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

(a) In solids, the average energy of attraction between molecules is much greater than the average kinetic energy of molecules. In liquids, they are comparable, while in gases, the average kinetic energy of molecules is much greater than the average energy of attraction. (b) Increasing temperature raises the average kinetic energy of molecules, causing a solid to melt into a liquid and a liquid to boil into a gas as the kinetic energy overcomes the intermolecular forces. (c) Under extremely high pressure, a gas's molecules are forced closer together, increasing density and potentially causing a phase transition to a liquid or solid if the energy of attraction between molecules becomes significant enough to overcome their kinetic energy.

Step 1: Part (a): Comparison of molecular energies in different states

In the three states of matter, the balance between the average kinetic energy of molecules and the average energy of attraction between molecules is different. 1. In solids: The average energy of attraction between molecules is much greater than the average kinetic energy of molecules. This is because the molecules are tightly held in a fixed pattern resulting in a rigid structure. 2. In liquids: The average energy of attraction between molecules is comparable to the average kinetic energy of the molecules. This allows the molecules to slide past one another, giving liquids their characteristic fluidity. 3. In gases: The average kinetic energy of molecules is much greater than the average energy of attraction between molecules. In this state, the molecules are far apart and move freely at high speeds.

Step 2: Part (b): Phase changes with increasing temperature

Increasing the temperature of a substance causes it to change in succession from a solid to a liquid to a gas because: 1. Solid to liquid transition (melting): As the temperature increases, the average kinetic energy of the molecules also increases. When the kinetic energy becomes sufficient to overcome the energy of attraction between the molecules, the substance transitions from a solid to a liquid state. 2. Liquid to gas transition (boiling): As the temperature of a liquid further increases, the average kinetic energy of the molecules continues to increase. Once the kinetic energy surpasses the energy of attraction between the molecules to an even greater extent, the substance transitions from a liquid to a gas state.

Step 3: Part (c): Effect of extremely high pressure on gases

When a gas is subjected to extremely high pressure, the molecules are forced closer together. This results in the following effects: 1. Increased density: With the molecules confined to a smaller volume, the density of the gas increases significantly. 2. Phase transition: Under certain conditions, a gas subjected to extremely high pressure may condense into a liquid or even solidify. This transition occurs when the applied pressure forces the molecules close enough for the energy of attraction between them to become significant and overpower their kinetic energy.