Salts in SolutionHydrolysis and Buffers

IntroductionStrong acids added to water produce a weak conjugate base.

HCl(g) + H2O(l) ➜ Cl-(aq) + H3O+(aq)strongacid

weakbase

Strong bases added to water produce a weak conjugate acid.

NaOH(s) ➜ Na+(aq) + OH-(aq)strongbase

weakacid

IntroductionWeak acids added to water produce a relatively strong conjugate base.

HNO2(aq) + H2O(l) ➜ NO2-(aq) + H3O+

(aq)weakacid

strongbase

Weak bases added to water produce a relatively strong conjugate acid.

NH3(aq) + H2O(l) ➜ NH4+(aq) + OH-(aq)

weakbase

strongacid

Salt HydrolysisWhen acids and bases become involved in neutralizations, they form salts and water.

HNO2(aq) + NaOH(aq) ➜ NaNO2(aq) + H2O(l)nitrous

acidsodium

hydroxide

HCl(aq) + NH3(aq) ➜ NH4Cl(aq)hydrochloric

acidammonia

sodiumnitrite

water

ammoniumchloride

HCl(aq) + NaOH(aq) ➜ NaCl(aq) + H2O(l)hydrochloric

acidsodium

hydroxidesodiumchloride

water

Salt HydrolysisWhen the salts themselves are dissolved in water, they hydrolyze.

NaNO2(aq) ➜ Na+(aq) + NO2-(aq)

sodiumnitrite

sodium

NH4Cl(aq) ➜ NH4+(aq) + Cl-(aq)

ammoniumchloride

ammonium

nitrite

chloride

NaCl(aq) ➜ Na+(aq) + Cl-(aq)sodiumchloride

sodium chloride

Salt HydrolysisThe ions that are weak conjugate acids and bases have no other effect on the solution.

cations(weak conjugate acids)

anions(weak conjugate bases)

sodium, Na+ chloride, Cl-

potassium, K+ nitrate, NO3-

calcium, Ca2+ bromide, Br-

strontium, Sr2+ chlorate, ClO3-

Salt HydrolysisThe ions that are relatively strong conjugate acids and bases have effects on the solution.

cations(strong conjugate acids)

anions(strong conjugate bases)

ammonium, NH4+ acetate, CH3COO-

methylaminium, CH3NH3+ nitrite, NO2

-

pyridinium, C5H5NH+ fluoride, F-

ethylaminium, CH3CH2NH3+ hypochlorite, ClO-

Salt HydrolysisStrong conjugate acids hydrolyze in solution, donate hydrogen ions, and lower the pH.

NH4+(aq) + H2O(l) ➜ NH3(aq) + H3O+(aq)

Strong conjugate bases hydrolyze in solution, accept hydrogen ions, and raise the pH.

CH3COO-(aq) + H2O(l) ➜ CH3COOH(aq) + OH-

(aq)

Salt HydrolysisIf we have a salt which results from the neutralization of a strong acid and a strong base

the resulting solution is neutral.

NaCl(aq) ➜ Na+(aq) + Cl-(aq)

Na+(aq) + H2O(l) ➜ no reaction

Cl-(aq) + H2O(l) ➜ no reaction

Salt HydrolysisIf we have a salt which results from the neutralization of a strong acid and a weak base

the resulting solution is acidic.

NH4Cl(aq) ➜ NH4+(aq) + Cl-(aq)

NH4+(aq) + H2O(l) ➜ NH3(aq) + H3O+(aq)

Cl-(aq) + H2O(l) ➜ no reaction

Salt HydrolysisIf we have a salt which results from the neutralization of a weak acid and a strong base

the resulting solution is basic.

NaClO(aq) ➜ Na+(aq) + ClO-(aq)

Na+(aq) + H2O(l) ➜ no reaction

ClO-(aq) + H2O(l) ➜ HClO(aq) + OH-(aq)

Salt HydrolysisIf we have a salt which results from the neutralization of a weak acid and a weak base

the resulting solution may be acidic, basic, or neutral.

It depends on the relative strengths of the acid and base.

NH4ClO(aq) ➜ NH4+(aq) + ClO-(aq)

NH4+(aq) + H2O(l) ➜ NH3(aq) + H3O+(aq)

ClO-(aq) + H2O(l) ➜ HClO(aq) + OH-(aq)

BuffersA buffer is a solution in which the pH remains relatively constant when small amounts of acid or base are added.

A buffer is prepared with a solution of

a weak acid and one of its salts

CH3COOH and NaCH3COO

a weak base and one of its salts

NH3 and NH4Cl

BuffersBuffers are better able to resist pH changes than is pure water.

Add 10 mL of 0.1 M HCl to 50 mL of

pure water

pH goes from 7.00 to 1.78 (∆pH = 5.22)

acetic acid/acetate buffer

pH goes from 4.74 to 4.57 (∆pH = 0.18)

BuffersThe equilibrium set up between the acetic acid (CH3COOH) and its acetate salt (CH3COO-) allows the solution to absorb excess acid or base.

H3O+ + CH3COO- ⇄ CH3COOH + H2O

OH- + CH3COOH ⇄ CH3COO- + H2O

The concentrations of the acid and the salt act as reservoirs of neutralizing power.

BuffersA buffer cannot control pH when too much acid or base is added.

The reservoirs of neutralizing power are used up.

When this happens, we exceed the buffering capacity of the system.

Our bodies keep blood at pH = 7.35-7.45 using

carbonic acid/hydrogen carbonate

dihydrogen phosphate/hydrogen phosphate