CHEM 121 Module 7

CHEM 121 Foundations of General Chemistry

pH scale

measures the acidity or basicity of an aqueous solution as determined by the number of hydrogen ions (H⁺)

hydronium ions (H₃O⁺)

pH can be determined by the number of — in solution

acids

low numbers on the scale (0-6.9).

0 being the strongest acid.

bases

higher numbers (7.1-14).

14 being the strongest base.

alkaline

synonymous with basic

neutral

pH of 7.

combining acids and bases of similar strengths will neutralize each other.

buffer

a substance that is resistant to changes in pH.

important for maintaining chemically stable environments in the tissues and cells of living things.

Brønsted-Lowry

concept of acids and bases is defined by the transfer of hydrogen ions, H⁺ (often referred to as protons)

Brønsted-Lowry acid

a compound that donates a proton to another compound

Brønsted-Lowry base

a compound that accepts a proton

acid-base reaction

a proton is transferred from a donor (acid) to an acceptor (base)

conjugate pairs

useful in describing Brønsted-Lowry acid-base reactions (and other reversible reactions)

 conjugate base 

an acid donates H⁺

conjugate acid

a base accepts H⁺

acid ionization

reaction between a Brønsted-Lowry acid and water.

the process by which neutral molecules are divided into charged ions when exposed to a solution.

base ionization

a species occurs when it accepts protons from water molecules

amphiprotic or amphoteric

species capable of either donating or accepting protons

autoionization

process in which like molecules react to yield ions

 ion-product constant for water, K_w

extent of the water autoionization process is reflected in the value of its equilibrium constant

neutral

it contains equal concentrations of hydronium and hydroxide ions

 acidic

it contains a greater concentration of hydronium ions than hydroxide ions

 basic

it contains a lesser concentration of hydronium ions than hydroxide ions

pH

a solution is the p-function of the molar concentration of hydrogen ions (H⁺) in solution

pOH

hydroxide ion (OH^-) molarity

Acidic Relative Ion Concentrations

[H₃O⁺] > [OH^-]

Neutral Relative Ion Concentrations

[H₃O⁺] = [OH^-]

Basic Relative Ion Concentrations

[H₃O⁺] < [OH^-]

Acidic pH at 25 C

pH < 7

Neutral pH at 25 C

pH = 7

Basic pH at 25 C

pH > 7

Perchloric Acid.

Hydrochloric Acid.

Hydrobromic Acid.

Hydroiodic Acid.

Nitric Acid.

Sulfuric Acid.

6 Strong Acids

HCIO₄ 

Perchloric Acid

HCI 

Hydrochloric Acid

HBr

Hydrobromic Acid

HI

Hydroiodic Acid

HNO₃

Nitric Acid

H₂SO₄ 

Sulfuric Acid

Lithium Hydroxide.

Sodium Hydroxide.

Potassium Hydroxide.

Calcium Hydroxide.

Strontium Hydroxide.

Barium Hydroxide.

6 Strong Bases

LiOH 

Lithium Hydroxide

NaOH

Sodium Hydroxid

KOH

Potassium Hydroxide

Ca(OH)₂

Calcium Hydroxide

Sr(OH)₂

Strontium Hydroxide

Ba(OH)₂

Barium Hydroxide

acid ionization constant, K_a

equilibrium constant for an acid

percent ionization

another measure of the strength of an acid

base ionization constant (K_b)

acids, the relative strength of a base is reflected in the magnitude — in aqueous solutions

leveling effect

describes the limit on the acid strength of solutes in a solution

oxyacid

nonmetallic elements that form covalent compounds containing acidic -OH groups

salts

ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water

monoprotic acid

contains one ionizable hydrogen atom in each molecule

diprotic acids 

 contain two ionizable hydrogen atoms per molecule; ionization of such acids occurs in two steps

stepwise ionization

process occurs for all polyprotic acids

triprotic acid 

an acid that has three ionizable H atoms

diprotic base

can accept two protons, as shown below

buffer

a solution containing appreciable amounts of a weak conjugate acid-base pair

buffer capacity 

the amount of acid or base that can be added to a given volume of a buffer solution before the pH changes significantly, usually by one unit

Henderson-Hasselbalch equation

calculate the pH of buffer solutions

titrations

can be used to quantitatively analyze solutions for their acid or base concentrations

titrant

a solution of known concentration

analyte

a solution of known volume and unknown concentration

indicator

a solution that changes color in the presence of a certain pH, is added to signal when the endpoint of the reaction has been reached

equivalence point

when the volume of titrant has sufficiently neutralized the analyte

titration curve 

a plot of the pH of the analyte solution versus the volume of titrant

initial state

(added titrant volume = 0 mL): pH is determined by the acid being titrated; since the two acid samples are equally concentrated, the weak acid will exhibit a greater initial pH

Pre-equivalence point

(0 mL < V < 25 mL): solution pH increases gradually and the acid is consumed by reaction with added titrant; composition includes unreacted acid and the reaction product, its conjugate base

Equivalence point

(V = 25 mL): a drastic rise in pH is observed as the solution composition transitions from acidic to either neutral (for the strong acid sample) or basic (for the weak acid sample), with pH determined by ionization of the conjugate base of the acid

Post-equivalence point

(V > 25 mL): pH is determined by the amount of excess strong base titrant added; since both samples are titrated with the same titrant, both titration curves appear similar at this stage

acid-base indicators

either weak organic acids or weak organic bases

color change interval or pH interval

an acid-base indicator is defined as the range of pH values over which a change in color is observed, and for most indicators this range is approximately pK_a ± 1

coordinate covalent bond or dative bond

occurs when one of the atoms in the bond provides both bonding electrons

Lewis acid-base chemistry

describes reactions involving the formation of coordinate covalent bond

Lewis base

the species donating the electron pair that compose the bond

Lewis acid

the species accepting the electron pair

Lewis acid-base adduct

the product of the reaction

ligand

an ion or molecule attached to a metal atom through coordinate bonding