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Dissociation. Water. Acids and Bases. Ions in Solution Water Acids, Bases, and pH Scale Salts. Lecture 20. Chapter 10.5  10.14. Dissociation. Ionic compounds (e.g., NaCl ) consist of negative and positive ions dissolve in highly polar liquids (e.g., H 2 O ).

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Lecture 20 l.jpg

Dissociation. Water. Acids and Bases.

Ions in Solution

Water

Acids, Bases, and pH Scale

Salts

Lecture 20

Chapter 10.5  10.14


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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

Faraday’s 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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Only 3% of the world water is fresh.

2/3 of it is trapped in ice of Arctic and Antarctic.

Water

Seawater has a concentration of salts of 3.5%.

85% of the salinity is due to Na+Cl.

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


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