1. 17 Equilibrium (AHL)
DP Chemistry
R. Slider

2. Liquid-Vapour Equilibrium
Definitions
Vapourisation
The movement of liquid particles into the gas phase.
This process is endothermic as it requires energy
Condensation
The movement of gas particles into the liquid phase.
This process is exothermic as it releases energy
Vapour Pressure
The pressure created by vapour particles above a liquid in a closed container

3. Liquid-Vapour Equilibrium
Some molecules at the surface of a liquid gain enough kinetic energy to escape from the surface
Vapour particles spread out in the container depending upon the temperature of the system. Some vapour particles now lose energy and move back to the liquid phase.
With enough time, there are equal numbers of particles moving from liquid to vapour and back again.
Source:
LV Equilibrium - when the rate of condensation and vapourisation is equal.

4. Measuring Vapour Pressure
The diagram to the left shows a vessel that can be used to measure vapour pressure.
The U-tube contains liquid mercury and clearly shows that a pressure is created by vapour particles filling the vessel.
Note that vapour pressure is temperature dependent, but is not affected by volume or surface area
Note: a lower surface area will not affect the equilibrium vapour pressure, but can affect how long it takes to reach equilibrium. A smaller surface area will take longer to reach equilibrium.
Source:

5. Vapour Pressure vs. Temperature
Vapour pressure is temperature dependent
Vapour pressure increases exponentially with increased temperature.
The graph shows that those substances with a lower boiling point have a higher vapour pressure.
WHY?
This is due to differences in forces holding molecules together in the liquid phase. Water with H-bonding is held more strongly than ether which has only weak Dispersion forces.

6. Vapour Pressure vs. Temperature
Vapour pressure is temperature dependent
Kinetic energy increases with an increase in temperature. This shifts the Maxwell-Boltzmann distribution and leads to more particles having enough energy to move into the vapour phase. More vapour particles in the same volume leads to greater vapour pressure

7. Vapour Pressure vs. Temperature
These graphs show phase diagrams for water and CO2. “Normal” b.p. values are measured at 1atm.
Liquid water can exist up to 3740C with sufficient pressure. Above, it only exists as a gas. This is known as the critical temperature (D on the graph)
CO2 can only exist as a liquid above 5.11 atm. This graph also shows why CO2 sublimes at atmospheric pressure (1atm)

8. Enthalpy of Vapourisation (ΔHvap)
The amount of energy required to convert 1 mole of a substance to the gas phase is known as the enthalpy of vapourisation in kJ.mol-1.
When a substance such as water boils, there is no change in temperature. This can be seen in the graph between D and E.
The added energy goes into breaking the attractive forces between particles.
The stronger the attractive forces:
The higher the enthalpy of vapourisation
The lower the vapour pressure at a given temperature
The higher the boiling point

9. Boiling Point
This is the temperature where the vapour pressure is equal to the atmospheric pressure.
Normal boiling point: the boiling temperature at atmospheric pressure (1atm or 101.3kPa)
Source:
Source:
At high elevations, water will boil at a lower temperature due to lower atmospheric pressure.
A pressure cooker allows water to boil at a higher temperature. This means food cooks much faster.

10. Equilibrium Law 𝐾𝑐= [𝐶] 𝑐 [𝐴] 𝑎 [𝐷] 𝑑 [𝐵] 𝑏
Recall that for a system at equilibrium, we can calculate the equilibrium constant Kc
For AHL, you will be expected to solve for K, but may also be expected to solve for an equilibrium or starting concentration based on given data.
You must practise many different types of problems until you feel comfortable solving these types of questions.
𝐾𝑐= [𝐶] 𝑐 [𝐴] 𝑎 [𝐷] 𝑑 [𝐵] 𝑏

11. Reaction quotient 𝑄= [𝐶] 𝑐 [𝐴] 𝑎 [𝐷] 𝑑 [𝐵] 𝑏
𝑄= [𝐶] 𝑐 [𝐴] 𝑎 [𝐷] 𝑑 [𝐵] 𝑏
Q (the reaction quotient)is similar to K as it is calculated the same way except that the concentrations are taken before equilibrium has been reached.
So,
If Q < K, the reaction is proceeding forward
If Q > K, the reaction is proceeding in reverse
You will need to practise these types of problems as well.
See your text and your teacher…