EXOTHERMIC ENDOTHERMIC Why upon mixing do some solutions get cold while other release heat?
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Transcript of EXOTHERMIC ENDOTHERMIC Why upon mixing do some solutions get cold while other release heat?
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EXOTHERMIC
ENDOTHERMIC
Why upon mixing do some solutions get cold while other release heat?
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Mixture
Solutions
Pure SolventPure Solute
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When copper chloride is dissolved in water, the ions are cloaked in water molecules
SolvationSolvation
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Heat of Solution: Hsoln
The amount of heat involved in the process of solution formation
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Net energy (Hsoln) is the sum of the parts
1. Break ions apart
2. Hydrate ions
Hsoln = 1in + 2out = 821 – 819 = 2 kJ/mol
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Hsoln = Hs + Haq
Neg. = out ( T)
Positive = in (T)
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Heat of Solution
If the enthalpy of formation of the solution is more
negative that that of the solvent and solute,
the enthalpy of solution is negative.
The solution process is
exothermic!
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Energy of a SystemEnergy of a SystemDepends on the polarity of the solvating molecule, size of the ions and the charge
The higher the charge, the stronger the attraction (Mg+2 vs. Li+1 and K+1)
The smaller the ion, the stronger the attraction (Li+1 vs. K+1)
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Heat of Solution: Hsoln
Determines the solubility of a substance:
If the Hsoln is very positive, water does not have enough energy to break ions apart and it will NOT
dissolve
If the Hsoln is negative, water will gain energy from ions and it will
dissolve
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Hydration Energy1. Explain why the enthalpy of hydration of Na+ is more negative than that of Cs+.
2. Explain why the enthalpy of hydration of Mg2+ is more negative than that of Cs+.
3. Which should have a more negative enthalpy of hydration F-or Cl- ? Explain
Na+ is smaller size
Mg2+ has greater charge
F- is smaller size
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13MEASRUING Heat of Solution
Ba(OH)2
The solution process is
exothermic
What is the Molar
Hsoln ?
NH4(OH)2
The solution process is
endothermic
What is the Molar
Hsoln ?
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Solutions can be classified as
saturated or unsaturated.
Definitions
A saturated solution contains the maximum quantity of solute
that dissolves at that temperature.
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Solubility: Maximum amount of solute per 100 g solvent
UNSATURATED SOLUTIONSSATURATED SOLUTIONS
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Dissolving Effects: c34Dissolving Effects: c34 Saturated SolutionSaturated Solution- maximum amount - maximum amount
of solute in solutionof solute in solution Any extra solute added just sits on bottom.Any extra solute added just sits on bottom.
UnsaturatedUnsaturated- less than max. solute than - less than max. solute than then can be dissolve.then can be dissolve.
SupersaturatedSupersaturated- A solution containing - A solution containing more solute than normal for a given more solute than normal for a given temperature.temperature. Must heat a saturated solution up. Must heat a saturated solution up.
Then return it to room temperature.Then return it to room temperature.
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For a Supersaturated solution, when more solute is
added, the extra solute will settle to the bottom
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SupersaturatedSupersaturatedSodium AcetateSodium Acetate
• One application of a supersaturated solution is the sodium acetate “heat pack.”
• Sodium acetate has an ENDOthermic heat of solution.
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Temperature effects the solubility of a substance
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Solubility Curves
How much LiCl will dissolve into 100 ml of water at a temperature of 20 C?
How much KCl will dissolve into 500 ml of water at a temperature of 80 C?
How much RbCl will fall out of solution if 200 ml of solution is cooled from a temperature of 80 C to 20 C?
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Dissolving EffectsDissolving Effects SolubilitySolubility- the quantity of solute - the quantity of solute
that will dissolve in 100 g of that will dissolve in 100 g of solvent at a set temperaturesolvent at a set temperature
If solution is holding the MAX. If solution is holding the MAX. Amount of solute called Amount of solute called saturatedsaturated
SolidSolid ionic compounds:ionic compounds: more in more in with higher temperaturewith higher temperature
GasesGases:: More in at lower More in at lower temperaturestemperatures
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Dissolving Gases & Dissolving Gases & PressurePressure
Gas solubility can be Gas solubility can be changed by both the changed by both the
temperature and temperature and PRESSUREPRESSURE..
When the can of soda is When the can of soda is opened the pressure (opened the pressure (PPgasgas) )
drops and causes the drops and causes the solubility to decrease. solubility to decrease.
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Henry’s LawHenry’s LawGas solubility (mol/L) = Sg
Sg = kH • Pgas
kH = Henry’s law constants
for O2 = 1.66 x 10-6 M/mmHg
When Pgas increases, solubility
increases.
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Henry’s LawHenry’s LawWhat is the concentration of oxygen in fresh water
when the air pressure is 1 atm?
Sg = kH • Pgas
kH for O2 = 1.66 x 10-6 M/mmHg
O2 = 21% so 0.21(760 torr) = Pgas
Sg = 1.66 x 10-6 M/mmHg • 160 torr = 2.66 x 10-4 M
What is the concentration of oxygen in milligrams/ liter? O2 = 8.5 mg/L
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