Unit 7: Equilibrium And Acids

Section 6: Misc Facts About Acids-Bases

• Equilibrium Expression
• Strength Of Acids
• Monoprotic, Diprotic, Triprotic

Equilibrium Expression

The equilibrium expression is exactly the same thing as the equilibrium constant earlier in this unit, except it is called the acid dissociation constant and has the variable (K_a when it is for an acid and K_b when it is for a base.)

So for the reaction:

the acid dissociation constant is:

(Note₁: Liquids don't get put into the expression) Note ₂: H₃O⁺ is the same thing as H⁺

Strength of Acids

It is important to know the next few facts. First, acids are strong when they dissociate a lot in water (most of the acid breaks up.) Secondly, acids are weak when then don't dissociate in water (only small amounts of the acid actually break up.) Third, the stronger the acid the weaker the conjugate base. Fourth, the weaker the conjugate acid the stronger the conjugate base.

Monoprotic, Diprotic, Triprotic

A monprotic acid is an acid that only gives up one H⁺ ion. For instance, HCl is a monoprotic acid. A diprotic acid is an acid that gives off the H⁺ ions. For example, H₂SO₄ breaks up, first into H⁺ and HSO₄ then breaks up into H⁺ and SO₄^-2. (Note: It takes two steps to break up H₂SO₄.) A triprotic acid is an acid that gives up 3 H⁺ ions. For example, H₃PO₄, first breaks up into H⁺ and H₂PO₄^- then breaks up into H⁺ an HPO₄^-2 the breaks up into H⁺ and PO₄^-3. (Note: It takes three steps to break up H₃PO₄.)