CHAPTER_15 - Acid-Base Concepts The Brnsted-Lowry Theory...

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2/21/2009 1 Acid-Base Concepts: The Brønsted-Lowry Theory Arrhenius Acid : A substance that dissociates in water to produce hydrogen ions, H + . Arrhenius Base : A substance that dissociates in water to produce hydroxide ions, OH - . M + ( aq ) + OH - ( ) MOH( ) H + ( ) + A - ( ) HA( ) Brønsted-Lowry Base : A substance that can accept hydrogen ions, H + . In other words, a proton acceptor. Brønsted-Lowry Acid : A substance that can transfer hydrogen ions, H + . In other words, a proton donor. Acid-Base Concepts: The Brønsted-Lowry Theory Acid-Base Concepts: The Brønsted-Lowry Theory Conjugate Acid-Base Pairs : Chemical species whose formulas differ only by one hydrogen ion, H + . Brønsted-Lowry Base : A proton (H + ) acceptor. Brønsted-Lowry Acid : A proton (H + ) donor. HA + B BH + + A - H + donor Acid H + acceptor Base H + donor Acid H + acceptor Base Conjugate Acid-Base Pairs Acid-Base Concepts: The Brønsted-Lowry Theory Acid-Dissociation Equilibrium Hydronium ion = H 3 O + + + HA(aq) + H 2 0( l ) H 3 0 + (aq) + A - (aq) Acid Base Acid Base H A ●● ●● ●● H O H ●● A ●● ●● - + + H O H H + ●● Conjugate Acid-Base Pairs
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2/21/2009 2 H O H ●● ●● + H O - N + + + + NH 3 (aq) + H 2 0( l ) OH - (aq) + NH 4 + (aq) Base Acid Base Acid Conjugate Acid-Base Pairs Acid-Base Concepts: The Brønsted-Lowry Theory Base-Dissociation Equilibrium Acid Strength and Base Strength H 3 O + ( aq ) + A - ( ) HA( ) + H 2 O( l ) Base Acid Base Acid With equal concentrations of reactants and products, what will be the direction of reaction? Weaker acid Stronger acid Stronger base Weaker base + + Acid Strength and Base Strength Weak Acid : An acid that is only partially dissociated in water and is therefore a weak electrolyte. Strong Acid HA Weak Acid HA Very weak Acid HA HO 3 + A - HA HO 3 + A - HA HA HO 3 + A - ~ 100% Dissociation Partial Dissociation ~ 0% Dissociation The extent of dissociation is nearly 100% for a strong acid The extent of dissociation is considerably less than a 100% for a weak acid The extent of dissociation is considerably is nearly 0% for a very weak acid Relative Strength of Conjugate Acid-Base Pairs
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2/21/2009 3 Hydrated Protons and Hydronium Ions H + ( aq ) + A - ( ) HA( ) [H(H 2 O) n ] + For our purposes, H + is equivalent to H 3 O + . n = 4 n =2 n = 3 Due to high reactivity of the hydrogen ion, it is actually hydrated by one or more water molecules. n = 1 H 3 O + H 5 O 2 + H 7 O 3 + H 9 O 4 + H 3 O + ( ) + OH - ( ) 2H 2 O( l ) Dissociation of Water: Dissociation of Water K w = [H 3 O + ][OH - ] Dissociation of Water K w = (1.0 x 10 -7 )(1.0 x 10 -7 ) = 1.0 x 10 -14 at 25°C: [H 3 O + ] = [OH - ] = 1.0 x 10 -7 M H O H ●● ●● + H O - + H O H ●● H O H H + ●● Acid Base Acid Base Ion-Product Constant for Water: K w = [H 3 O + ][OH - ] Conjugate Acid-Base Pairs Dissociation of Water K w = [H 3 O + ][OH - ] = 1.0 x 10 -14 [H 3 O + ] = [OH - ] 1.0 x 10 -14 K w = (1.0 x 10 -7 )(1.0 x 10 -7 ) = 1.0 x 10 -14 [H 3 O + ] = [OH - ] = 1.0 x 10 -7 M Since [OH - ] = [H 3 O + ] 1.0 x 10 -14 H 3 O + ( ) + OH - ( ) 2H 2 O( l )
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This note was uploaded on 11/13/2011 for the course CHEM 1b taught by Professor Cabrera during the Spring '10 term at San Jose City College.

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CHAPTER_15 - Acid-Base Concepts The Brnsted-Lowry Theory...

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