Question

Describe the changes (if any) in potential energy and in kinetic energy among the molecules when gaseous \(\mathrm{PCl}_{3}\) condenses to a liquid at a fixed temperature.

Short answer

Kinetic energy decreases, and potential energy decreases.

Step-by-step solution

Understand the Phase Transition

Identify the phase transition occurring. In this case, gaseous \(\mathrm{PCl}_{3}\) is condensing to a liquid. Condensation is the process where a gas turns into a liquid.

Identify Changes in Kinetic Energy

During condensation, molecules slow down as they move from a gaseous state to a liquid state. This means that there is a decrease in the average kinetic energy of the molecules because temperature is held constant and the movement of particles is reduced.

Identify Changes in Potential Energy

As gaseous \(\mathrm{PCl}_{3}\) condenses, the molecules come closer together, resulting in a decrease in potential energy. This is due to the fact that intermolecular forces become stronger and potential energy is lowered when distances between molecules decrease.

Summarize the Energy Changes

When gaseous \(\mathrm{PCl}_{3}\) condenses to a liquid, kinetic energy decreases because molecular motion slows down, and potential energy decreases due to the stronger intermolecular attractions.

Key concepts

kinetic energy

Kinetic energy is the energy that an object possesses due to its motion. In the context of molecular motion, it refers to how fast and energetically the molecules are moving.
When gaseous \(\text{PCl}_3\) condenses into a liquid, the molecules slow down. This means the average kinetic energy of the molecules decreases.

• In a gas, molecules move rapidly and randomly, resulting in higher kinetic energy.
• When the gas condenses to a liquid, the molecules move less energetically and more slowly.
• In a liquid, the motion is more restricted due to closer interactions between molecules.

Hence, as \(\text{PCl}_3\) changes from gas to liquid, its molecules lose kinetic energy because they are moving less vigorously.

potential energy

Potential energy is the energy stored in an object due to its position or arrangement. In molecules, this mainly refers to the energy associated with intermolecular forces and distance between molecules.

• In a gaseous state, molecules are far apart, so they have higher potential energy due to weaker intermolecular forces.
• When a gas condenses, the molecules come closer together, decreasing the distance between them.
• As a result, intermolecular forces become stronger and the potential energy decreases.

For \(\text{PCl}_3\), condensation brings molecules closer, thereby reducing potential energy as intermolecular attractions strengthen.

condensation

Condensation is the process where a gas turns into a liquid. During this phase transition, there are key changes in the energies of the molecules involved.

• In condensation, the energy of motion (kinetic energy) decreases because the molecules slow down as they approach each other.
• Potential energy decreases as the molecules move closer together and stronger intermolecular forces take effect.
• The overall energy of the system decreases, often releasing heat to the surroundings.

In the case of \(\text{PCl}_3\), as it moves from a gaseous to a liquid state, these energy transformations ensure that the molecules pack more closely and interact more strongly, facilitating the phase change.