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May 24, 2015
May 24, 2015
Acids and Bases
Acids Bases
contain ... in molecule
pH of aqueous solution
general formula
dissociation in water (example)
so far:
examples
learned so far:
A substance IS an acid because it releases an H+,
or a substance IS a base because it releases OH-.= view according to Arrhenius theory of acids and bases
dissociation in water (general)
neutralization:
May 24, 2015
review: Arrhenius theory of ACIDS and BASES
• An acid is a substance that dissociates in water to release hydrogen ions (H+)
• A base is a substance that dissociates in water to
release hydroxide ions (OH-)
➧ Arrhenius acids increase the H+ concentration, so they must contain an H atom
➧ Arrhenius bases increase the OH-‐ concentration, so they must contain an OH group
➧ Neutralization according to Arrhenius:
An acid and a base react to produce a SALT and WATER; reaction takes place in water:
➧ net ionic equation:
May 24, 2015
Corrections to the Arrhenius theory:
➧ does H+ exist on its own in water?
remember: water is a polar molecule
H+
-‐-‐> has a positive and a negative pole
Hydrogen ions (H+ = protons) attach themselves
to a water molecule and form hydronium (H3O+) ions in water.
what really happens:
remember: H+ is only a proton
May 24, 2015
Corrections to the Arrhenius theory:
baking soda: acid or base?
chemical formula: NaHCO3
test: dissolve some in water and measure pH:
reactions in water:
1) dissolution: NaHCO3 Na+ + HCO3-
HCO3- + H2O
2) reaction as base:
pH > 7
May 24, 2015
Brønsted-‐Lowry theory of acids and bases
An acid is a substance in which a proton (=hydrogen ion) can be removed. An acid is a PROTON DONOR.
A base is a substance that can remove a proton from another substance. A base is a PROTON ACCEPTOR.
➧ Like Arrhenius acids, Brønsted-‐Lowry acids must contain an H atom
➧ All negative ions, not only (OH-‐), and also neutral molecules can be Brønsted-‐Lowry bases.
➧ Neutralization is the transfer of a proton from one substance to another.
➧ A substance behaves as an acid or a base rather than is an acid or a base.
May 24, 2015
Example:
NH3 + H2O NH4+ + OH-
acidbase conjugate acid
conjugate base
compound formed after an acid released a proton is called the conjugate base of that acid
compound formed after base accepted a proton is called the conjugate acid of that base
conjugate acid-‐base pair
May 24, 2015
Example 2:
HCL + H2O H3O+ + Cl-
> identify acid, base, conjugate base and conjugate acid
P.338 # 1,2
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Review
May 24, 2015
May 24, 2015
pH and pOH
The pH is a measure of the H+ / H3O+ concentration of a solution:
pH = -‐ log[H+]
Examples:
[H+] = 1.0 x 10-7 mol/L
[OH-] = 1.0 x 10-5mol/L
[H+] = 1.0 x 10-3.7mol/L
[H+] = 2.0 x 10-4 mol/L
The pOH is a measure of the HO- concentration of a solution:
pOH = -‐ log[OH-]
A solution has a pH = 4.5 -‐ what is the H+ concentration?
A solution has a pOH = 3 -‐ what is the OH- concentration?
May 24, 2015
waterhas a pH of
➧
To a very small degree water dissociates /ionizes into H+ and OH-‐ according to the following EQUILIBRIUM:
Equilibrium using hydronium ion:
Equilibrium constant
Constant called: ion product constant of water (KW)
KW = [H+]·∙[OH-‐] = 10-14
If equilibrium is disturbed by addition of H+, what will happen?
[H+]·∙[OH-‐] = 10-14
2 H2O(l) H3O+(aq) + OH-‐
(aq)
• initially
10-‐7 mol/L+H+
• reverse reaction is favoured
• new equilibrium
10-‐7 mol/L
10-‐2 mol/L ?
What is the new OH-‐ concentration if [H+] = 10-‐2?
May 24, 2015
Rule between pH and pOH:
if KW = [H+]·∙[OH-‐] = 10-14
➔ pH + pOH = 14
Examples:pH of a solution is ..., what is its pOH?
pH = 10 -‐-‐> pOH =pH = 4 -‐-‐> pOH =
pOH = 2.7 -‐-‐> pH =pOH =9 -‐-‐> pH =
pH
pOH
1 142 7 9
13 12 7 5 0pHpOH+ 14 14 14 14 14
Example calculations:
if [H+] = 2 × 10-‐4 mol/L, what is the [OH-‐] concentration?
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compare the following two acid solutions
HCl
HA A-‐ H3O+
vinegar
HA A-‐ H3O+
+ H2O +HA A-‐ H3O+
+ H2O ++
weak and strong acids
May 24, 2015
quantifying the strength of an acid
Ionization percentage:1.)
Ioniza'on percentage =[H3O+][HAtotal]
×100
[H3O+] = equilibrium concentra'on of H3O+
[HAtotal] = concentra'on of acid (ini'al concentra'on)
example:
A 0.1 M vinegar solution has a pH of 3; what is the ionization percentage of vinegar?
May 24, 2015
2.) Acidity constant:
quantifying the strength of an acid
HA(aq) + H2O(l) H3O+(aq) + A-‐
(aq)
Ka =
Ka = acidity constant
[A-‐] = equilibrium concentra'on of A-‐
[H3O+] = equilibrium concentra'on of H3O+
[HA] = concentra'on of non dissociated HA at equilibrium (ICE-‐table required!!!)
➔ the stronger the acid the larger Ka!
May 24, 2015
At 25 ºC an acetic acid solution of 1 mol/L has a pH of 2.38. What is the acidity constant?
CH3COOH(aq) + H2O H3O+(aq) + CH3COO-‐
(aq)⇌ICE table:
initial
change
equilibr.
CH3COOH H3O+ CH3COO-‐
example:
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strong acids weak acids
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strong/weak bases -‐ Basicity constant
strong bases -‐ predominantly contain OH-‐ and fully dissociate into ions in water:
B(OH)(aq) B+ + OH-‐
weak bases -‐ usually don't contain OH-‐ and attain a dissociation equilibrium in water:
B(aq) + H2O(l) ⇌ HB+ + OH-‐
Kb =
Kb = basicity constant
[HB+] = equilibrium concentra'on of HB+
(=conjugate acid)
[OH-‐] = equilibrium concentra'on of OH-‐
[B] = concentra'on of base B at equilibrium (ICE-‐table required!!!)
The weaker the base, the smaller Kb
May 24, 2015
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