Dissolution Process

Homogeneous Mixtures

• Solutions

– Can be of any phase (solution of sugar in water; sterling

silver; brass; air)

• Component

– Substances in the solution– Substances in the solution

• Solvent

– The component present in the greatest amount

• Solute

– Other components in the solution

The Dissolution ProcessAbility of substances to form solutions depend on:

– Intermolecular interactions involved

– Tendency of substances to diffuse into large volumes

The Effect of Intermolecular Forces

– Solutions form when magnitudes of IFAs between solute

and solvent particles are comparable with those between and solvent particles are comparable with those between

solvent-solvent and solute-solute interactions


• Solvation/Hydration*

– Process of surrounding and dispersing solute particles by the solvent

– Stabilizes ions in solution; prevents anions and cations from

recombining


• Indicate the type of solute-solvent interaction that should be

most important in each of the following solutions: (a) CCl4 in

benzene; (b) methanol in water; (c) KBr in water; (d) HCl in

acetonitrile

• Why doesn’t NaCl dissolve in nonpolar solvents such as

hexane, C6H14?

Energy Changes and Solution Formation

• Consider mixing CCl4 (nbp: 77oC) and C6H14 (nbp:

69oC)– Comparable magnitudes of IFAs

– Dissolution occurs spontaneously (energy considerations and the tendency to spread out into larger volumes�

Solution Formations, Spontaneity and Entropy

and the tendency to spread out into larger volumes�entropy considerations)

Processes in which the energy content of the system decreases tend to occur spontaneously.

Processes occurring at a constant temperature in which the randomness in space of the system increases tend to occur

spontaneously.

• Solution process involves two factors

– Change in enthalpy (heat, energy)

– Change in entropy (disorder)

Solution Formations, Spontaneity and Entropy

AgCl is essentially insoluble in water. Would you expect a AgCl is essentially insoluble in water. Would you expect a

significant change in the entropy of the system when 10g of

AgCl is added to 500 mL water?

• Crystallization

– Process opposite of solution

formation

Saturated Solutions and Solubility

• Saturated solution• Saturated solution

– the rates of dissolution and

crystallization are equal

– There can be no further increase

in the amount of solute in the

solution

– Amount of solute in a saturated

solution is given by the

solubility

The solubility is the maximum amount of solute that will dissolve in a given amount of solvent at a specified

temperature.

Saturated Solutions and Solubility

Example

Solubility of NaCl in water at 0oC is 35.7 g/100 mL water

1. Nature of Solvent and Solute• The stronger attractions are between solvent and solute

particles, the greater the solubility

Factors Affecting Solubility

• Polar liquids tend to dissolve readily in polar solvents

• Nonpolar liquids tend to be insoluble in polar solvents

• Miscible vs. Immiscible liquids

– Acetone and water

– Gasoline and water


Substances with similar intermolecular attractive forces tend

to be soluble in one another


Ex. Predict whether each of the following substances is more

likely to dissolve in carbon tetrachloride or water: (a) C7H16;

(b) Na2SO4; (c) HCl; (d) I2.

2. Pressure Effects

• Important to consider in solutions

with gaseous solutes

• The gaseous solute is in dynamic

equilibrium between ‘free’ gas and


equilibrium between ‘free’ gas and

solution phases

• The solubility of a gas increases in direct

proportion to its partial pressure above

the solution

2. Pressure Effects

• Henry’s Law: Sg = kPg

• Calculate the concentration of CO2 in a soft drink that is

bottled with a partial pressure of CO2 of 4.0 atm over the

liquid at 25oC. The Henry’s Law constant for CO2 in water

at this temperature is 3.1 x 10-2 mol/L-atm


at this temperature is 3.1 x 10-2 mol/L-atm

3. Temperature Effects


The solubility of most solid solutes in water increases as

the temperature of the solution increases.

The solubility of gases in water decreases with increasing The solubility of gases in water decreases with increasing

temperature.

Dissolution Process

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