T-x-y & P-x-y Diagrams
What does a T-x-y diagram show?
It shows how temperature changes with the liquid and vapor compositions at constant pressure.
What does Raoult’s Law state?
Partial pressure = mole fraction × pure component vapor pressure.
What is a non-ideal solution?
A solution that does not obey Raoult’s Law.
What is volatility?
How easily a liquid turns into vapor.
What happens to vapor pressure if temperature increases?
Vapor pressure increases.
What is a bubble point on a T-x-y diagram?
The temperature where liquid just starts to boil for a given composition.
What is an ideal solution?
A solution that exactly follows Raoult’s Law.
What is meant by positive deviation?
When the mixture boils at a lower temperature (higher vapor pressure) than expected.
What does relative volatility show?
It compares how easily two liquids can evaporate.
What is the benefit of lowering the pressure during distillation?
It lowers the boiling point of the liquid, so it can be distilled at a lower temperature.
What is a dew point on a T-x-y diagram?
The temperature where vapor just starts to condense for a given composition.
When does Raoult’s Law apply best?
For mixtures of similar molecules, like benzene and toluene.
What is meant by negative deviation?
When the mixture boils at a higher temperature (lower vapor pressure) than expected.
What does it mean if relative volatility is high?
It means it is easy to separate the liquids by distillation.
How does lowering pressure affect boiling point?
It lowers the boiling point.
Why is the area between bubble and dew point curves important?
It shows where both liquid and vapor exist together in equilibrium.
Why don’t all solutions follow Raoult’s Law?
Because of differences in sizes and forces between different molecules.
What is an azeotrope?
A mixture that boils at a fixed temperature and composition, like a pure substance.
What if relative volatility is close to 1?
Then the liquids are hard to separate by distillation.
How can changing pressure help separate azeotropes?
Changing pressure may shift or remove the azeotrope, allowing separation.
How can you use a T-x-y diagram to find vapor composition if liquid composition and temperature are given?
Draw a horizontal line at the temperature to meet the dew point curve (tie line) curve to find y.
Calculate partial pressure if x = 0.3 and Psat = 90 kPa.
P = 0.3 × 90 = 27 kPa.
Why can’t azeotropes be separated by simple distillation?
Because the vapor has the same composition as the liquid at that point, so distillation stops separating them.
Why does relative volatility change with temperature and pressure?
Because vapor pressures of the components change differently with temperature and pressure.
How do temperature and pressure together affect distillation?
They change vapor-liquid equilibrium and relative volatility, which decide how many stages are needed and how easy the separation will be.