Solutions

Solutions

A homogenous mixture of 2 or more chemicals with the substance in excess being the solvent and any other substance being the solute

Solids in Liquids

When a solid is dissolved in the liquid a number of properties, also known as colligative properties of the liquid are changed:

Boiling point increases

Freezing point decreases

Vapour pressure is lowered

Osmotic pressure is increased

The magnitude of such a change is proportional to the number of moles of the solute introduced.

Osmosis

The net movement of water molecules through a semi-permeable membrane from a region oh high concentration of the solvent to a region of low concentration of the solvent until evenly distributed.

Reverse Osmosis

Reverse osmosis is the process of purification of salty water against a concentration gradient. The water is passed through at a high pressure allowing it to pass through a semi-permeable membrane leaving the salt behind (the water would be passing from an area of high pressure to an area of low pressure). This process requires a lot of energy.

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What is a Boiling Point?

Boiling occurs when the pressure of the vapour state of the liquid is equal to the atmospheric pressure.

  1. Liquids are in equilibrium with their vapour state

When the pressure decreases liquids will boil at a lower temperature because it would be easier to reach the surrounding pressure,w hile when the pressure increases liquids will boil at a higher temperature.

Immiscible Liquids

These are two liquids that do not mix together.

Steam Distillation

Steam distillation is the process of purifying two immiscible liquids from each other by heating the mixture. The mixture would have a saturated vapour pressure (SVP) which would be the total of the two vapour pressures.

A number of statements that can be said about steam distillation are:

  • SVPtotal = SVPA + SVP B
  • SVPmixture > SVP A
  • Boiling point of mixture < Boiling point of A
  • SVPmixture > SVPB
  • Boiling point of mixture < Boiling point of B

This means that the mixture will have a boiling point which is lower than both the boiling point of pure A and the boiling point of pure B.

This method is used in order to:

Boil a high boiling point liquid at a lower temperature

Avoid decomposing the high boiling point liquid.

Save energy

Partition Coefficient

The partition coefficient is a ratio of the dissolution of solute x into solvents A nd B which are immiscible together. This is used to purify organic compounds from inorganic compounds, with organic compounds dissolving in organic solvents while inorganic solutes dissolving in water.

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Some important things to note are:

  • Solute X must dissolve in solvent B more than it does in solvent A
  • Solute X must not react with either of solvent B nor solvent A
  • Solvent B must not react with solvent A
  • Solvent B must be immiscible with solvent A
  • Small amounts of solvent B should be used to maximise the extraction of solute X.

Miscible Liquids

When two or more liquids completely mix together.

Ideal Solutions

Ideal solutions are a solution that when mixed together have the same type of bonding as the 2 original liquids. Therefore no bonds are neither created nor destroyed. This is observed when the two liquids have similar properties.

Raoult’s Law

Raoult’s law states that the total vapour pressure is equal to the mole fraction of A multiplied by the vapour pressure of pure A added to the vapour pressure of B multiplied by the vapour pressure of pure B.

P-X diagram ideal solution

Raoult’s law can be easily be put in a graph of vapour pressure against composition.

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P-X diagram non-ideal solutions

Not all solutions are ideal, and most of the solutions either increase or decrease the inter-molecular onding between the molecules, and these would, therefore, deviate from Raoult’s law.

Positive deviation

The positive deviation occurs when the molecules would have less intermolecular attraction and therefore they are moving much faster than in the original liquids. This would require energy to compensate for this extra movement, and therefore these are endothermic solutions.

Since here are fewer attractions there will be more vapour pressure then for an ideal solution, since the particles can leave the solution easier.

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Negative deviation

In contrast to the positive deviation, in negative deviations there are more intermolecular bonding, therefore the particles will start to move slower giving out heat to the surroundings (this wold be the extra energy available when the particles have slowed down).

Since there are more attractions less vapour pressure would be present then ideal solutions since the particles can attract each other more.

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Temperature-Composition diagram

The vapour pressure – composition diagrams are not very useful in aiding purification of the compounds, and therefore the temperature composition diagrams are prepared. The vapour state and the liquid state in a solution have different composition and this can be used to purify a mixture of two miscible liquids. (This is explained in more detail in a fractionating column in separation techniques)

Ideal solutions

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Positive deviation

Both positive and negative deviations have an azeotropic point, which is a point where the vapour and the liquid composition are the same. In a positive deviation the azeotropic point it will result in the distillate always being at the azeotrope concentration since this is the lowest boiling point of the whole mixture.

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Negative deviation

The azeotropic point of this temperature-composition diagram is the highest point, meaning it is the highest temperature. This means that the distillate is always a pure liquid, with the resulting solution being the azeotrope.

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