Solution properties

• Definitions– Solution– Solvent– Solute– Solubility– Saturated

solution• Concentration

expression

• Matter– Solid, liquid, gas

• Vapor pressure• Vapor pressure

of liquid– Escaping

tendency– Ideal solution

• Rauolt’s law

– Real solution

Solution

•Mixture of two or more components that –form a single phase, – homogeneous down to the molecular level

• Solvent– Component

that determines the phase of the solution

– constitutes the largest proportion of the system

•Solute–dispersed as

• molecules or • Ions

– throughout the solvent; i.e. dissolved in the solvent

•Solubility–Is the amount of solute that passes into solution when an equilibrium is established between the solution and excess, i.e. undissolved substance

•Saturated solution–The solution obtained when maximum amount of solute is dissolved in a solvent under given conditions

Methods of Expressing Concentration of solutions•Quantity per quantity•Percentage (%)•Parts (p)•Molarity•Molality•Mole Fraction•Milliequivalents & Normal

Solutions

Methods of Expressing Concentration1.Quantity per quantity:•Concentrations are often expressed simply as the weight or volume of solute that is contained in a given weight or volume of the solution. w/v (majority), w/w, v/v

Methods of Expressing Concentration

•Percentage (%)• % w/v = weight of solute/volume of solution

x 100

• % w/w = weight of solute/weight of solution x 100

• % v/v = volume of solute/volume of solution x 100

3.Parts (p):• The number of 'parts' of solute

dissolved in a stated number of 'parts' of solution.– solid in a liquid parts by weight (g) of

solid in parts by volume (ml) of solution– liquids in liquids parts by volume of

solute in parts by volume of solution – gases in liquids parts by weight of gas in

parts by weight of solution.

4.Molarity:• the number of moles of solute in

1 liter of solution.• unit of molarity is mol/l. • Molar concentration and its

symbol M describe the molarity of a solution

• Nr. of moles = wt / mol.wt• 1 M = mol wt / liter

5.Molality:•number of moles of solute divided by the mass of the solvent, i.e. mol/kg.

6.Mole Fraction:• number of moles of solute

divided by the total number of moles of solute and solvent, i.e

• mole fraction of solute= •n1 and n2: numbers of moles of

solute and solvent, respectively.

7.Milliequivalents & Normal Solutions:

• 1 millimole = one thousandth of a mole

• 1 milliequivalent (mEq) of an ion [in case of electrolytes]– one thousandth of the gram

equivalent of the ion • the ionic weight in grams divided by the

valency of the ion.

Normality•The equivalent weight of the solute, expressed in grams, in 1 liter of solution.

•1 N = mol.wt. / valency / liter

Solution properties

• Definitions– Solution– Solvent– Solute– Solubility– Saturated

solution• Concentration

expression

• Matter• Vapor pressure• Vapor pressure

of liquid– Ideal solution

• rauolt’s law– Real solution

Kinetic theory of matter

Kinetic theory of matter

•gaseous state: The thermal motions of molecules of a substance can overcome the attractive forces that exist between the molecules, – molecules undergo a completely random movement within the confines of the container

Kinetic theory of matter

• liquid state: Van der Waals forces of attraction, lead to some degree of coherence between the molecules of liquids.– Consequently,

liquids occupy a definite volume.

Kinetic theory of matter

•solid state: The intermolecular forces are

so strong that a high of order,

–hardly influenced by thermal motions

Vapor pressure

Vapor pressure• Although solids and

liquids are condensed systems with cohering molecules some of the surface molecules in these systems will occasionally acquire sufficient energy to overcome the attractive forces exerted by adjacent molecules and so escape from the surface to form a vaporous phase.

Vapor pressure

•Definition•The pressure exerted by the vapor at equilibrium is referred to as the vapor pressure of the substance.

Vapor pressure

•All condensed systems have the inherent ability to give rise to a vapor pressure.

Vapor pressure

surface loss of vapor from liquids by the process of evaporation is more common than surface loss of vapor from solids via sublimation.

Vapor pressure

•However, the vapor pressures exerted by solids are usually much lower than those exerted by liquids, why?– because the intermolecular forces in

solids are stronger than those in liquids so that the escaping tendency for surface molecules is higher in liquids.

Solution properties

• Definitions– Solution– Solvent– Solute– Solubility– Saturated solution

• Concentration expression

• Matter– Solid, liquid, gas

• Vapor pressure– definition

• Vapor pressure of liquid– Tendency of

escaping– Ideal solution

• rauolt’s law– Real solution

Vapor pressure of liquid

• In the case of a liquid solvent containing a dissolved solute

(solution):– molecules of both solvent and solute may show a tendency to escape from the surface and so

contribute to the vapor pressure.

Vapor pressure of liquid

•The relative tendencies to escape will depend:

–on the numbers of the different molecules in the surface of the solution

–on the strengths of the attractive forces between adjacent solvent molecules

–on the strengths of the attractive forces between solute and solvent molecules

•The intermolecular forces between solid and liquid molecules are relatively strong – such solute molecules do not generally escape from the surface of a solution and contribute to the vapor pressure.

•The solute is non-volatile so the vapor pressure is due to–the dynamic equilibrium between the rates of evaporation and condensation of solvent molecules in the solution.

• In a mixture of miscible liquids, i.e. a liquid in liquid

solution, – the molecules of both

components are likely to evaporate and contribute to

the overall vapor pressure exerted by the solution.

Ideal solutions; Raoult's Law:

Francois Marie Raoult (1830 - 1901)

Ideal solutions; Raoult's Law:• In the model it is

assumed that the strengths of all intermolecular forces are identical, i.e.

• solvent-solvent, solute-solvent and solute-solute interactions are the same and are equal to the strength of the intermolecular interactions in the pure solvent or pure solute.

• Because of this equality the tendencies of solute and solvent molecules to escape from the surface of the solution will be determined only by their relative numbers in the surface.

• Since a solution is homogeneous by definition then the relative numbers of these surface molecules will be reflected by the relative numbers in the whole of the solution which can be expressed by the mole fractions of the components

Binary solution (with two components)• The total vapour pressure P exerted by

such a binary solution • P = P1 + P2 = P1oX1+

P2oX2• X1 & X2 : mole fractions of the solute and

solvent, respectively. • P1 & P2 : partial vapour pressures

exerted above the solution by solute and solvent, respectively,

• P1o & P2o: vapour pressures exerted by pure solute and pure solvent respectively

Definition of ideal solution:• The solution which obeys Raoult's

law. • Ideal behaviour should only be

expected from systems having chemically similar components, why? – because it is only in such systems that

the condition of equal intermolecular forces between components, that is assumed in the ideal model, is likely to be satisfied.

Raoult's law is obeyed over an appreciable concentration range by relatively few systems in reality

• e.g. –Mixtures of benzene + toluene, –n-hexane + n-heptane – ethyl bromide + ethyl iodide

2-Real or Non-Ideal Solutions• The majority of real solutions do not

exhibit ideal behaviour, why?– because solute-solute, solute-solvent

and solvent-solvent forces of interaction are unequal.

• The effective concentration of each component cannot be represented by normal expression of concentration, e.g. the mole fraction x but by the so-called activity or thermodynamic activity

Real or Non-Ideal Solutions• P1 = P1oa1• This equation is applicable to all

systems whether they are ideal or non-ideal.

• In ideal solution a = x, in real solutions a ≠ x

• The ratio of activity/concentration is termed the activity coefficient (f) and it provides a measure of the deviation from ideality.

Positive deviation from Raoult`s law

•If the attractive forces between solute - solvent molecules are weaker than solute- solute or solvent – solvent–then the components will have little affinity for each other.

• The escaping tendency of the surface molecules in such a system is increased when compared with an ideal solution.

• i.e. P1, P2 and P are greater than expected from Raoult's law

• thermo dynamic activities of the components are greater than their mole fractions, i.e. a1 > X1 and a2 > X2.

e.g.

•alcohol + benzene –small deviation

•water + diethyl ether –less miscible (greater + D)

•benzene + water– immiscible (very large +D)

Negative deviation from Raoult`s law

• If the solute and solvent have a strong affinity for each other that results in the formation of a complex or compound – negative deviation from Raoult's law

occurs. P1, P2 and P are lower than expected

and a1< X1 and a2< X2. • e.g.

– chloroform + acetone,– pyridine + acetic acid and – water + nitric acid.