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Lecture

CHEM 1AA3 Lecture Notes - Buffer Solution, Conjugate Acid, Acid Strength


Department
Chemistry
Course Code
CHEM 1AA3
Professor
Pippa Lock

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Chapter 17: Additional Aspects of Acid-Base Equilibria
Acid-Base Review
BL acid H+ donor, BL base H+ acceptor
Conjugate acid/base pairs
Strong acids/bases 100% ionization in water
Weak acids/bases equilibrium reaction with water (Ka and Kb values)
o Weak acid Ka < 1, pKa > 1
o Weak base Kb < 1, pKb > 1
pH + pOH = 14 = pKa + pKb
pH of salt acidic (NH4Cl), neutral (LiBr), basic (NaCH3COO)
Common-Ion Effect in Acid-Base Equilibria
A solution of a weak acid or weak base initially contains a second source of one of the
ions produced in the ionization of the acid or base
The added ions are said to be common to the weak acid or base
Solutions of Weak Acids and Strong Acids
o Consider a solution that is at the same time 0.100 M CH3COOH and 0.100 M HCl
o When both are ionized, H3O+ is formed in both reactions, we say that it is a
common ion
o Adding a strong acid to a weak acid greatly reduces the concentration of the
anion (CH3COO-)
o Increasing the concentration of one of the products of the reaction (the common
ion), shifts the equilibrium condition in the reverse direction
o The common-ion effect is the suppression of the ionization of a weak electrolyte
caused by adding more of an ion that is a product of this ionization
Solutions of Weak Bases and Strong Bases
o Consider a solution that is at the same time 0.100 M NH3 and 0.100 M NaOH
o When a strong bases supplies the common ion OH-, the equilibrium shifts to
form more NH3
Solutions of Weak Acids and Their Salts
o The salt of a weak acid is a strong electrolyte, its ions become completely
dissociated from one another in aqueous solutions
o One of the ions (anion) is an ion common to the ionization equilibrium of the
weak acid
o The presence of this common ion suppresses the ionization of the weak acid
o Ex. NaCH3COO Na+ + CH3COO-
o When a salt supplies the common ion CH3COO-, the equilibrium shifts to form
more CH3COOH
Solutions of Weak Bases and Their Salts
o Ex. NH4Cl NH4+ + Cl-

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o When a salt supplies the common anion NH4+, the equilibrium shifts to form
more NH3
Buffer Solutions
Buffer solutions are those whose pH values change only slightly on the addition of small
amounts of either an acid or a base
Buffer solutions require two components
o One component is able to neutralize acids
o The other is able to neutralize bases
o However, the two components must not neutralize each other
o The last requirement rules out mixtures of a strong acid and a strong base
Instead, common buffer solutions are described as combinations of
o A weak acid and its conjugate base
o Or a weak base and its conjugate acid
o And both concentrations > 100 x Ka
Consider a solution that as equilibrium concentrations [CH3COOH] = [CH3COO-]
o [H3O+] = Ka = 1.8 X 10-5 M
o As a result, pH = 4.74
Imagine adding a small amount of a strong acid to this buffer solution
o A reaction occurs in which a small amount of CH3COO- is converted to its
conjugate acid
o After the neutralization of the added H3O+, we find that [CH3COOH] has
increased slightly and [CH3COO-] has decreased slightly
o The ratio [CH3COOH]/[CH3COO-] is only slightly greater than 1 and [H3O+] has
barely changed
o The buffer solution has resisted a change in the pH following the addition of a
small amount of acid
Imagine adding a small amount of a strong base to the original buffer solution
o A reaction occurs in which a small amount of the weak acid is converted to its
conjugate base
o Here, we find that [CH3COOH]/[CH3COO] is only slightly smaller than 1 and [H3O+]
has barely changed
o The buffer solution has resisted a change in pH following the addition of a small
amount of base
If a solution is to be an effective buffer, the assumptions (c x) = c and (c + x) = c will
always be valid
o The equilibrium concentrations of the buffer components will be very nearly the
same as their stoichiometric concentrations
Three ways to make a buffer
o Mix weak conjugate acid and base together
HCOOH and HCOONa or NH3 and NH4Cl
o Titrate a weak acid with a limiting amount of strong base
HCOOH + NaOH or NH4Cl + NaOH
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