CHEM 105bL Chapter 15: Chapter 15 - Acids and Bases

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5 Feb 2017

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Chapter XV: Acids and Bases
I. The Nature of Acids and Bases
- Acids: sour taste, ability to dissolve many
metals, neutralize bases
- Bases: bitter taste, slippery feel, neutralize acids
II. Definitions of Acids and Bases
- The Arrhenius definition: acids produce H+ ions
in aqueous soln; bases produces OH- ions in
aqueous soln
- The H3O+ ion is called the hydronium ion
- The Bronsted-Lowry definition: acids donate
protons; bases accept protons
- A base accepts a proton to become a conjugate
- An acid donates a proton to become a
conjugate base
III. Acid Strength and Acid Ionization Constant (Ka)
- A strong acid completely ionizes in a solution
- A weak acid partially ionizes
- A monoprotic acid contains one ionizable
- A diprotic acid contains two ionizable protons
- The strength of an acid is dependent on the
attraction between H+ and A-; the stronger acid
the weaker the conjugate base
- The acid ionization constant (Ka) is an
equilibrium constant for the ionization rxn of a
weak acid
- 
- The smaller the constant, the weaker the acid
IV. Autoionization of Water and pH
- Water is amphoteric: can act as a base or an
- Water acts as an acid and a base with itself in a
process called autoionization
- The equilibrium constant is called the ion
product constant for water (Kw)
- 
- In pure water, the concentrations of H3O+ and
OH- are equal and soln is neutral
- An acidic soln contains an acid that creates
H3O+ ions
- A basic soln contains a base that creates OH-
- The pH specifies the acidity of a soln
-   
- The # of significant digits in the concentration of
the acid = the # of decimal places for the pH
- pH < 7 acidic
- pH = 7 neutral
- pH > 7 basic
- The pOH specifies the basicness of a soln
-   
-    
- The pKa is used to quantify the strength of a
weak acid
-  
V. Finding the [H3O+] and pH of Strong and Weak
Acid Soln
- Except in extremely dilute acid solns, the
autoionization of water contributes a negligibly
small amount of [H+] (can be ignored)
- The [H+] is equal to the concentration of the
strong acid (complete ionization)
- For weak acids, use the Ka to determine the
concentration of H+
- The percent ionization is the ratio of the
ionized acid concentration to the initial acid
-   
- The equilibrium H+ concentration of a weak
acid increases with increasing initial
concentration of the acid
- The % ionization of a weak acid decreases with
increasing concentration of the acid (H+ is
increased much slower than the concentration)
- In a mixture of a strong acid and weak acid, the
strong acid completely ionizes and the H+
formed suppresses the formation of additional
H+ from the weak acid
- In a mixture of two weak acids, the stronger
acid (or water) contributes the most to [H+] so
other sources (the weaker acid or water) are
VI. Base Soln
- A strong base completely dissociates in soln
Ca(OH)2, Ba(OH)2
- The base ionization constant quantifies the
extent of ionization of a weak base
- 
- Finding the [OH-] of a base is analogous to
finding the [H+] of an acid
VII. The Acid-Base Properties of Ions and Salts
- Cations tend to form acidic or neutral soln
- Anions tend to form basic or neutral soln
- An anion that is the conjugate base of a weak
acid is a weak base
- An anion that is the conjugate base of a strong
acid is pH neutral
- To determine the pH of a solution, you need to
know the Kw of the soln
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