Unit 11

Acids and Bases

Acid vs Base All aqueous solutions contain H+

and OH- ions. Relative amounts determine

whether the solution is acid, base, or neutral Acid soln – more H+ (Hydronium ion) Basic soln- more OH-

Neutral- equal amounts of each

Basic Acidic Neutral

H+

H+

H+OH-

OH-

OH-

Solution Solution Solution

Which solution is Acidic? Basic? Neutral???

Arrhenius Model Acid is a substance that contains

hydrogen and ionizes to produce hydrogen ions in aqueous solns

Base is a substance that contains a hydroxide group and dissociates to produce a hydroxide ion in aqueous solns

Shortcomings- NH3 (Exception)

Acids and Bases

Produce H+ ions in water

Have a sour taste Break down metals Formula starts with

H Poisonous and

corrosive to skin pH less than 7

Produce OH- ions in water

Have a bitter taste and a slippery feel

Break down fats and oils

Formula ends with OH

Poisonous and corrosive to skin

pH greater than 7

Brønsted-Lowry Model Acid- hydrogen ion donor Base- hydrogen ion acceptor

HX (aq) + H20 (l) H30+ (aq) + X- (aq)Base ConjugateAcidAcid ConjugateBase

Conjugates Conjugate Acid

Species produced when a base accepts a hydrogen ion from an acid

Conjugate Base Species that results when an acid donates a

hydrogen ion to a base Conjugate acid- base pair

Consists of 2 substances related to each other by donating and accepting of a single H+

Conjugates HF + H2O H3O+ + F- (H3O+ Conjugate acid)

(F- Conjugate base)

NH3 + H20 NH4+

+ OH- (NH4+ Conjugate acid)

(OH- Conjugate base) Amphoteric- substances that can act as both

acids and bases Monoprotic- HCl, HF Polyprotic- H2SO4, H3PO4

Acid Strength Strong acids – ionize completely Weak acids- do not ionize completely

Ka =

HCN + H2O H3O+ + CN-

Ka =

][.]..][.[

AcidBCAC

][]][[ 3

HCNCNOH

Practice Problems Write an ionization equation and

acid ionization constant expression for Nitrous Acid.

HNO2

HNO2 + H2O H3O+ + NO2-

Ka = ][

]][[2

23

HNONOOH

One More Practice Problem Write an ionization equation and

acid ionization constant expression for Chlorous Acid.

HClO2

HClO2 + H2O H3O+ + ClO2-

Ka = ][]][[

2

23

HClOClOOH

Base Strength Strong Bases- completely

dissociate into metal ions and hydroxide ions

Weak bases- partially dissociateBase ionization constant

Kb = ][.]..][.[

BaseBCAC

Write ionization equations and base ionization constant expressions for the carbonate ion.

CO32-

CO32- + H2O HCO3

- + OH-

Kb =

Practice Problems

][

]][[2

3

3

CO

OHHCO

One More Practice Problem Write ionization equations and base

ionization constant expressions for the hydrogen sulfite ion.

HSO3-

HSO3- + H2O H2SO3 + OH-

Kb = ][

]][[

3

32

HSO

OHSOH

pH Measure of H+ ions in soln pH = -log[H+] Acidic solutions have a pH below 7 Basic solutions have a pH above 7 pH 7 is neutral Change of 1 pH unit represents a

tenfold change. (exponential)

pOH Measures concentration of OH- ion

pOH = - log [OH-]

pH + pOH = 14.00

Practice Problems Calculate the pH and pOH of

aqueous solutions having the following ion concentrations.

[OH-] = 6.5 x 10-6

pOH = -log[OH-] pH = 14.00 – pOHpOH = -log[6.5 x 10-6] pH = 14.00 –

5.19pOH = -[log 6.5 + log 10-6] ph = 8.81pOH = -[0.81 + (-6)]pOH = 5.19

One more, one more time! Calculate the pH and pOH of aqueous

solutions having the following ion concentrations.

[H+] = 3.6 x 10-9

pH = -log[H+] pOH=14.00 – pHpH = -log[3.6 x 10-9] pOH=14.00-8.44pH = -[log 3.6 + log 10-9] pOH=5.56pH = -[0.56 + (-9)]pH = 8.44

Buffers A buffer is a mixture of a weak acid

and its conjugate base OR, a weak base and its conjugate acid.

This mixture resists changes in pH. The amount of acid or base a

buffer can absorb without significant change in pH is called the buffer capacity.

Neutralization Reactions

When an acid is added to a base, the end products are always salt and water. (neutral)

A salt is defined as the neutral end product of an acid/base reaction.

ACID + BASE SALT + WATER H2S + Ca(OH)2 CaS + H2O What is wrong with this equation???

Balance the final equation!

H2S + Ca(OH)2 CaS + H2O

1 Ca 11 S 14 H 22 O 1

H2S + Ca(OH)2 CaS + 2 H2O

Neutralization Reactions

Try another example:Acid + Base Salt + Water

H2SO4 + NaOH Na2SO4 + H2O1 Na 21 SO4 13 H 21 O 1

H2SO4 + 2 NaOH Na2SO4 + 2 H2O

Take it one step further…

Sulfurous acid and sodium hydroxide yields sodium sulfite and water.

H2SO3 + NaOH Na2SO3 + H2O

1 Na 21 SO3 1

3 H 21 O 1

H2SO3 + 2NaOH Na2SO3 + 2H2O

One Last Step

Hydrosulfuric acid and calcium hydroxide yields what???

H2S + Ca(OH)2

1) One product will always be water. H2S + Ca(OH)2 H2O +

2) The other product will be the + ion of the base bonded with the – ion of the acid.

H2S + Ca(OH)2 2H2O + CaS

pH Indicators A chemical substance that changes color in

the presence of an acid and/or a base.1) pH paper – Dip the paper, match color to

scale on vial to determine numeric pH.pH<7 = acid, pH>7 = base, pH = 7 neutral

2) Litmus – Dip one red and one blue paper.Red stays red, blue turns red AcidBlue stays blue, red turns blue BaseRed stays red, blue stays blue Neutral

pH Indicators

3) Bromthymol Blue – Add a few drops of bromthymol blue to the substance.

If the blue color turns to yellow AcidIf the blue color stays blue Base

4) Phenolphthalein – Add a few drops of phenolphthalein to the substance.

If the clear liquid turns to pink BaseIf the clear liquid remains clear Acid