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Acids - Bases. Arrhenius : Acid …. A substance that increases the hydrogen ion, H + , concentration when dissolved in H 2 O. Eg. HCl, H 2 SO 4 , HC 2 H 3 O 2 (CH 3 COOH), etc. Base…. A substance that increases hydroxide ion, OH - , concentration. Eg. NaOH, NH 4 OH, etc. - PowerPoint PPT Presentation
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Acids-BasesArrhenius:
Acid…. A substance that increases the hydrogen ion, H+, concentration when dissolved in H2O. Eg. HCl, H2SO4, HC2H3O2 (CH3COOH), etc.
Base…. A substance that increases hydroxide ion, OH-, concentration.
Eg. NaOH, NH4OH, etc.
Bronsted-Lowry (1923) (not worried about increasing/decreasing H+ or OH- concentrations)
Acid … a “proton donor”, p+ = H+, any formula HA.
Eg. HCl, H2SO4, HC2H3O2 (CH3COOH), etc.
Base …. A “proton acceptor” Eg. OH-, NH3 ; Cl-
Note: HOH can do both!
Really no different from Arrhenius.
This is different from Arrhenius!
Acid solutions:
HCl(g) + H2O(l) H3O+(aq) + Cl-
(aq)
H3O+(aq) = H+
(aq)
H :Cl:....
+
:O:..
..H
H ..
..H
H
:O:
H ++ :Cl:
..
..-
Hydrogen ion, H+, or hydronium ion, H3O+ has formed
HCl(g) + H2O(l) H+(aq) + Cl-
(aq)
Bronsted and Lowry would say the HCl donated a p+ (H+ion), and the water accepted a p+ (H+ion). Either way the HCl is an acid. The water is a base ….it was a proton acceptor!
Coordinate covalent bond
Arrhenius would say the concentration of H+ has increased.
Strength of an acid can be determined by conductivity.
12 M HCl
Good or poor electrolyte? Large or small # of ions?
good Large
HCl(g) + H2O(l) H3O+(aq) + Cl-
(aq) Initially: 100 0 0
@equil: 0 100 100
Ka = [H3O+] [Cl-]
[HCl]= Large value Strong Acid
Acids with Ka values greater then “1” are considered strong acids. They 100% ionize.
Note: 100% pure HCl is a poor conductor. Why? No ions!
Strength of an acid can be determined by conductivity.
Good or poor electrolyte? Large or small # of ions?Pure HC2H3O2 no conduction none
Diluted HC2H3O2 poor small
HC2H3O2 (g) + H2O(l) H3O+(aq) + C2H3O2
-(aq)
Initially: 100 0 0
@equil: 97 3 3
< 5% ionization….. Very weak acid
Weak acid
Ka = [H3O+][C2H3O2
-]
[HC2H3O2]= a Very small # = 1.76 X 10-5
Properties of Acids:
1. Conduct electricity if they ionize.
2. React with metals to form H2
3. Neutralize bases.
4. Turn litmus red
5. Taste sour
More Bronsted-Lowry Acid/Base info:General acid/base reaction:
HA + B = A- + HB+
acid1 base2 base1 acid2
conjugateAcid/base
pair
HCl(aq) + NH3(aq) = NH4+
(aq) + Cl-(aq)
acid1 base2 acid2 base1
Which direction is favored?
Which is a stronger acid? HCl…. Ka is larger
At equilibrium there is more ________________ product
Titration problems
What is the volume of a 0.325 M NaOH solution needed to just neutralize 65.2 ml of 1.37 M HNO3?Balanced equation:
1 NaOH + 1 HNO3 = 1 HOH + 1NaNO3
65.2ml x ml
L
1000
1 x L
molHNO
1
37.1 3 x31
1
molHNO
molNaOHx
molNaOH
L
325.0
1 x
L
ml
1
1000= 274.8 = 275ml NaOH
Or using: MaVa#H’s = MbVb
#OH’s
(1.37M)(65.2ml)(1) = (0.325M)(Vb)(1)
Vb = 275ml
Since mole ratio of H’s and OH’s is 1:1,
Basic Solutions:Strong Arrhenius bases:
1. Group IA hydroxides. Eg. NaOH, KOH
2. Lower Group IIA hydroxides. Eg. Sr(OH)2, Ba(OH)2
100% “Dissociate”: NaOH(S) Na+(aq) + OH-
(aq)
1.0 M Ba(OH)2 = 1.0mol Ba+2 + 2.0mol OH-
The Kb for these bases would be very large; > than 1.
They would also be good conductors of electricity.
3. Acid anions of weak acids: eg. C2H3O2-, from HC2H3O2
or OH-, from HOH
Strong Bronsted-Lowry bases:
More Bronsted-Lowry Acid/Base info:Bases react with water to produce OH-
General equation: B(g) + H2O(l) = BH+(aq) + OH-
(aq)
NH3(g) + H2O(l) NH4+
(aq) + OH-(aq)
Base1 acid2 acid1 base2
Kb =
[NH4+][OH-]
[NH3]= 1.79 x 10-5
Which direction is favored in the reaction?
Hydrolysis
Salt + H2O Acidic solution
Neutral solution
Basic solution
In water: H2O + H2O H3O+ + OH-
Kw = 1 x 10-14 Equal amounts of ions, but very little!
Because the [H3O+] = [OH-]…. Water is neutral
If one of the ions from a salt removes one of the ions from the water….the other ion from the water will be in excess!
NaClO + H2O :
NaClO is a salt of a base NaOH and an acid HClO
NaOH…strong base, OH- ion still present in solution
HClO…. Weak acid,
Solution will be ___________basic
Na+ + OH- vs H+ + ClO-
H+ ion removed from solution
Hydrolysis (continue)
Lewis Acid/Base
Lewis acid: electron pair acceptor
Lewis base: electron pair donor
H+ + :N:....
H
HH
Lewis
acid Lewisbase
..
..H
H
H
H : :N
+
“adduct” … the product of a L.acid/base rxn
Acid-Base EquilibriaReview:
In pure water & all aqueous solutions:
H2O + H2O = H3O+ + OH-
Kw = [H3O+][OH-] = 1 x 10-14
[H3O+] = [OH-] = 1 x 10-714101
x or =
[H3O+] > [OH-] = Acidic solution
[H3O+] < [OH-] = Basic solution
[H3O+] = [OH-] = Neutral solution
Solutions of Strong Acids/ Bases:0.10 M HNO3 actually consists of 0.10 M H3O+ or [H+] = 0.10 M
0.10 M NaOH…. Adds 0.10 OH- to the water solutionIonization & dissociation are 100% for strong acids & bases
HNO3 + H2O H3O+ + NO3-
NaOH + H2O Na+ (aq) + OH-
(aq)In 0.10M Ba(OH-)2 [OH-] =
GONEGONE
In 0.20 M H2SO4 [H+] =
0.20M
0.40 M
Solutions of Weak acids/bases0.10 M HC2H3O2 : a weak acid - when it reacts with
water it does not ionize 100%. What is the [H3O
+] for this solution? Ka = 1.8 x 10-5
Equation for the ionization of HC2H3O2 in water:
HC2H3O2 + H2O = H3O+ + C2H3O2-
Write the MAE, equilibrium expression of this reaction:
Ka = [H3O+] [C2H3O2
-]
[HC2H3O2]= 1.8 x 10-5
Ka = [H3O+] [C2H3O2
-]
[HC2H3O2]= 1.8 x 10-5
[H3O+] = [C2H3O2-] =
[HC2H3O2 ] =
x
x
10.
2= 1.8 x 10-5
ignore
“rule of thumb”… if %ionization < 5%, the value for x is so small it can be ignored. (otherwise you would need to use quadratic equation to solve).
x
0.10 - x
Weak acids (continue)
% ionization < 5% when: [HA] / Ka > 100
0.10 1.8 x 10-5 = 5,556 !
x
x
10.
2= 1.8 x 10-5
So….
x = 1.3 x 10-3 = [H3O+] Also = [C2H3O2- ]
% ionization = 1.3 x 10-3
.10x 100 = 1.3%
< 5% as predicted
Weak acids ionize very little….
there is very little H3O+ present.
Weak acids (continue)
Weak bases….
B + H2O = BH+ + OH-
NH3 + H2O = NH4+ + OH-
Given: 0.35M NH3 solution; Kb = 1.8 x 10-5
What is the concentration of OH- ?
xx35.
2
=[NH4+][OH]
[NH3]=
ignore…<5%
1.8 x 10-5
x = 2.5 x 10-3 = [OH-]
If the [OH-] = 2.5 x 10 -3 , what is the [H3O+]?
Given: [H3O+][OH-] = 1 x 10-14
[H3O+][2.5 x 10 -3] = 1 x 10 -14
[H3O+] = 4.0 x 10-12
Are we saying that this solution has both H3O+ & OH - in it? Yes
All acid or base solutions have them….Why? They have H2O !
Why is this solution Basic? [OH-] > [H3O+]
Weak bases (continue)
pHA simple scale for ranking the H3O+ concentrations of dilute acid/base solutions. (Sorenson).
pH = - Log [H3O+ ]
The logarithm of a number is that number expressed as an exponent of the base 10.
For example, the logarithm of 1 is 0, 1 x 100.
If [H3O+] = 0.001 =
pH = -log 0.001 =If this is 1
This is the pH1x10-3
3
pH Scale
neutral
pH 7
[H3O+] 1x10-7
[OH-] 1x10-7
pOH 7
Acidity increases
10-6
6
10-8
8
0 1 2 3 4 5
100 10-1 10-2 10-310-410-5
10-12
12
[H3O+][OH-]= 1x10-14
pH = - log[H3O]
pH + pOH = 14
Formulas to remember
1M HA
8
10-8
10-6
6
Basicity increases
9 10 11 12 13 14
10-9 10-10 10-11 10-12 10-13 10-14
10-3
3
1M BOH
100
[H3O+] = 3.25 x 10- 4
Given a 3.25 x 10-4 M HNO3 , solve for:
a. [H3O+]
b. pH =
c. [OH-] =
HNO3 is a strong acid. Thus, 3.25 x 10-4M HNO3 =
-log [3.25 x 10-4]
prediction
= -(-3.488) = 3.49
1 x 10-14
3.25 x 10-4= 3.08 x 10-11
d. pOH = -log [OH-] = -log [3.08 x 10-11] = 10.5
Or pOH = 14.00 –3.49 = 10.51
More About the pH Scale
What is the pH of a 0.10 M HC2H3O2 solution?
Ka = [H3O+] [C2H3O2
-]
[HC2H3O2 ]= 1.8 x 10-5
[H3O+] = [C2H3O2
-]
[HC2H3O2 ] =
x
x
10.
2
= 1.8 x 10-5
ignore
= x
0.10 - x
x = 1.3 x 10-3 = [H3O+]
pH = - Log [1.3 x 10-3] = 2.9
Sample Problem