L7, (Strength of Acids and Bases) Chemistry B Unit 2: Solutions, Acids, and Bases
FOR AN ACID IN WATER
K_a= ([H+][A-]/[HA])
a strong acid has a higher ? and a ? dissociation constant.
[H3O+], large
a weak acid has a low ? and a ? dissociation constant.
[H3O+], small
A 0.1000M solution of ethanoic acid is only partially ionized. From measurements of the pH of the solution, [H+] is determined to be 1.34 x 10^-3 M. What is the acid dissociation constant (K_a) of ethanoic acid?
[ethanoic acid]=.1000M [H+]=1.34 x 10^-3M CH3COOH(aq)+H2O(l)<->(H3O+)(aq)+(CH3COO-)(aq) K_a=([H+] x [CH3COO-])/[CH3COOH] ((1.34 x 10^-3) x (1.34 x 10^-3))/.0987= 1.82 x 10^-5
indicates the ability of a weak base to compete with the very strong base OH- for hydrogen ions
base dissociation constant (K_b)
strong acids
completely ionize in aqueous solution. (hydrochloric acid, sulfuric acid)
indicate how much of an acid or base is dissolved in solution. refer to the number of moles of the acid or base in a given volume.
concentrated, dilute
strong bases
dissociate completely into metal ions and hydroxide ions in aqueous solution. (some are not very soluble in water, calcium hydroxide and magnesium hydroxide)
[H+]
hydrogen ion
acid dissociation constants are sometimes called ?.
ionization constants
Weak acids
ionize only slightly in aqueous solution. (ethanoic acid, boric, cabonic acid)
A ? value of K_a means the dissociation or ionization of the acid is more complete.
larger
The pH of .10 M solution of formic acid, HCOOH, at 25'C is 2.38. Calc K_a for formic acid at this temp.
pH= 2.38 .10 M solution of formic acid HCOOH HCOOH+H2O<--> (H3O+)+(COOH-) K_a=([H30+][COOH-])/[HCOOH] R [HCOOH]+[H2O]<->[H3O+]+[COOH-] I ||||||0.1|||||||||||||||||||||||||0||||||||||||0 C ||-.004||||||||||||||||||||||+.004||||||+.004 E. ||.096||||||||||||||||||||||||| .004|||||||.004 pH= -log[H+]=2.38 10^-2.38=.004 [H3O]=.004 K_a= ([.004][.004])/[.096]=1.67 x 10^-4
weak bases
react with water to form the hydroxide ion and conjugate acid of the base. (ammonia)
Weak acids have ? K_a (acid dissociation constant) values.
small
if the value of the dissociation constant is small, then the degree of dissociation or ionization of the acid in the solution is ?.
small
refer to the extent of ionization or dissociation of an acid or base. how many of the particles ionize or dissociate into ions.
strong and weak
[HA]
the concentrations of the acid
Acids are classified as strong or weak depending on...
the degree to which they ionize in water.
[A−]
the negative ion from the dissociation of the acid
conjugate acid
the particle formed when a base gains a hydrogen ion; NH4+ is the conjugate acid of the base NH3
conjugate base
the particle that remains when an acid has donated a hydrogen ion; OH- is the conjugate base of the acid water
base dissociation constant (K_b)
the ratio of the concentration of the conjugate acid times the concentration of the hydroxide ion to the concentration of the conjugate base K_b= ([conguate acid] x [OH-])/(conjugate base)
acid dissociation constants
the ratio of the concentration of the dissociated (or ionized) form of an acid to the concentration of the undissociated (nonionized) form.
acid dissociated constant
the ratio of the concentration of the dissociated form of an acid to the undissociated form; stronger acids have larger K_a values than weaker acids.
the smaller the value of K_b the ? the base
weaker
