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Topic 8: Acids & Bases

Friday, October 14, 2011

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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 (H2).• React with carbonates and bicarbonates

to produce carbon dioxide gas (CO2)

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

Bases


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Properties of Acidsþ Produce H+ (as H3O+) ions in water (the hydronium ion is

a hydrogen ion attached to a water molecule)

þ Taste sour

þ Corrode metals

þGood Electrolytes

þ React with bases to form a salt and water

þ pH is less than 7

þ Turns blue litmus paper to red “Blue to Red A-CID”


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Properties of BasesGenerally produce OH- ions in water

Taste bitter, chalky

Are electrolytes

Feel soapy, slippery

React with acids to form salts and water

pH greater than 7

Turns red litmus paper to blue “Basic Blue”


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What is an acid or base? Arrhenius defined them as:

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

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

Categorical definition – easy to sort substances into acids and bases

Problem – many bases do not actually contain hydroxides like NH3


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Brønsted-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-

Ex HCl + H2O → H3O+ + Cl-

Acid Base Conj Acid Conj BaseOperational definition - The classification depends

on how the substance behaves in a chemical reaction


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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.

H

Conjugate Acid-Base Pairs


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Note: Water can act as acid or base (Amphoteric) Acid Base Conjugate Acid Conjugate BaseHCl + 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 Brønsted-Lowry Acid Base Systems


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Lewis defined them as: Acid - an electron pair acceptorBase - an electron pair donor

G.N. Lewis Definition


Try it!

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Try it!

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HomeworkIdentify the conjugate acid-base pairs. Connect as shown.

NH2- + H2O ↔ NH3 + OH-

CN- + H2O ↔ HCN + OH-

HClO4 + CH3COOH ↔ ClO4- + CH3COOH2+

HCN + H2O ↔ H3O+ + CN-

HSO4- + HCl ↔ H2SO4 + Cl-

SO42- + HNO3 ↔ HSO4- + NO3- 13


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