5 Steps to a 5 AP Chemistry

 Le Cha^telier’s principle says that if an equilibrium system is stressed, it will reestablish

equilibrium by shifting the reactions involved. A change in concentration of a species

will cause the equilibrium to shift to reverse that change. A change in pressure or tem-

perature will cause the equilibrium to shift to reverse that change.

 Strong acids completely dissociate in water, whereas weak acids only partially

dissociate.

 Weak acids and bases establish an equilibrium system.

 Under the Brønsted–Lowry acid–base theory, acids are proton (H+) donors and bases are

proton acceptors.

 Conjugate acid–base pairs differ only in a single H+; the one that has the extra H+is the acid.

 The equilibrium for a weak acid is described by Ka, the acid dissociation constant.

It has the form:. Know how to apply this equation.

 Most times the equilibrium concentration of the weak acid, [HA], can be approximated

by the initial molarity of the weak acid.

 Knowing Kaand the initial concentration of the weak acid allows the calculation of the

[H+].

 Water is an amphoteric substance, acting either as an acid or a base.

 The product of the [H+] and [OH−] in a solution or in pure water is a constant, Kw,

called the water dissociation constant, 1.0 × 10 −^14. Kw=[H+] [OH−] = 1.0 × 10 −^14 at

25 °C. Know how to apply this equation.

 The pH is a measure of the acidity of a solution. pH =−log[H+]. Know how to apply

this equation and estimate the pH from the [H+].

 On the pH scale 7 is neutral; pH >7 is basic; and pH <7 is acidic.

 pH +pOH =pKw=14.00. Know how to apply this equation.

 Kbis the ionization constant for a weak base.. Know how to

apply this equation.

 Ka×Kb=Kwfor conjugate acid–base pairs. Know how to apply this equation.

 Buffers are solutions that resist a change in pH by neutralizing either an added acid or

an added base.

 The Henderson–Hasselbalch equation allows the calculation of the pH of a buffer

solution:. Know how to apply this equation.

 The buffer capacity is a quantitative measure of the ability of a buffer to resist a change

in pH. The more concentrated the acid–base components of the buffer, the higher its

buffer capacity.

 A titration is a laboratory technique to determine the concentration of an acid or base

solution.

pH p

A

a HA

=+

−

K log

[]

[]

Kb

HB OH

HB

=

[][ ]+−

[]

Ka

HO A

HA

=

[][]+−

[]

3

Equilibrium  239