Strong and Weak Acids and Bases

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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 water
• If 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 ltlt 0.10 mol ltlt
  0.10 mol
• 5 ionization at 25?Cmany 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 ltlt 0.10 mol ltlt
  0.10 mol
• 5 ionization at 25?C
• 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 Bases
• HClO4 perchloric acid LiOH lithium hydroxide
• HCl hydrochloric acid NaOH sodium hydroxide
• HNO3 nitric acid KOH potassium hydroxide
• H2SO4 sulfuric acid RbOH rubidium hydroxide
• HBr hydrobromic acid CsOH cesium hydroxide
• HI 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
• use a conductivity apparatus test (light bulb
  will be brighter for a strong acid).
• measure conductivity of solutions (strong acid
  will have a higher conductivity).
• react the two acids with a metal like magnesium
  (stronger acid will react faster, more bubbling
  as H2 is formed)
• 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
• use a conductivity apparatus test (light bulb
  will be brighter for a strong base)
• measure conductivity of solutions (strong base
  will have a higher conductivity)
• react the two bases with a chemical and observe
  the rate of the reaction (stronger base will
  react faster)
• 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) ? SO42-(aq) H3O(aq)

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Acids Concentration vs. Strength
DILUTE
CONCENTRATED
H A - H A - HA A -
H A - H A H A -
H A - H A - HA H
A - H A - H A - H
H A- H A- H A- H A- HA A- 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
STRONG ACIDS Dissociate nearly 100
HA H1 A- WEAK
ACIDS Dissociate very little HA
H1 A-
STRONG
HA HA HA HA HA HA
HA H A - HA HA HA
HA H A HA H A HA HA
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
WEAK
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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
10.0 M CH3COOH
Dilute, strong acid
Concentrated, weak acid
2.0 M HCl
Concentrated, strong acid
OR Dilute, strong, acid
12.0 M HCl
Concentrated, strong acid
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Review and Practice

• Page 558-559
• 1-2,4, 6-11