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 \(\mathrm{NaCl}\) so soluble in water?

Short answer

NaCl is soluble in water primarily due to its strong hydration energy. When NaCl dissolves, the ionic bonds in the crystal lattice are broken, and the ions are surrounded by water molecules. The dissolution process involves two energy changes: the endothermic lattice energy and the exothermic hydration energy. In the case of NaCl, the strong hydration energy, which results from the attractions between the ions and water molecules, compensates for the energy required to break the ionic bonds in the crystal lattice. This makes the dissolution process energetically favorable, allowing NaCl to dissolve in water.

Step-by-step solution

Understand the dissolution process

When a salt dissolves in water, the crystal lattice of the salt is broken apart and the ions are surrounded by water molecules. This process involves overcoming the attraction between the ions in the crystal lattice and the formation of new attractions between the ions and the water molecules.

Consider the energy changes during dissolution

Dissolving a salt in water involves two main energy changes: 1. The energy required to break the ionic bonds in the crystal lattice (lattice energy) - an endothermic process that consumes energy. 2. The energy released when ions form new attractions with water molecules (hydration energy) - an exothermic process that releases energy.

Factors affecting the solubility of NaCl in water

The solubility of NaCl in water is affected by the balance between the lattice energy and the hydration energy. When the hydration energy is greater than or equal to the lattice energy, the process becomes energetically favorable, and NaCl will dissolve in water.

Discuss the specific interactions that make NaCl soluble in water

NaCl is soluble in water primarily because of the strong hydration energy. When NaCl dissolves in water, the positively charged Na+ ions are attracted to the partially negative oxygen atoms of the water molecules, while the negatively charged Cl- ions are attracted to the partially positive hydrogen atoms of the water molecules. These attractions result in the formation of a hydration shell around the ions, which stabilizes them in the solution. This strong hydration energy compensates for the energy required to break the ionic bonds in the crystal lattice, making the dissolution process energetically favorable.

Key concepts

Dissolution process

The dissolution process is the sequence of steps that takes place when a substance, such as sodium chloride (NaCl), dissolves in a solvent like water. Picture a crystal of NaCl; it consists of positive sodium ions (\(\text{Na}^+\)) and negative chloride ions (\(\text{Cl}^-\)) arranged in a solid structure, known as a crystal lattice. During dissolution, this lattice is broken apart as water molecules surround each individual ion.

For NaCl to dissolve, water molecules use their partial charges to interact with the ions. The oxygen end of a water molecule, which is slightly negative, attracts the\(\text{Na}^+\)ions, while the hydrogen end, being slightly positive, attracts the\(\text{Cl}^-\)ions. This interaction weakens the ionic bonds between the\(\text{Na}^+\)and\(\text{Cl}^-\)ions, leading to the breakdown of the crystal lattice and forming what is known as a solution. This process crucially involves overcoming the strength of the ionic bonds initially holding the salt together.

Lattice energy

Lattice energy is a crucial concept when understanding how ionic compounds like NaCl dissolve in water. It refers to the amount of energy needed to disassemble a crystal lattice into gaseous ions. This process is typically endothermic, meaning it absorbs energy from the surrounding environment.

For NaCl, its strong crystal lattice, maintained by the electrostatic forces between opposite charges of \(\text{Na}^+\)and\(\text{Cl}^-\), requires significant energy to break apart. The lattice energy is quite high due to these strong ionic interactions. It tends to work against the dissolution process since this energy must be provided for the ions to separate. However, in the presence of water, this energy is compensated by another factor, hydration energy, making dissolution possible.

Hydration energy

Hydration energy plays a pivotal role in the solubility of salts, like NaCl, in water. It is the energy released when water molecules surround and stabilize individual ions after they have separated from the crystal lattice. This process is exothermic, releasing energy to the surrounding system.

When NaCl dissolves, water molecules form a 'hydration shell' around each sodium and chloride ion. The partial charges on the water molecules align with the opposite charges on the ions: the partially negative oxygen atoms of water are attracted to \(\text{Na}^+\)ions, while the partially positive hydrogen atoms are drawn to \(\text{Cl}^-\)ions. This formation of the hydration shell is crucial, as it not only stabilizes the ions in solution but also releases enough energy to offset the energy required to break the lattice structure. This compensation essentially drives the dissolution process forward, rendering the solution energetically favorable.

Ionic bonds

Ionic bonds are the strong electrostatic attractions between positively charged ions, like sodium (\(\text{Na}^+\)), and negatively charged ions, like chloride (\(\text{Cl}^-\)). These bonds form the backbone of the crystal lattice structure in salts such as NaCl.

These bonds provide notable stability and require substantial energy to break, as reflected in the high lattice energy of sodium chloride. Ionic bonds create a tightly packed, organized structure that gives salts their solid form. During dissolution, water molecules disrupt these ionic bonds, allowing the individual ions to disperse and become surrounded by water molecules.

• The strong forces of attraction between \(\text{Na}^+\)and\(\text{Cl}^-\)ions make the ionic bonds in NaCl significant in energy considerations.
• Understanding these forces is vital to grasp why dissolution requires energy and how it's facilitated by the subsequent energy release during hydration.