Chapter 7Acids and Bases

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Introduction to Acids and Bases

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The pH Scale

Role of water and the properties of acids

Role of water and the properties of alkalis

Strong and Weak Acids Strong and Weak Alkalis

Mind Map

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Meaning of Acid

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An acid is a chemical substance which ionises in water to produce hydrogen ions, H+.

The acidic properties of an acid is due to the presence of hydroxonium ions, H3O+

. However, this ion can be written as H+, hydrogen ion.

Hydroxonium ion

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Basicity of Acid

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Basicity of an acid is the maximum number of hydrogen ions that is produced when one molecule of the acid ionises in water.

Monoprotic acid is an acid which produces one hydrogen ion when one molecule of the acid ionises in water.

Nitric Acid

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Diprotic acid is an acid which produces two hydrogen ions when one molecule of the acid ionises in water.

For example : Sulphuric Acid, H2SO4

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A alkali is a chemical substance which ionises in water to produce hydroxide ions, OH- .

Bases

AlkalisAlkalis are bases which are soluble in water.

Bases are oxides/hydroxides of metals which react with acids.

All alkalis are bases but not all bases are alkalis.

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Na+ OH-

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Substance Uses

Acids1. Sulphuric acid To make detergents, paints, fertilisers

As electrolyte in lead-acid accumulator.

2. Hydrochloric acid To clean metals before electroplating in industry.

3. Nitric acid To make fertilizers, plastics, explosives.

4. Ethanoic acid To make vinegar.

5. Methanoic acid To coagulate latex in the rubber industry.

6. Carbonic acid •To make fizzy drinks

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Substance Uses

Bases and Alkalis1. Ammonia (aqueous) To make fertilisers, cleaning

agents.To manufacture nitric acid.To keep latex in liquid form.

2. Magnesium hydroxide To make gastric tablets and tooth paste.

3. Calcium hydroxide To make cement, limewater.To neutralise acidity of soil.

4. Sodium hydroxide •To make detergents, soaps, fertilisers and bleaching agents.

Uses of Acids, Bases and Alkalis

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ROLE OF WATER ON THE PROPERTIES OF ACID

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Role of Water

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• An acid only shows it acidic properties when water is present.

Liquid glacial ethanoic acid

Aqueous ethanoic acid

Ethanoic acid in dry propanone

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Role of Water

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• A pure acid in the dry state does not exhibit its acidic properties.

• Dry acid consists of covalent molecules only. • Thus, hydrogen ions are not present.

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Role of Water

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• The presence of water enables the acid molecules to ionise to produce hydrogen ions.

• Thus it exhibits acidic properties.

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Properties of Acids

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Acid + base salt + water Acid + metal salt + hydrogen gas

Acid + metal carbonate salt + water + carbon dioxide gas

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ROLE OF WATER ON THE PROPERTIES OF ALKALI

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Role of Water

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• When water is present, an alkali will dissolve and ionises to produce freely moving hydroxide ions.

• Thus the alkali exhibits alkaline properties.

For Example:

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Role of Water

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• Without water, a pure alkali in the dry state does not exhibit alkaline properties because it does not contain freely moving hydroxide ions.

For example : Aqueous Ammonia Solution

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Role of Water

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Alkali + acid salt + water Alkali + metal ion insoluble metal hydroxide

Alkali + ammonium salt salt + water + ammonia gas

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Role of Water

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Alkali + acid salt + water Alkali + metal ion insoluble metal hydroxide

Alkali + ammonium salt salt + water + ammonia gas

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pH Scale

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The pH scale is used to indicate the degree of acidity or alkalinity of a solution. It consists of pH values that range from 0 to 14.

pH Paper pH Meter

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pH Value

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• The pH value is a measure of the concentration of hydrogen ions, H+ and hydroxide ions, OH- .

•The lower the pH value, the higher the concentration of hydrogen ions.

•The higher the pH value, the higher the concentration of hydroxide ions.

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pH

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The strength of an acid depends on the degree of ionisation or dissociation of the acid in water.

A strong acid is a chemical substance that dissociates completely in water to produce high concentration of hydrogen ions, H+ .

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H+ + Cl- (100% ionisation )

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Weak acids are chemicals that dissociate partially (incomplete dissociation) in water to produce low concentration of hydrogen ions.

An example of weak acid is dilute ethanoic acid, CH3COOH.

Most of the ethanoic acid exist as molecules, few as CH3COO- and H+ ions.

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Partial dissociation O CH3 C -OH

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Strong Alkalis

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A strong alkali is a chemical substance that dissociates completely in water to produce high concentration of hydroxide ions, OH-.

For example: Sodium hydroxide, NaOH

dissociation dissociation

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Weak Alkalis

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A weak alkali is a chemical substance that dissociates partially (incomplete dissociation) to produce low concentration of hydroxide ions, OH- in water.

For example: Aqueous Ammonia Solution, NH3

High pH value

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Dissociation of Strong and Weak Alkalis

in Water

NH3(aq) NH3(aq)

NH3(aq)