Chem1811 lab talk: Enthalpy of vapourization from vapour pressure measurements Part 2

Main Ideas

• Volatility describes how readily a substance enters the vapor phase. It has no single numerical value at a given temperature and pressure, so it is commonly compared using vapor-pressure curves or boiling points.
• Vapor pressure rises as temperature increases. Ethyl ether, ethanol, water, and ethylene glycol have different vapor-pressure-versus-temperature profiles.
• Boiling point links vapor pressure to external pressure. A liquid boils when its vapor pressure reaches the surrounding pressure. At atmospheric pressure, ethyl ether boils near body temperature, ethanol around 78–79°C, and water at 100°C. A lower boiling point generally indicates a higher vapor pressure—and greater volatility—at a given temperature.
• Pressure changes can be hazardous. Rapidly lowering the pressure around a hot, pressurized substance can cause its vapor pressure to suddenly meet or exceed the external pressure, creating a risk of rapid vaporization or explosion.

Why Volatility Matters

• Evaporation and persistence: Highly volatile liquids, such as rubbing alcohol, evaporate quickly. Low-volatility materials, such as engine oil, remain liquid over a wide temperature range and can persist in soil or water, making spills difficult to clean up.
• Fire safety: Volatile, flammable liquids—including gasoline, paint thinner, organic solvents, and high-alcohol mixtures—can create ignitable vapors. A fire may spread beyond the area where the liquid was spilled. Keep such vapors away from open flames and static discharges.
• Quick-drying products: Volatile solvents are useful in varnishes, wood stains, nail polish, paints, and glues because their evaporation leaves the desired product behind. Solvent removal can also concentrate components and, in some processes, promote polymerization.
• Volatile solids: Volatility can apply to solids that sublime, not just liquids. Dry ice and iodine are examples. Their vapors can also produce recognizable smells. The speaker notes that smell can provide warnings about substances and is connected to taste.

Vapor Pressure and Partial Pressure

Vapor pressure is related to a substance’s partial pressure in a gas mixture. The speaker connects partial pressure with a gas’s mole fraction and the system’s total pressure.

Applications mentioned include:

• Combustion engines: Fuel must vaporize and mix with air for combustion. Poor mixing can contribute to knocking or other damaging processes.
• Gas exchange in the body: The relative partial pressures of oxygen and carbon dioxide are involved in hemoglobin’s transport of these gases.
• Gas sensors and detectors: These can detect hazardous or flammable gases based on their pressure-related behavior.

Lab Context

The talk introduces the relationship between volatility and vapor pressure as background for a later experiment measuring vapor pressures. It does not provide a step-by-step experimental procedure in this segment.

Speaker and Source

One lecturer or narrator presents the material. No other speakers or named source material are featured.