Chapter 12

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Chapter 12
Solutions
Types of Mixtures
1. Solutions
– AKA Homogeneous Mixtures/can only be separated
by evaporation
– Atoms, molecules, or ions are thoroughly mixed,
resulting in a mixture that has the same composition
and properties throughout
– Made by having one substance dissolved into another
• Soluble: capable of being dissolved
• Insoluble: unable of being dissolved
• Components of solutions
– Solvent: the dissolving medium
• Water is the universal solvent (aqueous solutions or hydrated
solutions)
• Alcohol is also used frequently as solvent (tinctures)
• Solvent is greatest quantity
– Solute: the substance that is dissolved
• Usually designated as the component that is of lesser
quantity
• Once dissolved, the solute particles can no longer be seen
(usually less than 1 nm in diameter)
• As long as conditions do not change, solutes will remain in
solvent indefinitely
• Types of solutions
Gas-gas: air
Gas-liquid: carbon dioxide and water (pop)
Liquid-liquid: fruit juice
Liquid-solid: dental amalgam
Solid-liquid: sugar water
Solid-solid: alloys
2. Suspensions
•
•
•
•
Occur when particles are too large to remain
in the solvent unless constantly agitated
Particles are usually over 1000nm in
diameter
Would be identified as heterogeneous
mixture
Can be separated by a filter
3. Colloid
– Particles are intermediate of solutes and suspensions
– Particles are between 1-1000nm
– Particles become dispersed in a dispersing medium
(think fog)
– See table 2 page 404 for ex.
– Can not be filtered, but will scatter light (tyndall effect)
– Also see brownian motion (dust particles in air)
Electrolytes versus nonelectrolytes
• An electrolyte is a substance that will conduct an
electric current when in an aq. Soln.
– Ionic compounds and polar covalent molecules in
water generally are electrolytes
• A nonelectrolyte is a substance that will not
conduct an electric current when in an aq. Soln.
– Nonpolar covalent molecules generally are
nonelectrolytes
Factors affecting the rate of
dissolution
• Increase the surface area of the solute
– Crushing/grinding will increase the amount of
surface area a solvent affects
• Agitate the solution
– Think KMT, faster moving particles have more
collisions, more collisions increase the
amount of solvent affecting the solute
• Heating the solvent
– KMT
Solubility
• Soln equilibrium: the physical state in which the
opposing processes of dissolution and crystallization of a
solute occur at equal rates
– When in equilibrium in a closed system, the solution is saturated
(holds the max. amount of solute at given conditions)
– When a soln has less than max, it is said to be unsaturated
– When holds more solute than a saturated soln has under the
same conditions, called supersaturated
• Requires heating and allowing soln to cool to saturated point
• As long as undisturbed, solute will remain dissolved. Disruptions
will cause excess to precipitate (rock candy)
• Solubility of a substance is the amount of the substance
required to form a saturated soln with a specific amount
of solvent at a specified temperature (if gas, pressure
must also be stated)
• The rate of dissolution is not related to the solubility (at
that temperature) Max amount of solute that dissolves to
reach equilibrium is always the same under same
conditions
• Solubility is related to polarity.
– “like dissolves like” (polar dissolves polar, nonpolar dissolves
nonpolar, nonpolar doesn’t dissolve polar)
• Liquid solvents and solutes
– If the liquids are not soluble in each other, use
term immiscible instead of insoluble
– If the liquids are soluble in each other, use
term miscible instead of soluble
Effects of Pressure on Solubility
• Changes in pressure have little effect on
the solubilities of liquids or solids
• Pressure increases the ability of gas to
dissolve in liquid (increase P, increase
solubility)
Henry’s Law
• chemical law stating that the amount of a gas
that dissolves in a liquid is proportional to the
partial pressure of the gas over the liquid,
provided no chemical reaction takes place
between the liquid and the gas.
• It is named after William Henry (1774–1836), the
English chemist who first reported the
relationship
•
An everyday example of Henry's law is given by carbonated soft drinks such
as colas (and also by beers and sparkling wines).
•
Before the bottle or can is opened, the gas above the drink is almost pure
carbon dioxide at a pressure slightly higher than atmospheric pressure.
•
The drink itself contains dissolved carbon dioxide. When the bottle or can is
opened, some of this gas escapes, giving the characteristic hiss
•
Because the pressure above the liquid is now lower, some of the dissolved
carbon dioxide comes out of solution as bubbles----this is called
effervescence (think Mentos and diet pop)
•
If a glass of the drink is left in the open, the concentration of carbon dioxide
in solution will come into equilibrium with the carbon dioxide in the air, and
the drink will go "flat"
• Henry's law can be put into mathematical terms
(at constant temperature) as
–
• where p is the partial pressure of the solute, c is
the concentration of the solute and kH is a
constant with the dimensions of pressure divided
by concentration.
• The constant, known as the Henry's law
constant, depends on the solute, the solvent and
the temperature.
•
Pressure-Solubility Law (Henry's Law)
The concentration of a gas in a liquid at any given temperature is
directly proportional to the partial pressure of the gas on the solution.
•
•
Cg = kgPg
where Cg = the concentration of the gas, kg is the proportionally
constant for that gas and Pg is the partial pressure of the gas above
the solution. Constant temperature is assumed. This equation is true
only at relatively low concentrations and pressure and for gases that
do not do react with the solvent.
•
A more useful expression of Henry's Law is
•
C1= C2
P1
P2
•
• At 20oC the solubility of N2 in water is 0.0150 g/L when the
partial pressure of nitrogen is 580 torr. What will be the
solubility of N2 in water at 20oC when its partial pressure is 800
torr?
• C1 = 0.0150 g/L
P1 = 580 torr
C2 = ?
P2 = 800 torr
C2 = C1 X P2
P1
= 0.0150 g/L X 800 torr
580 torr
= 0.0207 g/L
The solubility under the higher pressure is 0.0207 g/L
Enthalpy of solution
• The enthalpy of solution (or enthalpy of
dissolution) is the enthalpy change
associated with the dissolution of a
substance in a solvent at constant
pressure.
• We will expand on this later
Solution Composition
• Perhaps the most important property of a
solution is its concentration
• In order to quantify the concentrations of
solutions, chemists have devised many
different units of concentration each of
which is useful for different purposes.
• Weight percent = wt of solute
wt of solution x 100%
• What is the weight percent of glucose in a solution made
by dissolving 4.6 g of glucose in 145.2 g of water?
• Determine total weight of solution:4.6 g
+ 145.2 g
149.8 glucose-water solution
• Calculate percent:
• Weight % glucose = 4.6 g glucose x 100 = 3.1%glucose
149.8 g solution
• How would you prepare 400. g of a 2.50% solution of sodium
chloride?
• Analysis:
We need to find out how much salt is needed and how much water
is needed.
• Determine weight of salt:
400. g x 2.50% salt = 10.0 g salt
400. g solution x 2.50 g salt
100 g solution = 10.0 g salt
• Determine weight of water:
400. g total
- 10. g salt
390. g water
Answer:
Dissolve 10.0 g salt in 390. g water.
Molarity
• The term concentration is used to indicate
the amount of solute dissolved in a given
quantity of solvent or solution.
• The most widely used way of quantifying
concentration in chemistry is molarity.
• The molarity (symbol M) of a solution is
defined as the number of moles of solute in a
liter volume of solution:
Molarity
• abbreviated M.
• For example, calculate the volume of a
1.5 M solution of HCl necessary to
completely react with 0.32 moles of NaOH:
•
•
• What is the molarity of a solution made by
dissolving 20 grams of NaCl in 100 mls of
water?
•
M solution
• Calculate the number of moles of CaCl2 in
0.78 liters of a 3.5 M solution:
• How many liters of a 2.0 M solution of
HNO3 do we need to have 5 moles of
HNO3?
• Dilution alters the molarity (i.e. concentration) of the
solution but not the total number of moles of molecules
in the solution (in other words, dilution does not create or
destroy molecules).
• One of the standard equations for determining the
effects of dilution upon a sample is to set up an equation
comparing (concentration)*(volume) before and after
dilution
• Since (concentration)*(volume) gives us the total number
of moles in the sample, and since this does not change,
this value before and after dilution are equal:
• (concentration)*(volume) = (concentration)*(volume)
• (moles/liter)*(liter) = (moles/liter)*(liter)
• moles = moles
• How much of a 5 M stock solution of NaCl
will you need to make up 250 mls of a 1.5
M solution?
• X liters = 0.075 liters (or 75 mls)
• What is the concentration of water?
– Molecular weight of H2O = 18.0g/mole
– Density of H2O = 1g/ml or 1000g/L
– Pure water is 55.6M H2O
Molality
• An alternative unit of concentration to molarity is
molality.
• The molality of a solute is the number of moles
of that solute divided by the weight of the solvent
in kilograms.
• For water solutions, 1 kg of water has a volume
close to that of 1 liter, so molality and molarity
are similar in dilute aqueous solutions.
• molality (M) = moles solute/kg of solution
• To convert a volume and a molality of a
solution to moles of solute, simply solve
the above equation for moles of solute:
– moles solute = molality * kg of solution
• If you have 10.0 grams of Br2 and dissolve it in
1.00 L of cyclohexane, what is the molality of the
solution? The density of cyclohexane is 0.779
kg/l at room temperature.
– 10 g / (159.8 g/mole) = 0.063 moles BR2
– 1.00 L * 0.779 kg/l = 0.779 kg (convert the volume of solvent
to the weight of solvent using the density)
– 0.063 moles Br2/ 0.779 kg cyclohexane = 0.080 molal
Normality
• When you need to compare solutions on the
basis of concentration of specific ions or the
amount of charge that the ions have, a different
measure of concentration can be very useful. It
is called normality
• Normality, the number of molar equivalents of
solute per liter of solution, has the units
equivalents / L which are abbreviated N.
• The normality of a solution is simply a
multiple of the molarity of the solution.
Generally, the normality of a solution is
just one, two or three times the molarity. In
rare cases it can be four, five, six or even
seven times as much.
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