Topic 17: Equilibrium

Use this outline in conjunction with the IB syllabus.

17.1 Liquid-vapor equilibrium

17.1.1

Liquid and its vapor in a closed system will create liquid-vapor equilibrium.  

Quick particles of liquid water can hit the surface and leave the liquid – evaporate. Slowly moving particles of a vapor can hit the surface and become liquid again – condensate. The liquid-vapor equilibrium is established when the rate of evaporation equals the rate of condensation. The system will seek for equilibrium because as the rate of evaporation increases, the vapor pressure rises, there are more particles above a liquid, so the probability that some of them will hit the surface of the liquid increases.

H₂O(l) → H₂O(g)   ΔH=positive

Factors that do affect vapor pressure (equilibrium position):

1. Temperature shifts the equilibrium toward right because it always favors an endothermic reaction. Temperature equals the average kinetic energy of the particles in Kelvin. Thus, increasing temperature rises the average energy of the particles in a manner that more particles have sufficient energy to leave the liquid and evaporate. Accordingly, as the temperature increases the vapor pressure also rises because the rate of evaporation becomes quicker. 
2. The intermolecular forces must be overcome in order for a particle to leave a liquid. Thus, weaker intermolecular forces mean that particles leave the liquid more easily, so the vapor pressure will be higher. Stronger intermolecular forces reduce the magnitude of the vapor pressure.

Other factors that do not affect the equilibrium vapor pressure:

1. Larger surface area does not affect the vapor pressure (position of equilibrium) because the evaporation and condensation happen on the equal area.  
2. Volume of the liquid does not affect its vapor pressure because the evaporation/condensation happens on its surface. 

17.1.2 Vapor pressure vs. temperature graphs

The vapor pressure exponentially increases as the temperature rises since at higher temperatures, more particles have a sufficient energy to leave the liquid (evaporate). In addition, there are less slowly moving particles in the vapor, which would otherwise condensate, because the overall energy of the particles is higher.

17.1.3 Enthalpy of vaporization, boiling point and intermolecular forces

Enthalpy of vaporization is the energy required by a one mole of particles to have to overcome the intermolecular forces at standard pressure. Vaporization is a synonym of boiling. High intermolecular forces make a boiling point higher and the enthalpy of vaporization increases correspondingly. 

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

Boiling occurs when the vapor pressure reaches the pressure of the surroundings. Thus, when the intermolecular attractions are weak, the system can reach the sufficient vapor pressure more easily, so the boiling point is lower. When the intermolecular attractions are strong, it takes more energy to reach the sufficient vapor pressure, so the boiling point is high.

17.2 The equilibrium law

17.2.1

If a forward reaction has an equilibrium constant K_c then, at the same temperature and pressure, the reversed reaction has the equilibrium constant of 1 ∕ K_c (K_c inverse). 

Calculation of the yield:

Reaction A + B  C + D has a known equilibrium constant of K₀ and the concentrations of A, B, C and D are a, b, 0 and 0 respectively.

After the equilibrium was reached the concentrations of all the substances are as follows:

[A] = a - x

[B] = b – x

[C] = x

[D] = x

where x is the change between the initial concentration and the equilibrium concentration.

Thus, we can calculate x by setting up the equation: