Definitions: The Hydronium Ion and Acid-Base Chemistry

Vocabulary terms covering acid-base definitions (Arrhenius and Br%nsted-Lowry), polyprotic acids, conjugate pairs, acid strength, equilibrium predicting rules, and water's autoionization properties.

Hydronium Ion ($H_3O^+$)

The ion formed when an acid reacts with water.

Arrhenius Acid

A substance that produces hydronium ions ($H_3O^+$) when dissolved in water.

Arrhenius Base

A substance that produces hydroxide ions ($OH^-$) when dissolved in water.

Br%nsted-Lowry Acid

Any substance able to give a hydrogen ion ($H^+$) to another molecule or ion, serving as a proton donor.

Br%nsted-Lowry Base

A substance that accepts $H^+$ from an acid; it must have a lone pair of electrons to form a bond with the incoming proton.

Monoprotic Acid

An acid that contains one proton, such as hydrochloric acid ($HCl$).

Polyprotic Acids

Acids that contain more than one proton, such as diprotic sulfuric acid ($H_2SO_4$) or triprotic phosphoric acid ($H_3PO_4$).

Conjugate Acid-Base Pairs

Pairs of chemical species found on opposite sides of a chemical reaction whose formulas differ by exactly one hydrogen ion ($H^+$).

Strong Acids

Acids that give up a proton easily and are essentially $\sim 100\%$ dissociated in solution.

Weak Acids

Acids that give up a proton with difficulty, resulting in a dissociation of much less than $\ll 100\%$, where equilibrium favors the undissociated form.

Stepwise Dissociation

The process where polyprotic acids lose protons one at a time; the second stage occurs to a much lesser extent because separating a positive $H^+$ from a negative anion is electrostatically difficult.

Seesaw Rule

The inverse strength relationship stating that a stronger acid has a weaker conjugate base, and a weaker acid has a stronger conjugate base.

Acid Dissociation Constant ($K_a$)

The equilibrium constant expression for the reaction of a weak acid with water, calculated as $K_a = \frac{[H_3O^+] [A^-]}{[HA]}$.

Amphoteric

A substance, such as water, that can react as either an acid (donating a proton) or a base (accepting a proton).

Ion-Product Constant ($K_w$)

The constant for the autoionization of water, defined as $[H_3O^+][OH^-]$, which equals $1.0 \times 10^{-14}$ at $25^\circ C$.

Acidic Solution

A solution where the hydronium ion concentration is greater than $10^{-7}$ and the hydroxide ion concentration is less than $10^{-7}$.

Basic Solution

A solution where the hydronium ion concentration is less than $10^{-7}$ and the hydroxide ion concentration is greater than $10^{-7}$.