Henry's Law and Raoult's Law

Pre: Solutions

Enthalpy of a Solution

Definition

The enthalpy of a solution is the amount of heat that is released or absorbed during the dissolving process

Solubility of Gasses in Liquid

Solubility

Definition

Solubility refers to how much of a substance will dissolve in a given amount of water, expressed in .

More generally, it is the maximum concentration of a solute that can dissolve in a solvent at a given temperature.

Info

Not everything is soluble. All solutes have a different degree of solubility

  1. Soluble ()
  2. Slightly soluble ()
  3. Insoluble ()

Whether or not a solute dissolves and to what extent will depend on the many forces of attraction which exist between the following:

  • Solvent particles
  • Solute particles
  • Solvent and solute particles

Like dissolves like

Info

  • Polar dissolves polar
  • Charged dissolves charged
  • Non-polar dissolves non-polar
    • E.g oil and water don't dissolve

Factors Affecting Solubility

List

  1. Molecule size
    • As size , solubility
  2. Temperature
    • For gasses in liquids: as temperature , solubility
    • For solids in liquids: as temperature , solubility
  3. Pressure
    • For gasses in liquids: as pressure , solubility
  4. Type of Solvent

Real world examples:

  • Coke retains gas better in the fridge than if you leave it out
  • Fish survive better in cold water.
  • When a can of coke is opened, pressure decreases and is released from the solution.
Info

At equilibrium, concentration = solubility

Henry's Law

Generally applicable to gas-liquid solutions.

Law

where:

  • is the partial pressure of the gas above the solution
  • is Henry's constant
  • is the mole fraction of the liquid
Law

where:

  • is the concentration of gas in the liquid solution
  • is Henry's constant
  • is the partial pressure above the solution

The value and units of Henry's constant depends on the solute, solvent, temperature, and units of and .
Henry's constant if often given as:

Example

Natural gas consists of about 90% methane (). Assume that the solubility of natural gas at and gas pressure is about the same as that of , water. If a sample of natural gas under a pressure of is kept in contact with of water, what mass of natural gas will dissolve?

solution
From Henry's law, we know will be the same, so we can write:

Since the mass of water is , we can find the mass of natural gas:

Example

When a scuba diver rapidly ascends to the surface, nitrogen gas originally dissolved in the blood returns to a gaseous state, forming gas bubbles trapped in tissues and the blood stream, a condition known as “the Bends.”

What volume of nitrogen gas, measured at and , is released from blood when a scuba diver rapidly ascends from a depth of , where the pressure of air is , to sea level, where the pressure of air is ?

Data:

  • The solubility of in blood is approximately at
  • Moles and Volume of blood in an average adult is approximately
  • Air is approximately

solution
Calculate Henry's constant using the solubility of and the fact that at equilibrium, solubility = concentration:

Then use Henry's law again, knowing Henry's constant, to calculate the change in concentration:

Using the volume of blood, calculate how much leaves the blood:

Calculate using Ideal Gas Law:

Raoult's Law

The presence of a dissolved solute lowers the vapour pressure of the solvent.

Law

where:

  • is the partial pressure above the solution
  • is the mole fraction of the component in the solution
  • is the vapour pressure of the pure component

Applies to:

4.1B

4.5

An ideal solution contains 50 mol% A and 50 mol% B. You know A is more volatile than B. Which of the following is true of the vapour in equilibrium with this solution?

  4. Not enough info

answer: 2

4.6
Positive because we exceeded the highest given pressure (205 > 200)

If we have A and B both with 50% mole fraction, then the vapour pressure:

false