CHEM 217 Lecture Notes - Lecture 12: Lattice Energy, Supersaturation, Scattering
Unit 12
1 2 . 1 T H I R ST Y S O L U T I O N S
i.e. seawater – one that draws more water to itself
seawater is a solution – homogeneous mix of 2 or more substances
majority typically called solvent and minority called solute
unless it is highly favourable energetically, substances tend to combine to uniform muxtures
seawater thirsty because of tendency to mix
otravels along cells in digestive tract that also have dissolved ions but less concentrated
cells here are basically fluid and membrane – nature’s tendency to want
uniform mixture and selective cell permeability ‘uniformize’ all water and ions
so there is more fluid in the intestine (diarrhea) and more salt in cells
1 2 . 2 T Y P E S
can be of all states (s+l, g+l, l+l)
aqueous: water is solvent and l,g,s is solute
particular solvenst don’t dissolve all possible solutes
solubility of a substance is the amount of the substance that will dissolve in a given amount of
solvent
Formation of solution does not necessarily lower potential energy
oTendency to mix not related to lowering potenetial energy but to entropy
Entropy: measure of energy randomization or energy dispersal in a system
above 0K, gas particles all have kinetic energy
oPervasive tendency for energy to spread whenever it isnt’ restrained is the reason for
mixing
oInsetad of molecules being spread, consider a tool of iron – you heat one end, thermal
energy will want to be spread out over larger # of particles
EFFECT OF INTERMOLECULAR FORCES
Two substances mix without intermolecular forces
w/forces can promote or prevent forming solution
forces exist between (a)solvent and solute particles (b) solvent itself (c)solute itself
Solution always forms if solvent-solute interactions are comparable or stronger than each’s own
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If sv-sl are waker, i.e. if molecules inteart more strongly with their own kind, solution may still
form depending on relative disparities between interactions
oDisparity small: tendency to mix even if process is energetically uphill
oDisparity large: no solution
General rule in predicting: like dissolves like
1 2 . 3 E N E R G E T IC S O F S O L U T I O N F O R M A T IO N
Heat can be evolved/absorbed in forming solns
Understand energy changes with three step process forming:
o1. Separating solute into its constituent particles
always endothermic (positive ^H) because energy required to overcome forces
o2. Separating solvent particles to make room for solute
also endothermic
o3. Mixing solute – solvent
exothermic
Hess’ law : enthalpy of soln
o^solnH = ^soluteH + ^solvent + ^mixH
SUMMARY
overall sign of ^soln depends on magnitudes
if sum of endothermic terms is about equal to exothermic then ^ solnH is ~0 and tendency of
entropy drives formation of soln and energy levels remain nearly constant
If endothermic is smaller then ^soln negative and process is exothermic
If endothermic is greater hten ^soln is positive and endothermic. If ^soln not too large, entropy
can still drive soln formation
AQUEOUS SOLNS AND HEATS OF HYDRATION
Many common solns contain ionic compounds dissolved in water
In these ^solvent and ^mixH can be comined into single term called heat of hydration (^hydH)
It is the enthalpy change that occurs when 1 mol of the gaseous solute ions are dissolved in
waeer
Bc ion-dip interactions that occur are much stronger than H bonds in water, ^ydH is always
largely negative (exothermic) for ionic compounds
Using ^hydH can write enthalpy soln as a sum of two term (1 endo and 1 exo)
o^solnH=^solute + ^hydH
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for ionic compounds, ^solute is negative of the solute’s lattice energy
for ionic aqueous, overall enthalpy of soln depends on relative magnitudes of
^soluteH & ^hydH
o1. ^solute H < ^HydH - ^soln negative, process exothermic
o2. “>” - ^soln positive – endothermic
o3. “~” - ^soln approximately 0 and neither appreciably endo/exo
1 2 . 4 E Q U I L I B R I U M / F A C T O R S A F F E C T IN G S O L U B I L I T Y
dissolution of solute in solvent – equilibrium process
NaCl- initially rate of dissolution far exceeds rate recrystallization but as concentration of
dissolved sodium increases, so does recrystallization
oEventually, rates become equal >>> dynamic equilibrium
Soln in equilibrium is a saturated soln. if you add additional solute, it will NOT dissolve
Soln containing less than the equilibrium amount of solute is an unsaturated soln and if you add
solute it will dissolve
Certain circumstances – supersaturated soln with more than equilibrium amount may form but
such are unstable and excess solute normally precipitates
TEMPERATURE DEPENDENCE OF SOLID SOLUBILITY
Solubulity of most solids in water increases with temperature
Common way to purify a solid is recrystallization
Minimum amount of suitable recrystallization solvent at an elevated temp is added to solid to
create a saturated soln. as soln cools, becomes supersaturated and excess precipitates
oIf soln cools slowly, solid forms crystals and structure tends to reject impurities
FACTORS AFFECTING SOLUBILITY OF GASES IN WATER
Temperature
oSolubility (unlike solids) of gases decreases with increasing temperature
oInverse relationship between gas solubility and temperature is the reason warm pop
bubbles more than cold popo when you open it and warm beer goes flat faster than cold
beer
Pressure
oSolubility – higher pressure of a gas above a liquid, the more soluble the gas in the liquid
oA sealed can of pop – co2 maintained by high P of O2 within the can
oWhen opened, pressure released and solubility of CO2 decreases ->bubbling
oWhen pressure is increased, more dissolves but equilibrium restores and more is
dissolved
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Document Summary
1 t h i r s t y s o l u t i o n s. I. e. seawater one that draws more water to itself. Seawater is a solution homogeneous mix of 2 or more substances. Majority typically called solvent and minority called solute. Unless it is highly favourable energetically, substances tend to combine to uniform muxtures. Can be of all states (s+l, g+l, l+l) Aqueous: water is solvent and l,g,s is solute. Particular solvenst don"t dissolve all possible solutes. Solubility of a substance is the amount of the substance that will dissolve in a given amount of solvent. Formation of solution does not necessarily lower potential energy o. Tendency to mix not related to lowering potenetial energy but to entropy. Entropy: measure of energy randomization or energy dispersal in a system above 0k, gas particles all have kinetic energy o o. Pervasive tendency for energy to spread whenever it isnt" restrained is the reason for mixing.
