Dissociation. Water. Acids and Bases.

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Lecture 20

• Dissociation. Water. Acids and Bases.

Chapter 10.5 ? 10.14

• Ions in Solution
• Water
• Acids, Bases, and pH Scale
• Salts

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Dissociation
Ionic compounds (e.g., NaCl) consist of negative
and positive ions dissolve in highly polar
liquids (e.g., H2O).
The separation of a compound into ions when it
dissolves is called dissociation.
The released ions are the same as those in the
crystal.
Substances that separate into ions by
dissociation are called electrolytes. Soluble
covalent compounds that do not dissociate are
nonelectrolytes.
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Dissociation
Faradays explanation electric current causes a
substance in solution to breakup into ions.
Arrhenius explanation the ions are free in a
dissolving electrolyte.

• Evidence
• Reactions between electrolytes take place
  instantaneously in solutions, while dry
  electrolytes do not react
• Low freezing points and higher boiling points of
  electrolyte solutions

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Ions in Solutions
Properties of ions in solutions are different
from properties of neutral elements.
Properties of electrolytes are the sum of
properties of their ions.
Atoms with unfilled outer shells are more active
than ions with closed outer shells.
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Water

• Only 3 of the world water is fresh.
• 2/3 of it is trapped in ice of Arctic and
  Antarctic.

Seawater has a concentration of salts of
3.5. 85 of the salinity is due to NaCl?. Water
begins to taste salty at a salinity of 1.2.
Half of the available fresh water is being
employed by humans. Every 20 years the demand for
water doubles.
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Hard and Soft Water
Hard water contains dissolved minerals. When
heated, hard water forms deposits (scale) inside
heaters, pipes, boilers, etc.
The deposits can be dissolved in acids.
Groundwater becomes hard by flowing through
limestones. Limestone (mainly CaCO3) dissolves in
the presence of C02 as follows CaCO3 CO2
H2O ? Ca2 2HCO3?
? Bicarbonate ion
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Acids
Acids are hydrogen-containing substances whose
water solutions taste sour. They are combinations
of hydrogen and one or more nonmetals.
There are strong acids (H2SO4 , HCl), which
completely dissociate, and weak acids (HC2H3O2),
which dissociate only slightly.
HCl H2O ? H3O Cl?
? Hydronium ion
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Bases
Bases are substances containing hydroxide groups
(OH?). Their solutions in water increases the
number of hydroxide ions present.
NaOH ? Na OH?
Some substances react with water and add OH?
groups NH3 H2O ? NH4 OH?
Substances called alkali give basic water
solutions. This the historic reason for naming
group 2 elements alkali metals.
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The pH Scale
Pure water dissociates a little H2O ? H OH?
In an acidic solution the concentration of H is
greater than in pure water, while in a basic
solution there is more OH? than in pure water.
The pH scale is a method to express the exact
degree of acidity or basicity of a solution.
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Salts

• When mixing an acid and a base, neutralization
  occurs.
• The reaction occurs with no visible effect, but
  sometimes with energy liberation.

HCl NaOH ? H2O NaCl
H Cl? Na OH? ? H2O Na Cl?
Salts are prepared by mixing appropriate acids
and bases and evaporating the solution to dryness.
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Summary
Properties of electrolytic solutions are
determined by properties of the constituent
ions Pure fresh water is a valuable substance
which may become rarity in the nearest
future Acids, bases, and salts are three
important classes of electrolytes