Problem 67 - Introductory Chemistry Atoms First

Question

Which has the greater hydration energy, \(\mathrm{AlCl}_{3}\) or NaCl? Explain your answer.

Step-by-Step Solution

Answer

AlCl₃ has a greater hydration energy than NaCl, as the Al³⁺ ion is smaller and more highly charged than the Na⁺ ion, leading to stronger electrostatic interactions between the ion and water molecules. This results in a larger amount of energy being released during the hydration process.

Step 1: Define hydration energy

Hydration energy is the energy that is released when one mole of gaseous ions combines with water to form hydrated ions. It is an exothermic process and its value depends on the size and charge of the ions being hydrated. Smaller and more highly charged ions typically have higher hydration energies.

Step 2: Identify the ions in AlCl₃ and NaCl

In AlCl₃, we have the aluminum ion, Al³⁺, and the chloride ion, Cl⁻. In NaCl, we have the sodium ion, Na⁺, and the chloride ion, Cl⁻.

Step 3: Compare the size and charge of Al³⁺ and Na⁺ ions

The aluminum ion, Al³⁺, is smaller and has a higher charge (3⁺) than the sodium ion, Na⁺ (1⁺). This means that Al³⁺ ions will exhibit stronger electrostatic interactions with water's polar molecules due to their higher charge, leading to a larger hydration energy.

Step 4: Compare the hydration energies of AlCl₃ and NaCl

Since the hydration energy is determined mainly by the cations (Al³⁺ and Na⁺) in the salts, we can compare their hydration energies based on these cations' properties. As mentioned earlier, Al³⁺ has a higher charge and is smaller than Na⁺. This indicates that AlCl₃ will have a higher hydration energy compared to NaCl.

Step 5: Conclusion

AlCl₃ has a greater hydration energy than NaCl, as the Al³⁺ ion is smaller and more highly charged than the Na⁺ ion, leading to stronger electrostatic interactions between the ion and water molecules. This results in a larger amount of energy being released during the hydration process.

Key Concepts

Ionic Compounds

Ionic compounds are fascinating structures formed when metals combine with non-metals to create a lattice of charged particles, known as ions. These compounds result from the complete transfer of electrons between atoms.
For example, in the formation of sodium chloride (NaCl), a sodium atom donates an electron to a chlorine atom. This forms sodium ions, Na⁺, and chloride ions, Cl⁻.
Some key features define ionic compounds:

• They are generally solid at room temperature.
• They have high melting and boiling points.
• They dissolve easily in water, thanks to the interaction between the ions and the polar water molecules.
• Their crystal structures are arranged in a repeating pattern that maximizes attractive forces and minimizes repulsive forces.

Understanding ionic compounds is essential for grasping how different substances interact with their environment, especially in a solution setting like water.

Charge and Size of Ions

The charge and size of ions are crucial factors in determining their chemical behavior, particularly in solutions. The charge of an ion is essentially its oxidation state, indicated by the number of electrons lost or gained from its parent atom.
Ions with a higher charge, like Al³⁺ or Fe³⁺, have stronger interactions with opposite charges, than those with a lower charge, like Na⁺ or K⁺. The highly charged ions are more prone to lead complex chemical behaviors, such as higher hydration energies.
Additionally, the size of ions plays a significant role in determining their interaction strength. Smaller ions, like Li⁺ compared to K⁺, can pack more tightly and bond more strongly with other ions and molecules in their surroundings.

• Small ions with high charges typically have greater attraction to opposite charges, leading to higher hydration energies.
• These factors altogether influence properties such as the solubility and stability of ionic compounds.

Electrostatic Interactions

Electrostatic interactions are the forces that occur between charged particles or ions. These interactions are pivotal in the understanding of molecular and ionic behaviors, especially in aqueous environments.
In aqueous solutions, water molecules interact with ions predominantly through electrostatic forces. Water, being polar, has a partial negative charge on the oxygen and partial positive charges on the hydrogens.

• The attraction between the water molecules and anions, like Cl⁻, occurs at the hydrogen ends of the water molecules.
• Meanwhile, cations, like Na⁺, attract the oxygen end, leading to hydration.

This is why smaller ions or ions with higher charges, such as Al³⁺, enhance electrostatic interactions, resulting in greater hydration energies. Electrostatic interactions are critical in many biological and chemical processes, defining how substances dissolve and react in water.