Chapter 17 - Aqueous Ionic Equilibrium and Buffer Solutions

Buffer Systems

Solutions that resist pH changes upon addition of acid/base.

Common Ion Effect

Equilibrium shift due to added ion from another solute.

Le Chatelier's Principle

System shifts to counteract changes in equilibrium.

Henderson-Hasselbalch Equation

Calculates pH of buffer solutions based on concentrations. pH = pKa + log([base]/[acid]) or pH = pKa + log [A−]/[HA].

Titration

Process of adding titrant to determine concentration of analyte.

Neutralization Reaction

Reaction between acid and base producing salt and water; neutralize each other.

Equivalence Point

Point in titration where moles of acid equal moles of base.

pH at Neutralization Point

pH depends on strength of acid and base involved.

Weak Acid

Acid that partially ionizes in solution, establishing equilibrium. Partially dissociates in solution, e.g., formic acid.

Strong Acid

Acid that completely ionizes/dissociates in solution, no equilibrium established. e.g., HNO3. Completely dissociates in solution, e.g., HCl.

Weak Base

Base that partially ionizes/dissociates in solution, establishing equilibrium. e.g., NH3.

Strong Base

Base that completely ionizes in solution, no equilibrium established. Completely dissociates in solution, e.g., NaOH.

Ionization

Process of an acid or base dissociating into ions.

Acetic Acid

Weak acid represented as CH3CO2H in reactions. Weak acid with Ka = 1.8 x 10⁻⁵.

Formic Acid

Weak acid represented as HCO2H in reactions. Weak acid with 2.5% ionization at 0.30 M. used in buffer calculations.

Acetate Ion

Conjugate base of acetic acid, suppresses ionization.

Formate Ion

Conjugate base of formic acid, suppresses ionization.

Spectator Ion

Ion that does not participate in the reaction.

Hydronium Ion

H3O+ ion, represents acidity in solution.

pH Calculation

Determining acidity level based on hydronium concentration. using concentrations of acid and base. pH = 14 - pOH.

Acid-Base Pair

Conjugate acid and base related by proton transfer.

Buffer Capacity

Ability of buffer to resist pH changes.

pH of Buffer Solution

Depends on ratio of acid to base concentrations.

Buffer Solution

Contains weak acid and conjugate base. Solution that resists pH changes upon acid/base addition.

Conjugate Base

Species formed when an acid donates a proton. Product of weak acid after neutralization.

Conjugate Acid

Species formed when a base accepts a proton. Product of weak base after neutralization.

pKa

Negative logarithm of the acid dissociation constant (Ka). pH at midpoint of buffer region.

Hydrolysis Reaction

Reaction of an ion with water, affecting pH.

pH Change Resistance

Buffers maintain pH despite added acids or bases.

Salt of Weak Acid

Conjugate base formed from weak acid.

Salt of Weak Base

Conjugate acid formed from weak base.

Buffer Preparation Steps

Choose acid, substitute in H-H equation.

Significant Concentration

Concentration ≥ 0.1 M for effective buffering.

Hydrofluoric Acid (HF)

Weak acid with Ka = 3.5 x 10⁻⁴.

Molarity Ratio

Ratio of concentrations of base to acid.

Hypochlorous Acid

Weak acid (HClO) with Ka = 3.5 x 10⁻⁸.

Hydrocyanic Acid

Weak acid (HCN) with Ka = 4.9 x 10⁻¹⁰.

Buffer Solution Example

0.10 M HF and 0.10 M NaF form buffer.

Iodic Acid

Strong acid with formula HIO3 and Ka 1.7 x 10−1.

Acetylsalicylic Acid

Weak acid with pKa of 3.48 in buffer system.

Blood pH

Normal blood pH maintained around 7.40.

Bicarbonate Buffer System

Maintains blood pH using HCO3− and H2CO3.

Ka1 for H2CO3

First dissociation constant, Ka1 = 4.3 x 10−7.

Ammonia Buffer

Buffer solution of NH3 and NH4Cl.

Kb for NH3

Base dissociation constant, Kb = 1.8 x 10−5.

Kw

Ion product of water, Kw = 1.0 x 10−14.

pKa for NH4+

Acid dissociation constant for NH4+, pKa = 9.25.

Stoichiometric Calculation

Calculation of moles before and after reaction.

Equilibrium Calculation

Determining concentrations at equilibrium after reaction.

NH3 Moles in Buffer

Initial moles of NH3 = 0.16 mol.

NH4+ Moles in Buffer

Initial moles of NH4+ = 0.20 mol.

Final Buffer pH

New pH calculated after acid addition.

Mole Ratio

Ratio of moles of NH3 to NH4+. Proportional relationship between reactants in moles.

NH3

Ammonia, a weak base in buffer solutions.

NH4+

Ammonium ion, a weak acid in buffers.

HCl

Strong acid used to adjust buffer pH.

pH

Measure of acidity or basicity of a solution.

Stoichiometry

Calculation of reactants and products in reactions.

Concentration ratio

Ratio of acid to base in buffer solutions.

HCO2H

Chemical formula for formic acid.

HCO2K

Potassium formate, a conjugate base in buffer.

Effective buffer

Buffer that maintains pH within a specific range.

pH range

Effective range of buffer, pKa ± 1.

H3O+

Hydronium ion, indicates acidity in solution.

pH change

Difference in pH before and after addition.

Titration Curve

Graph of pH versus added titrant volume.

Strong Acid-Strong Base Titration

Simple titration with sharp pH change at equivalence.

Weak Acid-Strong Base Titration

Titration involving hydrolysis of conjugate base.

Salt

Product of acid-base reaction, can affect pH.

Hydrolysis

Reaction of salt with water affecting pH.

pH at equivalence point

pH varies: 7 for strong, >7 for weak acid. pH equals 7 for strong acid-base titration.

Region 1

Before titrant added; pH from initial acid.

Region 2

Before equivalence point; acid in excess.

Region 3

At equivalence point; moles of acid equal base.

Region 4

After equivalence point; base in excess.

Sharp pH change

Rapid pH variation near equivalence point.

Weak Base-Strong Acid Titration

Involves hydrolysis of conjugate acid.

Acidic solution

pH < 7; occurs in weak acid-strong base titration.

Basic solution

pH > 7; occurs in weak base-strong acid titration.

Moles HA = moles A−

Condition at midpoint of weak acid-strong base titration.

Moles BH+ = moles B

Condition at midpoint of weak base-strong acid titration.

Initial pH > 7

Indicates basic solution at start of titration.

Neutral Salt

Salt formed from strong acid and strong base.

Initial pH

pH before titration begins, > 7 for basic.

Molarity (M)

Concentration of solute in moles per liter.

Moles of NaOH

Calculated as volume (L) × concentration (M).

Moles of HNO3

Calculated as volume (L) × concentration (M).

OH- Concentration

Moles of OH- left divided by total volume.

pOH

Calculated from [OH-] using pOH = -log[OH-].

Half-Equivalence Point

Point where half of acid is converted to base.

KNO2

Conjugate base of weak acid HNO2.

H-H Equation

pH = pKa + log([A-]/[HA]).

Volume at Equivalence Point

Volume of titrant needed to neutralize analyte.

pH after Adding KOH

pH calculated based on remaining acid and base.

Total Volume

Sum of initial solution and added titrant volumes.