EXOTHERMIC

ENDOTHERMIC

Why upon mixing do some solutions get cold while other release heat?

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Heat of Solution: Hsoln

The amount of heat involved in the process of solution formation

Net energy (Hsoln) is the sum of the parts

1. Break ions apart

2. Hydrate ions

Hsoln = 1in + 2out = 821 – 819 = 2 kJ/mol

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!

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

Mixture

Solutions

Pure SolventPure Solute

When copper chloride is dissolved in water, the ions are cloaked in water molecules

SolvationSolvation

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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.

Solubility: Maximum amount of solute per 100 g solvent

UNSATURATED SOLUTIONSSATURATED SOLUTIONS

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.

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.

Temperature effects the solubility of a substance

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?

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

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.

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