Module 4 and 5

bronsted lowry acid

proton donor

bronsted lowry base

proton acceptor

conjugate acid base pair

pair of molecules that differ by a single hydrogen

amphiprotic

both an acid and a base

lewis acid

electron pair acceptor

lewis base

electron pair donor

what is Kw

equilibrium constant for the self ionization of water → 1.01 × 10^-14 for 25C, increases with temp

if pH increases by 1 unit, what happens to [H₃O]

decreases 10 fold

acids in water

protonates water molecules when dissolved in solution, produces H₃O

bases in water

deprotonate water molecules when dissolved in solution, produces OH^- ions

pH and pOH relationship for pure water

pH = pOH = ½ pKw

pH and pOH relationship for acidic solution

pH < pOH → pH < ½ pKw

pH and pOH relationship for basic solution

pOH < pH → pH > ½ pKw

pKw of pure water at 25C

14.00

Ka

equilibrium constant for an acid reacting with water. acid must have a hydrogen

Kb

equilibrium constant for a base reacting with water. base must contain a lone pair

bond between a base and a proton

coordinate covalent bond → both electrons come from the base

strong acid

Ka »1 → reaction goes to basically completion

weak acid

Ka « 1 → only some molecules ionize

common strong acids

HI, Hbr, HCl, HBrO4, HIO4, HClO4, HNO3, H2SO4

special thing about H2SO4

diprotic, has a Ka associated with each ionization

common strong bases

hydroxide salts of group 1 and group 2 metals, H^-, O^2- and S^2-

weak acid Ka or week base Kb

less than 1, larger than 10^-14

if Ka/Kb are less than 10^-16…

acid/base too weak to affect pH → pH of 7 at 25C

4 factors to consider when assessing acid strength

atom, resonance, induction, orbitals

what does acid strenght generally increase with

• increasing H-A bond length

• increasing electron withdrawing character of A

• increasing stability of conjugate base (A-)

questions of ario: a

what atom is the acidic proton bonded to

questions of ario: r

does the conjugate base have resonance

questions of ario: i

are there inductive effects to be considered

questions of ario: 0

are there orbital effects to be considered

ARIO → how does atom the acidic proton is bonded to affect acid strength

dictates length of HA bond and conj base stability

• comparing atoms in the same row → strength increases with electronegativity

  • comparing atoms in the same column → strength increases with size

ARIO → how does resonance of the conjuage affect acid strength

delocalized negative charge is more stable than localized one → more resonance = more stable = stronger acid

acid strength ___ with increasing pKa

decreases

what is induction

pulling of electron density through sigma bonds

ARIO → how does induction affect acid strength

more electronegative atoms → increase S+ of proton and stabilizes conjugate base → stronger acid

ARIO → how does orbitals affect acid strength

sp orbitals have highest s character → held closest to nucleus → most stabilized negative charge → stronger acid

for weak polyprotic acids, do the second and third ionizations contribute to pH

no, only consider the first ionization

what is the only polyprotic acid you need to consider Ka2/3 of

H2SO4 → strong acid therefore its second ionization will affect pH

stronger the acid, the ___ the conjugate base

weaker

conjugate of weak is

weak

conjuage of strong is

very weak

conjugate of very weak is

strong

are salt solutions always pH 7

no, can have acidic or basic properties

cations can potentially be

acids

anions can potentially be

bases

special case about acid/base capacity of anions of a polyprotic molecule

may act as an acid OR a base → amphiprotic → compare Kb and Ka

when will a metal ion affect pH of a solution

if it has a larger charger (+3 or higher)

when will polyatomic cations act as acids

conjugates of bases → NH4+ and RNH3+ are conjugate acids

when will an anion affect pH

if it is the conjugate of an acid, will act as a base (but consider its strenght)

common ion effect

le chateliers principle → ionization of a weak acid (HA) is significantly suppressed by the addition of a strong acid or A^-

neutralization between weak and strong

neutralization goes to completion → when calculating pH, think neutralization first then equilibrium with whats left

whats a buffer

maintains relatively constant pH even when adding small amounts of strong acid or strong base

how does a buffer work

has appreciable amounts of both weak acid and weak base → replaces strong acid with weak acid; strong base with weak base

shortcut for buffer calcs

you have the conjugate in an appreciable amount → suppressed ionization → equilibrium [ ] = initial [ ] use Henderson Hasselback eqn

buffer capacity

number of moles of strong acid/base required to raise pH of 1L of solution by 1 unit

titrant

solution of known concentration

end point

titration observed to be complete → colour change

equivalence point

when moles of base = moles of acid

pH at half neutralization point of weak acid strong base titration

buffer ratio is 1 → the pH is equal to pKa of the acid

pH passed the equivalency of weak acid strong base titration (excess strong base)

only the strong base controls the pH even though there’s technically some weak base as well

is pH at equivalence neutral

only if it is a strong acid strong base.

weak acid + strong base → slightly basic

weak base + strong acid → slightly acidic

using henderson hasselbach eqn with a base

convert pKb into pKa and then plug in the base/acid

rules of assigning oxidation states

1. elemental from = 0

2. sum must equal total charge

3. group 1 metals = +1, group 2 metals = +2, F =-1

4. H → +1 unless with group 1 or 2 metal

5. O → -2 unless bonded to itself (peroxide) or fluorine

6. Cl Br and I → -1 unless earlier rule changes it

what is the oxidation state defined as

hypothetical charge if the bonding electrons between each pair of atoms belonged to the atom with greater EN

oxidation vs reduction

oxidation → losing electrons

reduction → gaining electrons

do reactants need to be in direct contact for electron transfer

no

basic components of an electrochemical cell

2 half cells and a salt bridge, with conducting material for electrons to flow

what makes up a half cell in an electrochemical cell

electrode (surface for electron transfer) immersed in a solution

what is the salt bride in an electrochemical cell

tube filled with very concentrated salt solution → prevents charge build-up in the solutions

anode vs cathode

anode → site of oxidation, losing mass

cathode → site of reduction, gains mass

2 main types of electrochemical cells

galvanic and electrolytic

2 main types of electrochemical cells: galvanic cell

aka voltaic cell. spontaneous redox rxn → produces energy

• eg a battery

2 main types of electrochemical cells: electrolytic cell

non-spontaneous redox rxn, external energy source forces the rxn → consumes energy

• good for converting metallic ions into metallic solid

general form of shorthand notation

anode first, cathode second → electrode | solution || solution | electrode

general form of shorthand notation: what does the single line represent

phase barrier

general form of shorthand notation: what does the double like represent

salt bridge

cell potential

electrical work divided by the charge transfered → E = w/Q = w/nF

what does cell potential depend on

• oxidizing or reducing strength of each half cell

• conditions of the cell → temp, pressure, concentrations

what is the accepted reference half cell for comparing all possible half cells

SHE → standard hydrogen electrode

what is the general structure of the standard hydrogen electrode

solution → 1M H⁺

electrode → flat Pt metal for electron surface (inert)

1 bar H₂ flows into solution

why are some element’s voltage potentials negative

implies it has a lesser tendency to reduce than SHE → electrons flowed toward the SHE

what is a volt defined as

joules/coulombs

rules of calculating standard cell potential (E^o_cell)

• reverse rxn → change E^o sign

• multiply rxn → no change

• adding 2 half reactions → add E^o (ensure one is E^o_ox and one is E^o_red)

standard conditions of an electrochemical cell

25^o, 1 bar for all gases, 1M for all solutions

sign of E^o_cell in relation to spontaneity

positive → spantaneous → galvanic cell

negative → non-spantaneous → electrolytic cell

• EXCEPT if some products are present, reverse rxn occurs spontaneously

oxidizing agent vs reducing agent

oxidizing agent → gets reduced → very +ve reduction potential

reducing agent → gets oxidized → very +ve oxidation potential

note about oxidizing agents and reducing agents from the table of reduction potentials

good oxidizing agents are the reactants at the top, good reducing agents are the products at the bottom

what is the nernst eqn used for

relates cell potential to reaction quotient Q → can calculate cell potentials for non-standard conditions

what does n represent in the nernst equation

the number of electrons transferred (be sure half reactions are balanced and they cancel out)

what is a concentration cell

same species involved in both anode and cathode, but different concentration/pressures. once [ ] equilibrates on both side, the battery dies

what happens to Ecell when equilibrium is reached in the half cells (Q=K)

voltage drops to 0