94270 acids and bases

8/3/2019 94270 Acids and Bases

1/22

The Arrhenius Theory of Acids and Bases

In 1887, Svant Arrhenius published a theory to explain the nature of acids and bases. It is

called the Arrenhius theory of acids and bases:

An acid is a substance that dissociates (breaks apart) in water to produce one or more

hydrogen ions, H+.

A base is a substance that dissociates (breaks apart) in water to form one or more

hydroxide ions, OH-.

Arrhenius Acids:

1. HCl (aq)

H+ (aq) + Cl- (aq)2. HBr (aq) H+ (aq) + Br- (aq)

3. H2SO4 (aq) 2H+ (aq) + SO42- (aq)

4. HClO4 (aq) H+ (aq) + ClO4- (aq)

Arrhenius Bases:

1. NaOH (aq) Na+ (aq) + OH- (aq)

2. LiOH (aq) Li+ (aq) + OH- (aq)

3. KOH (aq) K+ (aq) + OH- (aq)

4. Ba(OH)2 (aq) Ba2+ (aq) + 2OH- (aq)

U N I T 6 : A C I D S A N D B A S E S P A G E 1 O F 2 2


2/22

Questions for the Arrhenius Theory of Acids and Bases

1. Which of the following compounds is an acid according to the Arrhenius theory?

(a). H2O acid orbase (amphoteric)

(b). Ca(OH)2 base

(c). H3PO3 acid(d). HF acid

2. Which one of the following compounds is a base according to the Arrhenius theory?

(a). KOH base

(b). Ba(OH)2 base

(c). HClO acid

(d). H3PO4 acid



3/22

The Brnsted-Lowry Theory of Acids and Bases

In 1923, two chemists working independently of each other proposed a new theory of acids and

bases. Johannes Brnsted in Copenhagen, Denmark, and Thomas Lowry in London, England,

proposed what is called the Brnsted-Lowry theory of acids and bases. This theory overcame

the limitations of the Arrhenius theory:

An acid is a substance from which a proton (H+ ion) can be removed.

A base is a substance that can remove a proton (H+ ion) from an acid.

Brnsted-Lowry Acids and Bases:

1. HCl (acid) + H2O (base)

Cl- + H3O+ (hydronium)2. CH3COOH (acid) + H2O (base) CH3COO- + H3O+

3. HBr (acid) + H2O (base) Br- + H3O+

* 4. NH3 (base) + H2O (acid) NH4+ + OH-

* indicates a special case question. Water, H2O, is amphoteric, meaning it can act as an acid or

a base; it can donate a proton (H+ ion), or accept a proton (H+ ion).

Things to Remember:

Acids will always donate, while bases will always accept!



4/22

Conjugate Acid-Base Pairs

1. Conjugate acid-base pair: an acid and a base that differ by one proton.

2. Conjugate acid of a base (gains H+ ion): the particle that remains when the base receivesa proton from an acid.

3. Conjugate base of an acid (loses an H+ ion): the particle that remains when a proton is

removed from an acid.

Things to Remember:

Conjugate simply means linked together.

Conjugate acids gain an H+ ion.

Conjugate bases lose an H+ ion.



5/22

Questions for Conjugate Acid-Base Pairs

1. When perchloric acid dissolves in water:

HClO4 (acid) (aq) + H2O (base) (l) CLO4- (conj. base) (aq) + H3O+ (conj. acid) (aq)

Identify the conjugate acid-base pairs:

2. Sodium acetate is a good electrolyte. In water, the acetate ion reacts as follows:

3. Name and write the formula of the conjugate base of each molecule or ion.

(a). HCl Cl-

(b). HCO3- CO3-2

(c). H2SO4 HSO4-

(d). N2H5+ N2H4

4. Name and write the formula of the conjugate acid of each molecule or ion.

(a). NO3- HNO3

(b). OH- H2O

(c). H2O H3O+

(d). HCO3 H2CO3

5. Write the equations to show that hydrogen sulfide, HS-, can be classified as amphoteric. First

show the ion acting as an acid. Then show the ion acting as a base.

Acid: HS- H+ + S-2

Base: HS- + H+ H2S



6/22

Strong and Weak Acids and Bases

1. Strong acid: an acid that dissociates (breaks apart) completely into ions in a solution.

2. Weak acid: an acid that dissociates (breaks apart) only slightly into ions in a solution.

3. Strong base: a base that dissociates (breaks apart) completely into ions in a solution.

4. Weak base: a base that dissociates (breaks apart) very slightly into ions in a solution.

Strong Acid:

HCl H+ + Cl-(1.0 mol/l) (1.0 mol/l) (1.0 mol/l)

Weak Acid:1%

CH3COOH H+ + CH3COO-(1.0 mol/l) (> 1.0 mol/l) (> 1.0 mol/l)

Strong Base:

NaOH Na+ + OH-(1.0 mol/l) (1.0 mol/l) (1.0 mol/l)

Weak Base:

NH3 + H2O NH4+ + OH-(1.0 mol/l) (> 1.0 mol/l) (> 1.0 mol/l)



7/22

Strong and Weak Acids and Bases

Acid Base

HCl Cl-

H2SO4 HSO4-

HN03 NO3-H3O+ H2O

HSO4- SO4-2

H2SO3 HSO3-

H3PO4 H2PO4-

HF F-

CH3COOH CH3COO-

H2CO3 HCO3-

H2S HS-

HSO3- SO3-2

H2PO4- HPO4-2

NH4+ NH3HCO3- CO3-2

HPO4-2 PO4-3

H2O OH-

HS- S-2

OH- O-2

The strongest acids appear at the top of the list on the left, and the strongest bases appear at

the bottom of the list on the right. Stronger acids have a weaker conjugate base, and stronger

bases have a weaker conjugate acid.



8/22

Strong Acids and Bases

Ionization for H2O:

H2O (l) + H2O (l) H3O+ (aq) + OH- (aq)

Kw = [H3O+ (aq)][OH- (aq)]

OR:

Kw = [H+ (aq)][OH- (aq)]

1 x 10-14 = [H+ (aq)][OH- (aq)]

Acidic Solutions:

[H+] > [OH-]

Basic Solutions:

[OH-] > [H+]

Neutral Solutions:[OH-] = [H+]

[H3O+] and [OH-] in Aqueous Solutions at 25C:In an acidic solution, [H3O+] is greater than 1.0 x 10-7 mol/l and [OH-] is less than 1.0 x 10-7

mol/l. In a basic solution, [H3O+] is less than 1.0 x 10-7 mol/l and [OH-] is greater than 1.0 x 10-7

mol/l. In a neutral solution, both [H3O+] and [OH-] are equal to 1.0 x 10-7 mol/l

Things to Remember:

Kw stands for the ionization of water.

Kw is always 1 x 10-14!



9/22

Questions for Strong Acids and Bases

1. Find [H3O+] and [OH-] in each solution.

(a). 2.5 mol/l nitric acid.

(b). 0.16 mol/l barium hydroxide.

1(a). HNO3 H+ + NO3- [H+] = 2.5 mol/l

2.5 mol/l 2.5 mol/l 2.5 mol/l [OH-] = ? 4 x 10-5 mol/l

Kw = [H+][OH-]

1 x 10-14 = [2.5 mol/l][OH-]

1 x 10-14 [2.5 mol/l] = [OH-]

4 x 10-5 mol/l = [OH-]

1(b). HNO3 H+ + NO3- [H+] = ? 3.125 x 10-4 mol/l

0.16 mol/l 0.16 mol/l 0.32 mol/l [OH-] = 0.32 mol/l

Kw = [H+][OH-]

1 x 10-14 = [H+][0.32 mol/l]

1 x 10-14 [0.32 mol/l] = [H+]

3.125 x 10-4 mol/l = [H+]

2. [OH-] is 5.6 x 10-14 mol/l in a solution of hydrochloric acid. What is the molar concentration of

HCl (aq)?

HCl (aq) H+ + Cl- [H+] = ? 0.18 mol/l

0.18 mol/l 0.18 mol/l 0.18 mol/l [OH-] = 5.6 x 10-14 mol/l

Kw = [H+][OH-]

1 x 10-14 = [H+][5.6 x 10-14 mol/l]

1 x 10-14 [5.6 x 10-14 mol/l] = [H+]0.18 mol/l = [H+]

3. [H3O+] is 1.7 x 10-14 mol/l in a solution of calcium hydroxide. What is the molar concentration

of Ca(OH)2 (aq)?



10/22

Ca(OH)2 Ca+2 + 2OH- [H+] = 1.7 x 10-14 mol/l

0.295 mol/l 0.295 mol/l 0.59 mol/l [OH-] = ? 0.59 mol/l

Kw = [H+][OH-]

1 x 10-14 = [1.7 x 10-14 mol/l][OH-]1 x 10-14 [1.7 x 10-14 mol/l] = [OH-]

0.59 mol/l = [OH-]

U N I T 6 : A C I D S A N D B A S E S P A G E 1 0 O F 2 2


11/22

pH Scale

0 . . . . . . 7 . . . . . . 14

acid neutral base

pH = -log[H+] [H+] = 10-pH

pOH = -log[OH-] [OH-] = 10-pOH

pH + pOH = 14.00

Things to Remember: The closer you get to zero, the stronger the acid.

The closer you get to fourteen, the stronger the base.

Everything else is weak or neutral (7).

Round two places after the decimal.

There is no unit for pH and pOH.

[H+] = 10-pH and [OH-] = 10-pOH are sometimes referred to as inverse logs.



12/22


13/22

[H+] = 1.48 x 10-10 mol/l



14/22

Questions for pH Scale Continued

3. At normal body temperature, 37C, the value of Kw for water is 2.5 x 10-14. Calculate [H30+]

and [OH-] at this temperature. Is pure water at 37C acidic, neutral, or basic?

Kw = [H+][OH-]

2.5 x 10-14 = [H+][OH-]

2.5 x 10-14 = (x)(x)

2.5 x 10-14 = x2

2.5 x 10-14 = x2

+ 1.58 x 10-7 = x

+ 1.58 x 10-7 = x = [H+] mol/l = [OH-] mol/l

- 1.58 x 10-7 = x = [H+] mol/l = [OH-] mol/l

pH = -log[H+]pH = -log(1.58 x 10-7)

pH = 6.8 (acidic orneutral)



15/22

Dilutions of Acids and Bases

Ci Vi = Cf Vf

Ci = initial concentrationVi = initial volume

Cf = final concentration

Vf = final volume

Things to Remember:

Always use final concentration values when calculating pH and/or pOH; you should never

use initial concentration unless asked!



16/22


17/22


18/22


19/22

The Acid and Base Dissociation (Ka and Kb)

If [HA or HB] Ka or Kb > (greater than) 500, the change in the initial concentration, x, isnegligible and can be ignored; you do not use x!

If [HA or HB] Ka or Kb < (less than) 500, the change in the initial concentration, x, may not benegligible; this means you use x in the equation.

Things to Remember: This is used only for weak acids and bases.



20/22


21/22


22/22

The Base Dissociation Constant, Ka (Weak Bases)

B (aq) + H20 (l) BH+ (aq) + OH- (aq)

Kb = [BH+ (aq)][OH- (aq)][B (aq)]

Example: Ammonia, NH3 (0.100 mol/l).

NH3 (aq) NH4+ (aq) + OH- (aq)0.100 mol/l X X

(weak base)

Concentration (mol/l) NH3 NH4+ + OH-

Initial 0.100 0 0

Change - x + x + x

Equilibrium 0.100 - x x x

0.100 1.34 x 10-3 1.34 x 10-3

Kb = 1.8 x 10-5 [weak base] Kb = 0.100 1.8 x 10-5 = 5555.55

5555.55 > 500 therefore the change is insignificant (you do not use the x for the

reactant(s)).

Kb = [NH4+ (aq)][OH- (aq)] 1.8 x 10-5 = (x)(x) 1.8 x 10-5 = x2

[NH3 (aq)] (0.100) (0.100)

1.8 x 10-5

(0.100) = x2

1.8 x 10-6

= x2

1.8 x 10-6

= x2

1.34 x 10-3

= x = [OH-]

pOH = -log[OH-] pH + pOH = 14.00

pOH = -log(1.34 x 10-3) pH = 14.00 2.87

pOH = 2.87 pH = 11.13


94270 acids and bases

Documents