c13 Chemistry

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Chapter 13

Solutions


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How do Solutions Work?

Compounds are either polar or

non-polar.A polar solvent only will

dissolve a polar solute.

A non-polar solvent will onlydissolve a non-polar solute

Like dissolves like


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Why Like dissolves like?

Polar compounds will hang tightly to

their own kind (like magnets). They will not surround (as a solvent)

or allow themselves to be

surrounded (as a solute) by non-polar compounds


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Concentrate on the Solution

Concentration is a measure of how

much solute is present in the solution. Qualitative: Described without

numbers

Quantitative: Described WITHnumbers.


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Qualitative Concentrations

Unsaturated:The solvent can still

dissolve more solute. Saturated:The solution contains the

maximum amount of solute.

Supersaturated: The solution hasdissolved more than the normal

maximum amount of solute

VERY unstable!


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Chapter 13

Separating Mixtures


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Separating Mixtures

Mixtures are separated based

on their physical properties.

Mixtures may be separated by

decanting, pouring off theliquid, with a centrifuge,

filtering, or evaporation


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Mixtures

If the boiling points of the components aredifferent, distillation can separate thembased on their boiling points.

As one component reaches its boiling point,it evaporates from the mixture and is allowedto cool and condense. This is called adistillate.

This process continues until all the desiredcomponents have been separated from themixture.


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Chapter 13

Concentration and

Molarity


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Concentration

In a solution, the solute is distributed evenly

throughout the solvent. This means that anypart of a solution has the same ratio of solute

to solvent as any other part of the solution.

This ratio is the concentration of the solution.

The concentration is the amount of a particular

substance in a given quantity of a solution


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Chapter 13

Physical Properties

of Solutions


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Electrical Conductivity

Some substances conduct electricity andsome cannot.

The conductivityof a substance isdescribed as its ability to conduct an electric

current.

The conductivityof a substance depends onwhether it contains charged particles, and

these particles must be able to move.


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Electrons move freely within a metal,

thus allowing it to conduct electricity.An aqueous solution of ionic

compounds such as NaCl contains

charged ions, which can move about.Solutions of ionic compounds conduct

electricity.

Pure water does not conduct electricity.


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An electrolyteis a substance that dissolvesin a liquid solvent and provides ions thatconduct electricity.

Strong electrolytes completely dissociate intoions and conduct electricity well.

Weak electrolytes provide few ions insolution.

Covalent compounds may be strongelectrolytes, weak electrolytes, or

nonconductors.


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The extent to which electrolytes dissociate into ionsis indicated by the conductivity of their solutions.

The sugar sucrose does not ionize at all in solution. It is a nonelectrolyteand does not conduct electricity.

A nonelectrolyteis a liquid or solid substance thatdoes not allow the flow of an electric current, eitherin solution or in its pure state, such as water orsucrose.


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Colligative Properties

Thephysical properties of water are changedwhen substances dissolve in it.

Salt can be added to icy sidewalks to meltthe ice. The salt actually lowers the freezing point of

water. Ice is able to melt at a lower temperature than it

normally would.

This change is called freezing-pointdepression.


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Nonvolatile solutes such as salt also

increase the boiling point of a solvent.

This change is called boiling-point elevation.

For example, glycol in a cars radiator

increases the boiling point of water in the

radiator, which prevents overheating.

It also lowers the freezing point,

preventing freezing in cold weather.


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Any physical effect of the solute on the

solvent is a colligative property.

A colligative propertyis a property ofa substance or system that isdetermined by the number of particlespresent in the system but independentof the properties of the particles

themselves.


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Any solute, whether an electrolyte or anonelectrolyte, contributes to the colligative

properties of the solvent.

The degree of the effect depends on theconcentration of solute particles (either

molecules or ions) in a certain mass ofsolvent.

The greater the particle concentration is, thegreater the boiling-point elevation or the

freezing-point depression is.


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Chapter 14

Reversible

Reactions and

Equilibriums


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Complete Reactions

If enough oxygen gas is provided for the followingreaction, almost all of the sulfur will react:

S8+ 8O2 8SO2

Reactions such as this one, in which almost all of the

reactants react, are called completion reactions.

In other reactions, called reversible reactions,theproducts can re-form reactants.


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Reversible Reactions

2 2-

4 4Ca ( ) SO ( ) CaSO ( ) aq aq s

Solid calcium sulfate, the product, can

break down to make calcium ions and

sulfate ions in a reaction that is thereverse of the previous one.


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Reversible Reactions

The reactions occur at the same rate

after the initial mixing of CaCl2andNa2SO4.

The amounts of the products and

reactants do not change. Chemical equilibriumis a state of

balance. the rate of a forward reaction

equals the rate of the reverse reaction.


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Equilibrium

In equilibrium, an atom may change frombeing part of the products to part of the

reactants many times.

But the overall concentrations of productsand reactants stay the same.

For chemical equilibrium to be maintained,the rates of the forward and reverse

reactions must be equal.


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Chapter 14

Systems and Stress


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STRESS


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Le Chateliers Principle

Stress is another word for something thatcauses a change in a system at equilibrium.

Chemical equilibrium can be disturbed by astress, but the system soon reaches a newequilibrium.

Le Chteliers principlestates that when asystem at equilibrium is disturbed, thesystem adjusts in a way to reduce the

change.


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Chemical equilibria respond to three kinds ofstress: changes in the concentrations of reactants or

products

changes in temperature

changes in pressure

When a stress is first applied to a system,equilibrium is disturbed and the rates of theforward and backward reactions are no

longer equal.


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The system responds to the stress by

forming more products or by forming more

reactants.

A new chemical equilibrium is reached when

enough reactants or products form.

At this point, the rates of the forward and

backward reactions are equal again.


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Concentrate

Increase the amount of reactants, the

equilibrium will shift towards products. Increase the amount of products, the

equilibrium will shift towards the

reactants.


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Getting Hot!

If the forward reaction is exothermic,

cooling the system will force thereaction forward.

If the forward reaction is endothermic,

heating the system will force thereaction forward.


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Feel the Pressure

Increasing pressure will shift the

equilibrium to the side with less atoms.


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Practical Le Chatelier

The chemical industry makes use of Le

Chteliers principle in the synthesis of ammonia

by the Haber Process.

High pressure is used to drive the following

equilibrium to the right.

2 2 3N ( ) 3H ( ) 2NH ( ) g g g


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Chapter 15

Acids and Bases


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Acids

These substances can recognized as acidic

by their tart, sour, or sharp taste.

These substances contain dissolved

compounds that chemists describe as acids.

Many other acids, such as sulfuric acid or

hydrochloric acid, are highly caustic and

should not be put to the taste test.


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Acids

Acids are electrolytes, so their solutions inwater are conductors of electric current.

Like other electrolytes, hydrogen chloridedissociates to produce ions.

HCl(g) + H2O(l)

H3O+(aq) + Cl(aq)

The hydronium ion, H3O+, is able to transfercharge through aqueous solutions much

faster than other ions do.


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Acids

Another property shared by aqueoussolutions of acids is that they react with many

metals.

All metals that are above hydrogen in theactivity series react with acids to producehydrogen gas.

2H3O+(aq) + Zn(s)2H2O(l) + H2(g) + Zn2+(aq)


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Acids

Some electrolytes are strong and

others are weak, depending on whetherthey dissociate completely or partially.

When a weak acidis dissolved in

water, only a small fraction of itsmolecules are ionized at any given

time.


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Bases

Bases are another class of electrolytes.Unlike acids, which are usually liquidsor gases, many common bases aresolids.

Solutions of bases are slippery to the

touch, but touching bases is an unsafeway to identify them.

The slippery feel comes about becausebases react with oils in your skin,

converting them into soaps.


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Bases

Some bases, such as magnesium hydroxide,Mg(OH)2, are almost insoluble in water.

Other bases, such as potassium hydroxide,are so soluble that they will absorb watervapor from the air and dissolve in the water.

A base that is very soluble in water is calledan alkali, a term that describes the Group 1

metals of the periodic table.


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Bases

The alkali metals react with water to form

hydroxides that are water-soluble alkalis.

These are called basic orAlkaline.

Just as acids may be strong or weak

depending on whether they ionize completelyor reach an equilibrium between ionized and

un-ionized forms, bases are also classified

as strong or weak.


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Bases

Both strong and weak bases generate

hydroxide ions when they dissolve inwater.

Many oxides, carbonates, andphosphates are bases, too.


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Classifications

Arrhenius acidProduces Hydronium

ions

Arrhenius BaseProduces Hydroxide

ions


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Classification

Brnsted-Lowry acidsDonate

Protons Brnsted-Lowry acidsAccept Protons


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Conjugate Acids and Bases

Look again at the equation for the reversible

reaction of ammonia, NH3, with water:

-

3 2 4NH ( ) H O ) NH ( ) OH ( ) aq (l aq aq

base acid c. acid c. base

Water donates a proton to ammonia, so it is an

acid.

Ammonia accepts the proton, so it is a base.


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Amphoteric

Some species are both an acid and a base

and can both donate and accept protons.

Such species are described as amphoteric.

Amphoteric describes a substance, such as

water, that has the properties of an acid and

the properties of a base.


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Self-Ionization

Water is both an acid and a base. It both gives

and receives protons.

-

2 2 3H O( ) H O( ) H O ( ) OH ( ) l l aq aq

[H3O+] = [OH] = 1.00 107M


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Self-Ionization

An equilibrium-constant expression

relates the concentrations of speciesinvolved in an equilibrium.

The relationship for the waterequilibrium is simply

[H3O+][OH] = Kw


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The value of Kw can be found from the

known concentrations of the hydroniumand hydroxide ions in pure water.

Kw = (1.00 107

)(1.00 107

) = 1.00 1014

The product of these two ion

concentrations is always a constant.


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Chapter 15

pH, or more math!


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Meaning of pH

When acidity and basicity are exactly

balanced such that he numbers ofH3O+and OHions are equal, we say

that the solution is neutral.

Pure water is neutral because it

contains equal amounts of the two ions.


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Meaning of pH

A solution made by dissolving 0.100 mol of

NaOH in 1.00 L of water has a hydroxide ion

concentration

of 0.100 M.

The hydronium ion concentration can becalculated using Kw.

+ - 14+ 133

3 -

[H O ][OH ] 1.00 10[H O ] 1.00 10

[OH ] 0.100


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Meaning of pH

In 1909, Danish chemist Sren Srensenproposed using the negative logarithm of [H3O

+]

as the index of basicity and acidity.

He called this measure the pH or power ofHydrogen.

pH can be calculated by the followingmathematical equation:

pH = log [H3O+

] OR [H3O+

] = 10pH

Section2 Acidity, Basicity, and pHChapter 15


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The Meaning of pH, continued

Calculating pH from [H3O+

], continued The pH equation may be rearranged to calculate the

hydronium ion concentration from the pH.

[H3O+

] = 10pH

Because pH is related to powers of 10, a change in

one pH unit corresponds to a tenfold change in the

concentrations of the hydroxide andhydronium ions.

p


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