Principles of Chemical Science_Acid-Base Equilibrium Is MIT Water Safe to Drink - Lec21
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Course
Chemistry
Institution
Chemistry
Lecture 21: Acid-Base Equilibrium: Is MIT Water Safe to Drink?
1. Definitions and Relationships between pKw, pH, and pOH
2. Strengths of Acids and Bases
3. Equilibrium Acid-Base Problems (Weak Acids and Weak Bases)
5.111 Lecture Summary #21 Wednesday, October 29, 2014
Reading for Today: Sections 11.7-11.9, 11.11-11.12 (10.7 -10.9, 10.11 – 10.12 in 4th ed.)
Reading for Lecture #22: Sections 11.13, 11.18-11.19, 12.1-12.3 (10.13, 10.18-10.19, 11.1-
11.3 in 4th ed.)
Topics: I. Definitions and Relationships between pKw, pH, and pOH
II. Strengths of Acids and Bases
III. Equilibrium Acid-Base Problems (Weak Acids and Weak Bases)
I. Definitions and Relationships between pKw, pH, and pOH
Autoionization of water and definition of pKw
2H2O (l) H3O+ (aq) + OH- (aq) or H2O (l) + H2O (l) H3O+ (aq) + OH- (aq)
acid base acid base
How much H2O is in a glass of water?
2H2O (l) H3O+ (aq) + OH- (aq) ∆G° = +79.89 kJ/mol
- ( 7.989 x 104 J/mol)
ln K = -∆G°/RT = = -32.24
(8.3145 J/Kmol)(298.0 K)
K= at 298 K
This very value indicates that only a small proportion of water
molecules are ionized. Concentration of ions due to autoionization of water is very low,
about 1 molecule in 200 million.
K = [H3O+][OH-] This K is called Kw.
Because Kw is an equilibrium constant, the product of [H3O+][OH-] is always 1.0 x 10-14
at 298 K.
Note: Because the concentration of the solvent, H2O, does not change significantly in a dilute
solution, it does not enter the equilibrium expression. The solvent, water, is very nearly pure,
and pure liquids and pure solids are not included in equilibrium expressions.
Definitions of pH and pOH
pH Function: pH = -log
pOH Function: pOH = -l og
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, Relationship between pH, pOH and pKw
Kw = [H3O+][OH-] = 1.0 x 10-14 at 25°C
logKw = log[H3O+] + log[OH-]
-logKw = -log[H3O+] - log[OH-]
pKw = + = 14.00 at 25°C
II. Strength of Acids and Bases
pH of pure water pH = -log (1.0 x 10-7) = 7.00
pH of an acid solution is
pH of an base solution is
EPA defines waste as "corrosive" if the pH is lower than 3.0 or higher than 12.5.
Demo: let’s check out the acidity or basicity of common household products.
Acid Strength
CH3COOH (aq) + H2O (l) H3O+ (aq) + CH3COO- (aq)
The equilibrium constant for an acid in water is termed Ka (the acid ionization constant)
For this reaction, Ka =
The value for Ka is 1.76 x 10-5 at 25°C. This small value tells us that only a small number
of CH3COOH molecules donate their proton when dissolved in water (weak acid).
Generic expressions for acids in water:
HA (aq) + H2O (l) H3O+ (aq) + A- (aq) ACID (HA) IN WATER
BH+ (aq) + H2O (l) H3O+ (aq) + B (aq) ACID (BH+) IN WATER
A strong acid has a Ka >1 which means that the acid ionizes alm ost .
A weak acid has a Ka <1. The reaction with water does not produce many ionized
species before equilibrium is reached.
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