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