ACIDS & BASES

Arrhenius Theory 1. in aqueous solution 2. Acid: produces H + 3. Base: produces OH -

HA H 3 O + + A - Acid O H H HA + O H H H + + A -

HCl(g) + H 2 O  H 3 O + (aq) + Cl - (aq) CH 3 COOH(l) + H 2 O = H 3 O + (aq) + CH 3 COO - (aq)

careless, but often seen HCl  H + + Cl - CH 3 COOH  H + + CH 3 COO -

Base NaOH (s) Na + (aq) + OH - (aq)

Arrhenius acid/base reaction acid + base  H 2 O + a salt HA + MOH  HOH + MA

Monoprotic acid: HCl HCl(aq) + NaOH(aq)  H 2 O(l) + NaCl(aq) H + + Cl - + Na + + OH -  H 2 O + Na + + Cl - H + + OH -  H 2 O HCl 

Diprotic acid: H 2 SO 4 H 2 SO 4 (aq) + 2NaOH (aq)  2H 2 O(l) + Na 2 SO 4 (aq) H + + OH -  H 2 O H 2 SO 4 

Triprotic acid: H 3 PO 4 Polyprotic H 3 PO 4 (aq) + 3NaOH (aq)  3H 2 O(l) + Na 3 PO 4 (aq) H 3 PO OH -  3 H 2 O + PO 4 3- H 3 PO 4 

Bronsted-Lowry Theory 1. aqueous & nonaqueous solutions 2. Acid: species donating a proton HCl  H + + Cl - H 2 SO 4  H + + HSO 4 - CH 3 COOH  H + + CH 3 COO -

Bronsted-Lowry Theory 3. Base: species accepting a proton OH - + H +  HOH H 2 O + H +  H 3 O + NH 3 + H +  NH 4 +

Conjugate acid-base pairs acid 1 + base 1 acid 2 + base 2 conjugate pairs HF + HOH

Conjugate acid-base pairs conjugate pairs HF + HOH H 3 O + + F - acid 1 + base 1 acid 2 + base 2

ALL Arrhenius reactions are Bronsted-Lowry reactions HCl + NaOH  H 2 O + NaCl

NOT all Bronsted reactions are Arrhenius reactions CH 3 COOH + NH 3  NH CH 3 COO -

Amphiprotic = Amphoteric Can act as either an acid or a base HCl + HOH  H 3 O + + Cl - NH 3 + HOH  NH OH - NH 3 + OH -  NH HOH HOH + HOH  H 3 O + + OH -

ACID STRENGTH Relative ability of a compound to donate a proton Base strength is considered a result, not a cause

Strong acid 100% dissociation Weak acid <100% dissociation Notice this is NOT related to concentration REVIEW

Electronegativity is the most significant factor influencing the strength of acids & bases

HF > HCl > HBr > HI as acids in non- aqueous solvents, or as pure gases

Look at difference in electronegativities 2.1 H - F H - Cl H - Br H - I 2.5

Most “ionic” is the most acidic Nonpolar Polar Ionic E  0 E  1.7 E  4.0

However, as acids in aqueous solution HF < HCl = HBr = HI

2.1 H - O 3.5 competition! 2.1 H - F H - Cl H - Br H - I 2.5

Is methane acidic as a gas or in aqueous solution? 2.1 H - C 2.5

The strength of oxy-acids are also dependent on electronegativity.

Oxy-acids and bases have the same fundamental structure

NaOH: Na - O - H HClO: Cl - O - H

In water, the more “ionic” bond dissociates, forming the acid or base

NaOH: Na - O - H HClO: Cl - O - H

Are alcohols acids or bases? C - O - H

Acids in homologous series are of different strength

Acid Strength H 2 SO 4 > H 2 SO 3 HNO 3 > HNO 2 HClO 4 > HClO 3 > HClO 2 > HClO

Structurally H 2 SO 4 = O 2 S(OH) 2 H 2 SO 3 = OS(OH) 2

Need to examine formal charge of central atom.

Acid Strength CH 3 COOH> CH 3 CH 2 OH CF 3 COOH > CH 3 COOH

Need to examine inductive effect of neighboring atoms.

pH pK K a, K b, K w

2H 2 O H 3 O + + OH -

K eq [H 2 O] 2 = [H 3 O + ][OH - ] K w = [H 3 O + ][OH - ] where K w (25 o C ) = 1 x

in a neutral solution [H 3 O + ] = [OH - ] 1 x = [H 3 O + ] 2 = [OH - ] 2 [H 3 O + ] = [OH - ] = 1 x 10 -7

pX = -log X pK = -log K pH = -log [H 3 O + ] pOH = -log [OH - ]

leveling effect of H 2 O limits [H 3 O + ] & [OH - ] to that controlled by H 2 O

upper limit [H 3 O + ] = 1 lower limit [H 3 O + ] = 1 x

pH scale acid neutral base highest [H 3 O + ] on left lowest [H 3 O + ] on right

[H 3 O + ] and [OH - ] must be considered together

K w = [H 3 O + ][OH - ] -log K w = -log {[H 3 O + ][OH - ]} -log K w = {-log [H 3 O + ]} + {-log[OH - ]}

pK w = pH + pOH but K w = 1 x = pH + pOH

Relationship between conjugate acids & bases HA + H 2 O H 3 O + + A - A - + H 2 O HA + OH -

K a x K b = [H 3 O + ][OH - ] = K w

K a. K b = [H 3 O + ][OH - ] = K w K a. K b = K w

SUMMARY pH = -log [H 3 O + ] pOH = -log [OH - ] [H 3 O + ][OH - ] = 1 x pH + pOH = 14 K a. K b = K w

Applications of Acid-Base Concepts

for weak acids & bases, refer to Appendix H for K a & Appendix I for K b values in Kotz & Treichel

1. What is the pH of a solution that is M KOH?

2. What is the pH of a 0.20 M acetic acid solution?

mL of 0.10 M CH 3 COOH are mixed with 20.0 mL of 0.10 M NaOH. What is the pH of the solution?

4. Calculate the percent ionization of 0.10 M methylamine (CH 3 NH 2 ).

pH of Salts & Oxides

What effect does the addition of a salt to water have upon the pH of the water?

H 2 O equilibrium is the prime factor in the behavior of solutions.

pH of a salt solution is dependent upon the strength of the salt as an electrolyte.

Example 1 NaCl(s) + HOH NaOH(aq) + HCl(aq)

Example 1 NaCl(s) + HOH  NaOH (aq) + HCl(aq) strong base strong acid Na + + OH - + H + + Cl -  Na + + HOH + Cl -

thus, NaCl in water has NO effect on pH

Example 2 NaCN(s) + HOH NaOH(aq) + HCN(aq)

Example 2 NaCN(s) + HOH  NaOH (aq) + HCN(aq) strong base weak acid  Na + + OH - + HCN

CN - is the anion of the weak acid HCN CN - + HOH HCN + OH -

NaCN(s) + HOH  Na + + OH - + HCN strong base weak acid

thus, NaCN in water produces a/n ?? solution

thus, NaCN in water produces a BASIC solution

5. What is the pH of a M sodium cyanide solution?

Example 3 NH 4 Cl(s) + HOH  NH 4 OH (aq) + HCl(aq)

Example 3 NH 4 Cl(s) + HOH  NH 4 OH (aq) + HCl(aq) weak base strong acid  NH 4 OH + H + + Cl -

NH 4 + is the cation of the weak base NH 4 OH NH HOH NH 3 + H 3 O +

NH 4 Cl(s) + HOH -> NH 4 OH + H + + Cl - weak base strong acid

thus, NH 4 Cl in water produces a/n ?? solution

thus, NH 4 Cl in water produces an ACID solution

6. What is pH of a 0.10 M ammonium chloride solution?

Example 4 NH 4 CN(s) + HOH NH 4 OH (aq) + HCN(aq) weak base weak acid

NH 4 + is the cation of the weak base NH 4 OH NH HOH NH 3 + H 3 O +

CN - is the anion of the weak acid HCN CN - + HOH HCN + OH -

thus, NH 4 CN in water produces a/n ?? solution

The pH of a solution formed from the cation of a weak base and the anion of a weak acid is dependent on the relative strength of the weak acid and weak base.

K a (HCN) = 6.2 x [Text: Table 5.1] Appendix H A-23 K b (NH 4 OH) = 1.8 x [Text: Table 5.3] Appendix I A-25

thus, NH 4 CN in water produces a/n ?? solution

thus, NH 4 CN in water produces a BASIC solution, because the weak base is stronger (ionizes more) than the weak acid

Acidity of Oxides

SO 2 + HOH ??

SO 2 + HOH H 2 SO 3 [O 2 ] H 2 SO 4

SO 2 + HOH H 2 SO 3 [O 2 ] H 2 SO 4 Covalent oxides are acidic & are referred to as acid anhydrides

Na 2 O + HOH ??

Na 2 O + HOH 2NaOH (aq)

Ionic oxides are basic & are referred to as basic anhydrides

Lewis Acid-Base Theory

Acid substance capable of accepting an e - pair

Lewis acid must have an empty valence level orbital i.e. H + has an empty 1s orbital which can accept an e - pair

Thus, H + is an acid under all three theories Arrhenius Bronsted-Lowry Lewis

Lewis Acid-Base Theory Acid: substance capable of accepting an e - pair Base substance capable of donating an e - pair

Examples of Lewis bases OH -, NH 3, F - all have unbonded pairs of e - available for donation

Elements of Group 13 (3A) form compounds that make excellent Lewis acids

another “typical” Lewis acid-base reaction:

Reaction of a Lewis Acid and Lewis Base   New bond formed using electron pair from the Lewis base.   Coordinate covalent bond   Notice geometry change on reaction.

Formation of hydronium ion is also an excellent example. Lewis Acids & Bases Electron pair of the new O-H bond originates on the Lewis base.

Lewis Acid/Base Reaction

H 3 BO 3 + H 2 O H 2 BO H 3 O + ? NO!

H 3 BO 3 + 2H 2 O B(OH) H 3 O +

is Al(OH) 3 an acid or base?

Amphoterism of Al(OH) 3

This explains AMPHOTERIC nature of some metal hydroxides. Al(OH) 3 (s) + 3 H +  Al H 2 O Here Al(OH) 3 is a Brønsted base. Al(OH) 3 (s) + OH -  Al(OH) 4 - Here Al(OH) 3 is a Lewis acid. Lewis Acids & Bases

Transition metal ions also very good Lewis Acids

Other good examples involve metal ions. Lewis Acids & Bases

Zn 2+ + HOH ?.. Zn 2+ => [Ar] 4s 0 3d 10 4p 0 Zn(H 2 O)

Reaction of NH 3 with Cu 2+ (aq)

Formation of complex ions is a Lewis acid-base reaction

Lewis Acid-Base Interactions in Biology   The heme group in hemoglobin can interact with O 2 and CO.   The Fe ion in hemoglobin is a Lewis acid   O 2 and CO can act as Lewis bases Heme group

Lewis Bronstead Arrhenius Inclusiveness of the Acid/Base Definitions