Question

Is \(\Delta H\) positive or negative when \(1 \mathrm{~mol}\) of water vapor condenses to liquid water? Why? How does this value compare with \(\Delta H\) for the vaporization of 2 mol of liquid water to water vapor?

Short answer

\(\Delta H\) is negative for condensation. This value is equal in magnitude but opposite in sign to the \(\Delta H\) for the vaporization of 1 mol of water. For 2 mol of vaporization, \(\Delta H\) is twice as large and positive.

Step-by-step solution

Understand the Process

First, recognize the type of phase change occurring. Water vapor condensing to liquid water is a phase transition from gas to liquid.

Determine the Sign of \(\Delta H\)

When water vapor condenses, it releases energy because bonds are forming. The enthalpy change \(\Delta H\) for a bond-forming process is negative.

Compare to Vaporization

Vaporization is the opposite process of condensation. If the vaporization of 1 mol of liquid water requires energy, its \(\Delta H\) is positive. For vaporization of 2 mol of liquid water, the enthalpy change \(\Delta H\) is twice as large and also positive.

Key concepts

phase transition

In the world of chemistry, a phase transition refers to the change of a substance from one state of matter to another. The states of matter can be solid, liquid, or gas. This process happens due to changes in temperature or pressure. Let's take water as an example:

• Solid to Liquid: Melting
• Liquid to Gas: Vaporization
• Gas to Liquid: Condensation

Phase transitions are crucial because they involve significant changes in energy. When a substance changes its phase, it either absorbs or releases energy, which we measure as enthalpy change (\( \triangle H \)). Understanding these energy changes is vital in fields such as chemistry, physics, and even engineering. Knowing whether energy is absorbed or released helps us predict how a substance will behave under different conditions.

condensation

Condensation is the process where gas transforms into a liquid. It occurs when gas molecules lose energy, typically due to cooling, and come together to form a liquid. For instance, when water vapor (gas) cools down, it condenses to form liquid water. This change isn't just about the state of matter; it also involves energy changes. When water vapor condenses:

• Energy is released into the surroundings
• The enthalpy change (\( \triangle H \)) is negative

This negative \( \triangle H \) means that the process is exothermic. The release of energy is because forming new bonds between water molecules releases energy. This is why mornings can feel cooler when dew forms on the grass; the heat released during condensation is transferred to the environment.

vaporization

Vaporization is the opposite of condensation. It happens when a liquid turns into a gas. This can occur in two main ways: evaporation and boiling. Both processes involve liquid molecules gaining enough energy to break free and become gas. Let's recap the key points about vaporization:

• Energy is absorbed from the surroundings
• The enthalpy change (\( \triangle H \)) is positive

When water vaporizes, it requires energy to break the bonds between the water molecules. This energy absorption makes the process endothermic. For instance, when you heat water on a stove, you supply the energy needed for water to vaporize. If the vaporization of 1 mol of liquid water requires a certain amount of energy, vaporizing 2 mol would need precisely double that energy.