Prentice Hall © 2003 Chapter 13

Hlanganani Tutu, C403 School of Chemistry

Email: hlanganani.tutu@wits.ac.za

Prentice Hall © 2003 Chapter 13

• Solution - homogeneous mixture of solute and solvent.

• In solutions, intermolecular forces become rearranged.

The Solution ProcessThe Solution Process

Prentice Hall © 2003 Chapter 13

• gas in gas – e.g. air• gas in liquid -- e.g. soda• gas in solid -- e.g. gas on solid, catalyst• liquid in liquid• liquid in solid -- e.g. mercury amalgam• solid in liquid• solid in solid -- e.g. 14-karat gold, brass

Examples of solutions

Prentice Hall © 2003 Chapter 13

• Consider NaCl (solute) dissolving in water (solvent):– Interruption of water H-bonds,

– NaCl→Na+ + Cl-,

– ion-dipole forces form: Na+ … -OH2 and Cl- … +H2O.

– If water is the solvent, we say the ions are hydrated.

Prentice Hall © 2003 Chapter 13

Energy Changes and Solution Formation3 energy steps in forming a solution:

• separation of solute molecules (H1),• separation of solvent molecules (H2), and• formation of solute-solvent interactions (H3).

Prentice Hall © 2003 Chapter 13

Hsoln = H1 + H2 + H3.

Hsoln can be +ve or -ve depending on the intermolecular forces.

Prentice Hall © 2003 Chapter 13

Prentice Hall © 2003 Chapter 13

• “Rule”: polar solvents dissolve……………………….?

Non-polar solvents dissolve..............................................?

Prentice Hall © 2003 Chapter 13

Exercise: Why doesn’t gasoline dissolve NaCl?

Exercise: Why doesn’t water and octane mix well (immiscible)?

Remember: the resultant solution’s interactions must be stronger than the interactions in the original substance

Prentice Hall © 2003 Chapter 13

Solution Formation, Spontaneity, and Disorder

• When the energy of the system decreases (e.g. dropping a book and allowing it to fall to a lower potential energy), the process is spontaneous.

Prentice Hall © 2003 Chapter 13

Example: a mixture of CCl4 and C6H14 is less ordered than the two separate liquids. Therefore, they spontaneously mix

Prentice Hall © 2003 Chapter 13

Prentice Hall © 2003 Chapter 13

• There are solutions that form by physical processes and those by chemical processes.

Prentice Hall © 2003 Chapter 13

• Consider:

Ni(s) + 2HCl(aq) NiCl2(aq) + H2(g).

• When all the water is removed from the solution, no Ni is found only NiCl2·6H2O. Therefore, Ni dissolution in HCl is a chemical process.

Prentice Hall © 2003 Chapter 13

• Consider:

NaCl(s) + H2O (l) Na+(aq) + Cl-(aq).

• When the water is removed from the solution, NaCl is found. Therefore, NaCl dissolution is a physical process.

Prentice Hall © 2003 Chapter 13

• Dissolve: solute + solvent solution.• Crystallization: solution solute + solvent.• Saturation: crystallization and dissolution are in

equilibrium.

Saturated Solutions and Saturated Solutions and SolubilitySolubility

Prentice Hall © 2003 Chapter 13

Solubility: amount of solute required to form a saturated solution.Supersaturation: reached when more solute is dissolved than in a saturated solution.

Prentice Hall © 2003 Chapter 13

Prentice Hall © 2003 Chapter 13

Solute-Solvent Interaction

• Miscible liquids: mix in any proportions.• Immiscible liquids: do not mix.• Intermolecular forces are important• The more C atoms, the less the solubility in water.

Factors Affecting Factors Affecting SolubilitySolubility

Prentice Hall © 2003 Chapter 13

• The -OH groups in a molecule increase solubility in water….“like dissolves like”

Prentice Hall © 2003 Chapter 13

Prentice Hall © 2003 Chapter 13

Which of these two would be more soluble in water?

Prentice Hall © 2003 Chapter 13

• Network solids do not dissolve. Why?

Prentice Hall © 2003 Chapter 13

Pressure Effects• Solubility of a gas in a liquid is a function of the pressure

of the gas.

Prentice Hall © 2003 Chapter 13

Pressure Effects

Prentice Hall © 2003 Chapter 13

• The higher the pressure, the more molecules of gas are close to the solvent

Prentice Hall © 2003 Chapter 13

where: Sg - solubility of a gas, k is a constant, and Pg is the partial pressure of a gas

gg kPS Henry’s Law gives:

Prentice Hall © 2003 Chapter 13

Example

27g of acetylene, C2H2, dissolves in 1L of acetone at 1.0 atm pressure. If the partial pressure of acetylene is increased to 12 atm, what is the solubility in acetone?

Solution:

S1 = kP1…………(1)

S2 = kP2…………(2)

Ans: 3.2 x 102 g

Prentice Hall © 2003 Chapter 13

• Carbonated beverages are bottled with a partial pressure of CO2 > 1 atm.

• What happens when a bottle is opened?

Prentice Hall © 2003 Chapter 13

Temperature Effects• As temperature increases, solubility of solids generally

increases, e.g. sugar in warm water• Sometimes, solubility decreases as temperature increases

(e.g. Ce2(SO4)3).

Prentice Hall © 2003 Chapter 13

Temperature Effects• Gases - less soluble at high temperature

• Thermal pollution in dams and rivers – loss of O2

Prentice Hall © 2003 Chapter 13

Mass Percentage, ppm, and ppb• Definitions:

Ways of Expressing Ways of Expressing ConcentrationConcentration

100solution of mass total

solutionin component of masscomponent of % mass

Prentice Hall © 2003 Chapter 13

Example: How would you prepare 425 g of an aqueous solution containing 2.40% by mass of sodium acetate, NaC2H3O3?

Ans:Mass of NaC2H3O3 = 10.2 gMass of H2O = mass of solution - mass of NaC2H3O3 = 415 g

Prentice Hall © 2003 Chapter 13

610solution of mass total

solutionin component of masscomponent of ppm

Prentice Hall © 2003 Chapter 13

910solution of mass total

solutionin component of masscomponent of ppb

Prentice Hall © 2003 Chapter 13

solution of moles totalsolutionin component of moles

component offraction Mole

solution of literssolute moles

Molarity

Mole Fraction, Molarity, and Molality

Prentice Hall © 2003 Chapter 13

• Converting between molarity (M) and molality (m) requires density.

solvent of kgsolute moles

Molality, m

Exercise: 0.2 mol of ethylene glycol is dissolved in 2000 g of water. Calculate the molality

Prentice Hall © 2003 Chapter 13

Example: What is the molality of a solution containing 5.67 g of glucose, C6H12O6 (Mr = 180.2 g), dissolved in 25.2 g of water? (Calc. the mole fractions of the components as well).

Solution: Think about the solute!................glucose (express in moles) Think about the solvent!...............water (express in kilograms)

Ans: 1.25 m

Prentice Hall © 2003 Chapter 13

Example: Converting molarity to molality

An aqueous solution is 0.907M Pb(NO3)2. What is the molality of lead nitrate, Pb(NO3)2, in this solution? The density of the solution is 1.252 g/mL. (Molar mass of Pb(NO3)2 = 331.2 g)

Ans: 0.953 m Pb(NO3)2