Acids and BasesAcids and Bases

Chapter 19Chapter 19

Section 19.1Section 19.1

AcidsAcidsAcids and Bases are substances that are common in nature

Ants use formic acid to Limestonecommunicate danger caverns are

created by acids in

rainwater

acid wears away limestone making small micro-caverns. More acid rain = bigger cavernspH ~2-3Can be propelled up to 1 meter Orange juice gets it’s unique

taste from citric acid. pH ~3.5

BasesBasesCalcium hydroxide found

in

Drano is a base that unclogs

our pipes.

Sodium hydroxide found in soap

is a base that cleanses our hands.

The base forms a strong alkaline

solution when dissolved in water. The pH is ~14. The base

dissolves grease and hair clogs.

Aluminum hydroxide found in TUMS is a base

that relieves acid indigestion. The base

neutralizes the acid in your stomach by raising the pH

Acids produce H+ ions in a solutionBases produce OH- ions in a solution

As you have already learned in Chapter 15, water is the universal solvent. Whenever any substance (solute) is added to water, the polar covalent water molecule will separate the molecules and ions of the solute. If more hydrogen ions are released into the water, the solution will become an acid. If more hydroxide ions are released, the solution will become a base.

If a solution contains equal amounts of hydrogen and hydroxide ions, the solution is neutral.

Contact lens solutionMust be neutral to avoidirritating sensitive eye tissues.

Properties of Acids and Bases - Properties of Acids and Bases - BoyleBoyle

For thousands of years people have known that many foods such as lemons and vinegar tasted sour and the word acid comes from “acere”, which is the Latin word for sour. In the seventeenth century, Robert Boyle called bases alkalies and described the differences between acids and bases as:

Acids – taste sour, are corrosive to metals, change litmus (a dye extracted from lichen) red, and become less acidic when mixed with bases.

Bases – feel slippery, change litmus blue, and become less basic when mixed with acids

Robert Boyle

Properties of Acids and Bases - Properties of Acids and Bases - ArrheniusArrhenius

In the late 1800’s, the Swedish scientist Svante Arrhenius proposed that water can dissolve many compounds by separating them into their individual ions. Arrhenius suggested that acids are solutions that contain hydrogen ions and can dissolve in water to release hydrogen ions into solution.

For example: hydrochloric acid (HCl) dissolves in water as follows:

HCl + H2O -> H+ + Cl-

Properties of Acids and Bases - Properties of Acids and Bases - ArrheniusArrhenius

Arrhenius defined bases as substances that dissolve in water to release hydroxide ions (OH-) into solution

For example: a typical base according to the Arrhenius definition is sodium hydroxide (NaOH)

NaOH + H2O -> Na+ + OH-

The Arrhenius TheoryThe Arrhenius TheoryThe Arrhenius definition of acids and bases explains a number of things. Arrhenius’s theory explains why all acids have similar properties to each other (and conversely why all bases are similar) because all acids release H+ into solution (and all bases release OH-). The Arrhenius definition also explains Boyle’s observation that acids and bases counteract each other. This idea, that a base can make an acid weaker, and vice versa is called neutralization.

NaOH + HCl -> NaCl + H2O base + acid -> salt + water

Limits to Arrhenius’ TheoryLimits to Arrhenius’ TheoryThe definition does not explain why some The definition does not explain why some

substances, such as common baking soda substances, such as common baking soda (NaHCO(NaHCO33), can act like a base even though ), can act like a base even though they do not contain hydroxide ionsthey do not contain hydroxide ions

In 1923, the Danish scientist Johannes In 1923, the Danish scientist Johannes Bronsted and the Englishman Thomas Bronsted and the Englishman Thomas Lowry published independent, yet similar Lowry published independent, yet similar papers that refined Arrhenius’ theory. In papers that refined Arrhenius’ theory. In Bronsted’s words, Bronsted’s words, “…acids and bases are “…acids and bases are substances that are capable of splitting substances that are capable of splitting off or taking up hydrogen ions off or taking up hydrogen ions respectively.”respectively.” The Bronsted-Lowry The Bronsted-Lowry definition broadened the Arrhenius definition broadened the Arrhenius concept of acids and bases.concept of acids and bases.

Bronsted

Bronsted-Lowry Theory Bronsted-Lowry Theory Definition of AcidsDefinition of Acids

The Bronsted-Lowry definition of acids is very similar to the Arrhenius definition. Any substance that can donate a hydrogen ion is an acid. Under the Bronsted definition, acids are often referred to as proton donors because an H+ ion, hydrogen minus its electron is simply a proton.

For example, when HCl (g) dissolves in water, each HCl molecule donates a proton to a water molecule, and so qualifies as a Bronsted-Lowry acid. The water molecule, which accepts the proton is a Bronsted-Lowry acid.

H2O + HCl -> H3O+ + Cl-

base acid acid base

Bronsted-Lowry Definition of Bronsted-Lowry Definition of BasesBasesThe Bronsted definition of bases is quite different from the Arrhenius

definition. The Bronsted base is defined as any substance that can accept a hydrogen ion. In essence, a base is the opposite of an acid. NaOH and KOH would still be considered bases because they can accept an H+ from an acid to form water.

acid base water salt

HCl + NaOH -> H2O + NaClHBr + KOH -> H2O + KBr

However, the Bronsted-Lowry definition also explains why substances that do not contain OH- can act like bases. Baking soda (NaHCO3), for example, acts like a base by accepting a hydrogen ion from an acid as illustrated below:

acid base acid salt

HCl + NaHCO3 -> H2CO3 + NaCl

Conjugate Acids and BasesConjugate Acids and BasesEvery acid has associated with it a conjugate base, formed from the acid by the loss of a proton. The conjugate base is everything that remains of the acid molecule after a proton is lost. Every base has associated with it a conjugate acid, formed from the base by the addition of a proton. The conjugate acid is formed when the proton is transferred to the base. A conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single proton.

AmphotericAmphoteric

Notice that water sometimes acts as an acid, and sometimes it acts as a base. Substances that can act as both acids and bases are said to be amphoteric.

NH3 + H2O -> NH4+ + OH-

base acid conjugate conjugate acid base

H2O + HCl -> H3O+ + Cl-

base acid acid base