Chapter 14 - acids and bases
acids taste _____________; bases taste ___________________ and feel _________________. Bases
dissolve ______________. Acids dissolve certain _________________. Acids turn litmus
______________, while bases turn litmus _________________. Acids are ___________ in
phenolphthalein, while bases are _______________ in phenolphthalein. Acids have a pH below ____;
Bases have a pH above _______. More on pH later……Beyond these properties, how do we define what is
an acid or a base? Definition #1: Arrhenius (same as the factor guy) - acids give off H+ - bases give off OH-.
The problem with this definition is ammonia, NH3. Ammonia is a _______. So we had to come up with a
better definition. An English scientists named Lowry and a Danish scientist named Bronsted independently
came up with definition #2 - Acids are proton donors. Bases are proton _________________. Think
______ for proton. Thus ammonia is a base because of the reaction: NH 3 + H2O ⇌ NH4+ + OH-1 . Note
that in this
reaction, the water acts as a Bronsted-Lowry acid since it’s a proton __________________. Note that for
every B/L acid, there is a _________________ base (the conjugate is what is formed after the acid _______
its proton, and what is formed after the base accepts its proton).
TTP Find the conjugate base of:
a] HBrO3
b] H2C2O4
c] HCO3
d] HPO4-2
e] H2O
Find the conjugate acid of:
a] H2O
b] HCO3
c] HPO4-2
d] F-1
You can also look at a reaction and determine the B/L acid and base and their conjugate pairs. For example:
A]
HCO3 + F-1 ⇌ HF + CO3-2
B]
HPO4-2 + HIO3 ⇌ H2PO4-1 + IO3-1
C]
H2O + NO2-1 ⇌ HNO2 + OH-1
D]
C2H3O2-1 + HSO4-1 ⇌ HC2H3O2 + SO4-2
Are we done with the definitions?? Nope… Definition #3: Lewis definition (same guy as the electron dot
formula) - Acids are _______________ acceptors. Bases are ___________________ donors. This does
explain why aluminum chloride in water lowers the pH. The Al+3 acts as a Lewis ____________ because it
can accept _____________. The reaction is:
Another important example is ammonia and boron trihydride.
It is reasonable to assume that the _______________ definition is the most complete definition for acids
and bases.
To note how strong an acid is, we use our good friend the equilibrium constant. To denote an acid, we use a
special K for acids called Ka . The _________________ the Ka, the stronger the acid. Ka represents the
reaction: HX(aq) + H2O(l) ⇌ H3O+(aq) + X-(aq) . X is any ____________. H3O+ is called the
________________ ion. Most of the time we cheat on Ka and don’t include the water. So Ka is written as
the eq constant for the reaction: HX ⇌ H+ + X- . So Ka is :
Note: the conjugate base of a strong acid is a weak base. And the conjugate base of a weak acid is a strong
base.
To note how strong a base is, we use a similar method. Kb represents the strength of a base. The ________
the value of Kb, the stronger the base. The Kb reaction is: X(aq) + H2O(l) ⇌ HX+1(aq) + OH-(aq) .
NOTE: you must add water to a base. You don’t need to do so for an acid. But you must add water to
bases.
Thus Kb =
Use p. 663 for Ka for some acids. Note: the chart does not include strong acids. My chart does. Strong acids
have a Ka that is too large to be measurable.
Use p. 685 for a list of Kb. Note that strong bases are 1A metals plus OH. For example:
Because the amount of H+ can vary widely for acids and bases, the pH scale was developed to conveniently
measure the amount of acid or base there is in a solution.
pH =
Since water decomposes itself by the reaction:
we can write an equilibrium law relation for this reaction. It is:
This K is a special K called _______
At 25oC, Kw = 1.0 x 10-14
Thus, the [H+] =
at 25oC and the [OH-] =
Thus the pH of a neutral solution is 7.00
If the [H+] > [OH-], the solution is ____________
If the [H+] < [OH-], the solution is _____________
If the [H+] = [OH-], the solution is _____________
Note: there is no upper or lower ____________ to pH. pH ranges for negative infinity to pos infinity.
Note, a pH of 7 is neutral only at 25oC. If I change the temp, neutral pH is a different number. (see prob 39)
new equation: pOH =
putting 3 equations (pH, pOH, and Kw) together we get:
Using these four equations we can solve problems like the following:
G: [H+] = 4.5 x 10-3 M, Find: [OH-], pH, pOH, and if the solution is acidic
G: pH = 12.78, Find: [H+], [OH-], pOH, and if the solution is acidic or basic
G: pOH = 13.56, Find: [OH-], [H+], pH, and if the solution is acidic or basic
G: [OH-] = 8.9 x 10-5 M, find [H+], pH, pOH, and if the solution is acidic or basic
Finding the pH of strong and weak acids, and strong and weak bases:
Find the pH of a 0.079 M HCl;
Find the pH of a 2.7 M HC2H3O2;
Find the pH of a 0.038 M KOH;
Find the pH of a 1.4 M KCN
Note that for any given acid or base, Ka x Kb = ____ for a weak acid and its conjugate base, or for a weak
base and its conjugate acid.
Find the pH of the following solutions: 1] 1.28 M HF
4] 0.0033 M Ca(OH)2
2] 4.5 M KOCl
3] 0.096 M HNO3
Find the pH of
1] 0.0077 M NaOH
2] 12.0 M HCl
3] 2.6 M HCN
4] 0.93 M KF
Percent dissociation: this is a way to determine how much of a __________ acid has ____________.
It is also known as percent ionization.
The formula is:
TTP Find the percent dissociation for a 1.00 M HF solution and for a 0.100 M HF solution.
From the previous problem, you can see that as a solution becomes more ____________ the percent
dissociation ______________________. This makes sense if you just think of LeChatelier’s principle.
Find the percent dissociation for a 0.40 M HC2H3O2 and a 0.040 M HC2H3O2 solutions.
Find the value of Ka if a 0.14 M weak acid solution is 2.5% dissociated:
Find the value of Ka if a 0.056 M weak acid solution has a pH of 1.85.
Find the Ka value of a 0.034 M weak acid that has a 12.2% dissociation.
Find the Ka value of a weak acid if 0.0048 M solution has a pH of 2.64.
Polyprotic acids: these are acids that can produce _______________________________.
Examples are any diprotic, triprotic, or tetraprotic acid. How do you find the pH of a polyprotic acid???
The good news is that you really don't have to add all the H+’s together. Since the Ka values always
__________________ for each successive H+, you only have to consider the H+’s produced from the first
dissociation. Adding more H+’s from the successive dissociations won’t affect the overall pH. There is one
major exception: ________________ acid, ______________ (formula). The H’s from HSO4-1 are
significant to the overall total. Here is a sample calculation: Find the pH of a 0.0050 M H 2SO4 solution.
Salts are _______________ ionic compounds. Some are acidic, some are basic, and some are neutral.
Acidic salts have a metal that has a Ka larger than the Kb of its anion. Or they contain a +3 or greater metal
ion which will cause it to act as a ____________ acid. Examples:
Basic salts have an anion with a Kb that is larger than the Ka of its metal. Recall that 1A and 2A metals do
not act as acids. Examples:
Neutral salts have an anion with a Kb too small to be measured with a 1A or 2A metal cation. Another way
to say this is that a salt produces a neutral solution if the cation is the cation of a strong base, and the anion
is the anion of a strong acid. Examples of neutral salts:
There are oxides that produce acids and bases. ____________ oxides produce bases. Example:
______________ oxides produce acids. Example:
Effect of structure on Acid-Base properties.
The more polar the bond between H and X (some anion), the stronger the bond, so the _________ the acid.
For oxyacids (those that contain oxygen), the more oxygens, the stronger the acid. See HClO4, HClO3,
HClO2, and HOCl (same as HClO).
pH and solubility
Recall Ksp ??? I hope so. Using solubilities and Ksp’s, we can find pH’s and use pH’s to find Ksp’s. Wow!
TTP Find the pH of a saturated Mg(OH)2 solution at 25oC.
Find the pH of a saturated solution of AgOH at 25oC.
What is the Ksp value of a substance if the substance is made up of an alkali-earth metal and hydroxide if its
pH is 12.63?
What is the Ksp of a substance if the substance has a +1 metal ion with a hydroxide ion and its pH is 10.15?
BaF2, CaCO3, and Ag3PO4 would cause the pH of a solution to ______________. (WHY??)
AgCl, AgBr, and PbCl2 would cause the pH of a solution _________________. (WHY)
This means that the solubility of BaF2, CaCO3 and Ag3PO4 would be affected by a change in pH. The
solubility of AgCl, AgBr and PbCl2 would not be affected by a change in pH.