Problem 6 - Chemistry The Science in Context

Question

Which of the drawings in Figure P10.6, both of which are at constant temperature, most likely illustrates the pure liquid with the lower normal boiling point? Explain your choice.

Step-by-Step Solution

Answer

Answer: [Insert chosen liquid] Explanation: The chosen liquid has a higher vapor pressure at the given constant temperature compared to the other liquid. A higher vapor pressure means that the liquid has a greater tendency to evaporate at that temperature. Since a liquid boils when its vapor pressure equals the atmospheric pressure, the chosen liquid will have a lower boiling point due to its greater vapor pressure at the same temperature.

Step 1: Understanding vapor pressure and boiling point

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase. A higher vapor pressure indicates that the particles in the liquid have a greater tendency to escape into the vapor phase. When the vapor pressure of a liquid equals the atmospheric pressure, the liquid reaches its boiling point. Thus, a liquid with a higher vapor pressure at a given temperature will have a lower boiling point.

Step 2: Observing both drawings

Observe the drawings in Figure P10.6 carefully and determine the vapor pressure of both liquids compared to the atmospheric pressure. We are looking for a pure liquid with lower normal boiling point, which would relate to higher vapor pressure at the given constant temperature.

Step 3: Compare vapor pressure and select the liquid with lower boiling point

Based on the drawings in Figure P10.6, determine which liquid has a higher vapor pressure at the given temperature. The one with a higher vapor pressure is the pure liquid with the lower boiling point due to its tendency to evaporate more quickly at that temperature.

Step 4: Explain the choice

After comparing the vapor pressures and choosing the pure liquid with a lower boiling point, explain the reasoning behind the choice. Make sure to mention the relation between vapor pressure and boiling point in the explanation. Remember to provide a clear and concise explanation to make it easy for the student to understand the relationship between vapor pressure and boiling point in the context of this exercise.

Key Concepts

Vapor Pressure

The concept of vapor pressure is pivotal when discussing the physical properties of liquids. Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase at a given temperature. Understanding this concept is crucial when it comes to determining the boiling point of a liquid.

At any given temperature, the molecules of a liquid are in constant motion, and some possess enough energy to escape into the vapor phase. The pressure generated by these vapor molecules is what we know as vapor pressure. It is a measure of a liquid's tendency to evaporate; higher vapor pressure means that the liquid's molecules are escaping to the gas phase more readily.

Importance in Determining Boiling Point

When a liquid's vapor pressure equals the external atmospheric pressure, the liquid begins to boil. Thus, if we have two liquids at the same temperature, the one with the higher vapor pressure will have a lower boiling point because it means it will turn into vapor at a lower temperature when compared to a liquid with lower vapor pressure.

Equilibrium in Chemistry

Equilibrium in chemistry is a state in which the rate of the forward reaction equals the rate of the backward reaction, resulting in no overall change in the concentrations of reactants and products. This concept is vital when discussing vapor pressure because it involves the equilibrium between the liquid phase and its vapor.

At equilibrium, the amount of liquid evaporating is equal to the amount of vapor condensing. Such a system is said to be in a dynamic equilibrium since individual molecules are changing state, but the overall number of molecules in each phase remains constant.

Understanding Dynamic Equilibrium

In the context of vapor pressure, it's essential to note that even though the overall number of molecules in each phase is constant at equilibrium, the actual pressure exerted by the vapor can vary depending on the temperature and the nature of the liquid. The equilibrium vapor pressure is specific to each substance and increases with temperature due to increased molecular motion. This fundamental understanding helps explain why certain liquids boil at lower temperatures than others.

Phase Transitions

Phase transitions are transformations from one state of matter to another, such as from solid to liquid (melting) or from liquid to gas (evaporation). One critical phase transition that we often encounter is the boiling of a liquid, where it transitions from the liquid phase to the vapor phase.

The boiling point is a key factor in phase transitions and is influenced by the vapor pressure. When a liquid is heated, its vapor pressure increases until it matches the surrounding atmospheric pressure. This is when boiling occurs – the liquid's molecules have enough energy to form bubbles of vapor within the liquid, leading to a rapid transition to the gas phase.

Boiling Point and Atmospheric Pressure

It's crucial to note that the boiling point of a liquid is not a fixed temperature but rather depends on the atmospheric pressure. Lower atmospheric pressures, such as at high altitudes, result in lower boiling points. This is why understanding vapor pressure is essential when studying phase transitions, particularly boiling, as it provides insight into how different conditions affect the temperature at which a liquid will boil.