Acid-Base Strength:Ka, Kb, Kw

Mrs. Kay

Chemistry 12

Chapter 15 Pages: 583-584,587-597

Relative Strengths Of Binary AcidsH –XThe greater the tendency for the transfer of a proton from HX to H2O, the more the forward reaction is favored and the stronger the acid.

in a periodic group:• The weaker the bond, the stronger the acid.• The larger the resultant anion’s radius, the stronger is

the acid.• The strengths of binary acids increase from top to

bottom in a group of the periodic table.

Relative Strengths Of Binary Acids

H –Xin a periodic group:

Bond dissociation energy: the weaker the bond, the stronger the acid.Bond dissociation energy 569 > 431 > 368 > 297(kJ/mol) HF HCl HBr HI

Acid strength Ka 6.6x10-4 < ~106 < ~108 < ~109

Anion radius: the larger the anion’s radius, the stronger the acid.Anion radius (ppm) 136 < 181 < 195 < 216(kJ/mol) HF HCl HBr HI

Acid strength Ka 6.6x10-4 < ~106 < ~108 < ~109

The strength of binary acids increase from top to bottom in a group of the periodic table.

Relative Strengths Of Binary Acids

H –X

in a period:• The larger the electronegativity difference between H and X,

the more easily the proton is removed and the stronger is the acid.

EN 0.4 < 0.9 < 1.4 < 1.9

Acid strength CH4 NH3 H2O HF

• The strengths of binary acids increase from left to right across a period of the periodic table.

Representative Trends In Strengths of Binary Acids

The Acid dissociation constant, Ka

Ka

• A weak acid only ionizes to a small extent and

comes to a state of chemical equilibrium. • We can determine how much it ionizes by

calculating the equilibrium constant for this reaction, the ionization constant, Ka.

• The larger the Ka the more acid ions are found in solution and the stronger the acid because the more easily it donates a proton.

• The reverse is true for the smaller the Ka

HCOOH (aq) + H2O (l) < -- > H3O+ (aq) + HCOO- (aq)

• Ka = [H3O+][HCOO-]

[HCOOH]

• Notice how the Ka ignores the water since we are dealing with dilute solutions of acids, water is considered a constant, and when multiplied by both sides it is cancelled out.

Equilibrium In Solutions Of Weak Acids And Weak Bases

weak acid: HA + H2O H3O+ + A-

[H3O+][A-]

Ka =

[HA]

weak base: B + H2O HB+ + OH-

[HB+][OH-]

Kb =

[B]

You need to be able to write acid and base ionization equations!!!

Practice:

1. A solution of a weak acid, “HA”, is made up to be 0.15 M. Its pH was found to be 2.96. Calculate the value of Ka.

• Steps to follow:1. Write balanced equation

2. Calculate [H+] using 10-pH

3. Set up chart for equilibrium (ICE or i Δ f)

4. Solve using Ka expression

Answer

1. HA (aq) + H2O (l) < -- >H3O+ (aq) + A-(aq)

2. [H3O+]= 10 -2.96 = 0.0011 M3. Set up table:

HA H2O < -- > H3O+ A-

0.15 0 0

-.0011 +0.0011 +0.0011

0.139 0.0011 0.0011

i

Δ

f

4. Ka = [H3O+][ A-]

[HA]

= [0.0011][0.0011]

[0.139]

= 8.7 x 10 -6

Percent Dissociation

The fraction of acid molecules that dissociate compared with the initial concentration of the acid.

Percent Dissociation = [H3O+] x 100%

[HA i]

For the previous question:

Percent Dissociation = [0.0011] x 100% =0.73 %

[0.15]

Practice:

• The ionization constant, Ka, for a hypothetical weak acid, HA, at 25°C is 2.2 x 10-4.

a) Calculate the [H3O+] of a 0.20 M solution of HA.

b) Calculate the percent ionization of HA.c) Calculate the [A-].d) What initial concentration of HA is needed

to produce a [H3O+] of 5.0 x 10-3 M?

Answer

a) HA(aq) + H2O(l) < -- >H3O+(aq) + A-(aq)

Ka = [H3O+][A-] = 2.2 x 10-4

[HA]2.2 x 10-4 = (x)(x)

0.20 Mx2 = (2.2 x 10-4) (0.20 M)x = 0.0066 M

The [H3O+] is 0.0066 M.

Because it is a 1:1 ratio they are both the same concentration (x)

b) % ionization = 0.0066 M x 100% = 3.3%

0.20 M

c) From the stoichiometry of the reaction,

[H3O+] = [A-]

Therefore, [A-] = 0.0066 M

Found d) from table set up

d) 2.2 x 10-4 = (5.0 x 10-3 M)(5.0 x 10-3 M)x – 5.0 x 10-3 M

• 2.2 x 10-4(x – 5.0 x 10-3M) = 2.5 x 10-5

• 2.2 x 10-4x – 1.1 x 10-6 = 2.5 x 10-5

• x = (2.5 x 10-5 + 1.1 x 10-6) / 2.2 x 10-4

• x= 0.12 M• The initial concentration of HA required is

0.12 M.

Acid And Base Ionization Constants

weak acid: CH3COOH + H2O H3O+ + CH3COO-

[H3O+][CH3COO-]

Acid ionization constant: Ka =

[CH3COOH]

weak base: NH3 + H2O NH4+ + OH-

[NH4+][OH-]

Base ionization constant: Kb =

[NH3]

Acid and base ionization constants are the measure of the strengths of acids and bases.

Kb

• When using weak bases, the same rules apply as with weak acids, except you are solving for pOH and using [OH-]

Another relationship

• Useful to know:

Ka x Kb = Kw = 1.0 x 10-14

Buffer Solutions

• A buffer solution is a solution that changes pH only slightly when small amounts of a strong acid or a strong base are added.

• A buffer contains

a weak acid with its salt (conjugate base) or

a weak base with its salt (conjugate acid)

CH3COOH/CH3COONa

NH3/NH4Cl

Depicting Buffer Action

How A Buffer Solution Works

• The acid component of the buffer can neutralize small added amounts of OH-, and the basic component can neutralize small added amounts of H3O+.

• Pure water does not buffer at all.