Chapter 10: Acids & Bases

Ohio State University: BIOPHRM 3311

4 properties of ACIDS

SOUR

RED

rxn w/ some metals to produce hydrogen gas

rxn w/ bases to form salt & water

4 properties of BASES

SOUR

BLUE

slippery feeling on skin (rxn w/ fats/oils on skin)

rxn w/ acids to form salt & water (neutralization rxn)

dissociates means…

move away from

produce

give off

3 main rules / theories governing acid-base rxns

Arrhenius

Bronsted-Lowry

Lewis (not the structure)

arrhenius acid

molecular compound that produces / gives off a H+ ion when placed in H2O

what is a solvent

substance that dissolves other compounds - causes H+ / OH- ions to fall off from the other compounds (Arrhenius theory)

usually written above the rxn arrow (i.e., when H2O is the solvent, it is written above the rxn arrow(s))

gives acids their properties

hydrogen ions (H+)

gives bases their properties

hydroxide ions (OH-)

Conjugate Acid / Base pairs definition

two species that differ by one proton (H+) — i.e., an acid and its conjugate base or a base and its conjugate acid

Arrhenius base

molecular compound that dissociates / dissolves in H2O (solvent/dissolver) and gives off a hydroxide ion (OH-)

neutralization rxn

rxn between an acid and a base that produces H2O (neutral molecule) and salt (ionic compound)

ionic compound

molecular compound made up of oppositely charged atoms — cations and anions — held together by STRONG electrostatic attraction (think: balloon on wall)

cations

positively charged ions

anions

negatively charged ions

origin of hydrogen ion (H+)

hydrogen atom contains 1 e- and 1 proton and no neutrons

if this atom gives up its 1 e-, it becomes positively charged — i.e., H+ or hydrogen ion

hydrogen ion becomes PROTON

valence electrons are responsible for what?

covalent bonding between atoms

where are valence electrons found on the periodic table?

in the one’s place on the element’s group number (number at the top of each column; i.e., group 17 has 7 valence electrons, group 2 has 2 valence electrons, etc.)

Are atoms with unpaired electrons are more or less reactive with environment?

more reactive — more willing to form covalent bonds because a “buddy” electron creates a full octet on the atom’s outer shell (low-energy)

Arrhenius theory describes acid-base chemistry only within the confines of what?

an aqueous solution (H2O)

3 Arrhenius Theory Limitations

1. H+ cannot exist within H2O solution

2. Does not explain how ammonia (NH3), contains no OH-, and yet, acts as a BASE.

3. Theory only describes acid-base rxns in terms of H2O (aqueous) solutions [NH3 exists as a gas] — when base rxns can occur in other solvents (like methanol)

Bronsted-Lowry Acid

substance that is a proton (H+) donor

Bronsted-Lowry base

substance that is a proton (H+) acceptor

What Arrhenius acid-base theory limitation does Bronsted-Lowry ANSWER?

Explains why NH3 is a base without the existence of hydroxide ions (OH-)

[NH3 will act as a proton acceptor (i.e., B-L base)]

Coordinate bond

when both e- in a covalent bond come from the same atom

[example: creation of ammonium (NH4+) — H+ does not have e-

Lewis acid

substance is an electron pair acceptor

Hint: “Louis complains and does the opposite of Lestat (B-L acid)”

Lewis base

substance is an electron pair donor

Hint: “Louis complains and does the opposite of Lestat (B-L base)”

Acid-Base Identification Flowchart (know the info on here)

Acid-Base Theories — Summary Chart (KNOW)

Hydrochloric (HCl) strength & purpose

strong acid

crucial to proper digestion and defense against pathogens in the digestive system

Sulfuric acid (H2SO4) strength & purpose

strong acid

car battery acid

Phosphoric acid (H3PO4) strength & purpose

moderate strength acid

component in DNA or ATP

Acetic acid (CH3COOH) strength & purpose

weak acid

component of acetyl-coA (energy metabolism)

Sodium hydroxide (NaOH) strength & purpose

strong base

neutralize acids and soap making

Magnesium Hydroxide (Mg(OH)2) strength & purpose

strong base

laxatives and antacids

Acetate (CH3COO-) strength & purpose

weak base

product of acetic acid dissociation, precursor of biomolecules

Ammonia (NH3) strength & purpose

weak base

intermediate in protein degradation

polyprotic acids

refers to acids capable of donated >1 proton (H+) per molecule

there are mono- (1 H+), di- (2 H+), tri- (3 H+), tetra- (4 H+), penta- (5 H+), and hexa- (6 H+)

acids can differ based upon the number of ____ they can donate in an aqueous solution

protons (H+ ions)

monoprotic acids

acids that can only donate 1 proton per molecule in an aqueous solution

each monoprotic acid contains ____

1 acidic hydrogen

what is an acidic hydrogen

this is the hydrogen atom in an acid that forms an H+ ion when the acid dissociates (falls apart)

diprotic acids

acids that donate 2 protons per molecule during a 2-staged ionization rxn

why does a diprotic rxn need 2 ionization stages?

after the 1st ionization / dissociation, the resultant negatively charged anion will attract the remaining (not yet dissociated) H+ proton even more strongly — i.e., this 2nd proton (2nd ionization/dissociation) is harder to come off of the anion than the first dissociation

thicker arrow in reaction

means rxn occurs with more “ease”

thinner arrow in reaction

rxn occurs with less “ease” (harder) due to the increased negativity on the molecule you are pulling from

triprotic acids

acids that can donate 3 protons per molecule

why does triprotic acids rxn require 3 ionization stages?

similar reason to diprotic — the 1st stage is the easiest stage, and the third stage is the hardest stage (increasing negativity on main molecule)

T/F: The higher the degree of dissociation, the stronger the acid!

True

1st stage = weak vs. 3rd stage = strong

T/F: All hydrogen atoms in a molecule are acidic.

False

Whether a hydrogen atom can be donated as a H+ proton depends upon the nature of the atom or group to which it is bonded.

T/F: H bonded to C cannot be an acidic H!

True

electronegativity

ability of atoms to attract and hold electrons within chemical bonds

the higher the electronegativity of an atom, the greater the attraction towards the electrons

Electronegativity determines ____ and ____ of chemical bond!

polarity

type

nonpolar covalent bond’s electronegativity difference between atoms

0 — 0.4

polar covalent bond’s electronegativity difference between atoms

0.5 — 1.9

ionic bond’s electronegativity difference between atoms

2.0 and above

acidic hydrogens are H atoms that are bonded to _____

electronegative elements (such as N, O, F, Cl) in a molecule

conjugate acid-base pairs

occur within acid-base equilibrium rxns

an acid and its conjugate base, as well as its base and conjugate acid, differ only by 1 proton.

HA + B ←→ A- + HB+

[acid + base ←→ conjugate base + conjugate acid]

amphoteric compounds

molecule or ion that can react both as an acid and as a base

when HA accepts a proton, it acts as a base!

Zwitterion

neutral dipolar ion that has one positively charged group and one negatively charged group — canceling each other out!

When H2O reacts with a stronger acid, it acts a ____.

base

when H2O reacts w/ a stronger base, it acts as an ____.

acid

HSO₄^- acts as a ___ by accepting a proton from H₃O⁺ to form H₂SO₄^-

base

HSO₄^- acts as an ____ by donating a proton to hydroxide ion to form SO₄^2-

acid

In acidic solution, amino acids accepts a proton and acts as a ___.

base

In basic solution, amino acid act as an ___ by releasing a proton.

acid

Form:

NH₂ & COOH groups:

Condition [H⁺]:

Charge:

cation

protonated amino group

acidic (high)

positive

Form:

NH₂ & COOH groups:

Condition [H⁺]:

Charge:

zwitterion

protonated amino group & deprotonated carboxyl group

neutral

no net charge

Form:

NH₂ & COOH groups:

Condition [H⁺]:

Charge:

anion

deprotonated carboxyl group

basic (low)

negative

Strength of an acid refers to its relative ability to ____.

donate a proton

A strong acid will have a ____.

weak conjugate base

A weak acid will ____ only slightly in water.

ionize

A weak acid will have a ___.

strong conjugate base

If the forward rxn is favored, then there will be ___ conjugate base & conjugate acid at equilibrium.

more

If the reverse rxn is favored, then there will be ___ acid & base at equilibrium.

more

In every acid-base proton transfer rxn, the position of the equilibrium favors the transfer of the proton from the ___ acid to the ___ acid, or from a ___ base to a ___ base.

stronger, weaker

strong, weak

Single Forward Arrow (in rxns)

irreversible rxn

Equal Arrows (in rxns)

neither forward nor reverse rxn is strongly favored - balanced

Longer Forward Arrow (in rxns)

Products are more prominent, and forward rxn is favored

Longer Reverse Arrow (in rxns)

Reactants are more prominent, and the reverse rxn is favored