Gen Chem 2 Exam 4

the acidity of an aqueous soln depends on

concentration of hydronium ions

pH + pOH =

14.00

strong acids and bases

completely ionize in water

weak acids and bases

partially ionize in water

Ka

acid ionization constant ([A-][H3O+]/[HA]

larger Ka value means

stronger acid

for pH of weak acids

create a RICE table

Kb

base ionization constant, increases with base strength

• [BH+][OH-]/[B]

for pH of very dilute solns of strong acids, we must consider

that H20 → H+ and OH-, which means the pH=-log[H+] doesn’t make sense

polyprotic acids

more than 1 ionizable H atom per molecule

for polyprotic acids…

the first proton comes off readily (strong acid, amphoteric species is created) and the 2nd proton is harder to remove

typically only the first

proton significantly affects pH

more difficult to remove an H+ ion from

a negatively charged anion

Ka for polyprotic acids

Ka1 > Ka2 > Ka3

acidic and basic salts

how do they react in water?

NaF is a basic salt bc

the anion makes OH- in water and the cation makes nothing bc NaOH is a strong base that dissociates

• F- + H2O → HF + OH-

weak acids only

ionize to a limited degree in water

salt hydrolysis occurs when

ions produced by the dissociation of a salt reac with water to produce either hydroxide ions or hydronium ions)

basic salts are

conjugates of weak acids

acidic salts are

conjugates of weak bases

• NH4+ + H2O ←> NH3 + H3O+

• NH4+ is from NH4Cl

how do we estimate pH of salt solutions?

determine if it is an acidic or basic salt (or neither)

steps to determine acidic or basic salt

1. split salt into anion + cation

2. add OH- to the cation and H+ to the anion

3. which resultant acid or base is stronger?

metal oxides (left)

are basic

nonmetal oxides (right)

are acidic

semi-metal oxides (btwn)

are either acidic or basic

common ion effect

the shift in position of an equilibrium caused by the addition of an ion taking part in the rxn

result of common ion effect

anything that drives eq away from H+ makes solution more basic

buffer

a solution that contains a weak acid and its conjugate base (or vice versa)

oxidation

loss of electrons

reduction

gain of electrons

redox rxn

sum of oxidation ½ rxn and reduction ½ rxn

oxidation number

charge an atom would have if electrons will be transferred completely (can determine which species gains/loses)

ON of atoms in pure elements

0

ON of single atom ions

charge on PT

ON of F

always -1

ON of O

almost always -2

ON of H

almost always +1

ON of neutral compounds

sums to 0

acitivity series for metals

can be used to rank the strength of the metals as reducing agents

• a metal cation will oxidize any metal above it in the activity series

electrochem

branch of chem that examines transformations btwn chemical and electrical energy (electron flow)

reducing agent

is oxidized, reduces other species

oxidizing agent

is reduced, oxidizes other species

galvanic cells

use spontaneous rxns where electric current flows from anode → cathode to harness energy

anode

where oxidation occurs

cathode

where reduction occurs

salt bridges

connect solns and prevent short circuiting

cell diagram (electrochemical cell arrangement)

1. anode on left, cathode on right, double line in middle for salt bridge

2. vertical lines to indicate phase or symbol changes of ions

3. []s and pps if known

cell diagram example

Zn(s) l Zn²⁺(1.00M) ll Cu²⁺(1.00M) l Cu(s)

useful galvanic cells are thermodynamicaly favorable

delta G naught rxn is less than zero

standard reduction potential (E naught)

the potential of a reduction half rxn in which all reactants and products are in their std states at 25C

the more positive value of E naught…

the greater probability it will be reduced

reactants at the top of the activity series are

among the strongest oxidizin agents (more +)

reactants at the bottom of the activity series are

strongest reducing (less +)

E naught cath - E naught an =

E naught cell

more positive E naught value

cathode - reduction

less positive (more negative) E naught value

anode (oxidation)

n for E naught equations

moles of electrons transferred

R for e naught eqns

8.314 (energy!)

F for e naught eqns

Farraday’s constant (96,500)

nernst eqn

used to calculate E cell when non-std state conditions (Q = products/reactants)

henderson-hasselbach eqn is used for

determining pH of buffer solns

buffer

soln that resists changes in pH when small quantities of acids or bases are added to it

weak acid reacts with added base

HA + OH^- → H₂O + A^-

weak base (A-) reacts with added acid

A^- + H₃O⁺ → HA + H₂O

after H+ is consumed…

the system will react to reestablish equilibrium

for H-H calculations

look at concentrations after consumption of added acid or base!!!

buffer range

pH range within which a given buffer can provide pH protection

• pKa ± 1

buffer capacity

quantity of acid or base that a buffer can neutralize while maintaining pH within a desired range

• proportional to component concentrations

to make a buffer with a specific pH

1. pick a weak acid whose pKa is close to desired pH (±1)

2. substitute pH and pKa into eqn to obtain [conj base]/[weak acid] ratio

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

3. choose [buffer] (generally btwn 0.05M and 0.5M) and adjust pH w/ strong acid/base as necessary