Strength n Acids and Bases are classified acording to the degree to which they ionize in water: –Strong are completely ionized in aqueous solution; this means they ionize 100 % –Weak ionize only slightly in aqueous solution n Strength is very different from Concentration

Strong Acid Dissociation (makes 100 % ions)

Weak Acid Dissociation (only partially ionizes)

Measuring strength n Acid dissociation constant = K a n K a = concentrations of products / concentration of reactants n K a = [H + ][A - ] [HA] n Stronger acid = more products (ions), thus a larger K a (Note that water is NOT shown, because there is so much water that small reaction changes leave concentration of water virtually unchanged – treat as constant and leave out of K a )

What about bases? n Strong bases dissociate completely. n MOH + H 2 O  M + + OH - (M = a metal) n Base dissociation constant = K b n K b = [M + ][OH - ] [MOH] n Stronger base = more dissociated ions are produced, thus a larger K b.

Strength vs. Concentration n The words concentrated and dilute tell how much of an acid or base is dissolved in solution - refers to the number of moles of acid or base in a given volume n The words strong and weak refer to the extent of ionization of an acid or base n Is a concentrated, weak acid possible?

Practice n Write the K a expression for HNO 2 1)Equation: HNO 2 ⇆ H + + NO 2 - 2)K a = [H + ] x [NO 2 - ] [ HNO 2 ]

Question: Write the chemical reaction when lithium hydroxide is mixed with carbonic acid. Step 1: write out the reactants LiOH (aq) + H 2 CO 3 (aq)  Step 2: determine products LiOH (aq) + H 2 CO 3 (aq)  Li 2 CO 3 (aq) + H 2 O (l) Step 3: balance the equation 2LiOH (aq) + H 2 CO 3 (aq)  Li 2 CO 3 (aq) + 2H 2 O (l) lithium hydroxide + carbonic acid  lithium carbonate + water lithium hydroxide + carbonic acid  lithium carbonate + water Writing neutralization equations When strong acids & strong bases are mixed, a salt forms NaOH + HCl  H 2 O + NaCl base + acid  water + salt Ca(OH) 2 + H 2 SO 4  2H 2 O + CaSO 4

a) iron(II) hydroxide + phosphoric acid b) Ba(OH) 2 (aq) + HCl(aq) c) calcium hydroxide + nitric acid d) Al(OH) 3 (aq) + H 2 SO 4 (aq) e) ammonium hydroxide + hydrosulfuric acid f) KOH(aq) + HClO 2 (aq) Practice Write balanced chemical equations for these neutralization reactions. Under each compound give the correct IUPAC name.

Titration n Titration is the process of adding a known amount of solution of known concentration to determine the concentration of another solution n The equivalence point is when the moles of hydrogen ions is equal to the moles of hydroxide ions (= neutralized!) –An indicator is used to show when neutralization has occurred –Often we use phenolphthalein- because it is colorless in neutral and acid; turns pink in base

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Salt Hydrolysis n A salt is an ionic compound that: –comes from the anion of an acid –comes from the cation of a base –is formed from a neutralization reaction –some neutral; others acidic or basic n “Salt hydrolysis” - a salt that reacts with water to produce an acid or base

Buffers n Buffers are solutions in which the pH remains relatively constant, even when small amounts of acid or base are added –made from a pair of chemicals: a weak acid and one of it’s salts; or a weak base and one of it’s salts