Acid and Bases

ACID-BASE THEORIES 1 of 2 The most general theory for common aqueous acids and bases is the BRØNSTED - LOWRY theoryThe most general theory for common aqueous acids and bases is the BRØNSTED - LOWRY theory ACIDS DONATE H + IONSACIDS DONATE H + IONS BASES ACCEPT H + IONSBASES ACCEPT H + IONS

Conjugate acidConjugate base

Conjugate acid Conjugate base

Conjugate acid Conjugate base

Conjugate acid Conjugate base

Acetic Acid

PLAY MOVIE Lewis acid a substance that accepts an electron pair Lewis base a substance that donates an electron pair ACID-BASE THEORIES 2 of 2

A common way to express acidity is with pH pH = -log [H 3 O + ] = -log [H + ] = -log [H + ] In a neutral solution, [HO + ] = [OH - ] = 1.00 x at 25 o C [H 3 O + ] = [OH - ] = 1.00 x at 25 o C pH = -log (1.00 x ) = - (-7) = 7

PLAY MOVIE In pure water there is AUTOIONIZATION Equilibrium constant for autoion = K w K w = [H 3 O + ] [OH - ] = 1.00 x at 25 o C

Other pX Scales In generalpX = -log X and so pOH = - log [OH - ] K w = [HO + ] [OH - ] = 1.00 x at 25 o C K w = [H 3 O + ] [OH - ] = 1.00 x at 25 o C Take the log of both sides -log ( ) = - log [HO + ] + (-log [OH - ]) -log ( ) = - log [H 3 O + ] + (-log [OH - ]) pK w = 14 = pH + pOH pK w = 14 = pH + pOH

The pH Scale Adding Basic Substance Adding Acidic Substance Pure 25 o C

Strong Acids Generally divide acids and bases into STRONG or WEAK ones.Generally divide acids and bases into STRONG or WEAK ones. STRONG ACID: HNO 3 (aq) + H 2 O(l) H 3 O + (aq) + NO 3 - (aq) HNO 3 is about 100% dissociated in water. PLAY MOVIE

Strong Base: 100% dissociated in water.Strong Base: 100% dissociated in water. NaOH(s) Na + (aq) + OH - (aq) NaOH(s) Na + (aq) + OH - (aq) Strong Bases Other common strong bases include KOH and Ca(OH) 2 H2OH2O

Do Strong acid Strong base problems

Weak acids are much less than 100% ionized in water.Weak acids are much less than 100% ionized in water. One of the best known is acetic acid: CH 3 CO 2 H Weak Acids

Weak base: less than 100% ionized in waterWeak base: less than 100% ionized in water One of the best known weak bases is ammonia NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq) Weak Bases PLAY MOVIE

Equilibrium Constants for Weak Acids Weak acid has K a < 1 Leads to small [H 3 O + ] and a pH of Model Weak Acid

Equilibrium Constants for Weak Bases Weak base has K b < 1 Leads to small [OH - ] and a pH of Model Weak Base

Equilibrium Constants for Acids/Bases Acids ConjugateBases Increase strength

PLAY MOVIE Lewis acid a substance that accepts an electron pair Lewis base a substance that donates an electron pair ACID-BASE THEORIES 2 of 2 PLAY MOVIE

Many complex ions containing water undergo HYDROLYSIS to give acidic solutions. [Cu(H 2 O) 4 ] 2+ + H 2 O [Cu(H 2 O) 3 (OH)] + + H 3 O + Lewis Acids & Bases

Why is CH 3 CO 2 H an Acid? 1. The electronegativity of the O atoms causes the H attached to O to be highly positive. 2. The O—H bond is highly polar. 3. The H atom of O—H is readily attracted to polar H 2 O. – See Figure 17.12

Trichloroacetic acid is a much stronger acid owing to the high electronegativity of Cl.Trichloroacetic acid is a much stronger acid owing to the high electronegativity of Cl. Cl withdraws electrons from the rest of the molecule.Cl withdraws electrons from the rest of the molecule. This makes the O—H bond highly polar. The H of O— H is very positive.This makes the O—H bond highly polar. The H of O— H is very positive. Acetic acid Trichloroacetic acid K a = 1.8 x K a = 0.3