Post on 17-Dec-2015
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The strength of an acid is determined by The strength of an acid is determined by the extent to which it ionizes, its percent the extent to which it ionizes, its percent ionization, not the concentration of the ionization, not the concentration of the acid, the concentration of its hydronium acid, the concentration of its hydronium ions, or its ability to react with a metalions, or its ability to react with a metal
Strong AcidStrong Acid
An acid that nearly An acid that nearly completely completely dissociates dissociates All molecules of the All molecules of the acid break up to form acid break up to form the ions soluble in the ions soluble in waterwaterIf more than one proton is If more than one proton is being removed, not all being removed, not all steps need to be steps need to be complete dissociation.complete dissociation.
Weak AcidWeak Acid
An acid that only An acid that only slightly dissociates in slightly dissociates in a water solutiona water solution
Only a small percent Only a small percent of acid molecules of acid molecules donate their donate their hydrogen, and most hydrogen, and most remain the same.remain the same.
Example: CHExample: CH33COOHCOOH
A A strong acidstrong acid essentially ionizes 100%. essentially ionizes 100%.
An example of a strong acid is hydrochloric acid, An example of a strong acid is hydrochloric acid, HCl HCl (aq)(aq)
HClHCl(g)(g) + H + H22OO(l)(l) H H33OO++(aq)(aq) + Cl + Cl--(aq)(aq)
0.10 mol0.10 mol 0.10 mol0.10 mol 0.10 mol0.10 mol
100% ionization100% ionization
few moleculesfew molecules many ionsmany ions
An example of a weak acid is acetic acid, An example of a weak acid is acetic acid, CHCH33COOH.COOH.
CHCH33COOHCOOH(l)(l) + H + H22OO(l)(l) H H33OO++(aq)(aq) + CH + CH33COOCOO--
(aq)(aq)
0.10 mol0.10 mol << 0.10 mol << 0.10 mol << 0.10 mol << 0.10 mol
5% ionization at 255% ionization at 25CCmany moleculesmany molecules few ionsfew ions
Strong BaseStrong Base
A base that A base that dissociates almost dissociates almost completely into its completely into its ions.ions.
All oxides and All oxides and hydroxides of group 1 hydroxides of group 1 and 2 are strong and 2 are strong bases.bases.
Ex: NaOHEx: NaOH
Weak BaseWeak Base
Most bases are weakMost bases are weak
They dissociate only They dissociate only slightly in a water slightly in a water solutionsolution
Example: NHExample: NH33
Strong acids are strong electrolytes and Strong acids are strong electrolytes and weak acids are weak electrolytes weak acids are weak electrolytes
A A strong basestrong base dissociates 100%. dissociates 100%.
An example of a strong base is sodium An example of a strong base is sodium hydroxide, NaOH.hydroxide, NaOH.
NaOHNaOH(s)(s) + H + H22OO(l)(l) Na Na ++(aq)(aq) + OH + OH--
(aq)(aq)
0.10 mol0.10 mol 0.10 mol 0.10 mol 0.10 mol 0.10 mol
100% dissociation100% dissociationfew formula units (NaOH) many ionsfew formula units (NaOH) many ions
A weak base ionizes to a small extent. A weak base ionizes to a small extent.
An example of a weak base is NH3(g).An example of a weak base is NH3(g).
NHNH3(g)3(g) + H + H22OO(l)(l) NH NH44++
(aq)(aq) + OH + OH--(aq)(aq)
0.10 mol << 0.10 mol << 0.10 0.10 mol << 0.10 mol << 0.10 molmol
5% ionization at 255% ionization at 25CC
many molecules few ionsmany molecules few ions
Strong bases are strong electrolytes and Strong bases are strong electrolytes and weak bases are weak electrolytes. weak bases are weak electrolytes.
Examples of Strong Acids and Examples of Strong Acids and BasesBases
Strong AcidsStrong Acids Strong BasesStrong BasesHClO4 perchloric acidHClO4 perchloric acid LiOH lithium hydroxideLiOH lithium hydroxideHCl hydrochloric acidHCl hydrochloric acid NaOH sodium hydroxideNaOH sodium hydroxideHNO3 nitric acidHNO3 nitric acid KOH potassium hydroxideKOH potassium hydroxideH2SO4 sulfuric acidH2SO4 sulfuric acid RbOH rubidium hydroxideRbOH rubidium hydroxideHBr hydrobromic acidHBr hydrobromic acid CsOH cesium hydroxideCsOH cesium hydroxideHI hydriodic acidHI hydriodic acid Ca(OH)2 calcium hydroxideCa(OH)2 calcium hydroxide
Sr(OH)2 strontium Sr(OH)2 strontium hydroxidehydroxide
Ba(OH)2 barium hydroxideBa(OH)2 barium hydroxide
To experimentally distinguish strong acids To experimentally distinguish strong acids from weak acids; and strong bases from from weak acids; and strong bases from weak bases:weak bases:
compare a strong acid to a weak acid of compare a strong acid to a weak acid of equal concentrationequal concentration
– more hydronium ions and anions will be more hydronium ions and anions will be present in the strong acid solutionpresent in the strong acid solution
compare a strong base with a weak base of compare a strong base with a weak base of equal concentrationequal concentration
– more hydroxide ions and cations will be present more hydroxide ions and cations will be present in the strong base solution in the strong base solution
Therefore, we could compare a strong acid and a weak Therefore, we could compare a strong acid and a weak acid of equal concentration by:acid of equal concentration by:
a)a) use a conductivity apparatus test (light bulb will be use a conductivity apparatus test (light bulb will be brighter for a strong acid). brighter for a strong acid).
b)b) measure conductivity of solutions (strong acid will have measure conductivity of solutions (strong acid will have a higher conductivity). a higher conductivity).
c)c) react the two acids with a metal like magnesium react the two acids with a metal like magnesium (stronger acid will react faster, more bubbling as H(stronger acid will react faster, more bubbling as H22 is is formed)formed)
d)d) measure the pH of the solutions using a pH meter or measure the pH of the solutions using a pH meter or indicators (strong acid has a lower pH)indicators (strong acid has a lower pH)
A strong base can be distinguished experimentally from A strong base can be distinguished experimentally from a weak base of equal concentration by:a weak base of equal concentration by:
a)a) use a conductivity apparatus test (light bulb will be use a conductivity apparatus test (light bulb will be brighter for a strong base)brighter for a strong base)
b)b) measure conductivity of solutions (strong base will have measure conductivity of solutions (strong base will have a higher conductivity)a higher conductivity)
c)c) react the two bases with a chemical and observe the react the two bases with a chemical and observe the rate of the reaction (stronger base will react faster)rate of the reaction (stronger base will react faster)
d)d) measure the pH of the solutions using a pH meter or measure the pH of the solutions using a pH meter or indicators (stronger base has a higher pH)indicators (stronger base has a higher pH)
Dissociation EquationDissociation Equation
A balanced chemical equation showing all A balanced chemical equation showing all ions produced when an ionic compound ions produced when an ionic compound dissolvesdissolves
Example:Example:
HSOHSO44--(aq)(aq) + H + H22OO(l)(l) SO SO44
2-2-(aq)(aq) + H + H33OO++
(aq)(aq)
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
Strong vs. Weak AcidStrong vs. Weak Acid
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 508
Comparing StrengthsComparing Strengths
Tables tend to list strong acids towards the Tables tend to list strong acids towards the top, and strong bases towards the bottomtop, and strong bases towards the bottom
Figure 14.12 in text page 563Figure 14.12 in text page 563
Strengths Of Conjugate Acid-Base PairsStrengths Of Conjugate Acid-Base Pairs
The stronger an acid, the weaker is its The stronger an acid, the weaker is its conjugate base.conjugate base.
The stronger a base, the weaker is its The stronger a base, the weaker is its conjugate acid.conjugate acid.
An acid-base reaction is favored in the An acid-base reaction is favored in the direction from the stronger member to the direction from the stronger member to the weaker member of each conjugate acid-weaker member of each conjugate acid-base pair.base pair.
Concentrated vs. DiluteConcentrated 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