Which Is Predicted To Have The Lowest Vapor Pressure

Jane Haynes

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08-12-2024

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Predicting which substance will exhibit the lowest vapor pressure requires understanding the factors influencing vapor pressure. Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The lower the vapor pressure, the weaker the tendency of a substance to transition from its liquid or solid state to the gaseous state.

Several key factors determine a substance's vapor pressure:

• Intermolecular Forces: Stronger intermolecular forces (like hydrogen bonding, dipole-dipole interactions, or London dispersion forces) lead to lower vapor pressure. This is because stronger forces hold molecules together more tightly in the liquid or solid phase, making it less likely for them to escape into the gaseous phase.

• Molecular Weight: Higher molecular weight generally correlates with higher London dispersion forces and thus lower vapor pressure. Larger molecules have more electrons, leading to stronger temporary dipoles and stronger attractions between molecules.

• Temperature: Vapor pressure increases with temperature. Higher temperatures provide molecules with more kinetic energy, increasing their ability to overcome intermolecular forces and enter the gas phase.

Therefore, to predict which substance has the lowest vapor pressure, we need to compare the strengths of the intermolecular forces and molecular weights of the substances in question. Without knowing the specific substances being compared, a definitive answer is impossible.

Example:

Let's consider three hypothetical substances:

• Substance A: A small molecule with weak London dispersion forces.
• Substance B: A larger molecule with strong dipole-dipole interactions.
• Substance C: A large molecule with hydrogen bonding.

In this hypothetical scenario, Substance C would likely have the lowest vapor pressure because hydrogen bonding is a very strong intermolecular force. Substance B would have a higher vapor pressure than C but lower than A because dipole-dipole interactions are stronger than the weak London dispersion forces in Substance A.

In conclusion: To accurately predict which substance will have the lowest vapor pressure, you must provide the specific substances for comparison, allowing for an assessment of their intermolecular forces and molecular weights.