Chapter 16 Flashcards
What is a weak acid?
A weak acid ionizes only partially, generating an equilibrium mixture containing the intact weak acid, hydronium ion, and the conjugate base of the weak acid.
What is the general equilibrium expression for a weak acid, HA?
HA(aq) + H2O(l) ⇄ H3O+(aq) + A-(aq)
What symbol is used for the equilibrium constant of a weak acid?
K_a
What is the relationship between K_a and the concentration of water in the equilibrium expression?
K_a = K_c [H2O]
What is the value of K_a for HNO2?
K_a = [H3O+][NO2-] / [HNO2]
What is the value of K_a for HC2H3O2?
K_a = [H3O+][C2H3O2-] / [HC2H3O2]
Why are K_a values often small for weak acids?
Weak acid equilibria usually have fewer products than reactants at equilibrium.
What is pK_a?
pK_a = -log K_a
What is the percent ionization of a weak acid solution?
% ionization = (h30+ eq / initial concentration) x 100
How does the concentration of a weak acid affect percent ionization?
As the concentration of weak acid solutions decreases, the percent ionization increases.
What assumption can be made if the percent ionization is small?
The equilibrium concentration of the weak acid is almost the same as the starting concentration.
What is the threshold for using the assumption about percent ionization?
Based on the number of significant figures needed for your answers.
What is the formula for the equilibrium constant K for a weak acid and its conjugate base?
K_a x K_b = K_w
K_w is the ion product of water, typically 1.0 x 10^-14 at 25˚C.
At 25˚C, what is the relationship between pK_a, pK_b, and pK_w?
pK_a + pK_b = pK_w = 14.000
How is the K_b value calculated for the conjugate base of a weak acid?
K_b = K_w / K_a
Example: For C2H3O2^- with K_a = 1.75 x 10^-5, K_b = 5.71 x 10^-10.
In weak polyprotic acids, how is each H+ loss treated?
Each H+ loss is treated as an independent event.
What is the trend in the equilibrium constants for successive H+ losses in weak polyprotic acids?
K_a1 > K_a2 > K_a3 …
Why might the differences in equilibrium constant values between steps in polyprotic acids be significant?
Often the differences are 10^3 or greater.
What is the equilibrium expression for the first dissociation of a weak polyprotic acid H2A?
H2A(aq) + H2O(l) ⇄ H3O+(aq) + HA^-(aq)
K_a1 represents the equilibrium constant for this reaction.
What is the equilibrium expression for the second dissociation of a weak polyprotic acid HA^-?
HA^-(aq) + H2O(l) ⇄ H3O+(aq) + A^2-(aq)
K_a2 represents the equilibrium constant for this reaction.
