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Welter Class Notes
2.0 Strong and Weak Acids & Bases
Strong acids produce many H+^ ions (or H 3 O+^ ions); weak acids produce few H+^ ions. The stronger the acid, the more H+^ ions are produced.
Strong bases produce many OH-^ ions; weak bases produce few OH-^ ions. The stronger the base, the more OH-^ ions are produced.
Strong acids and bases are essentially one-way reactions - the acid or base breaks down completely to produce ions.
HCl(aq) → H+(aq) + Cl-(aq) or NaOH(aq) → Na+(aq) + OH-(aq)
Weak acids and bases, however, do not ionize completely. For weak electrolytes, equilibrium lies to the left side of the equation (the reactant side). The double arrow is commonly used to indicate the partial ionization of the solution.
HC 2 H 3 O2 (aq) ↔ H+(aq) + C 2 H 3 O 2 - (aq) NH 2 CH3(aq) + H 2 O(l) ↔ NH 3 CH 3 +(aq) + OH-(aq)
2.1 Ionization Constants: Ka, Kb, and Kw
Because acid/base solutions are systems at equilibrium, we can write equilibrium constant expressions for these systems, where the equilibrium constant for acids is Ka , Kb for bases, and Kw for water.
HCl(g) + H 2 O(l) → H 3 O+(aq) + Cl-(aq)
Ka =
[H 3 O+] [Cl-]
[HCl]
= 1.3×10^6
NH3 (aq) + H 2 O(l) ↔ NH 4 +(aq) + OH-(aq)
Kb =
[NH 4 +] [OH-]
[NH 3 ]
= 1.8×10-^5
Welter Class Notes
A large value of Ka means there are many H+^ ions in solution - in other words, a strong acid.
A large Kb indicates many OH-^ ions - a strong base.
We usually do not think of water as producing ions, but it does, just not very well. H 2 O(l) → H+(aq) + OH-(aq)
Kw = [H+] [OH-] = 1.0 × 10-^14
where [H+] = [OH-] = 1.0×10-
Values for Kw are given for 25°C.
Key Points: As long as temperature remains constant Kw does not change.
The value of Kw is very small, meaning that very few ions are present. Most water remains "intact" as H 2 O, and few ions form.
2.2 Calculating [H+] and [OH–]
Note: calculating ion concentrations depends on whether you have a strong acid (or base) or a weak acid (or base). * You should memorize the strong acids and bases. Look up any you are unsure of the Table of Acid and Base Strength. *
(a) Calculating Ion Concentrations for Strong Acids & Bases
Example: Calculate thehydrogen ion concentration in a 0.050 M solution of
hydrochloric acid.
Solution: Write the balanced reaction. Use equation ratios (since it is a strong acid it dissociates completely).
HCl(aq) → H+(aq) + Cl-(aq)
1 mole of HCl produces 1 mole of H+, therefore the concentration of H+^ will equal that of HCl. Answer: [H+^ ] = 0.050 M
Welter Class Notes
(c) Finding [OH-] in Acids & [H+] Bases
Use:
Kw = [H+] [OH-] = 1.0 × 10-^14
Example: Calculate thehydroxide ion concentration in a 0.050 M solution of
hydrochloric acid.
Solution:
Kw = [H+] [OH-]
1.0 × 10-^14 = [0.050] [OH-]
1.0 × 10-^14
(0.050) (x)
- 050
[x] = 2.0×10-^13 M
[OH-] = 2.0×10-^13 M
Some things to think about: In water, which is neutral (neither acidic nor basic), [H+] = 1.0×10-7M and [OH-] = 1.0×10-7M Acids increase [H+], so [H+] will be greater than 1.0×10-7^ and [OH-] will be less than 1.0×10-7M Bases increase [OH-], so [OH-] will be greater than 1.0×10-7, and [H+] will be less than 1.0×10-7M
Welter Class Notes
2.0 Strong and Weak Acids & Bases
Strong acids produce many H+^ ions (or H 3 O+^ ions); weak acids produce few H+^ ions. The stronger the acid, the ___________________ are produced.
Strong bases produce many OH-^ ions; weak bases produce few OH-^ ions. The stronger the base, the ___________________ are produced.
Strong acids and bases are essentially ___________________ - the acid or base breaks down completely to produce ions.
HCl(aq) → H+(aq) + Cl-(aq) or NaOH(aq) → Na+(aq) + OH-(aq)
Weak acids and bases, however, do not ionize completely. For weak electrolytes, equilibrium lies to the left side of the equation (the reactant side). The double arrow is commonly used to indicate the ___________________ of the solution.
HC 2 H 3 O2 (aq) ↔ H+(aq) + C 2 H 3 O 2 - (aq) NH 2 CH3(aq) + H 2 O(l) ↔ NH 3 CH 3 +(aq) + OH-(aq)
2.1 Ionization Constants: Ka, Kb, and Kw
Because acid/base solutions are systems at____________, we can write equilibrium constant expressions for these systems, where the equilibrium constant for __________is Ka , Kb for __________, and Kw for __________.
HCl(g) + H 2 O(l) → H 3 O+(aq) + Cl-(aq)
Ka =
[H 3 O+] [Cl-]
[HCl]
= 1.3×10^6
NH3 (aq) + H 2 O(l) ↔ NH 4 +(aq) + OH-(aq)
Kb =
[NH 4 +] [OH-]
[NH 3 ]
= 1.8×10-^5
Welter Class Notes
Example: Calculate thehydroxide ion concentration in a 0.010 M solution of barium
hydroxide, Ba(OH) 2.
Solution:
(b) Calculating Ion Concentrations for Weak Acids & Bases
- Write a _______________ equation for the reaction
- You will need to know the _______________ - if it is not given in the question, look it up.
- Set up the ______________________________. You will know the value of Ka (or Kb) and the concentration of the acid; you will be solving the equation for the concentration of the ions.
Example: Calculate thehydrogen ion concentration in a 0.10 M acetic acid solution,
HC 2 H 3 O 2 where the Ka for acetic acid is 1.8 ×10-5.
Solution: HC 2 H 3 O2 (aq) ↔ H+(aq) + C 2 H 3 O 2 - (aq)
Ka =
[H+] [ C 2 H 3 O 2 - ]
[HC 2 H 3 O 2 ]
Welter Class Notes
(c) Finding [OH-] in Acids & [H+] Bases
Use:
Kw = [H+] [OH-] = 1.0 × 10-^14
Example: Calculate thehydroxide ion concentration in a 0.050 M solution of
hydrochloric acid.
Solution:
Some things to think about: In water, which is neutral (neither acidic nor basic), [H+] = 1.0×10-7M and [OH-] = 1.0×10-7M Acids increase [H+], so [H+] will be greater than 1.0×10-7^ and [OH-] will be less than 1.0×10-7M Bases increase [OH-], so [OH-] will be greater than 1.0×10-7, and [H+] will be less than 1.0×10-7M
