Question

Why do liquids have a vapor pressure? Do all liquids have vapor pressures? Explain. Do solids exhibit vapor pressure? Explain. How does vapor pressure change with changing temperature? Explain.

Short answer

Liquids have vapor pressure because their particles have kinetic energy, resulting in evaporation. All liquids have vapor pressures, which depend on factors like temperature, intermolecular forces, and molecular weight. Solids also exhibit vapor pressure, but it is generally lower than that of liquids. Vapor pressure increases with increasing temperature as more particles can evaporate or sublime due to increased kinetic energy. This relationship is described by the Clausius-Clapeyron equation: \[ \ln P = -\frac{\Delta H_{vap}}{R} \left(\frac{1}{T}\right) + C\]

Step-by-step solution

1. Why do liquids have vapor pressure?

Liquids have vapor pressure because their particles have kinetic energy and are in constant motion. Due to this motion, some particles at the surface of the liquid escape into the gas phase, which is called evaporation. When the liquid is in a closed container, these evaporated particles cannot escape, and they create pressure in the space above the liquid. This pressure is called vapor pressure.

2. Do all liquids have vapor pressures? Explain.

Yes, all liquids have vapor pressures because particles in every liquid possess kinetic energy and are in constant motion. The extent of the vapor pressure depends on different factors like temperature, intermolecular forces, and molecular weight of the liquid. Liquids with weaker intermolecular forces or lower molecular weights tend to have higher vapor pressures compared to liquids with stronger intermolecular forces or higher molecular weights.

3. Do solids exhibit vapor pressure? Explain.

Yes, solids do exhibit vapor pressure, but it is generally much lower than that of liquids. This is because the particles in a solid have less kinetic energy and limited motion compared to liquids. Some particles at the surface of a solid can escape into the gas phase, a process called sublimation. The pressure exerted by these gaseous particles above the solid is called the vapor pressure of the solid.

4. How does vapor pressure change with changing temperature? Explain.

Vapor pressure increases with increasing temperature because the kinetic energy of the particles in a liquid or solid also increases with temperature. As the temperature increases, more particles possess higher kinetic energies and can overcome the intermolecular forces that keep them in the liquid or solid phase. As a result, more particles can evaporate or sublime, leading to an increase in vapor pressure. This relationship between vapor pressure and temperature can be mathematically described using the Clausius-Clapeyron equation: \[ \ln P = -\frac{\Delta H_{vap}}{R} \left(\frac{1}{T}\right) + C\] where \(P\) is the vapor pressure, \(\Delta H_{vap}\) is the enthalpy of vaporization, \(R\) is the gas constant, \(T\) is the temperature, and \(C\) is a constant that depends on the properties of the substance.