Chapter 15-Acids & Bases

Diprotic and Polyprotic Acids

Acids that can furnish more than one proton (H+ ion). Ionize in a stepwise manner; lose one proton at a time. An ionization constant expression can be written for each ionization stage. As a result , two or more equilibrium constant expressions must often be used to calculate the concentrations of species in the acid solution.

Strong Electrolyte

100% dissociation

Strong acid:

Ionization equilibrium lies far to the right. Yields a weak conjugate base.

Weak acid:

Ionize or dissociate partially, much less than 100% Ionization equilibrium lies far to the left. Weaker the acid, stronger its conjugate base.

Bases example

LiOH, NaOH, KOH, Ca(OH)2, NH3

Conjugate base is

everything that remains of the acid molecule after a proton is lost.

For a typical polyprotic acid in water, only the

first ionization step is important to pH.

For a given weak acid, the percent dissociation

increases as the acid becomes more dilute.

Weak Electrolyte

not completely dissociated

The ion-product constant (Kw) is

product of [H+] and [OH-] must always equal to 1.0 × 10-14 at 25°C.

A Brønsted base is

proton (H+ ion) acceptor

A Brønsted acid is

proton (H+ ion) donor

Conjugate acid is formed when

proton is transferred to the base.

Solving Weak Acid Equilibrium Problems:

1. Identify the major species that can affect the pH. • In most cases, you can ignore the autoionization of water. • Ignore [OH-] because it is determined by [H+]. 2. Use ICE to express the equilibrium concentrations in terms of single unknown x. 3. Write Ka in terms of equilibrium concentrations. Solve for x by the approximation method. If approximation is not valid, solve for x exactly. 4. Calculate concentrations of all species and/or pH of the solution

Weak acids, such as

HCOOH and NH3, produce stronger conjugate bases (HCOO− and NH2−, respectively).

Water is

amphoteric: Behaves either as an acid or as a base.

Neutral Solutions

Salts containing an alkali metal or alkaline earth metal ion (except Be2+) and the conjugate base of a strong acid (example: Cl-, Br-, and NO3-).

Basic Solutions

Salts derived from a strong base and a weak acid

Strong Acids are

strong electrolytes

As the initial concentration of a weak acid increases,

the percent ionization decreases quickly from 100% ionization, approaching 0% ionization as the initial acid concentration increases. At very low concentrations, both strong and weak acids ionize completely.

Predict the relative strengths of the oxoacids

watch youtube

Solving Weak Acid Equilibrium Problems

watch youtube -Calculate the pH of a 0.036 M nitrous acid (HNO2) solution -The pH of a 0.10 M solution of formic acid (HCOOH) is 2.39. What is the Ka of the acid? -What is the pH of a 0.40 M ammonia (NH3) solution -Calculate the concentrations of all the species present at equilibrium in 0.10 M Oxalic acid (H2C2O4) solution

In general, the stronger the acid,

weaker the conjugate base.