vacuum distillation
Summary
Vacuum distillation is distillation under reduced pressure, so the liquid boils at a lower temperature than its normal boiling point. The MCAT uses it for high-boiling or heat-sensitive compounds that could decompose if heated to their normal boiling point.
Detail
Meaning: Vacuum distillation is distillation at reduced pressure. A liquid boils when its vapor pressure equals the pressure above it, so pumping that pressure down lets it boil at a lower temperature. It suits compounds with very high normal boiling points or ones that decompose when heated that far. Components are still separated by differences in volatility.
Equation: ln(P2/P1) = −(ΔHvap/R)(1/T2 − 1/T1). P is vapor pressure (Pa), T is boiling temperature (K), ΔHvap is molar enthalpy of vaporization (J/mol), and R is 8.314 J/(mol·K). A lower P2 means a lower T2.
On the MCAT: A passage describes a heat-sensitive or high-boiling product and asks which technique to use. Or it gives a vapor pressure curve and asks for the boiling point at a lower pressure, which is lower. Expect comparisons with simple, fractional, and steam distillation.
Common confusion: The vacuum does not change the liquid's vapor pressure at a given temperature, and it does not weaken intermolecular forces. It lowers the external pressure, so the vapor pressure matches it at a lower temperature.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.