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Chapter 17

CHM110 Chapter 17

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University of Toronto Mississauga
Heather Miller

Chapter 17- Additional Aspects of Acid-Base Equilibria Legend: Text Book Mastering Chemistry • 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 blood 17.1-Common-Ion Effect in Acid-Base Equilibria + • When the concentrations of acid and base in a buffer solution are equal,3[H O ] =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 = HC H O 2 3 2 Conjugate Base = C 2 O3 2 • 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 H O 2 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 NaO+ with -he strong acid HCl. In solution, thand ions react to form 2 O, 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 ions. • 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 w2th H O to cre3te H O ions, acidifying the solution: • Similarly, if one of the ions that makes up the salt is a conjugate base, such an ion will reac2 with H O to create - OH ions, making the solution more alkaline: • It can also happen that both of the ions in the salt can react with H O. In this case, the pH of the solution can be 2 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. • For binary acids of elements in the same row of the periodic table, the polarity of the H-A bond increases from left to right (as the electronegativity of A increases), so acid strength increases. • For oxoacids, acid strength increases with the oxidation number of the central atom. If the oxidation number is the same, acid strength increases with the electronegativity of the central atom. 17.2-Buffer Solutions • Water has no buffer capacity • Buffer solutions- some aqueous solutions whose pH values change only very slightly on the addition of small amounts of either an acid or a base • Buffer solutions require 2 components: one component is able to neutralize acids, and the other is able to neutralize bases • A buffer solution that maintains a nearly constant pH outside this range requires different buffer components Changes in Buffers • A solution of a weak acid (HA) and its conjugate base (A), + • is called a buffer and will resist a change in pH. If acid is added, the reaction shifts to consume the added H , forming more HA. When base is added, the base will react with H , reducing its concentration. The reaction then shifts to replace H through the dissociation of HA into H and A. In both instances, H tends to remain constant. The Henderson-Hasselbalch Equation - pH = pK +alog [A /HA] only use for buffer solutions • This equation is only useful when
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