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Acids and BasesChapter 15

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Acids and Bases• The concepts acids and bases were loosely defined

as substances that change some properties of water.

• One of the criteria that was often used was taste. • Substances were classified – salty-tasting, sour-tasting, sweet-tasting, bitter-

tasting • Sour-tasting substances would give rise to the word

'acid', which is derived from the Greek word oxein, which mutated into the Latin verb acere, which means 'to make sour'

• Vinegar is a solution of acetic acid. Citrus fruits contain citric acid.

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Acids• React with certain metals to

produce hydrogen gas.• React with carbonates and

bicarbonates to produce carbon dioxide gas

• Have a bitter taste• Feel slippery. • Many soaps contain bases.

Bases

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Properties of Acids Produce H+ (or H3O+) ions in water (the hydronium ion is

a hydrogen ion attached to a water molecule)

Taste sour

Corrode metals

Good Electrolytes

React with bases to form a salt and water

pH is less than 7

Turns blue litmus paper to red

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Properties of Bases Generally produce OHGenerally produce OH-- ions in water ions in water

Taste bitter, chalkyTaste bitter, chalky

Are electrolytesAre electrolytes

Feel soapy, slipperyFeel soapy, slippery

React with acids to form salts and waterReact with acids to form salts and water

pH greater than 7pH greater than 7

Turns red litmus paper to blueTurns red litmus paper to blue

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Arrhenius DefinitionArrhenius

Acid - Substances in water that increase the concentration of hydrogen ions (H+).

Base - Substances in water that increase concentration of hydroxide ions (OH-).

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Bronsted-Lowry DefinitionAcid - neutral molecule, anion, or

cation that donates a proton.

Base - neutral molecule, anion, or cation that accepts a proton.

HA + :B HB+ + :A-

HCl + H2O H3O+ + Cl-

Acid Base Conj Acid Conj Base

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Conjugate Base - The species remaining after an acid has transferred its proton.

Conjugate Acid - The species produced after base has accepted a proton.

HA & :A- - conjugate acid/base pair

:A- - conjugate base of acid HA

:B & HB+ - conjugate acid/base pair

HB+ - conjugate acid of base :B

Conjugate Acid Base Pairs

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Note: Water can act as acid or base

Acid Base Conjugate Acid Conjugate Base

HCl + H2O H3O+ + Cl-

H2PO4- + H2O H3O+ + HPO4

2-

NH4+ + H2O H3O+ + NH3

Base Acid Conjugate Acid Conjugate Base :NH3 + H2O NH4

+ + OH-

PO43- + H2O HPO4

2- + OH-

Examples of Bronsted-Lowry Acid Base Systems

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Lewis

Acid - an electron pair acceptor

Base - an electron pair donor

G.N. Lewis Definition

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pH and acidityHNO3 + NaOH H2O + NaNO3

HC2H3O2 + H2O H3O+ + C2H3O2-

NH3 + HCl NH4+ + Cl-

HSO4- + HPO4

-2 H2SO4 + PO4-3

HSO4- + HPO4

-2 SO4-2 + H2PO4

-1

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Acid Strength Strong Acid - Transfers all of its protons to water; - Completely ionized or dissociated; - Strong electrolyte; - The conjugate base is very weak

Weak Acid - Transfers only a fraction of its protons to water;

- Only partly ionizes or dissociates; - Weak electrolyte; - The conjugate base is stronger As acid strength decreases, base strength

increases. The stronger the acid, the weaker its conjugate base The weaker the acid, the stronger its conjugate base

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Base Strength

Strong Base - all molecules accept a proton; - completely ionizes or dissociates; - strong electrolyte; - conjugate acid is very weak

Weak Base - fraction of molecules accept proton; - partly ionizes or dissociates - weak electrolyte; - the conjugate acid is stronger.

As base strength decreases, acid strength increases. The stronger the base, the weaker its conjugate acid. The weaker the base the stronger its conjugate acid.

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Common Strong AcidsStrong AcidsStrong Acids

Hydrochloric Acid, HCl

Hydrobromic Acid, HBr

Hydroioidic Acid, HI

Nitric Acid, HNO3

Sulfuric Acid, H2SO4

Chloric Acid, HClO3

Perchloric Acid, HClO4

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Common Strong BasesStrong BasesStrong Bases

Any Group 1 Hydroxide and any Group 2 Hydroxide below Mg.

Sodium Hydroxide, NaOH

Potassium Hydroxide, KOH

*Barium Hydroxide, Ba(OH)2

*Calcium Hydroxide, Ca(OH)2

*While strong bases they are not very soluble

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pH and acidity

The pH values of several common substances are shown at the right.

Many common foods are weak acids

Some medicines and many household cleaners are bases.

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The pH Scale

pH [H3O+ ] [OH- ] pOH

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pH and acidity1. Acidity or Acid Strength depends on Hydronium Ion

Concentration [H3O+]

2. The pH system is a logarithmic representation of the Hydrogen Ion concentration (or OH-) as a means of avoiding using large numbers and powers.

pH = - log [H3O+]

pOH = - log [OH-]

In pure water [H3O+] = 1 x 10-7 M

pH = - log(1 x 10-7) = 7

4. pH range of solutions: 0 - 14

pH < 7 (Acidic) [H3O+] > 1 x 10-7 M

pH > 7 (Basic) [H3O+] < 1 x 10-7 M

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pH and acidity Important Equations pH = - log [H3O+]

[H3O+] = 10-pH

pOH = - log [OH-] [OH-] = 10-pOH

pH + pOH = 14

Kw = [H3O+] [OH-] = 1.0 x10-14

In pure water: [H3O+] = [OH-] = 1.0 x10-7

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Calculating the pHpH = - log [H3O+]

Example 1: If [H3O+] = 1 X 10-10

pH = - log 1 X 10-10

pH = - (- 10)pH = 10

Example 2: If [H3O+] = 1.8 X 10-5

pH = - log 1.8 X 10-5

pH = - (- 4.74)pH = 4.74

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pH and acidity

If a substance has a pH of 3.5, determine: [H3O+], [OH-], pOH and whether it is acidic, basic, or neutral.

If a substance has an [OH-] of 8.7 x 10-5, determine: pH, pOH, [H3O+] and whether it is acidic, basic, or neutral.

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Indicators