3.1.11 Acids & Bases

Acids and Bases

Important in domestic, environmental and industrial contexts; acidity in aqueous solutions is caused by hydrogen ions.

pH

A logarithmic scale used to measure acidity, defined as pH = -log10[H+].

Buffer Solutions

Resist changes in pH and find many important industrial and biological applications.

Brønsted-Lowry Acid

An acid is a proton donor.

Brønsted-Lowry Base

A base is a proton acceptor.

pH Scale

A logarithmic scale used to measure the concentration of hydrogen ions in aqueous solution.

Kw

The ionic product of water, defined as Kw = [H+][OH-].

Ka

The dissociation constant for a weak acid.

pKa

Defined as pKa = -log10 Ka.

Acidic Buffer Solutions

Contain a weak acid and the salt of that weak acid.

Basic Buffer Solutions

Contain a weak base and the salt of that weak base.

Strong Acid

An acid which fully dissociates.

Weak Acid

An acid which only partially dissociates.

[H+]

[H+] = 10-pH.

Ionic Product of Water (Kw)

Kw = [H+][OH-].

Acid-Base Reaction Type 1 - OH

Acid + Metal → Salt + Hydrogen.

Acid-Base Reaction Type 2 - Metal Oxide

Acid + Metal Oxide → Salt + Water.

Acid-Base Reaction Type 3 - Metal OH

Acid + Metal Hydroxide → Salt + Water.

Acid-Base Reaction Type 4 - Metal Carbonate

Acid + Metal Carbonate → Salt + CO2 + Water.

Acid-Base Reaction Type 5

Acid + Ammonia → Ammonium salt.

Definition of a Salt

A chemical compound formed from an acid, when a H+ ion from the acid has been replaced by a metal ion or another positive ion.

Monoprotic Acid

An acid which only releases one H+ ion.

Diprotic Acid

An acid which releases two H+ ions.

Alkali

A base which releases OH- ions when dissolved in water.

Water as an Acid

Water can act as an acid by donating a proton.

Water as a Base

Water can act as a base by accepting a proton.

pH Calculation from [H+]

pH = -log10[H+].

pH Calculation from pH

[H+] = 10-pH.

Strong Acid Behavior

A strong acid fully dissociates in solution.

HCl

Strong acid that fully dissociates in solution.

HNO3

Strong acid that fully dissociates to H+ and NO3-.

H2SO4

Diprotic strong acid releasing 2 H+ ions.

pH of water

Neutral pH at 298K is 7.00.

Endothermic reaction

Reaction that absorbs heat, shifting equilibrium right.

Weak acid

Partially dissociates in solution, establishing equilibrium.

Dissociation of H2O

H2O ⇌ H+ + OH- in equilibrium.

Strong base

Fully dissociates in solution to release OH-.

NaOH

Strong base that dissociates to Na+ and OH-.

Temperature effect on pH

Higher temperatures lower water's neutral pH.

Hydrochloric acid

Strong acid represented as HCl.

Sulfuric acid

Strong acid represented as H2SO4.

Nitric acid

Strong acid represented as HNO3.

Phosphoric acid

Strong acid represented as H3PO4.

Carboxylic acids

All are weak acids, partially dissociating.

Methanoic acid

Weak acid with formula HCOOH.

Ethanoic acid

Weak acid with formula CH3COOH.

Benzoic acid

Weak acid with formula C6H5COOH.

Phenol

Weak acid with formula C6H5OH.

Dissociation equation

HA ⇌ H+ + A- for weak acids.

Kw expression

Kw = [H+][-OH] for water dissociation.

Calculating pH of weak acids

Use Ka and equilibrium concentrations to find pH.

pH of NaOH

Calculated using Kw and concentration of OH-.

pH calculation method

Steps to find pH from weak acid dissociation.

Acid strength

Stronger acids have lower pKa values.

Dilution

Adding solvent to reduce solute concentration.

Moles

Amount of substance measured in grams per mol.

Standard Solution

Solution with known exact concentration.

Burette

Equipment for delivering precise liquid volumes.

Pipette

Instrument for measuring and transferring liquid volumes.

Indicator

Substance that changes color at a specific pH.

Equivalence Point

Volume where acid completely neutralizes base.

End Point

Point in titration where indicator changes color.

Titration Curve

Graph showing pH change during titration.

Half-neutralisation Point

Volume where half of acid is neutralized.

Buffer Solution

Resists pH changes upon acid or base addition.

Acidic Buffer

Weak acid and its salt maintaining pH below 7.

Strong Base

Base with pH 12-14, fully dissociates in solution.

Weak Base

Base with pH 9-11, partially dissociates in solution.

Volume

Amount of space occupied by a substance, measured in cm³.

Concentration

Amount of solute per volume of solution.

Titration Steps

Procedure for determining solution concentration via reaction.

pH Meter

Device for measuring acidity or alkalinity of solutions.

Calibration

Adjusting pH meter using known pH solutions.

Swirling

Mixing technique during titration for uniform reaction.

Average Titre

Mean volume of titrant used at equivalence point.

Buffer

A solution that resists pH changes.

Salt

Ionic compound formed from acid-base reaction.

H+ Ion

Proton that increases acidity in solution.

OH- Ion

Hydroxide ion that increases alkalinity in solution.

Equilibrium Shift

Change in position of equilibrium due to concentration changes.

Basic Buffer

Contains weak base and its soluble salt.

ICE Box

Table used to track concentrations during reactions.

Half Neutralisation Point

Point where half of the acid is neutralised.

Dissociation

Process of a compound breaking into ions.

Change in pH

Difference in pH after adding acid or base.

Neutralisation Reaction

Reaction between acid and base forming salt and water.

Ethanoic Acid

Weak acid with Ka = 1.7x10-5.

CH3COO-

Conjugate base of ethanoic acid.

NH3

Weak base that can accept protons.

NH4Cl

Salt formed from weak base NH3.

Logarithm

Mathematical function used in pH calculations.

pH Calculation Method

Steps to determine pH from concentrations.

Concentration of H+

Determined from Ka expression in buffer calculations.

Buffer Saturation

Occurs when buffer capacity is exceeded.

pH Resistance

Ability of buffer to minimize pH changes.