1. Chapter 14 Acids and Bases
14.3 Strengths of Acids and Bases
Learning Goal Write equations for the dissociation of strong and weak acids; identify the direction of reaction.
Acids produce hydrogen ions in
aqueous solution.
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2. Strength of Acids and Bases
The strength of an acid is determined by the moles of H3O+ produced per mole of acid that dissociates.
The strength of a base is determined by the moles of OH− produced per mole of base that dissolves.
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3. Strong and Weak Acids
A solution of hydrochloric acid forms as H+ transfers from hydrogen chloride to water. Water is acting as a base in this reaction.
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4. Strong acids are strong electrolytes. They donate H+ easily
ionize completely in water
HCl(g) + H2O(l)  H3O+(aq) + Cl−(aq)
There are six strong acids:
Strong acid
Hydroiodic acid HI
Hydrochloric acid
HCl
Hydrobromic acid
HBr
Sulfuric acid
H2SO4
Perchloric acid
HClO4
Nitric acid
HNO3
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5. Strong versus Weak Acids
A strong acid such as HCl completely dissociates 100%.
Weak acids such as HC2H3O2 are only slightly ionized in water to form a solution.
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6. Strong versus Weak Acids
A weak acid, HC2H3O2 , slightly dissociates:
HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2−(aq)
A strong acid, HCl, dissociates 100%:
HCl (aq) + H2O(l)  H3O+(aq) + Cl−(aq)
Notice the different reaction arrows used.
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7. Weak acids are weak electrolytes. They
transfer only a small percentage of H+ ions
only partially ionize in water
have a strong conjugate base
have a forward and reverse reaction
have a strong reverse reaction
HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2−(aq)
Acetic acid
Acetate ion
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8. There are many weak acids:
citric acid, found in oranges and other citrus fruits
acetic acid, found in vinegar, used in salad dressings
phosphoric acid, found in many soft drinks
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9. Relative Strength of Acids
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10. donate one H+ ion at a time
Diprotic Acids
Diprotic acids
can donate two H+ ions
donate one H+ ion at a time
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11. Diprotic Acids
Sulfuric acid, H2SO4 , a strong acid, ionizes 100% to form HSO4− and H+. Bisulfate, HSO4−, is considered a weak acid; it only partially ionizes to form SO42−.
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12. Strong versus Weak Acids
Strong acids produce a higher concentration of H+ ions in solution than weak acids.
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13. Strong versus Weak Bases
Bases are found in many household products, such as those that remove grease and open drains.
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14. are strong electrolytes
Strong Bases
Strong bases
are strong electrolytes
dissociate completely in water to give metal ions and hydroxide ions
KOH(s)  K+(aq) + OH−(aq)
H2O
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15. Strong Bases
Strong bases include Group 1A (I) hydroxides and some Group 2A (II) hydroxides.
Strong Bases
Lithium hydroxide
LiOH
Sodium hydroxide
NaOH
Potassium hydroxide
KOH
Strontium hydroxide
Sr(OH)2
Calcium hydroxide
Ca(OH)2
Barium hydroxide
Ba(OH)2
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16. Strong Bases
High concentrations of hydroxide ions in household products can cause severe damage to skin and eyes. It is important to follow directions carefully when hydroxide ions are used in the home or a chemistry laboratory.
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17. Weak bases produce very few ions in solution and are poor electrolytes
poor acceptors of hydrogen ions
A typical weak base ammonia, NH3, used in window cleaners, reacts with water to form NH4+ and OH−.
NH3(g) + H2O(l) NH4+(aq) + OH−(aq)
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18. In acid−base reactions, one acid is stronger than the other acid
Direction of Reaction
In acid−base reactions,
one acid is stronger than the other acid
one base is stronger than the other base
In a conjugate acid−base pair,
the stronger the acid, the weaker its conjugate base
the stronger the base, the weaker its conjugate acid
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19. Direction of Reaction
Comparing the strengths of acids and bases in a chemical reaction, we can determine the direction of the reaction.
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20. Identify each of the following as a strong or weak acid or base: HBr
Learning Check
Identify each of the following as a strong or weak acid or base:
HBr
HNO2
NaOH
H2SO4
E. Cu(OH)2
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21. Identify each of the following as a strong or weak acid or base:
Solution
Identify each of the following as a strong or weak acid or base:
HBr strong acid
HNO2 weak acid
NaOH strong acid
H2SO4 strong acid
E. Cu(OH)2 weak base
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22. A. Identify the stronger acid in each pair. 1. HNO2 or H2S
Learning Check
A. Identify the stronger acid in each pair.
1. HNO2 or H2S
2. HCO3− or HBr
B. Identify the stronger base in each pair.
1. CO32− or NO2−
2. OH− or H2O
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23. A. Identify the stronger acid in each pair. 1. HNO2 or H2S HNO2
Solution
A. Identify the stronger acid in each pair.
1. HNO2 or H2S HNO2
2. HCO3− or HBr HCO3−
B. Identify the stronger base in each pair.
1. CO32− or NO2− CO32−
2. OH− or H2O OH−
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