Problem 24 - Chemistry An Atoms First Approach

Question

In order for sodium chloride to dissolve in water, a small amount of energy must be added during solution formation. This is not energetically favorable. Why is NaCl so soluble in water?

Step-by-Step Solution

Answer

Sodium chloride (NaCl) is soluble in water due to the polar nature of water molecules and the formation of ion-dipole interactions between Na+ and Cl- ions and water molecules. Although energy is required to break the ionic bonds in NaCl (endothermic process), the exothermic hydration energy released during the formation of ion-dipole interactions is sufficient to overcome this energy requirement, resulting in a net decrease in energy and making NaCl dissolution energetically favorable.

Step 1: Understand the components involved in the process

Sodium chloride, which is a positively charged sodium ion (Na+) and a negatively charged chloride ion (Cl-), is an ionic compound. Water, on the other hand, is a polar molecule with a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom.

Step 2: Describe the energetic process during dissolution

In order for sodium chloride to dissolve in water, the ionic bonds between the Na+ and Cl- ions must be broken, which requires the input of energy. This process is endothermic and therefore not energetically favorable on its own.

Step 3: Explain the role of water molecules in the dissolution process

Due to the polar nature of water molecules, they have a tendency to attract charged particles, such as the positively charged Na+ and negatively charged Cl- ions in sodium chloride. When water molecules come into contact with sodium chloride, the partial positive charge on the hydrogen atoms in water is attracted to the negatively charged chloride ions, whereas the partial negative charge on the oxygen atom in water is attracted to the positively charged sodium ions. These attractions between water molecules and sodium chloride ions create ion-dipole interactions that help to overcome the ionic bonds holding sodium chloride together.

Step 4: Describe the energetic changes during the dissolution process

As sodium chloride dissolves in water, energy is released due to the formation of ion-dipole interactions between water molecules and Na+ and Cl- ions. The energy released during this process is called hydration energy, and it is exothermic. The net energy change during the dissolution process is determined by the balance between the endothermic process of breaking ionic bonds and the exothermic process of forming ion-dipole interactions.

Step 5: Explain why NaCl is so soluble in water despite the energy requirement

The reason why sodium chloride is so soluble in water is because the exothermic hydration energy released during the formation of favorable ion-dipole interactions between the Na+ and Cl- ions and water molecules is sufficient to overcome the endothermic energy required to break the ionic bonds in sodium chloride. This results in a net decrease in energy, making the dissolution process energetically favorable.

Key Concepts

Ionic Compounds

Ionic compounds are made up of charged particles known as ions. These ions are the result of one atom losing electrons and another atom gaining them.
For instance, in sodium chloride (NaCl), sodium (Na) loses an electron to become positively charged (Na+), while chlorine gains that electron to become negatively charged (Cl-).
The attraction between these oppositely charged ions forms a strong ionic bond.

• Sodium Ion (Na+): This is a positively charged ion formed when sodium gives away one electron.
• Chloride Ion (Cl-): This is a negatively charged ion that accepts an electron to complete its outer shell.
• Ionic Bond: The electrostatic attraction between Na+ and Cl- ions holds them tightly in a lattice structure.

Breaking these bonds demands energy, but is essential for dissolution in water.

Polar Molecules

Water is an excellent example of a polar molecule. This means one end of the molecule carries a slight positive charge, while the other carries a slight negative charge.
This happens due to the difference in electronegativity between hydrogen and oxygen atoms in water. Oxygen is more electronegative and pulls the shared electrons towards itself, creating a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atom.

• Partial Charges: These occur because of the uneven distribution of electrons, leading to a positive and negative side.
• Water Molecules: They have a bent shape due to the angles between hydrogen and oxygen, enhancing polarity.

Polar molecules can attract ions or other polar molecules through their partial charges, which aids in the dissolution of substances like sodium chloride.

Ion-Dipole Interactions

Ion-dipole interactions are crucial in the dissolving process of ionic compounds in polar solvents like water.
These interactions occur between the charged ions of the compound and the polar molecules of the solvent.
In the case of sodium chloride dissolving in water, you can picture the water molecules orienting themselves to maximize attraction with the Na+ and Cl- ions.

• Sodium Chloride Dissolution: Water’s partial negative charge on oxygen atoms attracts Na+ ions.
• Chloride Ions Attraction: The partial positive charge on water's hydrogen atoms is drawn toward Cl- ions.

These forces help to pull apart the ionic lattice of salt, which is necessary for it to dissolve, even overcoming the energy needed to break ionic bonds.

Hydration Energy

When water molecules surround dissolved ions, creating a stable solution, this process is associated with the release of energy known as hydration energy.
As sodium chloride dissolves, the ion-dipole interactions formed release energy, offsetting the energy required to break the ionic bonds in the solid structure.

• Energy Release: Ion-dipole interactions release energy, helping dissolve the salt by stabilizing ions in solution.
• Exothermic Process: Hydration energy is exothermic, meaning it releases heat, which contributes to the overall favorability of the dissolution process.

Although input energy is required initially, the exothermic nature of hydration significantly contributes to the high solubility of sodium chloride in water.