.

Similarly, Ni²⁺ from NiCl₂ has a higher charge density compared to Rb⁺ from RbCl, which corresponds to a stronger attraction with water molecules and a more negative enthalpy of hydration.