# 11.1-11.2 Phase Change, Evaporation, Vapor Pressure, and Boiling Point

CHM 123Chapter 11

11.1-11.2 – Phase change, evaporation, vapor pressure, and boiling point

Phase Change (State Change): A change in physical form but not the chemical identity of a substance.

Heat (Enthalpy) of Fusion (ΔHfusion ): The amount of energy required to overcome enough intermolecular forces to convert a solid to a liquid.

Heat (Enthalpy) of Vaporization (ΔHvap): The amount of energy required to overcome enough intermolecular forces to convert a liquid to a gas.

• Depends on the amount of attractive (intermolecular )forces between molecules
• Stronger the force the higher ΔHvap (or boiling point)

For the solid  liquid phase change in water

ΔG = ΔH – TΔS = 0 at equilibrium

Or T = ΔH/ΔS

11.3 Evaporation, Vapor Pressure and Boiling point:

Vapor Pressure: The partial pressure of a gas in equilibrium with a liquid at a constant temperature

At equilibrium: rate (vap) = Rate (cond.)

Intermolecular forces / Enthalpy of vaporization / Boiling point / Vapor Pressure
Low / Low / Low / High
High / High / High / Low

Clausius-Clapeyron Equation

- this gives us a way of finding the heat of vaporization, the energy that must be supplied to vaporize a mole of molecules in the liquid state.

- Alternatively, once the heat of vaporization and the vapor pressure at one temperature are known, the vapor pressure at one temperature are known, the vapor pressure of the liquid at any other temperature can be calculated.

Clausius-Clapeyron equation leads to a convenient way to measure the heat of vaporization in the laboratory.

R = 8.314 J/mol •K

For water, Hvap=44.0 kJ/mol and its vapor pressure is 1.0 atm at 100.0 °C. What is vapor pressure of water at 25.0 °C?

Ether has Pvap = 400.0 mmHg at 17.9oC and a normal boiling point of 34.6oC. What is the heat of vaporization, ∆Hvap for ether in kJ/mol?

11.9– Phase diagram

- summarizes the effect of temperature and pressure on a substance in a closed container. Every point in this diagram represents a possible combination of temperature and pressure for the system. The diagram is divided into three areas, which represent the solid, liquid, and gaseous states of the substance

Normal Boiling Point: The temperature at which boiling occurs when there is exactly 1 atm of external pressure

Normal Melting Point: The temperature at which melting occurs when there is exactly 1 atm of external pressure

Critical Point: A combination of temperature and pressure beyond which a gas cannot be liquefied.

Critical Temperature: The temperature beyond which a gas cannot be liquefied regardless of the pressure.

Critical Pressure: The pressure beyond which a liquid cannot be vaporized regardless of the temperature.

Supercritical Fluid: A state of matter beyond the critical point that is neither liquid nor gas.

Triple Point: A point at which three phases coexist in equilibrium.

Water

Carbondioxide

Refer to the phase diagram below when answering the questions on this worksheet:

1) What is the normal freezing point of this substance? ______

2) What is the normal boiling point of this substance? ______

3) If I had a quantity of this substance at a pressure of 1.25 atm and a temperature of 3000 C and lowered the pressure to 0.25 atm, what phase transition(s) would occur?

4) At what temperature do the gas and liquid phases become indistinguishable from each other? ______

6)If I had a quantity of this substance at a pressure of 0.75 atm and a temperature of -1000 C, what phase change(s) would occur if I increased the temperature to 6000 C? At what temperature(s) would they occur?

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