Acids and Bases: An Introduction

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Introduction

Molecular Structure

Ionization Constants

Salts

Buffers

Lewis Theory

Autoionization of Water

If water can act as an acid or base, can a water molecule react with another water molecule, one acting as a base and accepting a proton from the other? The chemical equation for this reaction would be:

Pure water does conduct a very weak electrical current; this current occurs due to the above reaction producing H₃O⁺ and OH^- ions. This reaction is very reactant favored; the concentration of H₃O⁺ has been measured as only 1 x 10^-7 M. This measurement was done at 25º C and will vary slightly at other temperatures.

Why and how does [H₃O⁺] depend on temperature?

Since this reaction is an equilibrium, an equilibrium constant expression can be written:

K_w = [H₃O⁺] [OH^-]

Why isn't H₂O in K_w?

The equilibrium constant for this reaction is called K_w.

NOTE: Chemists often use H⁺ to represent H₃O⁺. With this notation, the ionization of water would be written:

It is important to remember that even if H⁺ (aq) is written, the proton is hydrated and present in solution as H₃O⁺.

What is the value of K_w at 25º C?

1 x 10^-7	1 x 10^-14	There is not enough information given.

K_w is equal to the H₃O⁺ concentration multiplied by the OH^- concentration. If the H₃O⁺ concentration is 1 x 10^-7 M, what must the OH^- concentration be? (HINT: The stoichiometry of the above equation tells you that one OH^- ion is formed every time an H₃O⁺ ion is formed.)

Good! Remember that this value for K_w is only correct at 25º C. However, it is often used at other temperatures as an approximate value.

If a strong acid is added to pure water, what will happen to the H₃O⁺ and OH^- concentrations?

[H₃O⁺] :	increase	[OH^-] :	increase
	decrease		decrease
	stay the same		stay the same

Adding acid to pure water will increase the H₃O⁺ concentration. In order for K_w to remain constant, what must happen to the OH^- concentration?

Remember that acids increase the H₃O⁺ concentration.

Good! This is an example of Le Châtelier's Principle; increasing the concentration of one of the products will cause the concentration of the other products to decrease to maintain equilibrium.

This equilibrium and the value of K_w allow you to calculate the concentration of either H₃O⁺ or OH^- if the concentration of the other is known.

When [H₃O⁺] and [OH^-] are equal, the solution is said to be neutral. If [H₃O⁺] is greater than [OH^-], the solution is acidic and if [OH^-] is greater than [H₃O⁺], the solution is basic:

Neutral Solution		[H₃O⁺] = [OH^-]
Acidic Solution		[H₃O⁺] > [OH^-]
Basic Solution		[H₃O⁺] < [OH^-]