strong and weak acids and bases
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Strong and Weak Acids and Bases. Pg 560-578. The strength of an acid is determined by the extent to which it ionizes, its percent ionization, not the concentration of the acid, the concentration of its hydronium ions, or its ability to react with a metal. Strong Acid. - PowerPoint PPT PresentationTRANSCRIPT
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Strong and Weak Acids and Bases
Pg 560-578
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The strength of an acid is determined by the extent to which it ionizes, its percent ionization, not the concentration of the acid, the concentration of its hydronium ions, or its ability to react with a metal
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Strong Acid
An acid that nearly completely dissociates All molecules of the acid break up to form the ions soluble in waterIf more than one proton is being removed, not all steps need to be complete dissociation.
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Weak Acid
An acid that only slightly dissociates in a water solution
Only a small percent of acid molecules donate their hydrogen, and most remain the same.
Example: CH3COOH
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A strong acid essentially ionizes 100%.
An example of a strong acid is hydrochloric acid, HCl (aq)
HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)
0.10 mol 0.10 mol 0.10 mol
100% ionization
few molecules many ions
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An example of a weak acid is acetic acid, CH3COOH.
CH3COOH(l) + H2O(l) H3O+(aq) + CH3COO-
(aq)
0.10 mol << 0.10 mol << 0.10 mol
5% ionization at 25Cmany molecules few ions
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Strong Base
A base that dissociates almost completely into its ions.
All oxides and hydroxides of group 1 and 2 are strong bases.
Ex: NaOH
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Weak Base
Most bases are weak
They dissociate only slightly in a water solution
Example: NH3
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Strong acids are strong electrolytes and weak acids are weak electrolytes
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A strong base dissociates 100%.
An example of a strong base is sodium hydroxide, NaOH.
NaOH(s) + H2O(l) Na +(aq) + OH-
(aq) 0.10 mol 0.10 mol 0.10 mol
100% dissociationfew formula units (NaOH) many ions
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A weak base ionizes to a small extent.
An example of a weak base is NH3(g).
NH3(g) + H2O(l) NH4+
(aq) + OH-(aq)
0.10 mol << 0.10 mol << 0.10 mol
5% ionization at 25C
many molecules few ions
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Strong bases are strong electrolytes and weak bases are weak electrolytes.
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Examples of Strong Acids and Bases
Strong Acids Strong BasesHClO4 perchloric acid LiOH lithium hydroxideHCl hydrochloric acid NaOH sodium hydroxideHNO3 nitric acid KOH potassium hydroxideH2SO4 sulfuric acid RbOH rubidium hydroxideHBr hydrobromic acid CsOH cesium hydroxideHI hydriodic acid Ca(OH)2 calcium hydroxide
Sr(OH)2 strontium hydroxide
Ba(OH)2 barium hydroxide
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To experimentally distinguish strong acids from weak acids; and strong bases from weak bases:
compare a strong acid to a weak acid of equal concentration– more hydronium ions and anions will be
present in the strong acid solution
compare a strong base with a weak base of equal concentration– more hydroxide ions and cations will be present
in the strong base solution
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Therefore, we could compare a strong acid and a weak acid of equal concentration by:
a) use a conductivity apparatus test (light bulb will be brighter for a strong acid).
b) measure conductivity of solutions (strong acid will have a higher conductivity).
c) react the two acids with a metal like magnesium (stronger acid will react faster, more bubbling as H2 is formed)
d) measure the pH of the solutions using a pH meter or indicators (strong acid has a lower pH)
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A strong base can be distinguished experimentally from a weak base of equal concentration by:
a) use a conductivity apparatus test (light bulb will be brighter for a strong base)
b) measure conductivity of solutions (strong base will have a higher conductivity)
c) react the two bases with a chemical and observe the rate of the reaction (stronger base will react faster)
d) measure the pH of the solutions using a pH meter or indicators (stronger base has a higher pH)
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Dissociation Equation
A balanced chemical equation showing all ions produced when an ionic compound dissolves
Example:
HSO4-(aq) + H2O(l) SO4
2-(aq) + H3O+
(aq)
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H+ A - H+ A - HA
A - H+ A - H+ A –
H+ A - H+ A - H+
A - HA H+ A -
H+ A - H+ A - H+
HA HA HA HA
HA HA HA
H+ A - HA HA
HA HA H + A –
HA H + A – HA HA
H+ A- H+ A- H+ A- H+ A- HAA- H+ A- H+ A- H+ A- H+ A -
H+ A- HA H+ A- H+ A- H+ A-
A- H+ A- H+ A- H+ A- H+ A- H+ H+ A - H + A - H + A - HA H + A -
A- H+ A- H+ A- H+ A- H+ A–
H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A-
HA A- H+ A- H+ A- H+ A- H+
HA HA H+ A- HA HA HA HA HA HA HA HA H+ A- H+ A- HA HA HA HA HA
HA HA H+ A- HA HA HA HA HA HA H+
A- HA HA H+ A- HA HA HA HA HA HA HA HA H+
A- HA HA H+ A- HA HA HA HA HA HA HA H+ A- HA HA HA
DILUTECONCENTRATED
ST
RO
NG
WE
AK
STRONG ACIDS
Dissociate nearly 100%
HA H1+ + A-
WEAK ACIDS
Dissociate very little
HA H1+ + A-
Acids: Concentration vs. Strength
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Strong vs. Weak Acid
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 508
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Comparing Strengths
Tables tend to list strong acids towards the top, and strong bases towards the bottom
Figure 14.12 in text page 563
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Strengths Of Conjugate Acid-Base Pairs
The stronger an acid, the weaker is its conjugate base.
The stronger a base, the weaker is its conjugate acid.
An acid-base reaction is favored in the direction from the stronger member to the weaker member of each conjugate acid-base pair.
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Concentrated vs. Dilute
0.3 M HCl
2.0 M HCl
12.0 M HCl
10.0 M CH3COOHDilute, strong acid
Concentrated, strong acidOR Dilute, strong, acid
Concentrated, strong acid
Concentrated, weak acid
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Review and Practice
Page 558-559
# 1-2,4, 6-11