Acid-Base Equilibria

Acid-Base Equilibria

Acids Bases Sour taste React with active

metals to release hydrogen gas

Change the color of indicators

Bitter taste Feel slippery Change the color of

indicators

Arrhenius Acid/Base Theory According to

Arrhenius, acids are substances that produce H+ (H3O+) ions in water.

Bases are substances that produce OH- ions in water.

Why does water affect acids? A water solution of HCl produces:

HCl(g) H+(aq) + Cl-(aq)

Acidic solutions are formed when an acid transfers a proton to water.

H+ + H2O H3O+

Brønsted-Lowry Acids/Bases Acids are substances capable of donating a

proton. Bases are capable of accepting a proton. B/L theory applicable to reactions that do

not occur in water Can include gas phase reactions NH3(g) + H2O(l) NH4

+(aq) + OH-(aq)

An acid and a base which differ by only one proton are conjugate acid/base pairs. Acid HI HNO` PH4

+ HSO4 -

Conj.

Base

Base HSO4 – H2O NH2

- F-

Conj.

Acid

The more readily an acid gives up a proton,the less readily the conjugate base will accept a proton.

The stronger the acid, the weaker its conjugate base and the weaker the acid, the stronger its conjugate base.

HCN is a weaker acid than HF. The conjugate base CN- is a stronger base than F-.

Diethylammonium ion (CH3)2NH2+ is a

weak acid.

Determine the conjugate base:

Is the conjugate base stronger or weaker thanthe conjugate base of HCl?

Strong Acids Ionize completely to form ions HCl

HBr

HI

HNO3

H2SO4

HClO4

Autoionization of Water Proton transfer from one water molecule to

another

H-O-H + H-O-H H3O+ + OH-

H2O H + + OH-

K = [H+][OH-]

H2O

Kw = [H+][OH-] = 1.0x10-14

[H2O] >> 55 M and remains constant not written in K

If solution is:neutral [H+] = [OH-]

acid [H+] > [OH-] base [H+] < [OH-]

[H+] = 2x10-5M[OH-] = 3x10 –9M[OH-] = 1 x 10-7M

pH Scale pH = -log[H+] Acidic solution pH < 7 Basic solution pH > 7 Neutral solution pH = 7 Number of decimal places in pH = number

of significant figures in concentration.

1.0x10-12 pH = 12.00

Strong Acids Strong electrolytes React completely with water to form H+

pH of a strong acid equals –log of hydrogen concentration

Weak Acids Weak electrolytes [molecules] greatest at equilibrium H atoms bound to C do not ionize while H

atoms bound to O will ionize

HX(aq) H+(aq) + X-(aq)

Ka = [H+][X-]

[HX]

0.10M HCOOH (formic acid)pH = 2.38

Calculate Ka and percent dissociation

In the reaction of a strong acid with metal the conductivity remains constant since all acid molecules ionize.

But, for a weak acid, conductivity will fluctuate since degree of ionization increasesAs [A] decreases.

Calculate %HF in 0.10 M HF 0.010 M HF

Polyprotic Acids Release more than one proton in H2O

H2SO3 H+(aq) + HSO3-(aq) Ka1

HSO3-(aq) H+(aq) + SO3

2-(aq) Ka2

Ka1 = 1.7 x 10-2

Ka2 = 6.4 x 10-8

If Ka constants differ by a factor of 103, consider Ka1 only. In other words, treatthe acid as if it were monoprotic.

The solubility of CO2 at 25oC and 0.10 atmequals 0.0037 M. All of the dissolved CO2

is as H2CO3. What is the pH of a 0.0037 Msolution of H2CO3?

Strong Bases Most common strong bases are heavy

Group IIA and all Group IA. Determine the pOH of a basic solution by:

pOH = -log [OH-]pH + pOH = 14

Determine the pH of a 0.010 M solution of Ba(OH)2.

Formation of Strong Bases Stronger bases than water are able to

remove an H+ ion from water:

O2- + H2O(l) 2OH-(aq)

H- + H2O(l) H2(g) + OH-(aq)

N3- + H2O(l) NH3(aq) +3OH-(aq)

Weak Bases Weak base + water acid + OH-

NH3 + H2O NH4+ + OH-

Kb = [NH4+][OH-]

[NH3]

Calculate the [OH-] in a 0.15 M solution of NH3. (Kb = 1.8 x 10-5)

Amines Weak nitrogen bases N-C bonds Due to lone pair on N, it is able to extract a

proton

H2N-CH3 + H2O [H3NCH3]+ + OH-

Anions of Weak AcidsWhen sodium salts dissolve in water, the Na+

ion merely acts as a spectator ion. The reaction occurs between the remaining anion and water.

C2H3O2- + H2O HC2H3O2 + OH-

Calculate the pH of a 0.010 M solution of

NaClO.

Relation Between Ka and Kb

NH4+(aq) NH3(aq) + H+(aq)

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

NH4+ and NH3 are conjugate pairs.

Ka = [NH3][H+] Kb = [NH4+][OH-]

[NH4+] [NH3]

Add Reaction 1 and Reaction 2.

H2O H+ + OH-

Multiply Ka x Kb

Ka x Kb = Kw

Ka and Kb are inversely related, as acid strengthincreases (Ka), base strength decreases (Kb)since product must equal Kw.

Calculate the Kb for F- and the Ka for NH4+.

Kb = 1.5 x 10-11

Ka = 5.6 x 10-10

Which of the following has the largest Kb?NO2

-, PO43-, N3

-

PO43-

Acid-Base Properties of Salts Completely ionized Nearly all salts are strong electrolytes Acid/base properties due to hydrolysis of

cations and anions Strong acids and bases produce ions that do

not hydrolyze

Anions of weak acids:NO2

-(aq) + H2O(l) HNO2(aq) + OH-(aq)

Cations of weak bases: NH4

+(aq) + H2O(l) NH3(aq) + H3O+(aq)

Anions of polyprotic acids act as either proton donors or acceptors depending on the value of Ka or Kb.

Predict whether Na2HPO4 will be acidic or basic.

pH of Salts Depends on parent acid and base. If acid and base are both strong, salt is neutral. If acid is strong and base is weak, salt is acidic. If acid is weak and base is strong, salt is basic. If parent acid and base are both weak, salt pH

depends on value of Ka and Kb.

List the following in order of increasing pH:

Co(ClO4)2 RbCN Sr(NO3)2 KC2H3O2

Acid/Base Character Any molecule containing H can act as a

potential acid, but bond must be polarized. Very strong bonds are less ionized than

weak ones.HF is a weak acid but acidic character increases with increasing atomic number making the remaining elements in the halogen family strong acids.

OxyAcids Acids in which OH groups and possibly

additional oxygen atoms are bound to a central atom.

Acid strength increases with increasing EN of Y.

HIO < HBrO < HClO

In a series of acids with the same Y, acidstrength increases with increasing oxidationnumber.

HClO < HClO2 < HClO3 < HClO4

Lewis Acids/Bases According to Lewis, an acid is an electron

pair acceptor. Acids will generally have incomplete octets.

A base is an electron pair donor. Bases will tend to have lone pairs.

BF3 + NH3 F3BNH3

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