Title: Chapter 16 : Acid-Base Equilibira
1Chapter 16 Acid-Base Equilibira
2Table of Contents
- 16.1 Review
- 16.2 Brønsted-Lowry Acids and Bases
- 16.3 Autoionization of Water
- 16.4 pH Scale
- 16.5 Strong Acids and Bases
- 16.6 Weak Acids
- 16.7 Weak Bases
- 16.8 Relationship Between Ka and Kb
- 16.9 Acid-Base Properties of Salt Solutions
- 16.10 Acid-Base Behavior and Chemical
Structure - 16.11 Lewis Acids and
- Bases
316.1 Review
- Acids
- Sour in taste
- Litmus paper turns red
- Bases
- Bitter, slippery
- Litmus paper turns blue
- When acids and bases mix, their properties
disappear!
4Arrhenius Acids and Bases
- Svante Arrhenius (1880)
- In aqueous solutions
- Acids will increase the concentration of H ions
when dissolved in water. - Bases will increase the concentration of OH-ions
when dissolved in water.
516.2 Brønsted-Lowry Acids and Bases
- 1923 Brønsted and Lowry made a more general
definition - Brønsted-Lowry Acid is a substance that can
transfer a proton. It must have a hydrogen atom
that can be lost as H. - Brønsted-Lowry Base is asubstance that can accept
a proton. Must have a nonbonding pair of
electrons to gain a H ion.
6Conjugate Acid-Base Pairs
- Conjugate base- Removal of proton from the acid
- Conjugate acid- Addition of proton to the base
7Relative Strengths of Acids and Bases
- The stronger the acid, the weaker its conjugate
base. - The stronger the base, the weaker its conjugate
acid. - 1. Strong acids completely transfer protons to
water. - 2. Weak acids partly dissociate in aqueous
solutions and exist as a mixture of acid
molecules and component ions. - 3. Negligible acidity contain Hydrogen but do
not demonstrate acidic behavior. Ex CH4 - Position of equilibrium favors transfer of proton
from stronger acid to stronger base.
816.3 Autoionization of Water
- Ion product of water
-
- 1.0 x 10 14 H OH-
- This is used to calculate concentrations of H
and OH- . - If H OH-, than neutral equation
- If H gt OH-, than acidic equation
- If H lt OH-, than basic equation
916.4 The pH Scale
- pH -log H
- pH of 7 is neutral
- Acidic solution 0 lt pH lt 7
- Basic solution 14 gt pH gt 7
- Other p scales are
- pOH -log OH-
- pOH pH -log Kw 14.0
10Examples on the pH Scale
11Measuring pH
- A pH meter consists of a pair of electrodes
connected to a meter which pH is generated when
placed in the solution. - An acid-base indicator turns a color if placed
in acid or base. Ex litmus paper
1216.5 Strong Acids and Bases
- Strong Acids
- 7 most common strong acids are
- HCl, HBr, HI, HNO3, HClO3, HClO4, and H2SO4
- In acidic reactions, equilibrium lies entirely to
the right side. - Completely dissociates
- Example
- HNO3 gt H NO3-
13Strong Bases
- Most common strong bases are ionic hydroxides of
alkali metals (1A) and heavier alkaline earth
metals (2A). Examples LiOH, RbOH, CsOH, NaOH,
KOH, and Ca(OH)2, Sr(OH)2, and Ba(OH)2. - Other strong bases react with water to form OH-
such as Na2O, CaO. - Also, anions O2-, H-, and N3- are stronger bases
than OH- and therefore remove a proton from H2O. - Example N3- H2O gt NH3 3OH-
1416.6 Weak Acids
- A weak acid only partially ionizes in aqueous
solutions. - General weak acid equation
- HX ? H X- where H is Hydrogen
- Many weak acids contain some Hydrogen atoms
bonded to carbon atoms and oxygen atoms (organic
compounds). - Ka is the acid dissociation constant.
- The larger the value of Ka , the stronger the
acid.
15Calculating Ka from pH
- Use and ICE box!
- Sample exercise
- A student prepared a .10 M solution of formic
acid and measures its pH which was 2.38. - A) calculate Ka for formic acid
- B) what percentage of the acid is ionized in the
.10M solution?
16Answer
- a) HCHO2 ? H CHO2-
- Ka HCHO2- HCHO2
- pH -logH
- 10 2.38 4.2 X 10-3M
- Ka 4.2 X 103 4.2 X 103 .10
- 1.8 X 10-4 4.2 X 103 4.2 X 103
.10 - b) Percent Ionization
- Concentration of H
- Initial concentration of component
- 4.2
HCHO2 ? H CHO-
I .10 M 0 M 0 M
C -4.2 X 103 4.2 X 103 4.2 X 103
E .10 - 4.2 X 103 4.2 X 103 4.2 X 103
17Using Ka to Calculate pH
- The best way to explain this is by an example.
- Calculate the pH of a .30 M solution of acetic
acid at 25o C. (Ka 1.8 X 10-5) - So HC2H3O2 ? H C2H3O2-
- Ka HC2H3O2- 1.8 X 10-5
- HC2H3O2
- What now?
18 HC2H3O2 ? H C2H3O2-
I .30 M 0 M 0 M
C -x x x
E .30-x x x
Ka (x)(x) 1.8 X 10-5 (.30
x) Either do the quadratic equation or in this
case you can take out x in the denominator.
H x 2.3 X 10-3 pH -log 2.3 X 10-3
2.64
19Polyprotic Acids
- Polyprotic acids have more than one ionizable
Hydrogen atom. - Example
- H2SO3 ? H HSO4-
- HSO4- ? H SO32-
- The second Ka (Ka2) is much smaller than Ka1
because it is easier to remove the first proton.
2016.7 Weak Bases
- Weak base water gt conjugate acid hydroxide
ion - Kb is the base-dissociation constant (equilibrium
in which base reacts when H2O to form conjugate
acid and OH- ion). - Types of weak bases
- Neutral substances that have atoms with a
non-bonding pair of electrons that can serve as a
proton acceptor. - Most of these contain amines, N-H which is
sometimes replaced with a bond between C or N Ex
NH2CH3 - Anions of weak acids
- Ex ClO- H2O ? HClO H
- ClO- is the weak base
2116.8 Relationship Between Ka and Kb
- Reaction 1 reaction 2 reaction 3
- Which leads to K1 x K2 K3
- Which leads to Ka x Kb Kw
- Kw is the ion-product constant for water
- Kw 1 x 10-14
- As the strength of the acid increases, the
strength of the base decreases and visa-versa. - pKa pKb pKw 14.00
2216.9 Acid-Base Properties of Salt Solutions
- Hydrolysis is the process at which ions react
with water and produce H or OH- - X- H2O ?HX OH-
- Anions of strong acids do not influence pH
- Ex NO3-
- Anions that still have ionizable protons are
amphoteric - Ex HSO3- from H2SO4
- Most cations (except 1A elements and Ca2, Sr2.
Ba2) act as weak acids in solution.
23Predicting the pH of a Solution
- 1. Salts derived from a strong acid and a strong
base makes a neutral pH (pH of 7). - NaOH HCl gt NaCl H2O
- 2. Salts derived from a strong base and a weak
acid will yield a pH of above 7 because the anion
hydrolyzes to produce OH- ions and the cation
does not hydrolyze. - NaOH HClO gt NaClO H2
- 3. Salts derived from a weak base and a strong
acid will result in a pH that is below 7 because
the cation hydrolyzes to produce H ions and the
anion does not hydrolyze. - Al(OH)3 3HNO3 gt Al(NO3)3 3H2O
24 - 4. Salts derived from a weak base and a weak acid
will yield a pH that is dependant on the constant
value of the constant dissociations (Ka and Kb). - if the base is more basic than the acid is
acidic, then the solution will have a pH that is
greater than 7. - If the acid is more acidic, than the pH will be
less than 7. - NH4 CN- ? NH4CN
- NH4 Ka 5.6 X 10-10
- CN- Kb 2.0 X 10-5
- Therefore, the pH of NH4CN is greater than 7
2516.10 Acid-Base Behavior and Chemical Structure
- Factors that effect acid strength
- If H-X bond is polarized (X is more
electronegative) the H acts as a proton acceptor. - Non-polar bonds (CH4) produce neutral solutions.
- Weaker bonds dissociate more easily than very
strong bonds. - HF is a weak acid because of this.
- The greater the stability of the conjugate base,
the weaker the acid. - Ultimately, there are three factors effecting
acid strength - Polarity of H-X bond
- Strength of H-X bond
- Stability of conjugate base, X-
26Binary Acids
- Binary acids are composed of Hydrogen and a
non-metal. - Ex HCl, HF, H2S, etc.
- The more polar the bond,the stronger it is
- The weaker the bond, the stronger the acid.
- Strength of the bond decreases (acidity
increases) as the element increases in size or
moves down a group. - Acid strength increases (acidity decreases)
moving from left to right
27 Group Group Group Group
4A 5A 6A 7A
Period 2 CH4 No acid or base properties NH3 Weak base H2O ------- HF Weak acid
Period 3 SiH4 No acid or base properties PH3 Weak base H2S Weak acid HCl Strong acid
Increasing base strength
Increasing acid strength
Increasing acid strength
Increasing base strength
28Oxyacids
- Oxyacids are acids with an OH group is bound to a
central atom. - Example H2SO4
-
29OH- Bonding
- To determine if an OH group acts as an acid or
base, consider this - If Y is a metal than sources of OH- behave as
bases. - If Y is a non-metal than the compound will not
readily lose the OH- ion. - The electronegativity will increase and so will
the acidity. - The increasing number of Oxygen atoms stabilizes
the conjugate base and thus increases the
strength of the acid.
30Oxyacid Rules of Thumb
- Oxyacids that have the same number of OH groups
and the same number of Oxygen atoms, acid
strength increases with increasing
elecronegativity of the central atom - Example HClO gt HBrO gt HIO (gt more acidic)
- 2. For oxyacids with the same central atom,
acid strength increases with increasing number of
Oxygen atoms that are attached. - Example HClO lt HClO2 lt HClO3 lt HClO4 ( lt less
acidic)
31Carboxylic Acid
- Carboxylic acids are organic compounds.
- -COOH is the
functional group - -R is either a
Hydrogen - or Carbon based group
- If an extra Oxygen is added than it stabilizes
the conjugate base and increases the acidity. - If conjugate base has resonance structures, it
spreads the negative charge evenly over the
compound. - Acid strength of carboxylic acid increases as the
number of electronegative atoms increase.
326.11 Lewis Acids and Bases
- G.N. Lewis proposed this
- Lewis Acids have an incomplete
- octet of electrons. Function as
- electron pair acceptors
- Lewis Bases act as electron pair
- donators
33Hydrolysis of Metal Ions
- Hydration is a process when when metals attract
unshared electron pairs of water molecules. - The metal acts as Lewis acid
- The water acts as Lewis base
- Ex Fe(H2O)63 ? Fe(H2O)5(OH)2 H
- So, general equation
- M(H2O)nc ? M(H2O)n-1(OH)c-1 H
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