Chapter 12 Study Guide

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17 Feb 2011
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Chapter 12 Solutions
-solution is homogenous mixture
-solutions are mixtures in which atoms and molecules intermingle on the molecular and atomic
scale
-examples include ocean water, gasoline, and air
Thirsty Solutions: Why You Should Not Drink Seawater
-drinking seawater causes dehydration because seawater draws water out of body tissues
-draws water as it passes through stomach and intestine, resulting in diarrhea and further
dehydration
-seawater is a thirsty solution, one that draws more water to itself
-seawater is a solution, homogeneous mixture of two or more substances or components
-majority component is solvent
-minority component is solute
-in seawater, water is solvent and sodium chloride is main solute
-seawater draws water to itself due to nature’s tendency toward spontaneous mixing
-substances tend to combine into uniform mixtures, not separate into pure substances, unless it is
highly unfavourable energetically
-substances spontaneously mix together to form a more dilute solution of uniform concentration
-fluids in the body are more dilute than seawater
-nature’s tendency toward mixing (produce solutions of uniform concentration) and selective
permeability of cell membranes (allow water to flow in and out, but restrict flow of dissolved
solids) causes a flow of solvent out of the body’s cells and into seawater
-2solutions become more similar in concentration
-body fluid becomes more dilute and solution in the cell becomes more concentrated
-accumulation of extra fluid in intestines causes diarrhea
-decreased fluid in cells causes dehydration
Types of Solutions and Solubility
-solution can be composed of solid and liquid, gas and liquid, two liquids
-in aqueous solutions, water is solvent and solid/liquid/gas is solute
-sugar water and salt water are both aqueous solutions
-alcohol and carbon dioxide dissolves in water to form aqueous solution
-solubility is the amount of substances that will dissolve in a given amount of solvent
-solubility depends on nature’s tendency toward mixing and types of intermolecular forces
Nature’s Tendency toward Mixing: Entropy
-physical systems tend toward lower potential energy
-formation of solution does not necessarily lower potential of its constituent particles
-example is the formation of homogeneous mixture (solution) of 2 ideal gases
-at low pressure and moderate temperature, noble gas behaves as ideal gas
does not interact with each other in any way
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there are no significant forces between their constituent particles
potential energy remains unchanged when 2 noble gases mix
-mixing of two ideal gases does not lower potential energy
-tendency to mix is related to entropy
-entropy is measure of energy randomization/ energy dispersal in a system
-gas at temperature above zero K has kinetic energy due to motion of atoms
-gas and its kinetic energy become spread out or dispersed over larger volume
-mixture of 2 gases has greater energy dispersal, or greater entropy than the separated
components
-2 ideal gases mix due to the pervasive tendency for energy to spread out or disperse, whenever it
is not restrained from doing so
-example of tendency toward energy dispersal is the transfer of thermal energy from hot to cold
places
-thermal energy at one will spontaneously spread along the entire body of object
-kinetic energy becomes dispersed over large volume because particles become dispersed
-tendency of energy to disperse is why thermal energy flows from hot to cold place, and not the
other way around
-energy does not spontaneously concentrate itself so one end is not hot and the other end is cold
does not take place
The Effect of Intermolecular Forces
-when intermolecular forces are absent, 2 substances spontaneously mix to form a homogeneous
solution
-intermolecular forces exist between
1) solvent and solute particles
2) solvent and solvent particles
3) solute and solute particles
Relative Interaction and Solution Formation
Solvent-solute interaction > solvent-solvent &
solute-solute interactions
Solution forms
Solvent-solute interaction = solvent-solvent &
solute-solute interactions
Solution forms
Solvent-solute interaction < solvent-solvent &
solute-solute interactions
Solution may or may not form, depending on
relative disparity
-miscible is when 2 substances are soluble in each other in all proportions due to similar
magnitude in all three interactions
-formation of solution is due to tendency toward mixing/ toward greater entropy
-if disparity is small, tendency to mix result, solution forms even though the process is
energetically uphill
-if disparity is large, solution will not form
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-energy require to pull water molecules away from one another is too great and too little energy
is returned from the interaction with solute molecules
-solution does not form when mixing non-polar molecules with water
-tendency to mix is strong, but it cannot overcome large energy disparity between strong solvent-
solvent interactions and weak solvent-solute interactions
-like dissolves like to predict formation of solutions
Energetics of Solution Formation
-energy changes in solution formation
1) Separation of solute into constituent particles
-always endothermic (positive H) because energy is required to overcome forces that hold
solute together
-Hsolute > 0
2) Separation of solvent particles from each other to make room for solute particles
-also endothermic because energy is required to overcome intermolecular forces among solvent
particles
-Hsolvent > 0
3) Mixing of solute particles with solvent particles
-exothermic because energy is released as solute particles interaction (through intermolecular
forces) with solvent particles
-Hesss law, overall enthalpy change in solution is enthalpy of solution (Hxoln)
Hsoln = Hsolute + Hsolvent + Hmix
Endo (+) endo (+) exo (-)
1) Hsoln = 0
-increasing entropy upon mixing drives formation of solution
-overall energy of the system remains nearly constant
2) Hsoln = negative
-solution process is exothermic
-tendency toward lower energy and tendency toward greater entropy drive formation of solution
3) Hsoln = positive
-solution process is endothermic
-if H is not too large, tendency toward greater entropy will still drive formation of solution
-if Hsoln is too large, solution will not form
Aqueous Solution and Heat of Hydration
-heat of hydration is the sum of Hsolvent and Hmix
-enthalpy change that occurs when 1 mol of gaseous solute ions are dissolved in water
-due to ion-dipole interaction that occur between dissolved ion and water molecules
-ion-dipole interaction is much stronger than hydrogen bond
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