ACIDS and BASES Chapter 19

pH indicators pH indicators are valuable tool for determining if a substance is an acid or a base. The indicator will change colors in solution.

Things to use… pH meter will indicate the numeric value of acid or base based on the pH range Chemical indicators: phenolphthalein, universal indicator… Natural indicators: poinsettia, red cabbage juice…

Properties of Acids and Bases Have a sour taste Change the color of many indicators Are corrosive (react with metals) Neutralize bases Conduct an electric current BASES Have a bitter taste Change the color of many indicators Have a slippery feeling Neutralize acids Conduct an electric current

The Arrhenius Theory of Acids and Bases

Arrhenius Theory of Acids and Bases: an acid contains hydrogen and ionizes in solutions to produce H+ ions: HCl  H+(aq) + Cl-(aq)

Arrhenius Theory of Acids and Bases: a base contains an OH- group and ionizes in solutions to produce OH- ions: NaOH  Na+(aq) + OH-(aq)

Neutralization Neutralization: the combination of H+ with OH- to form water. H+(aq) + OH-(aq)  H2O (l) Hydrogen ions (H+) in solution form hydronium ions (H3O+)

In Reality… H+ + H2O  H3O+ Hydronium Ion (Can be used interchangeably with H+)

Commentary on Arrhenius Theory… One problem with the Arrhenius theory is that it’s not comprehensive enough. Some compounds act like acids and bases that don’t fit the standard definition.

Bronsted-Lowry Theory of Acids & Bases

Bronsted-Lowry Theory of Acids & Bases: An acid is a proton (H+) donor A base is a proton (H+) acceptor

for example… Proton transfer HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq) Base Acid

another example… Water is a proton donor, and thus an acid. CONJUGATE BASE ACID NH3(aq) + H2O(l)  NH4+ (aq) + OH- (aq) BASE CONJUGATE ACID Ammonia is a proton acceptor, and thus a base

Amphoteric Substances A substance that can act as both an acid and a base (depending on what it is reacting with) is termed amphoteric. Water is a prime example.

Conjugate acid-base pairs Conjugate acid-base pairs differ by one proton (H+) A conjugate acid is the particle formed when a base gains a proton. A conjugate base is the particle that remains when an acid gives off a proton.

Examples: In the following reactions, label the conjugate acid-base pairs: H3PO4 + NO2-  HNO2 + H2PO4- CN- + HCO3-  HCN + CO32- HCN + SO32-  HSO3- + CN- H2O + HF  F- + H3O+ acid base c. acid c. base base acid c. acid c. base acid base c. acid c. base base acid c. base c. acid

SUMMARY OF ACID-BASE THEORIES Theory Acid Definition Base Definition Arrhenius Theory Any substance which releases H+ ions in water solution. Any substance which releases OH- ions in water solution Brǿnsted-Lowry Theory Any substance which donates a proton. Any substance which accepts a proton.

Strength of Acids and Bases A strong acid dissociates completely in sol’n: HCl  H+(aq) + Cl-(aq) A weak acid dissociates only partly in sol’n: HNO2  H+(aq) + NO2-(aq) A strong base dissociates completely in sol’n: NaOH  Na+(aq) + OH-(aq) A weak base dissociates only partly in sol’n: NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)

Acid-Base Reactions H+(aq) + OH-(aq)  H2O(l) Neutralization reactions: reactions between acids and metal hydroxide bases which produce a salt and water. H+ ions and OH- ions combine to form water molecules: H+(aq) + OH-(aq)  H2O(l)