Lecture 3 - Lecture 3 Chapter 8: Acids and Bases Chapter 9:...

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Anal Chem 251 Lect 3 1 Lecture 3 • Chapter 8: Acids and Bases • Chapter 9: Buffers
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Anal Chem 251 Lect 3 2 Summary of Acids,Bases and Buffers • pH = -log [H + ] • pOH = -log [OH - ] •pK a = -log [K a ] • pH + pOH = pK w = 14 at 25 C • pH of a buffer = pK a + log [A-]/[HA]
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Anal Chem 251 Lect 3 3 Classical Definition (Arrhenius) of Acids and Bases • In the classical definition, acids and bases are defined according to their formulas and their behavior in water: •A n a c i d is a substance that has H in its formula and dissociates in water to yield H 3 O + •Ab a s e is a substance that has OH in its formula and dissociates in water to yield OH - • When an acid and a base react, they undergo neutralization
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Anal Chem 251 Lect 3 4 The hydrated proton, hydronium ion, H 3 O + . Figure 4.4 The proton bonds covalently to one of the lone pairs of a water molecule’s O atom to form a hydronium ion, H 3 O + , which H bonds to several other water molecules.
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Anal Chem 251 Lect 3 5 Structure of hydronium ion, H 3 O +
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Anal Chem 251 Lect 3 6 The extent of dissociation for strong acids. Figure 18.1 Strong acid: HA( g or l ) + H 2 O( l ) H 3 O + ( aq ) + A - ( aq ) Note: single sided arrow.
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Anal Chem 251 Lect 3 7 Important to know for chemical analyses.
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Anal Chem 251 Lect 3 8 The extent of dissociation for weak acids. Figure 18.2 Weak acid: HA( aq ) + H 2 O( l ) H 3 O + ( aq ) + A - ( aq ) Note: double-sided arrow.
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Anal Chem 251 Lect 3 9 Strong acids dissociate completely into ions in water. K c >> 1 HA( g or l ) + H 2 O( l ) H 3 O + ( aq ) + A - ( aq ) Weak acids dissociate very slightly into ions in water. K c << 1 HA( aq ) + H 2 O( l ) H 3 O + ( aq ) + A - ( aq ) The Acid-Dissociation Constant stronger acid higher [H 3 O + ] larger K a K c = [H 3 O + ][A - ] [H 2 O][HA] K c [H 2 O] = K a = [H 3 O + ][A - ] [HA] smaller K a lower [H 3 O + ] weaker acid
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Anal Chem 251 Lect 3 10 The Effect of Molarity on the Percent Ionization of a Weak Acid • Fraction Ionized = [A - ]/([HA] + [A - ]) • % Ionized = fraction ionized x 100 • The higher the concentration of the weak acid, the lower the percent ionized. % Ionization M acid
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Anal Chem 251 Lect 3 11 ACID STRENGTH
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Anal Chem 251 Lect 3 12 Autoionization of Water and the pH Scale Water is an extremely weak electrolyte; this is the reason we can quantify H3O+( aq ) H 2 O( l ) H 2 O( l ) + OH - ( aq ) H 3 O + ( aq ) +
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Anal Chem 251 Lect 3 13 H 2 O( l ) + H 2 O( l ) H 3 O + ( aq ) + OH - ( aq ) K c = [H 3 O + ][OH - ] [H 2 O] 2 The Ion-Product Constant for Water K c [H 2 O] 2 =[ K w = H 3 O + ][OH - ] = 1.0 x 10 -14 at 25 0 C A change in [H 3 O + ] causes an inverse change in [OH - ]. In an acidic solution, [H 3 O + ] > [OH - ] In a basic solution, [H 3 O + ] < [OH - ] In a neutral solution, [H 3 O + ] = [OH - ]
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Anal Chem 251 Lect 3 14 The relationship between [H 3 O + ] and [OH - ] and the relative acidity of solutions. Figure 18.4 [H 3 O + ] [OH - ] Divide into K w [H 3 O + ] > [OH - ] [H 3 O + ] = [OH - ] [H 3 O + ] < [OH - ] ACIDIC SOLUTION BASIC SOLUTION NEUTRAL SOLUTION
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Anal Chem 251 Lect 3 15 SAMPLE PROBLEM 18.2: Calculating [H 3 O + ] and [OH - ] in an Aqueous Solution PROBLEM: A research chemist adds a measured amount of HCl gas to pure water at 25 0 C and obtains a solution with [H 3 O + ] = 3.0x10 -4 M.
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This note was uploaded on 04/27/2008 for the course CHEM 251 taught by Professor Vassilian during the Fall '06 term at Rutgers.

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Lecture 3 - Lecture 3 Chapter 8: Acids and Bases Chapter 9:...

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