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Acids and Bases

Chapter 16 Pages 614 - 652

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Arrhenius Definition of Acids & Bases

• Acids release H+ ions when dissolved in water.– HCl(g) H+(aq) + Cl-(aq)

• Bases release OH- ions when dissolved in water.– NaOH(s) Na+ (aq) + OH- (aq)

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HCl(g) H+(aq) + Cl-(aq)

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NaOH(s) Na+ (aq) + OH- (aq)

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Limitations of Arrhenius Concept: Restricted to aqueous solutions

• More general definition offered independently by Johannes Bronsted and Thomas Lowry (1923)

• Focuses on the transfer of H+ ions from one substance to another

• An H+ ion is simply a proton with no surrounding valence electrons; interacts strongly with nonbonding electons pairs of water molecules to form hydronium ions, H3O+ (aq)

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Reaction between HCl and NH3

(all gases)

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HCl acts as a Bronsted-Lowry Acid; Water is the base.

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Ammonia is a B-L base; Water is the acid.

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Bronsted-Lowry Acids & Bases

• An acid is a substance that can donate a proton to another substance.

• A base is a substance that can accept a proton.

• A tug-of-war between an acid & base for proton (in the form of H+)

• Because the emphasis is on proton transfer, can be applied to non-aqueous reactions, too.

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• A substance can function as an ACID only if another substance simultaneously behaves as a BASE.

• Acid – must have a H atom to lose as an H+ ion• Base – must have a nonbonding pair of

electrons to bind to H+ ion.

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Stupid Sports Analogy…

Acids are the quarterback, throwing away a PROTON (football) Bases are the receivers, catching the

PROTON, (football)

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Conjugate Acid-Base Pairs

• Consider a reversible reaction:

• HX (aq) + H2O X- (aq) + H3O+ (aq)

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• When reaction is reversed, X- acts as base and H3O+ acts as acid.

• HX (aq) + H2O(l) X- (aq) + H3O+ (aq) • Every acid has a conjugate base, formed by

removing a proton from the acid.HX (aq) acidX- (aq) conjugate base

H2O (l) baseH3O+ (aq) conjugate acid

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Conjugate Acid-Base Pairs

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Another Example

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And another,

Conjugate Acid-Base Pairs

• HBr(aq) + H2O (l)

• If HBr is the acid and H2O is the base, write the rest of the reaction…..

• Br-(aq) + H3O+ (aq)

• Br-(aq) is the conj. Base• H3O+ (aq) is the conj. Acid

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