Liquid State & Solutions: Mixtures, Dissolution, and Raoult's Law, Lecture notes of Chemistry

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CHM

Lecture 22

The Liquid State & Solutions

Mixtures

• Mixtures can be classified as either

i. heterogeneous

ii. homogeneous

• Heterogeneous mixture - mixing of components is visually non-uniform and has distinct regions of different composition e.g. sugar & salt, oil & water.
• Homogeneous mixture – mixing is uniform, (at least to the naked eye) and has a constant composition throughout e.g. seawater (sodium chloride with water) and brass (copper with zinc).
• Homogeneous mixtures can be classified according to the size of their constituent particles as either solutions or colloids

Solutions

• For solutions in which a gas or solid is dissolved in a liquid, the

dissolved substance is called the solute and the liquid is called

the solvent.

• The ability of a solid to go into solution depends on the

strength of attraction among particles.

• A supersaturated solution is one where the amount of solute dissolved is more than the amount normally dissolved at that temperature

Dissolution

• Solids in liquids : the smaller the magnitude of the solute-solute interactions, the more readily dissolution occurs.
• Solubility is the mass of a substance that can dissolve in 100 g of water at a given temperature. Usually more of a solid or liquid solute will dissolve in a solvent as the temperature is increased

Importance of Liquid Solutions

• When a substance is dissolved in a liquid it breaks down into its

constituent particles

• If it is covalent it breaks down into the molecules (e.g. Sugar)
• Ionic substances break down into the ions (e.g. NaCl into Na+^ andCl-

ions)

• This breaking up into small particles is very important for chemical

reactions

• It allows the particles to easily interact with each other to form new

compounds

• Additionally many particles can interact at the same time thereby

increasing the amount of individual reactions occurring

• It also helps the particles to arrange themselves in the appropriate

orientation to allow the reaction to occur

Vapour Pressure

• A vapour is the gaseous form of the liquid above a liquid (also a solid e.g. iodine)
• This vapour is in equilibrium with the liquid at a given temperature
• The amount of the vapour or pressure is inversely related to the strength of the intermolecular attraction of the molecules in the liquid
• As the temperature is increased the molecules will become more energetic and larger amount of molecules will come out of the liquid
• This leads to an increase in vapour pressure as temperature increases

Boiling Point

• A liquid is said to boil when its vapour pressure is equal to the

external (atmospheric) pressure

• The boiling point is the temperature at which the vapour

pressure is equal to the external (atmospheric ) pressure

• Therefore the boiling point of a pure liquid is controlled by the

external pressure exerted on the liquid

Raoult’s Law

• For a non volatile solute

PA = X A PA^

0

• PA = Vapour Pressure of A
• XA = Mole fraction of A
• P 0 = Vapour Pressure of pure A
• Mole Fraction:

XA = nA/(nTotal)

• nTotal = nA + nB + nC + …. nZ
• Generally
• XA + XB + XC + …. XZ = 1
• For a binary mixture
• XA + XB = 1
• XB = 1 – XA

Raoult’s Law

A B A B

• For a mixture of two volatile liquids PTotal = XA P 0 + XB P 0
• Based on Dalton’s Law of Partial Pressures PTotal = PA + PB PTotal = XA P 0 + ( 1 - XA) P 0
• The vapour above the mixture will tend to have a larger amount of the more volatile liquid - This concept is used to separate mixtures of liquids using distillation
• Composition of Vapour YA = PA/(PA+PB) Useful website: http://www.chemguide.co.uk/physical/phaseeqia/idealpd.html

Deviations from Raoult’s Law

• If the vapour pressure is greater than that of

the ideal then the deviation is said to be a

positive deviation

– This leads to a decrease in boiling temperature

• If the vapour pressure is less than that of the

ideal then the deviation is said to be a

negative deviation

– This leads to an increase in boiling temperature

Deviations from Raoult’s Law