CHM110H5 Lecture Notes - Buffer Solution, Conjugate Acid, Spectator Ion
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Chapter 17- Additional Aspects of Acid-Base Equilibria
•Water has no buffer capacity-that is, its pH changes sharply when even small quantities of
acids or bases are dissolved in it
•Buffer Solutions- solutions that can resist major change in pH when acids or bases are
added to them
•Most important buffer system to humans: buffer system that maintains the constant pH of
17.1-Common-Ion Effect in Acid-Base Equilibria
•When the concentrations of acid and base in a buffer solution are equal, [H3O+] =Ka and pH=pKa.
•When there is more acid than base, [H3O+] > Ka and pH < pKa.
Solutions of Weak Acids & Strong Acids
Strong Acid = HCl Weak Acid = HC2H3O2 Conjugate Base =
•Increasing the concentration of one of the products of a reaction-the common ion- shifts the
equilibrium condition in the reverse direction
•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
Solution of Weak Acids & Their salts
•Salt of a weak acid is a strong electrolyte-its ions become completely dissociated from one
another in aqueous solution H2O
Acid-Base Properties of Salt Solutions
•A salt is an ionic compound that is produced when a cation and an anion from an acid-base reaction combine. In
other words, the cation from a base replaces a proton on an acid.
•A simple example is the reaction of the strong base NaOH with the strong acid HCl. In solution, the and
ions react to form H2O, leaving a solution of salt ions, Na+and Cl-.
•In this example, the salt ions do not affect the pH of the solution. This is because the conjugate base of a strong acid
is always a weak base (and the conjugate acid of a strong base is always a weak acid). In this case, the and
counterions are so weak that they do not influence the pH of the solution at all. Such ions are said to be spectator
•However, often one of the salt ions is a conjugate acid or a conjugate base. If one of the ions that makes up the salt
is a conjugate acid, this ion will react with H2O to create H3O+ ions, acidifying the solution:
•Similarly, if one of the ions that makes up the salt is a conjugate base, such an ion will react with H2O to create
OH- ions, making the solution more alkaline:
•It can also happen that both of the ions in the salt can react with H2O. In this case, the pH of the solution can be
more difficult to predict, and it depends on the extent to which each of the ions hydrolyzes (reacts with water).
Acid Structure & Relative Acidity
•The strength of an acid, HA, is often determined by the strength and polarity of the H-A bond. In general, a weaker
and more polar bond leads to a stronger acid.
•For binary acids of elements in the same group of the periodic table, the H-A bond strength decreases down the
group, so acidity increases.
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