Ionization of Water, Equilibrium Constant Kw, and pH Calculations

Vocabulary flashcards covering the self-ionization of water, the equilibrium constant Kw, the pH scale, classifications of solutions, and logarithmic significant figure rules.

Self-Ionization of Water

An auto-ionization reaction occurring through an acid-base interaction between two identical liquid water molecules ($H_2O(l)$) acting as solvents.

Brønsted-Lowry Acid (Water Self-Ionization)

The water molecule in the self-ionization reaction that acts as a proton donor by releasing a hydrogen ion ($H^+$).

Brønsted-Lowry Base (Water Self-Ionization)

The water molecule in the self-ionization reaction that acts as a proton acceptor by accepting a released hydrogen ion ($H^+$).

Hydronium Ion ($H_3O^+$)

The cation produced during the auto-ionization of water when the proton-accepting water molecule gains an $H^+$ ion.

Hydroxide Ion ($OH^-$)

The anion produced during the auto-ionization of water when the proton-donating water molecule loses an $H^+$ ion.

Equilibrium Chemical Equation for Water Auto-Ionization

The balanced reversible equation: $2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$.

Equilibrium Constant for Water ($K_w$)

The equilibrium constant for the self-ionization of water, defined as $K_w = [H_3O^+][OH^-]$, where pure liquid water is excluded from the expression.

Value of $K_w$ at Standard Room Temperature

At $25\,^\circ\text{C}$, the numerical value of the equilibrium constant is $K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}\,M^2$.

Formula for Hydroxide Ion Concentration ($[OH^-]$)

The isolated expression derived from the equilibrium constant: $[OH^-] = \frac{1.0 \times 10^{-14}}{[H_3O^+]}$.

Formula for Hydronium Ion Concentration ($[H_3O^+]$)

The isolated expression derived from the equilibrium constant: $[H_3O^+] = \frac{1.0 \times 10^{-14}}{[OH^-]}$.

pH

A unitless (dimensionless) quantity mathematically defined as the negative base-10 logarithm of the hydronium ion concentration: $\text{pH} = -\log([H_3O^+])$.

Inverse Relationship Between $[H_3O^+]$ and pH

The inverse property stating that as $[H_3O^+]$ increases, pH decreases, and as $[H_3O^+]$ decreases, pH increases.

Neutral Solution

A solution where $\text{pH} = 7$ and the hydronium ion concentration is $[H_3O^+] = 10^{-7}\,M$.

Acidic Solution

A solution characterized by $\text{pH} < 7$ and a hydronium ion concentration greater than $10^{-7}\,M$.

Basic Solution

A solution characterized by $\text{pH} > 7$ and a hydronium ion concentration less than $10^{-7}\,M$.

Stomach Acid

A strongly acidic bodily fluid with a very low pH containing hydrochloric acid ($HCl$), which carries out chemical digestion.

Carbonated Beverages (pH Range)

Acidic commercial drinks, such as soft drinks, Coca-Cola, and carbonated water, that have typical pH values ranging from $2$ to $4$.

Fruit Juices (Typical pH)

Beverages such as orange juice and grapefruit juice that typically maintain low pH values around $3.2$.

Household Ammonia (pH Value)

A basic cleaning solution with an approximate pH value of $11.5$.

Lye (pH Value)

A strongly basic substance situated at the upper end of the pH scale with a maximum pH of $14$.

pH Meter

An electronic laboratory instrument utilized to acquire direct, precise pH readings.

Purple Cabbage Indicator

A natural dye extracted from purple cabbage used in experimental projects to estimate approximate pH values visually.

Significant Figure Rule for Logarithmic Calculations

The rule specifying that the number of significant figures in $[H_3O^+]$ dictates the number of decimal places (digits to the right of the decimal point) in the calculated pH value.