Acids and Bases Chapter 7 Zumdahl 6th Ed. The nature of Water. (Sections 7.1-
7.4). Acids in Water: Define and compare with Bases and Salts. Inorganic and ...
Acids and Bases Chapter 7 Zumdahl 6th Ed. The nature of Water (Sections 7.1-7.4) Acids in Water: Define and compare with Bases and Salts Inorganic and Organic Acids The Strength of an Acid (B-L definition of acids/bases) The Base conjugate to an Acid The action of Water (solvent) as an acid and a base Comparison of a strong and a weak acid Solve: pH of an Acid. (Do the strong and weak cases together). http://www.chemguide.co.uk/physical/acideqiamenu.html#top
Previously: Acids and Bases Water sustains acids and bases Acids in Water release protons H2O HA(aq) → H + (aq) + A − (aq)
(Sections 4.4, 4.9)
Bases in Water release hydroxides. BOH(aq) ↔ B+ (aq) + OH − (aq) Net reaction is the proton and hydroxide to make water:
H + (aq ) + OH − (aq )
↔
H − OH(aq )
Limiting reactant (eg proton) leaves hydroxide left over. (The solution is basic) A- and B+ are spectator ions (that are a salt, AB). This is all fundamentally correct: Now we add more to the story.
Fundamental Process of Dissociation HA ( aq ) + H 2O ( aq ) → H + ( aq ) + A− ( aq ) A proton is released from the acid (HA). The proton is taken up by the water, as is the anion, just like a salt dissociates (ionizes) in water. Unlike a salt the undissociated form stays dissolved in the water. Water, as a solvent is 55M. +
Notation : H ≡ H
+
A strong acid (most HA dissociated)
( aq ) ≡ H 3O
+
≡H
+
A weak acid (little dissociation)
( H 2O ) n
Carbonic Acid a diprotic inorganic acid
HCl, strong inorganic acid
HA ( aq ) + H 2O ( aq ) → H + ( aq ) + A− ( aq )
An acid is a molecule that can put a proton into water. This is both the Arrhenius and Bronsted-Lowry definition.
The Carboxyl Group, the essential form of an organic acid, R is anything organic
Organic Acids
Inorganic Acids
Various Acids (HA)
An Acid in water An Acid in water (aq) will partially ionize. Rxn: HA H + + A− NIC Table: Initially no dissociation (un-ionized form stays in solution) Then some dissociation, and come to equilibrium Species HA H + A−
Initial C A ≡ [ HA]o −X Change New
CA − X
0 X
0 X
X
X
Range on X: 0 < X < C A H + A− X2 = Equilibrium Condition: KA = QA =
[ HA]
CA − X
Strong vs Weak Acids K large
A + − HA ( aq ) + H 2O ( aq ) H aq + A ( ) ( aq ) K A small
Acid
Base
Acid
Strong Acid
Base
H+ is an acid, conjugate to the base Water A- is a base, conjugate to the acid HA Conjugate just means “partnered with” or “related to” Conjugate pairs only differ by a proton.
Weak Acid
H + ( aq ) ≡ H 3O + ≡ H + ( H 2O ) n
Compared to CA
Most Dissociates
Little Dissociates
Conjugate Acid/Base Pairs In this example, acetic acid donates a proton to water, leading to the formation of hydronium ion and acetate ion. H+ donor (acid)
H+ acceptor (base)
New acid formed
Hint: Any species with a positive charge can only be an acid. Any species with a negative charge can only be a base.
New base formed
(Strong or Weak) Bases K large
B + − BOH ( aq ) + H 2O ( aq ) B aq + OH ( ) ( aq ) K B small
Base
Acid
Acid
Base
OH − ( aq ) ≡ ( H 2O ) n OH −
OH- is a base, conjugate to the acid Water B+ is an acid, conjugate to the base BOH Two definitions of a base exist. A base can generate hydroxide ions (Arrhenius definition) or it can just absorb protons (Bronsted -Lowry- Definition). Alternative way to write the action of a base: K large
B + − B ( aq ) + H 2O ( aq ) HB aq + OH ( ) ( aq ) K B small
e.g : NH 3 ( aq ) NH 4+ ( aq ) + OH − ( aq )
Conjugate Acid/Base Pairs conjugate pair
B
+
Base
H—A Acid
B—H
+ A
Acid
Base
conjugate pair
Every acid has a base conjugate to it.
Every base has an acid conjugate to it.
These species differ by H+.
These species differ by H+.
When a base goes into water, water acts as the acid. When an acid goes into water, water acts as the base.
Relation of Acid and Conjugate Base Strength
HCL, strong acid, fully dissociated.
The Cl- ion is the base conjugate to the acid HCl. It is a very poor base because HCl is such a good acid.
HCl → H + + Cl −
This rxn does not happen: Cl − + H 2O ← HCl + OH −
HCN, weak acid, not dissociated.
HCN ← H + + CN −
Conversely, the CN- ion is a pretty good base. The equilibrium for this reaction is to the right.
CN − + H 2O → HCN + OH −
How we know Acid Strength
Great site for additional Kas, and its from UW. http://depts.washington.edu/eooptic/links/acidstrength.html
K A ( HCl ) = 1.3 ⋅106
K A ( H 2 SO4 ) = 1.0 ⋅103
K A ( HNO3 ) = 24.
Stronger acid: HF or HCN? In the table which is the weakest acid:? Stronger base: F- or CN-? In table, which is the strongest base?
pH
Why a pH scale: logs are a nice way to follow the exponents without having to write exponential notation. Hydrogen ion concentration changes by over 14 orders of magnitude, and we have a theory that can follow that change!!!!
([ ])
pA = − log A Definition of p: Log stands for log-base 10. Ln stands for log-natural base. e=2.7183 Can you take the log of a negative number? Can you take the log of a number with units? Can the log of a number ever be negative? What is the log of 0? Rule of logs: log ( A ⋅ B ) = log A + log B log Am = m ⋅ log A What is the log(1), log(2), log(5), log(10), log(55) A = 1 ⋅10
−5
pA = ?
A = 2 ⋅10−5
How to invert: given pA what is A? ALEKS and Sig Figs on Logs
pA = ?
A = 10− pA
pH Scale, Measured with a pH meter.
(
pH = − log10 H +
)
H + = ⋅10−10 ⇔ pH = 10 H + = ⋅10−5 ⇔ pH = 5
H + > 10−7 Base H + < 10−7 Acid
Neutral H + = 10−7
pH < 7
pH = 7
pH > 7
What is the pH of 10M Hcl? What is the pH of 1e-8M HCl? Compare Milk and Lemon Juice qulaitatively.
Auto-ionization of Water
Equivalent forms
{
2H 2O ( aq ) H 3O + ( aq ) + OH − ( aq ) H 2O ( aq ) H + ( aq ) + OH − ( aq )
Water at equilibrium on its own will have:
QW =
aH + aOH − aH 2 O
= H + ( aq ) OH − ( aq ) = KW = 1.0 ⋅10−14
Pure Water : H + ( aq ) = OH − ( aq ) ∴
pKW = 14
H + ( aq ) = KW
But this means that water must always satisfy the equiliribum when you throw in any other acids or bases into water: Always : H + OH − = KW
Auto-ionization of Water
The reaction is endothermic (because a bond is broken), therefore the equilibrium moves to the right as the temperature is increased, more acid is generated. The KW itself increases with temperature. T − 298 ) ( 1 pH = pKW pKW (T ) = 14 − 9.5* 2 T At T=50C pH=6.6. It is still neutral water and pH=pOH, but KW increased with the temperature. Therefore it is not acidic. http://www.chemguide.co.uk/physical/acidbaseeqia/kw.html#top What fraction of water molecules ionize? Water is 55 molar and pH=7. + H f = = 18 ⋅10−10 = 2 ppb [ H 2O ]
pH and pOH Scales KW = 1.0 ⋅10−14 = H + OH −
{
}
log10 ( KW ) = log10 (1.0 ⋅10−14 ) = log10 H + OH − pKW = 14 = pH + pOH ⇐ Always pOH = 14 − pH
⇐ Always
For pure water the proton and hydroxide concentrations must be equal; charge balance, charge neutral.
H + > 10−7 Base H + < 10−7 Acid
pH < 7
pOH > 7
pH > 7
pOH < 7
Is an acid hydroxide free? What is the pOH of 10M Hcl? What is the pOH of 1e-8M HCl?
The pH of an Acid (Approximate Treatment) What is the pH of an acid with concentration CA and equilibrium constant KA? NICE Table: HA H + + A− Species HA Initial C A = [ HA]0
H+ 0
A− 0
Change New
X X
X X
−X CA − X
What is the range on X? 0 < X < C A At Equilibrium: Acid Conservation:
H + A− X2 K A = QA = = CA − X [ HA]
A− + [ HA] = X + ( C A − X ) = C A = [ HA]o
The pH of an Acid (Approximate Treatment) + − H = A The answer is X because: = X The range on X: 0 < X < CA Defines two limits: A) X~0, B) X~CA Solve the Quadratic X2 KA = CA − X
⇒
X=
K A2 + 4 K AC A − K A 2
A) Weak Acid K A < CA K A = CA X =
X = 0.62C A
K AC A
Dissociation: Little
~half K
Large
B ) Strong Acid K A > CA X = CA
Almost All
+ − HA H + A K Small
The pH of an Acid: Specific Cases A) Weak Acid K A < CA X =
K AC A
B ) Strong Acid K A = CA
K A > CA
X = 0.62C A
X = CA
Determine [H+], and pH for the following examples. 1) HCl, at 0.02M, KA=10+6 (stomach acid) 2) Acetic acid, Vinegar, 0.5M, KA=1.6e-5? (Why can you put Vinegar on salad but HCL will send you to the hospital?) 3) HCl at 1e-8 M. 4) HF 1mM = 0.001M, KA=7.4 10-4. ans: (5.6 vs 8.6)e-4 An important observation, weak dissociation: H + = X = K AC A
+ K A ≤ H ≤ C A
pH =
1 2
( pK A + pC A )
Effect of Dilution on Percent Dissociation and [H+] of a Weak Acid Increasing the concentration of an acid increases all forms, and so the [H+] goes up. But how does the equilibrium partition? One way to see how concentration effects work is to look at the fraction of the acid that is dissociated vs the fraction that is not. The two fractions must sum to one. From our observation about the conservation of acid
− [ HA] A − C A = A + [ HA] or 1 = + CA CA
f A−
A− [ HA] = and f HA = CA CA
Astoundingly, The fraction (or percent) dissociation is : • INDEPENDENT of the concentration of the acid!!! • depends ONLY on the hydrogen ion concentration. • Decreases ALWAYS with increasing hydrogen ion concentration. (LeC)
