NOTES: 19.1 – Acid-Base Theories

Acids & Bases have distinctive properties…

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

Acids & Bases – Arrhenius Theory ● An acid is a compound that contains an H+, and produces hydrogen ion (H+) when dissolved in water. Example: HCl  H+ + Cl-

In Reality… HCl + H2O  H3O+(aq) + Cl-(aq) Hydronium Ion (can be used interchangeably with H+)

Acids & Bases – Arrhenius Theory ● A base is a compound that contains an OH- group and produces hydroxide ions (OH-) when dissolved in water. Example: NaOH  Na+ + OH-

Neutralization ● Neutralization: the combination of H+ with OH- to form water. ● H+(aq) + OH-(aq)  H2O(l)

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

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- acid base c. acid c. base base acid c. acid c. base

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

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.

The Lewis Theory of Acids and Bases

The Lewis Theory of Acids & Bases ● Lewis acid: a substance that can accept an electron pair to form a covalent bond (electron pair acceptor). ● Lewis base: a substance that can donate an electron pair to form a covalent bond (electron pair donor).

Example 1: .. .. .. .. .. Ionization of NH3: NH3 + H2O  NH4+ + OH- base acid + H .. H .. - .. ..  .. H N H + H O H H N H + O H H

Example 2: Auto-ionization of water: H2O + H2O  H3O+ + OH- + acid base .. .. .. .. - .. ..  .. H O H + H O H H O H + O H H

Example 3: Reaction of NH3 with HBr (a Lewis AND a Bronsted-Lowry acid-base reaction): NH3 + HBr  NH4+ + Br- base acid + .. .. H H - .. ..  .. H N H + H Br H N H + Br H

Acid Base Theories Lewis Bronsted-Lowry Arrhenius

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 (H+). Any substance which accepts a proton (H+). Lewis Theory Any substance which can accept an electron pair. Any substance which can donate an electron pair.