Problem 10 - Chemistry: The Central Science

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 gas if you put it under extremely high pressure?

Step-by-Step Solution

Answer

In solids, the average energy of attraction between molecules is greater than the average kinetic energy. In liquids, these energies are relatively close, with kinetic energy being slightly lower. In gases, the average kinetic energy of molecules is much greater than the energy of attraction between molecules. When the temperature increases, the kinetic energy of molecules also increases, causing a solid to transition to a liquid and then to a gas. Under extremely high pressure, a gas's molecules are forced closer together, increasing the energy of attraction, and may cause the gas to convert into a liquid, depending on the gas type and temperature.

Step 1: Understand the relationship between kinetic energy and energy of attraction

The kinetic energy of molecules is related to their motion while the energy of attraction is related to the intermolecular forces that hold them together. In general, the kinetic energy tends to dominate in gases, where molecules move more freely. In liquids, the kinetic energy and energy of attraction have a more balanced role, while in solids, the energy of attraction dominates, as the molecules are held closer together in a fixed lattice structure.

Step 2: Compare the average kinetic energy and attraction energy in different states of matter

In solids, the average energy of attraction between molecules is greater than the average kinetic energy. In liquids, the average kinetic energy is relatively closer to the energy of attraction between molecules, but it is still lower. In gases, the average kinetic energy of molecules is much greater than the energy of attraction between molecules.

Step 3: Explain the effect of increasing temperature on the states of matter

When the temperature increases, the kinetic energy of molecules also increases. For a solid substance, the increase in kinetic energy causes the molecules to vibrate more vigorously, eventually overcoming the energy of attraction between them and making the solid melt into a liquid. If the temperature continues to rise, the kinetic energy of the liquid's molecules increases further, overcoming the intermolecular forces and allowing the molecules to move more freely, leading to the transition from a liquid to a gas.

Step 4: Describe the effect of high pressure on a gas

When a gas is put under extremely high pressure, its molecules are forced to come closer together. As a result, the average distance between the molecules decreases and the energy of attraction between them becomes more significant. This compression may cause the gas to convert into a liquid under certain conditions, depending on the gas type and temperature.

Key Concepts

Kinetic Energy of Molecules

Understanding the kinetic energy of molecules is crucial for grasping the different behaviors of matter in its phases—solids, liquids, and gases. Kinetic energy refers to the energy that an object possesses due to its motion. In the context of molecules, it's the energy they have as they move and vibrate. The higher the temperature, the more kinetic energy molecules have.

In solids, molecules vibrate but don't have enough kinetic energy to move past each other, resulting in a definite shape. Liquids, on the other hand, have enough kinetic energy for the molecules to slide past each other, which is why they take the shape of their container. In gases, molecules have a great deal of kinetic energy, moving freely and rapidly in all directions, leading to no fixed shape or volume.

• Higher temperature = higher molecular kinetic energy.
• Solids have the least molecular movement due to low kinetic energy.
• Gases have the most molecular movement because of high kinetic energy.

Intermolecular Forces

Intermolecular forces are the attractive forces that occur between molecules. They are the glue holding matter together and are pivotal in understanding matter's physical properties and phase transitions. These forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonds.

In solids, intermolecular forces are strong enough to keep molecules locked in place. As molecules move to a liquid state, they still remain close due to these forces, but with increased mobility. In gases, intermolecular forces are the weakest because molecules are spread out and move independently.

• Strong intermolecular forces keep solids rigid.
• Liquids have intermolecular forces that allow some movement.
• Weak intermolecular forces in gases allow molecules to move independently.

Phase Transitions

Phase transitions occur when a substance changes from one state of matter to another, like ice melting to water (solid to liquid) or water boiling to steam (liquid to gas). These changes happen because of the balance between kinetic energy and intermolecular forces.

As temperature increases, so does the kinetic energy, which can overwhelm the intermolecular forces holding the molecules together in a solid. This is melting. Similarly, when a liquid turns into a gas, the molecules' kinetic energy is high enough to break free from the attraction they have for each other, turning into vapor in a process known as evaporation or boiling. Conversely, cooling a gas reduces kinetic energy, leading to condensation as intermolecular forces bring molecules closer, forming a liquid.

• Melting: Solid to liquid as kinetic energy overcomes intermolecular forces.
• Evaporation: Liquid to gas when kinetic energy allows molecules to escape intermolecular pull.
• Condensation: Gas to liquid due to a decrease in kinetic energy and strengthening of intermolecular forces.

Effect of Pressure on Gases

Pressure plays a significant role in the behavior of gases. Gases are compressible because the molecules are initially far apart with weak intermolecular forces. When a gas is subjected to high pressure, it's like squeezing a spring; the molecules are pushed closer together.

If the pressure is high enough and the temperature is appropriate, these compressed molecules can transition to a liquid state because the increased proximity enhances the effectiveness of intermolecular forces. This process is used in numerous applications, such as refrigeration, where gases are compressed and allowed to expand to facilitate cooling.

• High pressure squeezes gas molecules closer, possibly leading to liquification.
• Temperature and pressure must be right for gases to become liquid.
• This principle is employed in refrigeration technologies.