Ch. 18: Acids & Bases

Sec. 18.1:

Acids & Bases: An Introduction

Objectives

Identify the physical & chemical properties of acids & bases.

Classify solutions as acidic, basic, or neutral.

Compare the Arrhenius, Brønsted-Lowry, and Lewis models of acids & bases.

PropertiesAcidsTaste sourMay sting or burn on contactTurn blue litmus paper pinkAre electrolytesReact with most metals to

form H2 gas*

React with carbonates to form CO2 gas**

BasesTaste bitterFeel slipperyTurn pink litmus paper

blueAre electrolytes

Metal* & Carbonate** Acid Reactions

Recall that acid and metal reactions are single replacement reactions: Mg + HNO3 ?

Al + H2SO4 ?

Recall that acid and carbonate reactions are double replacement reactions with the immediate decomposition of H2CO3: HBr + CaCO3 ?

HCl + KHCO3 ?

Aqueous SolutionsAll aqueous solutions contain H+ and OH- ions

because water does dissociates slightly. The process is called self-ionization.

H2O H+ and OH-

If [H+] = [OH-], the solution is neutral. It is not acidic or basic. Pure water is neutral because [H+] has to equal [OH-].

In an acidic solution, the [H+] > [OH-].In a basic solution, [OH-] > [H+].

The Hydronium Ion

Since H+ ions will bond to water molecules in a solution, the solution does not really contain H+ ions. It contains H3O+ ions.

We will use the symbols H+ and H3O+ interchangeably to represent a hydrogen ion in a solution.

The Arrhenius ModelAn acid is a substance

that contains hydrogen and produces H+ ions in aqueous solution.

A base is a substance that contains a hydroxide group and produces OH- ions in aqueous solution.

Examples

An Arrhenius acid

HCl --> H+(aq) + Cl-(aq)

HCl is an acid.

When dissolved in water, the resulting solution

is acidic.

A Arrhenius base

NaOH --> Na+(aq) + OH-(aq)

NaOH is a base.

When dissolved in water,

the resulting solution is basic.

Brønsted-Lowry ModelTwo chemists independently

proposed a new model of acids & bases - one that

would recognize that some bases (like

NH3, for example) did not contain hydroxide yet

produced OH- ions in water.

Brønsted-Lowry Model

According to Bronsted-Lowry, ammonia was considered a base:

NH3 + H2O NH4+ + OH-

Brønsted-Lowry Model

An acid is a hydrogen ion (proton) donor.A base is a hydrogen ion (proton) acceptor.

Look at this general equation:

HX (aq) + H2O H3O+ + X-

the acid the base

Practice ProblemsIdentify the acid and base in the following

reactions:1 H3O+ + OH- H2O + H2O2 HCl + NH3 NH4

+ + Cl-

3 S-2 + H2O HS- + OH-

4 HS- + H2O S-2 + H3O+ 5 H2O + HC2H3O2 C2H3O2

- + H3O+ 6 C2H3O2

- + H3O+ H2O + HC2H3O2

Brønsted-Lowry ModelHX (aq) + H2O H3O+ + X-

On accepting the H+ ion, H2O becomes

H3O+ (which is an acid). WHY?? Because in the reverse reaction, the H3O+ donates its H+ ion to X-.

On donating its H+ ion, HX becomes X- (which is a base). WHY?? Because in the reverse reaction, the X- accepts the H+ ion.

Brønsted-Lowry ModelThe forward and reverse reactions are BOTH

reactions of an acid and base.The acid and base that react in the reverse reaction are

called the conjugate acid and base.The conjugate acid is the substance produced in the

forward reaction when the base accepts H+ from an acid.

The conjugate base is the substance leftover after the acid has donated H+ to a base in the forward reaction.

Practice Problems

Identify the conjugate acid and the conjugate base in the reactions from before:

1 H3O+ + OH- H2O + H2O2 HCl + NH3 NH4

+ + Cl-

3 S-2 + H2O HS- + OH-

4 HS- + H2O S-2 + H3O+ 5 H2O + HC2H3O2 C2H3O2

- + H3O+ 6 C2H3O2

- + H3O+ H2O + HC2H3O2

Brønsted-Lowry Model

HX (aq) + H2O H3O+ + X-

The hydronium ion is the conjugate acid of the base water.

The X- ion is the conjugate base of the acid HX.

A conjugate acid-base pair are two substances that are related to each other by the donating & accepting of a H+ ion.

Examples

HCO3- & H2CO3 are a conjugate acid-base pair.

H2O & OH- are also a conjugate acid-base pair.

Examples

Water

Water (& other substances) that can act as both an acid & a base are said to be amphoteric.

HF + H2O H3O+ + F-

BASENH3 + H2O NH4

+ + OH-

ACID

Practice Problems

Identify the conjugate acid-base pairs in the following reactions:

§ HSO4- + H2O H3O+ + SO4

-

§ CO3-2 + H2O HCO3

- + OH-

§ NH4+ + OH- NH3 + H2O

1. OH- + HC2H3O2 C2H3O2- + H2O

An acid that can donate only one hydrogen ion is a monoprotic acid.

Acids that can donate more than one hydrogen ion are polyprotic acids.

See Table 1 on p. 641. Which acids are monoprotic? polyprotic?

Monoprotic and Polyprotic Acids

The Lewis Model

According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron pair donor.

The Lewis model includes all the substances classified as Brønsted-Lowry acids and bases and many more.