Acido y base

Tadeusz Górecki

Ionic Equilibria

Acid-Base Equilibria
Brønsted-Lowry: an acid is a proton _______, a base is a ______________.
Acid ↔ Base + H +
+ ________________ (H3PO4, H2O), ______ ( NH 4 ) and _____ (H2PO4-) can all behave as acids.

Example:
+ NH 4 ↔ NH 3 + H +

Substances which can behave both as acids and as bases: ____________, or ______________ substances (e.g. H2O, SH-).SH ↔ H + + S 2 −
acid base −

H + + SH ↔ H 2 S
base acid

−

Free protons _________ in any solvent, thus the above reactions are ______________. In reality:
+ NH 4 + H 2O ↔ NH 3 + H 3O +

Energy required to dissociate _____ to ____ and __: _____ kcal/mol

Page 20

Tadeusz Górecki

Ionic Equilibria

Equilibrium constant for _______________: HA + H 2O ↔ H 3O + + A− [ H + ][ A− ]Ka = [ HA] ________________:
B + H 2O ↔ BH + + OH −

[ BH + ][OH − ] Kb = [ B] Relationship between Ka and Kb: K a ⋅ Kb = K w Ka = Kw Kb Kb = Kw Ka

Lewis: an acid is an ___________________; a base is an _________________. ____________________________________________________________

____ Strength of acids and bases
2 − HSO4 + H 2O ↔ H 3O + + SO4 − − H 2CO3 + H 2O ↔ H 3O + + HCO3

HCN +H 2O ↔ H 3O + + CN − acid1 + base2 ↔ acid 2 + base1
2 [ H 3O + ][ SO4 − ] = _______ Ka = − [ HSO4 ]

[ H 3O + ][ HCO3− ] Ka = = _______ [ H 2 CO3 ]
Page 21

Tadeusz Górecki

Ionic Equilibria

[ H 3O + ][CN − ] Ka = = ______ [ HCN ] _______ value of __ means that the acid is ________, thus: ______________________ ____________ of water:
H 2O ↔ H + + OH −

Equilibrium constant using__________: K0 = a H + aOH − a H 2O

Activity of water is by thermodynamic convention proportional to the ____ _________ of water in the solution. In dilute solutions it is close to __. Activity of water can be _____________________:

a H aOH = [ H + ]γ + [OH − ]γ − = ___
+ −

"________________" constant: [ H + ][OH − ] = K w Kw = K 0

γ +γ −

aH 2O

0 = K w / γ +γ −

Page 22Tadeusz Górecki

Ionic Equilibria

At 50°C, pKw = ______, and the neutral point is pH = ____. At 25°C in 3 M NaClO4 pKw = _____, and the neutral point is pH = ____.

Page 23

Tadeusz Górecki

Ionic Equilibria

____________ solvents:
+ − NH 3 + NH 3 ↔ NH 4 + NH 2

At -60°C, the equilibrium constant is:
+ − K = [ NH 4 ][ NH 2 ] = 10 −32

Thus, the pH scale (defined as -log[NH4+]) inliquid ammonia ranges from _ to __.

pH of a strong acid Initially PH, or "_________________", defined as PH = -log CH Today's definition of pH:
pH = pa H = − log a H = − log([ H + ]γ + )

General approach Example: HCl Mass balance: Ion product of water: Charge balance:

[Cl − ] = ____ [ H + ][OH − ] = K w = ____
[ H + ] = [OH − ] + [Cl − ] Kw + C HA [H + ]

Solution:

[H + ] =

Thisis a quadratic equation, which applies ________. When ____________, [H+] = ___ ([OH-] is _______________) At higher ionic strength, activity coefficient should be used.
Page 24

Tadeusz Górecki

Ionic Equilibria

Strong base: Example: NaOH Mass balance: Ion product of water: Charge balance:

[ Na + ] = ___
[ H + ][OH − ] = K w = 10−14 _____________________
[ H + ] + Cb = Kw [H + ]Solution:

Basic solution, thus ______________, and in general [H + ] = Kw Cb

___________________ ____________________________________ Example: pH of 2 ⋅ 10−7 M solution of NaOH [ H + ]2 + Cb [ H + ] − K w = 0
2 − Cb + Cb + 4 K w [H ] = 2 +

[ H + ] = 4.14 ⋅ 10 −8 mol / L

pH = _____

Simplified equation: pH = _____ ____________________________________

Page 25

Tadeusz GóreckiIonic Equilibria

pH of strong acid/base as a function of concentration:
0 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 2 4 6 8 10 12 14

log C

l C( d) og aci

l C( e) og bas

pH

Mixture of a strong acid and a strong base Example: HCl and NaOH Mass balance: Mass balance: Ion product of water: Charge balance:

[Cl − ] = ___ [ Na + ] = ___
[ H + ][OH − ] = K w = 10−14 [ H + ] + [ Na + ] = [Cl...