Química I (universidad) Tema soluciones

8/16/2019 Química I (universidad) Tema soluciones

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Contents:

Introduction. Solutes and Solvents. Concentration

Types of Solutions

Ideal Solutions

Definition and Properties

Binary Ideal Solutions

Raoult’s Law

P-x and T-x Diagrams. DistillationDiluted Ideal Solutions. Henry’s Law

Colligative Properties


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Solutions. Introduction.

A Solution is defined as an Homogeneous Mixture of two or moreComponents and composed of one single Phase.

A Phase is a System (portion of universe ) in which Physical Properties areessentially Uniform. A Component is one or more Substances with Uniform

Composition.


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Solutions. Introduction. Solute and Solvent.

A Solution is formed when a Substance (or more ) disperses uniformlythroughout another.

Examples of solutions are Liquid Solutions (e.g. a dye in water ) or Gaseous

(e.g. atmosphere ) or Solids (e.g. 18 carat gold, made with gold and 25% copper ).


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Solutions. Introduction. Solute and Solvent.

A Solution is formed when a Substance (or more ) disperses uniformlythroughout another.

Solutions are composed of one Solute (or more ) and a Solvent.

The Solvent is the component in major Proportion, which determines thePhase of the Solution (e.g. NaCl in water ). Solutes are the rest of theComponents in the solution.


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Solutions. Introduction.

Solutions are formed depending on:

- The Natural Tendency of Substances (solvent and solute ) to mix andspread into larger volumes, when not impeded somehow.

- The type of Intermolecular Interactions involved in the dissolution

process, and the accompanying Energy Changes.


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Solutions. Introduction. Why Solutions are Formed.

Molecules of Gases (e.g. O 2 and Ar ), when not physically separated, mixtogether (both gases occupying the overall volume ) spontaneously

(without any kind of energy input from outside the system ).

This (gas molecules randomly distributed throughout the system ) results inan increase of Entropy.


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Mixing of Gases is always spontaneous, as intermolecular forces are tooweak.

With Solids and/or Liquids, as intermolecular forces are important, therewill be Mixing assuming that Energetic Changes are Favourable.

Sodium Chloride dissolves in waterbecause intermolecular interactionsbetween ions and water molecules(ion-dipole ) compensate for the

attractions between ions in the solidcristal.

For the same reason, NaCl does notdissolve in gasoline.


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Three kinds of Intermolecular Interactions play in important role in SolutionFormation:

- Solute-Solute (e.g. Na + and Cl - ions in solid NaCl ). Those must beovercome so that solute particles are dispersed throughout the solvent,in order that solute dissolves.

-Solvent-Solvent (e.g. water molecules in the liquid state ). Thoseinteractions must be overcome to make room for the solute molecules(or ions ).

- Solute-Solvent (e.g. ion-dipole interactions between Na + and Cl - ions and

water molecules), that occur when the components mix. The Energy gainin this stage must compensate for the energy requirements in theprevious steps. Otherwise there will not be dissolution (e.g. insoluble salts, like AgCl in water ).


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Solutions. Introduction. Why Solutions are Formed. Energetics of theProcess of Dissolution.

The Dissolution Process requires firstly to overcome Solute-Solute and

Solvent-Solvent Intermolecular Interactions. In other words, to separatesolute and solvent particles (ions or molecules).

These two steps are endothermic (require energy input ).


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Solutions. Introduction. Why Solutions are Formed. Energetics of theProcess of Dissolution.

If the Energy Change associated to the buildup of solute-solvent

interactions is even greater than the two combined endothermic processesmentioned before, then heat (enthalpy ) is released in the overall dissolutionprocess (exothermic ).


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Solutions. Introduction. Types of Solutions.

Regarding the Solvent:

Aqueous

Non aqueous

Regarding Solute’s Phase:

Gas in Gas Air (O 2 , N 2 , Ar and other gases )

Gas in Liquid Carbonated water, soda (CO 2 in water )Gas in Solid H2 in palladium metal (catalyst for hydrogenation)

Liquid in Liquid gasoline (mixture of hydrocarbons )

Liquid in Solid Mercury in Silver (dental amalgam )

Solid in Liquid Sea Water (NaCl in water )Solid in Solid Metal Alloys (sterling silver, Ag + 7.5% Cu )

Regarding Solute’s Nature:

Electrolytes conduct electricity

Non electrolytes do not conduct electricity


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Solutions. Introduction. Concentration.

The Concentration of a Solution quantitatively indicates the amount ofsolute in a Solution:

It also means that the Components of the solution and the RelativeAbundances are known.


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Molarity (M ): it is the number of solute moles per volume (liters ) of solution.

Molality (m ): it is the number of solute moles divided per mass (kg ) ofsolvent

Mole Fraction (x A): it is the number of moles of a particular componentdivided by the total number of moles


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Percentage (% ): the amount of solute (in volume or mass ) per amount (involume or mass ) of solution referred to one hundred.

In case of very dilute solutions, the amount (mass ) of solute is referred to

one million or one billion parts of solution (respectively parts per million,ppm and parts per billion, ppb)


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The Concentration of a Solution quantitatively indicates the amount of

solute in a Solution:


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Solutions. Introduction. Concentration. Solubility.

Solubility is defined as the amount of solute (normally expressed in g ) thatcan be dissolved in a certain amount of a determined solvent (normally

expressed in liters or in 100 mL) at a given temperature.

e.g. solubility of NaCl in water is 35.7 g in 100 mL of water atr 25°C

At this point, it is said that a Saturated Solution is obtained (no more NaCl can be added to that amount of solvent at this temperature that resulted ina stable solution ).

Solubility is affected by the types of intermolecular interactions, by thetemperature, and, specially in gas solutions, also by the pressure.


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Solutions. Introduction. Concentration. Solubility. Effect of IntermolecularInteractions.

Solubility is affected by the types of intermolecular interactions, by the

temperature, and in gas solutions also by the pressure.

The strength of Intermolecular Interactions between solute and solvent willinfluence solubility. The stronger the solute-solvent interactions are, thehigher the solubility is.

For this reason, solutes are soluble in solvents with which they can build

intermolecular interactions similar to those existing in the pure form (like- dissolves-like, or Similia similibus solvuntur ).

Hydrogen bond between

two ethanol molecules

Hydrogen bond between

ethanol and water molecules


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Solutions. Introduction. Concentration. Solubility. Effect of Temperature.

In most cases with solids dissolving in liquids, solubility increases withtemperature, as dissociation of solid into particles is favored at higher

temperatures.

There are few exceptions to this rule (e.g. cerium(III) sulphate, Ce 2 (SO 4 ) 3 )


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Solutions. Introduction. Concentration. Solubility. Effect of Temperature.

But for Gases dissolving in Liquids (e.g. carbonated water ), solubilitygenerally decreases with temperature increasing, as more dissolved gas

goes to the gas phase at higher temperatures.


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Solutions. Introduction. Solubility. Effect of Pressure. Henry’s Law.

Pressure does not greatly affect Solubility in Liquid and Solid Solutions.

But for Gases dissolving in Liquids (e.g. carbonated water ), solubilitygenerally increases with Pressure, as the partial pressure of the gas abovethe solvent (liquid ) increases.

This is explained by Henry’s Law, which states that the solubility of a gas in

a liquid solvent is directly proportional to the partial pressure of the gas above the solvent .

S g = k P g


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The Henry’s Law states that the solubility of a gas in a liquid solvent isdirectly proportional to the partial pressure of the gas above the solvent.

S g = k P g where Sg is normally expressed as molarity (M )

and k depends on the gas, solvent and temperature

At Equilibrium, the rate of gas molecules dissolved in the liquid that go backto the gas phase equals that of gas molecules being incorporated into theliquid (being dissolved ).

If Pressure is increased, more gas molecules will be incorporated into theliquid solvent (solubility increases ), until a new equilibrium is reached, in

which solubility will have been increased.


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An Example of Henry’s Law are the bubbles that appear when opening abottle of a carbonated soft drink.

This is because those drinks are bottled at a CO2 partial pressure higherthan 1 atm, and thus more CO2 is dissolved in the water.

When the container is opened to the air, CO2 pressure on the liquid is thus

reduced, so is solubility, and excess CO2 (non dissolved ) escapes asbubbles.

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Química I (universidad) Tema soluciones