Question

One of the attractive features of ionic liquids is their low vapor pressure, which in turn tends to make them nonflammable. Why do you think ionic liquids have lower vapor pressures than most room-temperature molecular liquids?

Short answer

Ionic liquids have lower vapor pressures than most room-temperature molecular liquids because they have stronger intermolecular forces, primarily due to the significant Coulombic forces between their cations and anions. In contrast, room-temperature molecular liquids have weaker forces, such as dipole-dipole, hydrogen bonding, and van der Waals forces. Stronger intermolecular forces require more energy for evaporation, resulting in lower vapor pressures and making ionic liquids nonflammable.

Step-by-step solution

1. Understanding the composition of ionic liquids

Ionic liquids are composed of ions, specifically cations and anions. These ions are not well-coordinated, meaning that they are not highly ordered in their structure. In ionic liquids, the electrostatic forces between cations and anions, called Coulombic forces, are a significant contributor to the overall intermolecular forces present in the liquid.

2. Understanding the composition of room-temperature molecular liquids

Room-temperature molecular liquids consist of neutral molecules which interact through various types of intermolecular forces including dipole-dipole, hydrogen bonding, and van der Waals forces. These forces tend to be weaker than the forces present in ionic liquids.

3. The relationship between intermolecular forces and vapor pressure

Vapor pressure is influenced by the strength of the intermolecular forces present in a substance. A substance with stronger intermolecular forces requires more energy to evaporate, leading to a lower vapor pressure. Conversely, a substance with weaker intermolecular forces requires less energy to evaporate, resulting in a higher vapor pressure.

4. Comparing intermolecular forces in ionic and molecular liquids

As discussed earlier, ionic liquids have significant Coulombic forces acting between the ions that contribute to the overall intermolecular forces present in the liquid, making them stronger compared to the forces in room-temperature molecular liquids. Room-temperature molecular liquids have weaker intermolecular forces such as dipole-dipole, hydrogen bonding, and van der Waals forces, which are not as strong as the ionic forces in ionic liquids.

5. Connecting the strength of intermolecular forces to the vapor pressure of ionic liquids

The stronger intermolecular forces in ionic liquids mean that more energy is required for these substances to evaporate, or transition to their gas phase. As a result, ionic liquids have lower vapor pressures compared to room-temperature molecular liquids, which have weaker intermolecular forces and, therefore, higher vapor pressures. This explains why ionic liquids have lower vapor pressures and tend to be nonflammable compared to most room-temperature molecular liquids.