Problem 12 - General Chemistry: Principles and Modern Applications

Question

Surface tension, viscosity, and vapor pressure are all related to intermolecular forces. Why do surface tension and viscosity decrease with temperature, whereas vapor pressure increases with temperature?

Step-by-Step Solution

Answer

Surface tension and viscosity decrease with temperature because higher temperature means higher kinetic energy of particles which allows them to overcome the attractive intermolecular forces more easily, reducing the resistance to flow (surface tension and viscosity). Vapor pressure increases with temperature because higher temperature (kinetic energy) provides more molecules with the energy needed to escape from the liquid's surface and become a gas, which effectively increases the rate of evaporation (vapor pressure).

Step 1: Explanation of Surface Tension and Viscosity

Surface tension and viscosity are measures of the extent to which a liquid resists flow. Both of these properties result from the attractive intermolecular forces within the liquid. So, if the temperature increases, the kinetic energy of the particles also increases, they move more rapidly and can overcome the attractive forces more easily leading to a decrease in both surface tension and viscosity.

Step 2: Explanation of Vapor Pressure

Vapor pressure is a measure of a liquid's evaporation rate. When temperature increases, the kinetic energy of the particles also increases which translates to more molecules moving fast enough to escape from the liquid’s surface and become a gas, thus increasing the vapor pressure.

Key Concepts

Surface Tension

Surface tension is a phenomenon where the surface of a liquid behaves like a stretched elastic membrane. Imagine touching the surface of water gently, and you will notice that it can "hold" lightweight objects like a paperclip. This happens because molecules at the surface of a liquid are pulled together by strong intermolecular forces, primarily due to cohesive forces within the liquid. These forces tend to minimize the surface area, giving the liquid its "tension." When you heat the liquid, things start to change. The molecules gain more kinetic energy as temperature rises, which means they move around more vigorously. As these molecules move faster, they have a greater ability to break free from the tight embrace of their neighboring molecules. Thus, the increased movement makes it harder for them to maintain the strong surface tension, leading to a decrease in surface tension with higher temperatures.

Viscosity

Viscosity refers to a liquid's resistance to flow. Think of it as the "thickness" or "stickiness" of a liquid. Honey, for example, has a much higher viscosity than water because its molecules are more strongly attracted to each other. The reason why temperature affects viscosity stems from the same principle as surface tension. As temperature increases, the kinetic energy of the molecules rises. They move more energetically and can overcome the forces holding them together more swiftly.

• With increasing temperature, the molecules slide past each other more easily.
• There is less resistance to flow because the attractions between molecules are diminished.

So, the viscosity decreases as the liquid becomes warmer, making it easier for you to stir that honey in your tea!

Vapor Pressure

Vapor pressure is about how easily liquid molecules can "escape" into the air and become gas. Consider a pot of water left to simmer on a stove. As the water heats, you'll observe steam — that's the water molecules escaping into the gas phase. The increasing temperature results in greater kinetic energy. Molecules near the surface of the liquid move faster and can break free from the liquid phase, turning into vapor. This increase in the number of gas molecules above the liquid leads to a rise in vapor pressure. It's essential to understand that:

• Higher temperatures mean more molecules having enough energy to vaporize.
• This results in greater vapor pressure as more molecules enter the gaseous state.

In summary, as temperature increases, vapor pressure increases because more molecules are "taking the leap" into the air as gas.