Solutions I CHM 1010 PGCC Barbara A. Gage Characteristics.

Solutions I

CHM 1010 PGCC Barbara A. Gage

Characteristics

Solutions

• Homogeneous mixture : has no visible boundaries because the components are mixed as individual atoms, ions, and molecules.

• Solution : A homogeneous mixture is also called a solution. Solutions in water are called aqueous solutions. Although we normally think of solutions as liquids, they can exist in all three physical states.

CHM 1010 PGCC Barbara A. Gage

Components…

• Solute – substance that is dispersed or dissolved

• Solvent – substance that facilitates the dispersing or dissolving

• Entropy will generally make solution formation favorable!

CHM 1010 PGCC Barbara A. Gage

Solubility

• Maximum amount of solute that dissolves in a given amount of solvent at a set temperatureNH4Cl 37.2 g/100g H20 @20oC

CHM 1010 PGCC Barbara A. Gage

Factors Affecting Solubility

• Nature of solute and solvent…IMFs “Like dissolves like”If the forces are similar the

components will mix…

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

Figure 13.1 The major types of intermolecular forces in solutions.

CHM 1010 PGCC Barbara A. Gage

Figure 13.3 Like dissolves like: solubility of methanol in water.

water

methanol

A solution of methanol in water

CHM 1010 PGCC Barbara A. Gage

Figure 4.2 The dissolution of an ionic compound… Solvation

Dissolution of a polar compound…Solvation

CHM 1010 PGCC Barbara A. Gage

The slightly charged ends of polar solute molecules are attracted to the polar ends of polar solvents. The molecules are surrounded by solute particles. The solute particles stay intact.

CHM 1010 PGCC Barbara A. Gage

Solute IMFs Solvent IMFs

Dissolve or Mix?

Ion-ion Dipole-dipole yes

Ion-ion Dispersion no

Dipole-dipole Dipole-dipole yes

Dispersion Dipole-dipole no

Dispersion Dispersion yes

To determine solubility, decide what the IMFs of the solute and solvent are first.

CHM 1010 PGCC Barbara A. Gage

(c) Ethanol - Diethyl ether can interact through a dipole and dispersion forces. Ethanol can provide both while water would like to H bond.

(b) Water - Hexane has no dipoles to interact with the -OH groups in ethylene glycol. Water can H bond to the ethylene glycol.

SAMPLE PROBLEM 13.1

Predicting Relative Solubilities of Substances

SOLUTION:

PROBLEM: Predict which solvent will dissolve more of the given solute:(a) Sodium chloride in methanol (CH3OH) or in propanol

(CH3CH2CH2OH)(b) Ethylene glycol (HOCH2CH2OH) in hexane

(CH3CH2CH2CH2CH2CH3)

or in water.(c) Diethyl ether (CH3CH2OCH2CH3) in water or in ethanol (CH3CH2OH)

PLAN: Consider the intermolecular forces which can exist between solute molecules and consider whether the solvent can provide such interactions and thereby substitute.

(a) Methanol - NaCl is ionic and will form ion-dipoles with the -OH groups of both methanol and propanol. However, propanol is subject to the dispersion forces to a greater extent.

Factors Affecting Solubility

• Nature of solute and solvent…IMFs• Temperature As temperature increases, particle

motions and interactions increase.

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

Solubility Curves

What is the general trend for solids?

For gases?

Why are they different?

CHM 1010 PGCC Barbara A. Gage

Heats of solution and solution cycles

1. Solute particles separate from each other - endothermic

solute (aggregated) + heat solute (separated) Hsolute > 02. Solvent particles separate from each other - endothermic

3. Solute and solvent particles mix - exothermic

solvent (aggregated) + heat solvent (separated) Hsolvent > 0

solute (separated) + solvent (separated) solution + heat Hmix < 0

Hsoln = Hsolute + Hsolvent + Hmix

CHM 1010 PGCC Barbara A. Gage

Figure 13.4Solution cycles and the enthalpy components of the heat of solution.

Exothermic solution process

Endothermic solution process

Endo- and Exothermic Dissolvers• Endothermic dissolvers can be used

to form chemical cold packs because the solvent (water) becomes cold as it provides energy for the dissolving process.

• Exothermic dissolvers provide the basis for chemical hot packs since the energy released heats the solvent.

CHM 1010 PGCC Barbara A. Gage

Factors Affecting Solubility• Nature of solute and solvent…IMFs• Temperature• Molar Mass The larger the solute molecule the

more solvent molecules are required to keep the solute in solution.

CHM 1010 PGCC Barbara A. Gagehttp://www.theochem.rub.de/~axel.kohlmeyer/research.html

Solvation shell of water molecules for Na+ (left) and a larger peptide (right)

Factors Affecting Solubility

• Temperature• Nature of solute and solvent…IMFs• Molar Mass The larger the molecule the more

solvent molecules are required to keep the solute in solution.

• Pressure (for gases only)

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

The higher the pressure over the solution, the greater the solubility of the gas (Henry’s Law).

The “bends”…

Electrolytes

• If a solute conducts electricity when dissolved it is called an electrolyte.

• Solutes that dissolve but do not form ions form non-conducting solutions and are called non-electrolytes.

• A solute that dissociates partially and conducts electricity on a limited basis is called a weak electrolyte.

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

Figure 4.3 The electrical conductivity of ionic solutions.

Colloids and Suspensions

• When a solute is suspended in a solvent the result is a heterogeneous mixture that may appear homogeneous. Such mixtures are called colloids or colloidal suspensions. These generally do not settle in the short term.

• Suspensions are heterogeneous mixtures that do settle in the short term. They require shaking before dispensing.

CHM 1010 PGCC Barbara A. Gage

Colloids and Suspensions

• Both colloids and suspensions scatter light. A true solution will not scatter light because the dispersed solute particles are too small to affect the path of the light.

CHM 1010 PGCC Barbara A. GageColloid True solution

Types of Colloidal Suspensions

CHM 1010 PGCC Barbara A. Gage