Problem 10 - Chemistry The Central Science In Si Units

Question

(a) How does the average kinetic energy of molecules compare with the average energy of attraction between molecules in solids, liquids, and gases? (b) Why does increasing the temperature cause a solid substance to change in succession from a solid to a liquid to a gas? (c) What happens to a eas if you put it under extremelv high pressuret

Step-by-Step Solution

Answer

(a) Kinetic energy is lowest in solids and highest in gases compared to attraction. (b) Increasing temperature raises kinetic energy, causing state changes. (c) High pressure can liquify or solidify gases by compressing molecules closer together.

Step 1: Understanding Kinetic Energy vs. Attraction in Different States

In solids, the average kinetic energy of molecules is much lower compared to the energy of attraction between the molecules. The molecules vibrate around fixed positions. In liquids, the kinetic energy increases, almost equaling the energy of attraction, allowing molecules to move past each other while still being closely packed. In gases, the kinetic energy is much higher than the energy of attraction; hence, molecules move freely and are far apart.

Step 2: Influence of Temperature on State Changes

Increasing temperature adds energy to the system, increasing the kinetic energy of the molecules. This increase causes the molecules in a solid to vibrate more until they can overcome some of the attraction forces, leading to a liquid state (melting). Further temperature increase raises kinetic energy further, breaking all attractive forces, and molecules become gases (vaporization).

Step 3: Effect of High Pressure on Gases

Placing a gas under extremely high pressure compresses the molecules closer together, increasing the chances of attractive forces becoming significant again. This can result in the gas being compressed into a liquid or even solid state if the pressure is enough to overcome their kinetic energy and restore order.

Key Concepts

Kinetic Energy

Kinetic energy is the energy associated with the motion of molecules. It plays a crucial role in determining the state of matter. Molecules in a solid have the least amount of kinetic energy as they only vibrate in fixed positions. This limitation in movement is the reason they have a definite shape and volume, making solids rigid.
Molecules in liquids have more kinetic energy than solids, which allows them to slide past one another. This added motion contributes to the liquid's ability to take the shape of its container, yet maintain a fixed volume. Gases, on the other hand, contain molecules with the highest kinetic energy. This energy enables the gas molecules to move freely and occupy all available space, leading to no definite shape or volume.

Molecular Attraction

Molecular attraction refers to the forces that hold molecules together. In solids, these forces are very strong, keeping the molecules tightly packed and only capable of vibrating in one place.

• Strong attractions lead to definite structure and limited movement.

As we move to liquids, the molecular attractions are somewhat relaxed but still significant. This allows the molecules to move around while staying close, contributing to a liquid's ability to flow.
In gases, molecular attraction is minimal because the kinetic energy overwhelms these forces, allowing the particles to move rapidly and independently, resulting in expansion and diffusion.

Phase Transitions

Phase transitions are changes in the state of matter due to kinetic energy variations overcoming molecular attractions. For instance, when a solid absorbs heat, its molecules gain kinetic energy, vibrate more intensely, and break some of their bonds. This transition transforms a solid into a liquid, known as melting.
Further heating increases kinetic energy, resulting in vaporization, where the liquid molecules fully break their bonds and become a gas.

• Melting and vaporization illustrate how energy changes impact molecular interactions.

This process is vital in understanding how heat energy can transform matter from one phase to another.

Temperature Effects

Temperature is a key factor that influences the kinetic energy of molecules. When temperature increases, it directly raises the kinetic energy among molecules. This effect is observed as movement begins to overcome attractions, causing solids to melt into liquids and liquids to evaporate into gases.
The energetic particles at elevated temperatures are more likely to break free from their constraints, leading to a transition in phases. Lowering the temperature has the opposite effect, reducing kinetic energy and allowing attractive forces to dominate, causing gas to condense into liquid or liquid to freeze into solid.

Pressure Effects

Pressure is another important factor that impacts how kinetic energy and molecular attraction interact within a gas. High pressure compresses gas molecules, moving them closer together. When under such conditions, the attractive forces become more significant compared to the kinetic energy.
This increase in molecular proximity can cause gases to liquefy or even solidify if the pressure is high enough. Essentially, high-pressure conditions can force a gas to revert to a denser state of matter as the kinetic energy is trumped by the increased molecular attractions.