Acid-Base (Proton Transfer) Reactions
CHEMISTRY 4
CHAPTER 17
ACID-BASE (PROTON TRANSFER) REACTIONS
1 THE ARRHENIUS THEORY OF ACIDS AND BASES
An acid ( HCl) is a substance which produces hydrogen ions in water solution. The properties of an acid is the properties of the hydrogen ions.
A base (NaOH) is a substance which produces hydroxide ions in water solution. The properties of a base are the properties of the hydroxide ions.
The net reaction between a strong acid and a strong base is :
H+ + OH- H2O
The Arrhenius concept of acids and base is limited because it applies only to aqueous solutions.
2 THE BRONSTED-LOWRY THEORY OF ACIDS AND BASES
An acid-base reaction is a proton-transfer reaction in which the proton is transferred from the acid to a base with formation of another acid and base.. An acid is a proton donor, and a base is a proton acceptor.
The base formed when the acid has donated a proton is called the conjugate base of the acid.
Acid A ↔ Conjugate base of acid A + H+
The sign ↔ is used to show that the reaction is reversible.
The stronger the acid, the weaker the conjugate base, and the weaker the acid, the stronger the conjugate base.
The reaction when an acid is dissolved in water is
HCl + HOH → H3O+ + Cl-
HCH3COO + H2O ↔ H3O+ + CH3COO-
H3O+ is called hydronium ion. The conjugate base of acid HCl is Cl- . The conjugate base of acetic acid HCH3COO is the anion acetate CH3COO-.
Ammonia NH3is a base. When dissolved in water NH3 accepts proton from water molecule
NH3 + H2O ↔ NH4+ + OH-
The conjugate acid of the base NH3 is the cation NH4+ .
Water which can behave as an acid in one case and a base in another is said to be amphoteric.
3 RELATIVE STRENGTHS OF ACIDS AND BASES
An acid base reaction is a proton transfer reaction in which a proton is transferred from a stronger acid to a stronger base with formation of a weaker acid and weaker base.
Stronger Acid1 + Stronger Base2 ↔ Weaker Acid2 + Weaker Base1
HCl + NH3 ↔ NH4+ + Cl-
HCl + CH3COO- ↔ HCH3COO + Cl-
4 THE WATER EQUILIBRIUM
Pure water contains a tiny concentration of ions, due to the ionization of the water molecule:
H2O + H2O ↔ H3O+ + OH-
It can be shown that
[H3O+] [OH-] = Kw = 1.0 x 10-14
[H3O+] and [OH-] are molar concentrations of ions H3O+ and OH-. Kw is called autoprotolysis constant of water.
In pure water the concentration of H3O+ and OH- must be equal .
[H3O+] = [OH-] = 1.0 x 10-14
= 1.0 x 10-7 M
Solution in which [H3O+] = [OH-] = 1.0 x 10-7 M are neutral solution.
Solution in which [H3O+] >[OH-] is acidic. The solution is acidic if [H3O+]>1.0 x 10-7 M
Solution in which [H3O+] < [OH-] is basic. The solution is basic if [H3O+]<1.0 x 10-7 M
Example: Find the hydroxide ion concentration in a solution in which [H3O+] = 10-5 M.
Solution: [H3O+] [OH-] = Kw = 1.0 x 10-14
[OH-] = Kw / [H3O+] = 1.0 x 10-14 /[H3O+] = 1.0 x 10-14 /1.0 x 10-5= 10-9 M.
5 pH and pOH
By definition pH and pOH are given by :
pH ≡ -log [H3O+] and pOH ≡ -log [OH-]
[H3O+] ≡ antilog(-pH) ≡ 10-pH
[OH-] ≡ antilog(-pOH) ≡ 10-pOH
The negative sign in the definition of pH means that the higher the hydronium concentration, the lower the pH.
The relation between pH and pOH is
pH + pOH = 14
If pH = 7 (pOH = 7) the solution is neutral.
If pH > 7 (pOH < 7) the solution is basic.
If pH < 7 (pOH > 7 ) the solution is acidic.
Calculation with pH
Example: Calculate the pH of a solution if [H3O+] = 2.7 10-4
Solution: pH = -log(2.7 10-4) = - log(2.7) + -log (10-4 ) = -0.431364 + 4 = 3.568636 = 3.56
We keep two digits after the decimal because the number of significant digits in the hydronium concentration is two.
Example: Find the hydrogen concentration of a solution if its pH is 11.62
Solution: [H3O+] = antilog(-11.62) = 10-11.62 = 2.4 x 10-12 M
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Logarithms
The common logarithm of a number x is denoted log x and is the power to which 10 must be raised to equal x.
x = 10 log x
Example: the logarithm of 100 = 102 is 2. log 100 = 2
Example : the logarithm of 1.5 x 102 is 2.18 because 1.5 x 102 = 102.18
The number in front of the decimal point in the logarithm is called the characteristic, the decimal fraction is called the mantissa
The number of significant figures in the mantissa is equal to the number of significant figures in the decimal number.
The common antilogarithm of a number y is the number 10y . The antilogarithm of 2 is 102 .
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