Non-ideality - Henry's Law and Azeotropes | Chemistry Optional Notes for UPSC PDF Download

Introduction

• The proceeding discussion was based on the behaviors of ideal solutions of volatile compounds, and for which both compounds follow Raoult’s Law. Henry’s Law can be used to describe these deviations.
  p_B = kHp^o_B
• For which the Henry’s Law constant (kH) is determined for the specific compound. Henry’s Law is often used to describe the solubilities of gases in liquids. The relationship to Raoult’s Law is summarized in Figure  8.8.1.

Figure  8.8.1: The relationship between Raoult's Law and Henry's Law for a binary mixture.

Henry’s Law is depicted by the upper straight line and Raoult’s Law by the lower.

Solved Example

Example: The solubility of  CO₂(g) in water at 25 ^oC is 3.32 x 10^-2 M with a partial pressure of  CO₂ over the solution of 1 bar. Assuming the density of a saturated solution to be 1 kg/L, calculate the Henry’s Law constant for  CO₂
Ans: In one L of solution, there is 1000 g of water (assuming the mass of CO2 dissolved is negligible.)

The solubility of  CO₂ can be used to find the number of moles of  CO₂ dissolved in 1 L of solution also:

and so the mol fraction of  CO₂ is χb

And so
10⁵ Pa = 5.98 × 10⁻⁴ kH10
or
kH = 1.67 × 10⁹Pa

Azeotropes

An azeotrope is defined as the common composition of vapor and liquid when they have the same composition.

Figure  8.8.2: Phase diagrams for (left) a maximum boiling point azeotrope and (right) Ta maximum boiling point azeotrope.

Azeotropes can be either maximum boiling or minimum boiling, as show in Figure  8.8.2; left. Regardless, distillation cannot purify past the azeotrope point, since the vapor and quid phases have the same composition. If a system forms a minimum boiling azeotrope and also has a range of compositions and temperatures at which two liquid phases exist, the phase diagram might look like Figure  8.8.2; right

Figure  8.8.3: Phase diagram for a binary solution with the boiling point of a minimum boiling azeotrope that is higher that when components are miscible (single phase).

Another possibility that is common is for two substances to form a two-phase liquid, form a minimum boiling azeotrope, but for the azeotrope to boil at a temperature below which the two liquid phases become miscible. In this case, the phase diagram will look like Figure  8.8.3.

Solved Example

Example: In the diagram, make up of a system in each region is summarized below the diagram. The point e indicates the azeotrope composition and boiling temperature.

1. Single phase liquid (mostly compound A)
2. Single phase liquid (mostly compound B)
3. Single phase liquid (mostly A) and vapor
4. Single phase liquid (mostly B) and vapor
5. Vapor (miscible at all mole fractions since it is a gas)

Ans: Within each two-phase region (III, IV, and the two-phase liquid region, the lever rule will apply to describe the composition of each phase present. So, for example, the system with the composition and temperature represented by point b (a single-phase liquid which is mostly compound A, designated by the composition at point a, and vapor with a composition designated by that at point c), will be described by the lever rule using the lengths of tie lines ­l_A and l_B.