4.2
蒸馏：汽液平衡
蒸馏是一种利用混合物中元素沸点不同的特性所实现的分离技术。为了能够进行蒸馏，我们首先加热沸点差异显著（至少 20°C）的两种液体所组成的可混性混合物。当溶液加热并达到易挥发物质的泡点时，易挥发物质成分中的一些分子便会转变为气相并向上进入到冷凝器中，冷凝器是一个内部和外部分开的玻璃管。使得蒸汽能够进入…
蒸馏可分离沸点差异显著且具有不同气液平衡关系的互溶液体，这种气液平衡关系反映了各组分在气相与液相之间的分布情况。
当混合物被加热时，首先出现的蒸气泡对应于较易挥发组分的泡点。
蒸气被冷凝为液体，称为馏出液，并与混合物分离收集。早期的馏出液富含挥发性较高的组分，因为挥发性较低的组分仍倾向于留在液相中。
随着挥发性较强的组分不断蒸发，维持沸腾所需的温度逐渐升高。
因此，随着蒸馏的进行，较难挥发组分的蒸发速率增加，使得中间馏分成为更均匀的混合物。最终的馏分富含较难挥发的组分。
View the full transcript and gain access to JoVE Core videos
Q1: What is vapor-liquid equilibrium and why does it matter in distillation?
Vapor-liquid equilibrium describes how components distribute between vapor and liquid phases during distillation. It determines which component boils first and how the composition of the distillate changes throughout the process. Understanding this equilibrium helps predict boiling points and identify when distillation ends based on mole fractions of the mixture components.
Q2: Why does the temperature increase as distillation progresses?
As the more volatile component boils off, the mixture becomes enriched in the less volatile component, which requires higher temperatures to vaporize. The temperature rise reflects the changing composition of the remaining liquid. This temperature change is directly related to the vapor-liquid equilibrium of the binary mixture and helps indicate the distillation stage.
Q3: How does the composition of distillate change during a distillation experiment?
Early distillate is rich in the more volatile component because it vaporizes first. As distillation continues, the middle distillate becomes a more even mixture of both components. The final distillate is rich in the less volatile component. This compositional progression reflects the changing vapor-liquid equilibrium as the mixture composition shifts.
Q4: What is a vapor-liquid equilibrium diagram and how is it used?
A vapor-liquid equilibrium diagram plots equilibrium temperature against the mole fraction of components in a binary mixture. The x-axis shows mole fraction and the y-axis shows temperature. These diagrams, available in literature for common mixtures, allow chemists to determine boiling points, predict liquid and vapor composition at any temperature, and identify when distillation will end.
Q5: What minimum boiling point difference is required for effective distillation?
Distillation requires miscible liquids with a significant difference in boiling points, typically at least 20 degrees Celsius. This substantial difference ensures that the more volatile component vaporizes preferentially, allowing effective separation. Without sufficient boiling point difference, the vapor-liquid equilibrium relationships cannot provide adequate separation between components.
Q6: How does a condenser work in the distillation apparatus?
The condenser is a glass tube with separate inner and outer sections. Vapor travels into the inner section where it contacts cold water flowing through the outer section. This temperature difference condenses the vapor back to liquid, called distillate, which is then collected in a graduated cylinder or test tube for analysis.
Q7: What determines when a component appears in the vapor phase during distillation?
A component enters the vapor phase when the mixture reaches its bubble point temperature. The more volatile component has a lower bubble point and vaporizes first. As heating continues and composition changes, the bubble point rises, allowing the less volatile component to eventually vaporize. This behavior is governed by the states of matter and phase changes occurring in the mixture.