Weak acids only partially dissociate in water Hydrofluoric acid HF is the only halogen that forms a weak acid Learning Goal Write equations for the dissociation of strong and weak acids identify the direction of reaction ID: 592826
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Slide1
11.3 Strengths of Acids and Bases
Weak acids only partially dissociate in water.
Hydrofluoric acid, HF, is the only halogen that forms a weak acid.
Learning Goal Write equations for the dissociation of strong and weak acids; identify the direction of reaction.Slide2
Strong
and
Weak AcidsA strong acid completely ionizes (100%) in aqueous solutions.
HCl(g) + H2O(l) H3O+(aq) + Cl−(aq) A weak acid
dissociates only slightly in water to form a few ions in aqueous solutions.
H
2CO3(aq) + H2O(l) H3O+(aq) + HCO3− (aq)Slide3
Strong
Acids
In water, the dissolved molecules of HA, a strong acid, dissociate into ions 100
%.produce large concentrations of H3O+ and the anion (A−).The strong acid HCl dissociates completely into ions: HCl(g) + H2O(l)
H
3
O+(aq) + Cl−(aq) Slide4
Weak Acids
In weak acids
, only a few molecules dissociate.Most
of the weak acid remains as the undissociated (molecular) form of the acid.The concentrations of H3O+ and the anion (A−) are small.H2CO3 is a weak acid:
H
2
CO
3(aq) + H2O(l) H3O+(aq) + HCO3−(aq)Slide5
Relative Strength of Acids and BasesSlide6
Strong and Weak Acid Dissociation
In an
HCl solution, the strong acid HCl dissociates 100% to form H+ and Cl−
.A solution of the weak acid HC2H3O2 contains mostly molecules of HC2H3O2 and a few ions of H+ and C2H3O2−.Slide7
Strong and Weak Acid Dissociation
Figure 11.2 ▶ After dissociation in water,
(a) the strong acid HI has highconcentrations of H
3O+ and I–, and (b) the weak acid HF has a high concentration of HF and low concentrations of H3O+ and F. Slide8
Diprotic Acids: Carbonic Acid
Some
weak acids, such as carbonic acid, are diprotic acids that have two H+, which dissociate one at a time
. H2CO3(aq) + H2O(l) H3O+(aq) + HCO3−(aq)Because HCO3−
is also a weak acid, a second dissociation can take place to
produce another
hydronium ion and the carbonate ion, CO32−. HCO3−(aq) + H2O(l) H3O+(aq) + CO
32−(aq)Slide9
Diprotic Acids: Sulfuric Acid
Some strong
acids, such as sulfuric acid, are diprotic acids that have two H+,
which dissociate one at a time. H2SO4(aq) + H2O(l) H3O+(aq) + HSO4−(aq)
Because HSO
4
−
is a weak acid, a second dissociation can take place to produce another H+ and the sulfate ion, SO42− . HSO4−(aq) + H2O(
l) H3O+(aq
) +
SO
4
2
−
(
aq
)Slide10
Strong Bases
Strong bases
as strong electrolytesare formed from metals of Groups 1A (1) and 2A (2
).include LiOH, NaOH, KOH, Ba(OH)2, Sr(OH)2, and Ca(OH)2.dissociate completely in water. KOH(s) K+(
aq
)
+ OH
−(aq)are found in household products used to remove grease and unclog drains.Slide11
Weak Bases
Weak
bases are weak electrolytesthat are poor acceptors of H
+ ions.produce very few ions in solution.include ammonia. NH3(g) + H2O(l) NH4+(aq) + OH−(aq)
Ammonia
Ammonium hydroxideSlide12
Strong and Weak Bases
Strong
Bases
Lithium hydroxide LiOHSodium hydroxide NaOHPotassium hydroxide KOHRubidium hydroxide
RbOH
Cesium hydroxide
CsOH
Calcium hydroxide Ca(OH) 2*Strontium hydroxide Sr(OH) 2*Barium hydroxide Ba(OH)2**Low solubility, but they dissociate completely
Bases in Household Products
Weak
Bases
Window cleaner, ammonia, NH
3
Bleach,
NaOCl
Laundry detergent,
Na
2
CO
3
, Na
3
PO
4
Toothpaste and baking soda,
NaHC
3
Baking powder, scouring
powder, Na
2
CO
3
Lime for lawns and agriculture,
CaCO
3
Laxatives, antacids,
Mg(OH)
2
, Al(OH)
3
Strong
Bases
Drain cleaner, oven cleaner,
NaOHSlide13
Direction of Reaction
Strong acids have weak conjugate bases that do not readily accept H
+.
As the strength of the acid decreases, the strength of its conjugate base increases.In any acid–base reaction, there are two acids and two bases.However, one acid is stronger than the other acid, and one base is stronger than the other base.By comparing their relative strengths, we can determine the direction of the reaction.Slide14
Direction of Reaction: H
2
SO4Sulfuric acid, H2SO
4, is a strong acid that readily gives up H+ to water. H2SO4(aq) + H2O(l
) H
3
O
+(aq) + HSO4−(aq) Stronger Stronger Weaker Weaker acid base acid baseThe hydronium ion H3O
+ produced is a weaker acid than H2SO4
.
The conjugate base HSO
4
−
is a weaker base than water.Slide15
Direction of Reaction: CO
3
2−The carbonate ion from carbonic acid, H
2CO3, reacts with water.Water donates one H+ to carbonate, CO32− to form HCO3− and OH−.From Table 11.3, we see that HCO3−
is a stronger acid than H
2
O.
We also see that OH− is a stronger base than CO32−.To reach equilibrium, the strong acid and strong base react in the direction of the weaker acid and weaker base. CO32− (aq) + H2O(l
) OH−(aq) + HCO3
−
(
aq
)
Weaker
Weaker
Stronger Stronger
acid base base acidSlide16
Study Check
Identify each of the following as a strong or weak acid
or base:
A. HBr B. HNO2 C. NaOH D. H2SO4 E. Cu(OH)
2Slide17
Solution
Identify each of the following as a strong or weak acid
or base:
A. HBr strong acid B. HNO2 weak acid C. NaOH strong base D. H2SO4
strong acid
E. Cu(OH)
2
weak baseSlide18
Study Check
Using Table 11.3, identify the stronger acid in each pair.
A. HNO2
or H2S B. HCO3− or HBr C. H3PO4 or H3O
+Slide19
Solution
Using Table 11.3, identify the stronger acid in each pair.
A.
HNO
2
or H
2
S HNO2 is the stronger acid. B. HCO3−
or HBr
HBr
is the stronger acid.
C. H
3
PO
4
or H
3
O
+
H
3
O
+
is the stronger acid.