Arrhenius and Brönstedt acids and bases, pH, ionic product of water, pKs
(1) Arrhenius acids and bases and Brönstedt-Lowry acids and bases
In 1887, Svante Arrhenius concluded that acids base were substance which ionized in water
solution to produce hydrogen ions H+ (hydroxide ions OH-).
→
H3O+(aq) + Cl-(aq)
HCl(aq) + H2O(l)
→
OH- (aq) + NH4+(aq)
NH3(aq) + H2O(l)
Brönstedt and Lowry stated that in a chemical reaction, any substance which donates a proton is an
acid and any substance which accepts a proton is a base.
A general acid - base reaction:
+
A-(aq)
HA(aq) + B(aq) → HB+(aq)
acid
base
conjugate acid
conjugate base
All Arrhenius acids are B-L acids, but not all B-L acids are Arrhenius acids.
(2) Ionic product of water, pH values, pKs
There are many ways to characterize acids and bases, one of these is with the pH.
By definition: pH =–log [H+], pOH =–log [OH-]
Water is a typical example of an amphoteric substances and can act therefore both as a base and as
an acid.
H3O+ + OHH2O + H2O
In pure water at 25oC, the [H+] = [OH-] = 1.0 X 10-7 mol/L
and pH =7, Kw=1.0×10-14
Ks = Kw = [ H+] * [OH-]
Kw is the equilibrium constant for the ionic product of water.
(3) Calculation of the pH of strong and weak acids
The strength of acids and bases can be given by the equilibrium constant of their dissociation
reaction. The acidity constant Ka and the basicity constant Kb can be used to calculate the pKa and
pKb values of the corresponding acids and bases. Strong acids (and bases) have a pKa (pKb) value
lower than 3.5.
Strong acids completely dissociate into ions while weak acids partially dissociate.
For strong acids, we get that [HA] = [H+] and therefore : pH =–log [H+]
Calculation for the pH of a weak monoacid
HA + H2O
A− + H3O+
With some simplifications, the equilibrium constant for the reaction is specified by
Ka= [A-]*[ H+]/[HA]
A simplified expression to calculate the pH of a weak acid is:
pH = ½ * (pKa – log [HA]0) (pKa = -log Ka )
Diacids : Diacids have more than one acidic hydrogen atom and have therefore more than one
dissociation reactions, which have to be taken into account for pH calculations
Questions
1. Write the formula for the conjugate bases of (a) H2O, (b) H2SO3, (c) HClO3, and (d) HCO3-.
-5
2. Calculate the pH in a 0.010 mol/L CH3COOH solution. It is known that Ka = 1,8 *10