equilibrium

pH curve

A plot of solution pH against the volume of titrant added during a titration.

Equivalence point pH for strong acid-strong base titration

pH = 7 (determined solely by the autoionization of water).

Equivalence point pH for weak acid-strong base titration

pH > 7 (due to hydrolysis by the conjugate base).

Equivalence point pH for weak base-strong acid titration

pH < 7 (due to hydrolysis by the conjugate acid).

Order of steps for strong base-weak acid pH calculations

Perform stoichiometry calculations first, then equilibrium calculations.

Titrant

The solution in a burette during a titration.Standard solution

Endpoint

The point in a titration where a sharp change in a property occurs (such as an indicator changing color).

Difference between equivalence point and endpoint

Equivalence point is the theoretical stoichiometric completion of the reaction; endpoint is the observed physical change (like color change).

Function of a buffer

Resists drastic changes in pH when small amounts of acid or base are added.

Components of an acidic buffer solution

A weak acid and its conjugate base (or salt of its conjugate base).

Components of a basic buffer solution

A weak base and its conjugate acid (or salt of its conjugate acid).

Equilibrium shift when strong acid is added to a basic buffer

Shifts to consume the added H+ ions, converting weak base into conjugate acid.

Example reaction: Adding strong acid to an NH3 / NH4+ basic buffer

NH3(aq) + H+(aq) -> NH4+(aq)

Definition of the buffer zone

The flat plateau region on a titration curve where pH changes very slowly despite adding acid or base.

What occurs chemically within the buffer zone?

Significant concentrations of both the weak acid and its conjugate base coexist to neutralize incoming H+ or OH- ions.

Common ion effect

Lowering the solubility of an ionic compound by adding another common ion into the solution.

Solubility equilibrium

Dynamic equilibrium reached when the rate of dissolution equals the rate of precipitation (solution is saturated).

Reaction Quotient (Q)

An expression relating reactant and product concentrations at non-equilibrium conditions (q=k at equilibrium).

Effect of a catalyst on equilibrium

Lowers activation energy (Ea) to reach equilibrium faster without changing the equilibrium composition or position.

Effect of inert gases or pressure/volume changes on equilibrium

No shift occurs for pressure/volume changes; inert gases do not affect the system.

Factors affecting equilibrium constant (K)

Only temperature changes $K$; initial concentrations and non-reactive species do not affect $K$.

Relationship between forward and reverse equilibrium constants ($K_f$ and $K_r$)

They are reciprocals of each other ($K_f = 1 / K_r$).

Dynamic equilibrium

When opposing forward and reverse reactions continue at equal rates, maintaining constant concentrations.

Types of equilibrium

Phase equilibrium, Reaction equilibrium, and Solubility equilibrium.

Relationship between acid strength and conjugate base strength

The stronger an acid, the weaker its conjugate base (and vice versa).

Salt hydrolysis

The reaction of an ion from a salt with water to form an acidic or basic solution.

How to determine if a salt solution is acidic, basic, or neutral

1. Break salt into ions. 2. See how each reacts with water. 3. Spectator ions from strong acids/bases are neutral. 4. Compare $K_a$ and $K_b$ if both hydrolyze.

Criteria using $Q$ and $K$ for precipitation

$Q_{sp} > K_{sp}$ (precipitate forms); $Q_{sp} < K_{sp}$ (no precipitate); $Q_{sp} = K_{sp}$ (saturated).

When does a buffer work best?

When the pH is close to the $pK_a$ of the weak acid.

What determines buffer capacity?

Greater molar concentrations of the weak acid/conjugate base pair result in a higher buffer capacity.

Buffer response to added acid ($\text{H}^+$)

The basic component consumes the $\text{H}^+$ and the equilibrium shifts left.

Buffer response to added base ($\text{OH}^-$)

The acidic component neutralizes the $\text{OH}^-$ and the equilibrium shifts right.