unit 9 acids bases web -...
TRANSCRIPT
Properties of Acids
• sour or tart taste
• strong acids burn; weak acids feel similar
to H2O
• acid solutions are electrolytes
• acids react with most metals to release H2
• acids cause indicators to change color
– Acids turn litmus red
Properties of Bases
• bases taste bitter
• basic solutions do not burn
• basic solutions feel smooth and slippery
• basic solutions are also electrolytes
• bases usually do not react with metals
• bases also cause indicators to change
color
– Bases turn litmus blue
Three definitions
• Arrhenius
– Deals with H+ and OH-
• Bronsted Lowry
– Deals with protons
• Lewis
– Deals with pairs of electrons
Arrhenius Definition
• Arrhenius defined an acid as a substance
that ionizes in water to produce H+ ions
– Ex. HCl
– Any of our acids with H+ in the beginning
• Arrhenius defined a base as a substance
that ionizes in water to produce OH- ions
– Ex. NaOH
– Any of the metal hydroxides
Bronsted Lowry Definition
• NH3 is known to turn litmus blue, but it
does not have hydroxide ion
– Needed a new definition
– Substances do not need to be in water
• A B/L acid is any substance that can
donate a proton (H+)
• A B/L base is any substance that can
accept a proton (H+)
Conjugate pairs
HC2H3O2 + H2O → C2H3O2- + H3O
+
B/L acid B/L base conjugate base conjugate acid
NH3 + H2O → NH4+ + OH-
B/L base B/L acid conjugate acid conjugate base
Amphoteric Substances
• Notice in the examples, that H2O can act
as a B/L acid and as a B/L base.
• It is know as an amphoteric substance.
• Additional examples include
– HSO3-
– HCO3-
– H2PO4-
– HPO42-
These are the ions that result
when polyprotic acids release
H+ ions one at a time. Notice
that they can accept an H+ or
they can release an H+.
Lewis definition
• This is the most broad of the definitions
• A Lewis acid is an electron pair acceptor.
• A Lewis base is an electron pair donor.
• Example Lewis acid/base reaction….
Ion product constant for H2O
• Remember [H+][OH-] = 1.0 x 10-14 = Kw
– So if you know [H+] you can find [OH-] and
vice versa
Ex. If [H+] = 9.3 x 10-4 M, what is [OH-](9.3 x 10-4 M)[OH-] = 1.0 x 10-14
[OH-] = 1.1 x 10-11 M
pH or pOH
• pH = -log[H+] or pOH = -log[OH-]
• pH + pOH = 14
Ex. What is the pH of a solution that contains
4.9 x 10-9 M OH-? Find pOH first
pOH = -log (4.9 x 10-9)
= pOH 8.31
Since pH + pOH = 14, pH = 5.69
What is the [H+] of a solution that is pH4.82?
pH = -log [H+], rearranging for [H+]
[H+] = 10-pH
= 10-4.82
= 1.5 x 10-5 M H+
What is the [OH-] of a solution that is pH8.56?
If pH is 8.58, pOH is 5.42
[OH-] = 10-pOH
= 10-5.42
= 3.8 x 10-6 M OH-
You must understand that…
• When [H+] = [OH-], the solution is neutral, and the pH = 7
• When [H+] > [OH-], the solution is acidic, and the pH <7
• When [H+] < [OH-], the solution is basic, and the pH > 7.
• Increasing pH means decreasing [H+]…fewer H+
ions floating in solution….less acidic.
• Decreasing pH means increasing [H+]…more H+
ions floating in solution….more acidic.
Calculate the number of moles of LiOH in 8.75 mL of .150
M LiOH solution. What is the pH? pOH?
Moles = M x L
= (.150 M)(.00875 L) = .00131 moles LiOH
pOH = -log [OH-]
= - log .150 M
= pOH .824
pH = 14 - .824
= pH13.2
Acid Equilibrium problems
• Things to know
– All of our acids will be monoprotic, that is, give
off only one hydrogen ion.
– x will represent the amount of acid that
dissociates.
• Therefore x also represents the [H+]
– We will use ICE to determine concentrations
for the Ka expression
– We can use HA to represent the acid and A-
to represent the conjugate base.
Polyprotic acids
• Polyprotic acids are weak acids (except for H2SO4 which is a strong acid).
– They release their H+ ions one at a time.
– Each has its own Ka
– Ex. H3PO4
H3PO4 → H+ + H2PO4- Ka = 7.5 x 10-3
H2PO4- → H+ + HPO4
2- Ka = 6.2 x 10-8
HPO42- → H+ + PO4
3- Ka = 4.8 x 10-13
Kw revisited
• Water comes to equilibrium with its ions
according to the following reaction…
H2O(l) ↔ H+(aq) + OH-
(aq)
Kw = [H+][OH-] = 1.0 x 10-14
Kw = KaKb pKa + pKb = 14
Strong Acids/Bases
Strong Acids Strong Bases
HCl LiOH
HBr NaOH
HI KOH
HNO3 RbOH
HClO4 CsOH
H2SO4 Ba(OH)2
Sr(OH)2
Strong Acids/Base description
• Strong acids and bases completely
dissociate in water, therefore no Ka or Kb
– The dissociations do not reverse.
• Oxoacids are acids that contain oxygen.
– The greater the number of oxygen atoms
attached to the central atom in an oxoacid,
the stronger the acid.
• That’s because increasing the number of oxygen
atoms that are attached to the central atom
weakens the attraction that the central atom has
for the H+ ion.
Strong Acid/Base calculations
• Since these acids and bases completely
dissociate in water, the final concentration
of H+ ions is the same as the original
concentration.
• So you can always find the pH of a strong
acid solution directly from its
concentration.
– What is the pH of .20 M HCl?
pH = -log(.20) =
Titration
• When an acid and a base are mixed, a neutralization reaction occurs.– Acid + base → salt + water
• Neutralizations reactions are generally performed by titration, where a base of known concentration is slowly added to an acid (or vice versa)
• The progress of a neutralization reaction can be shown in a titration curve.
• The equivalence point is the point in the titration when exactly enough base has been added to neutralize all the acid that was initially present.
• An indicator will be used to mark the equiv. pt.
• Notice the shape of the curve if a strong
acid is added to a strong base
– The equivalence point is also pH7
pH of dissolved salts• If a salt is composed of the conjugates of a
strong base and a strong acid, its solution will be neutral. (NaCl)
• If a salt is composed of the conjugates of a weak base and a strong acid, its solutions will be acidic. (NH4Cl)
• If a salt is composed of the conjugates of a strong base and a weak acid, its solution will be basic. (NaC2H3O2)
• If a salt is composed of the conjugates of a weak base and a weak acid, the pH of its solution will depend on the relative strengths of the conjugate acid and base of the specific ions in the salt. (NH4C2H3O2)
Anhydrides
• An acid anhydride is a substance that
combines with water to form an acid.
– Generally nonmetal oxides.
• Ex. CO2 + H2O → H2CO3
• A basic anhydride is a substance that
combines with water to form a base.
– Generally metal oxides.
• Ex. Na2O + H2O → 2NaOH