Problem 20 - Chemistry and Chemical Reactivity

Question

You are comparing three different substances, \(A, B,\) and C, all liquids. The vapor pressure at \(25^{\circ} \mathrm{C}\) for substance A is less than the vapor pressure for \(B\) at this temperature. Substance C has the highest boiling point of the three substances. List the three substances \(\mathrm{A}, \mathrm{B},\) or \(\mathrm{C}\) in order of the strength of intermolecular forces, from least to greatest.

Step-by-Step Solution

Answer

Order: B, A, C (from weakest to strongest intermolecular forces).

Step 1: Identify Vapor Pressure Relationship

First, understand that vapor pressure is inversely related to the strength of intermolecular forces. The lower the vapor pressure, the stronger the intermolecular forces because molecules are less likely to escape into the gas phase. We know that substance A has a lower vapor pressure than B at 25°C, indicating that A has stronger intermolecular forces than B.

Step 2: Consider Boiling Points

The boiling point of a substance is directly related to the strength of its intermolecular forces. The higher the boiling point, the stronger the intermolecular forces needed to keep the molecules in the liquid phase. Since C has the highest boiling point, it must have the strongest intermolecular forces of the three substances.

Step 3: Rank Substances by Intermolecular Forces

Combining the information, we determine the order from weakest to strongest intermolecular forces. B has the weakest intermolecular forces because it has a higher vapor pressure than A and C. A is next since its vapor pressure is less than B but higher than C's inferred from the boiling point. C has the strongest intermolecular forces as inferred from its highest boiling point.

Key Concepts

Vapor Pressure

Vapor pressure is an important property when it comes to understanding liquids. It occurs when the molecules at the surface of a liquid absorb enough energy to become gaseous. This phenomenon is highly influenced by the intermolecular forces within the liquid. A lower vapor pressure indicates that fewer molecules have sufficient energy to escape into gas form.
When comparing substances with different vapor pressures, it's crucial to note that:

• A low vapor pressure signifies strong intermolecular forces since the molecules need more energy to vaporize.
• Conversely, a high vapor pressure suggests weaker intermolecular forces, as molecules can easily escape into the gas phase.

In the context of substances A, B, and C, we understand that the vapor pressure of substance A is lower than that of B. Therefore, substance A has stronger intermolecular forces than substance B.

Boiling Points

The boiling point of a liquid is another key indicator of the strength of its intermolecular forces. This is because boiling occurs when a liquid's vapor pressure equals the surrounding pressure, typically atmospheric pressure. A higher boiling point means that a substance's molecules require more energy to transition from liquid to gas, indicating stronger forces holding them together.
Let's keep in mind the following points:

• High boiling points indicate strong intermolecular forces, as much energy is required to break the forces that hold the molecules together in the liquid state.
• Low boiling points suggest weaker intermolecular forces, making it easier for molecules to separate and enter the gas phase.

Substance C, which has the highest boiling point among the three, hence possesses the strongest intermolecular forces in this group.

Substance Comparison

When tasked with comparing different substances based on their vapor pressures and boiling points, we're essentially judging the strength of their intermolecular forces. By linking these two properties, we can create a ranking system that accurately describes the substances:

• Substance B has the weakest intermolecular forces, as evidenced by its higher vapor pressure relative to A and C.
• Substance A, with a lower vapor pressure than B, indicates stronger forces than B but weaker than C.
• Substance C comes out strongest, given its highest boiling point, demanding more energy for its molecules to transition into gas.

This ordered comparison helps us understand how intermolecular forces impact the physical properties like vapor pressure and boiling points among different substances.