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 energy of attraction between molecules is greater than their kinetic energy, while in liquids, this energy of attraction is weaker but still considerable. In gases, the energy of attraction is negligible compared to kinetic energy. Increasing the temperature of a substance provides energy to the molecules, leading to a transition from solid to liquid and then to gas due to increased kinetic energy overcoming attractive forces. When a gas is subjected to extremely high pressure, it either compresses, or transitions into a liquid or solid, depending on the temperature and pressure conditions.

Step 1: (a) Comparing Kinetic Energy and Attraction Energy in Different States

In solids, liquids, and gases, the molecules exhibit different levels of attraction to each other and different amounts of kinetic energy. The kinetic energy of a molecule can be represented as:\( K.E. = \cfrac{1}{2}mv^2 \), where \(m\) is the mass of a molecule and \(v\) is its velocity. The energy of attraction between molecules can be represented as a negative value, as it leads to a decrease in the total potential energy of the system. In a solid, the energy of attraction between molecules is significantly higher than their kinetic energy. The molecules are closely packed and have restricted movements. In a liquid, the energy of attraction is weaker than in a solid but still considerable. The kinetic energy of molecules is higher than in solids, allowing them to move more freely and maintain short-range order. In a gas, the energy of attraction between molecules is negligible compared to their kinetic energy, resulting in random motion without any preferred direction or order.

Step 2: (b) Effect of Temperature on Changing States of Matter

Increasing the temperature of a substance provides energy to the molecules, which then increase their kinetic energy. As a result, the motion of the molecules increases, leading to changes in the state of the substance. 1. Solid to Liquid: As the temperature of a solid increases, the kinetic energy of the molecules also increases. The increased kinetic energy overcomes the energy of attraction between the molecules, causing them to move more freely and transition into a liquid state. This process is called melting. 2. Liquid to Gas: When the temperature of a liquid increases, the kinetic energy of the molecules increases further. Once the energy of attraction between molecules is considerably weaker than the kinetic energy, the molecules move independently, and the substance transitions into a gaseous state. This process is called boiling or vaporization.

Step 3: (c) Behavior of Gas under High Pressure

When a gas is subjected to extremely high pressure, the gas molecules are forced closer together, leading to a decrease in the volume of the gas. Depending on the pressure and temperature conditions, two possible scenarios can happen: 1. Gas Compression: As the pressure increases, the gas molecules come closer, and their volume decreases. This compressed state usually increases temperature due to the work done on the gas. Gas compression is a reversible process. 2. Gas to Liquid or Solid: If the pressure is high enough and the temperature is low enough, the gas molecules can come close enough to form intermolecular attractive forces. This may lead to a phase change, where the gas transitions into a liquid or a solid, depending on the specific conditions. This process is called liquefaction or solidification.