Explain how the addition of a solute affects the vapor pressure of the solvent.
One way is that the solute molecules interact with the solvent molecules. Normally the solvent molecules must overcome the effects of surface tension as well as the forces of neighboring molecules of solvent in order to "jump" from the liquid state to the vapor state. When solute particles are added the molecules must overcome that additional attraction which makes the vapor pressure a little lower.
the molecules water (H2O) and methane (CH4) hav similar masses, but methane changes from a gas to a liquid at -161 degees C. Water becomes a gas at 100 dgrees C. What could account for the difference?
When a solute is added to a solvent, it affects the vapor pressure of the solvent. To understand this, we need to consider the concept of vapor pressure and the behavior of molecules in a solution.
Vapor pressure refers to the pressure exerted by the vapor (gaseous state) of a substance in equilibrium with its liquid or solid state at a given temperature. In a pure solvent, the vapor pressure is determined solely by the solvent molecules escaping from the liquid phase.
When we add a solute to the solvent, the behavior of the solvent molecules changes. There are two main scenarios to consider:
1. Non-volatile solute: A non-volatile solute is one that has a negligible vapor pressure and does not evaporate easily. In this case, the addition of a non-volatile solute does not change the vapor pressure of the solvent significantly. The solvent molecules can still escape and form vapor above the solution.
2. Volatile solute: A volatile solute is one that has a significant vapor pressure and readily evaporates. In this case, the addition of a volatile solute affects the vapor pressure of the solvent.
The presence of solute particles, both non-volatile and volatile, affects the number of solvent particles at the surface of the solution available to escape into the vapor phase. The solute particles "get in the way" of the solvent particles, reducing their ability to escape. This reduction in solvent vaporization leads to a decrease in the vapor pressure of the solvent.
This phenomenon is known as vapor pressure lowering or Raoult's law. It states that the partial pressure of a solvent in a solution is proportional to the mole fraction of the solvent in the solution. Mathematically, the relationship can be expressed as:
Psolution = Xsolvent * Psolvent
where Psolution is the vapor pressure of the solution, Xsolvent is the mole fraction of the solvent, and Psolvent is the vapor pressure of the pure solvent.
By incorporating the vapor pressure lowering caused by the presence of solute particles, Raoult's law provides a quantitative understanding of how the addition of a solute affects the vapor pressure of the solvent.