3) Acids, Bases, Buffers

what is pH

negative log of the activity of H+ ions in solutions

pH is never concentration, always activity

details on neutral pH

change in T&P make neutral pH change
when pH is neutral, {H+} = {OH-}

dissociation of water

Kw = {H+}*{OH-} / {H2O} = 10^-14

assume {H2O} = 1, write it out first, state assumption, then cross off

{H+} = {H2O}*Kw / {OH-}

what is pka, pkb, pkw

pka: acid dissociation constant, giving an H+

pkb: base dissociation constant, giving an OH-

pkw: p=log, logKw = log(10^-14), pkw = 14

Ka for strong acid, HCl

Ka = {H+}*{Cl-} / {HCl(aq)} = >1 because very much going to pdts

ka for weak acid, acetic acid

Ka = {CH3COO-}*{H+} / {CH3COOH} = 1.8×10^-6, much less than 1

pka = -log(1.8×10^-6) = 5.74

Kb for weak base, ammonia

kb = {NH4}*{OH-} / {NH3}*{H2O} = 1.8×10^-6

pkb = -log(1.8×10^-6) = 4.74

pka = 14-4.74=9.26

what is threshold for strong acids/bases

pka/pkb <2 is strong

lower pka means?

stronger acid

explain this graph

total conc is 10^-3

H+ line is just plotting pH (10^-1 (y) = 1(x))

the OH- line is pkw-pH

dashed line is pk1, 9.2. going from HB to B-, HB=B-, after line HB decreases at slope of 1 and B- is constant

go through this example remove this card

HA <-> H+ + A-
Ka = {H+}*{A-}/HA

what are ways we can determine speciation?

speciation is how much of each ion is in solution

can determine with pH and Ka, measure CT, can calculate from that

or can look at graph, for certain pH (4), most will be HA, and very little will be A-

what happens at these scenarios

1: pH < pk, {H+} > K, (H+ + K = H+), HA

2: pH > pk, {H+} < K, (H+ + K = K), A-

3: pH = pk, H+ = K, H+ + K = 2H+

what is the proton balance condition

combined concentration of species that have donated protons must equal those that have accepted protons

example monoprotic acid HA

what is the pH of 10^-3 M HA solution

HA
H+ = A- + OH-
accepted = donated

accepted, HA can’t accept, so just put water accepting as H+
donated, HA can donate, and water as OH-

proton condition on graph (NaA)

where H+ and HA must equal OH-

Can’t be HA/A, its Ka
Can’t be A-/H+, it’s acid field and you’ve added Na
Can’t be H+/OH-, its neutral pH
Must be HA/OH-

H+ doesn’t matter in this case because where it crosses is so much lower than the other things in solution (it’s a log scale)
Only focus on the first things to cross from top down.

what are buffers

a solution capable of neutralizing both acids and bases with only small change in the pH of the system

chemistry of buffers

H+ + Ac- <-> HAc
HAc + OH- <-> Ac- + H2O

When Ac- is acting as HAc, it’s buffering

adding base or acid will buffer

When will pka = pH? (acids)

when activity of Ac- = activity of HAc

true for all acids

what is the effect of dilution on pH?

No effect

It dilutes Ac- and HAc equally, same for evaporation

Buffered solution prevents pH from changing due to dilution/evaporation
Only true for buffered solutions, and starting with equimolar concs of Ac- and HAc

where is buffering intensity highest?

at the pkas of the acid