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Hoa Dai Cuong A2
Chapter 3
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Acid-Base Equilibria
Hoa Dai Cuong A2
Chapter 3
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Acids and Bases:
Acid: vị chua, làm thuốc nhuộm đổi màu
Bases: vị đắng, cảm giác nhớt.
Arrhenius: acids làm tăng [H+], bases làm tăng [OH-] trong dung dịch.
Arrhenius: acid + base  salt + water.
Problem: định nghĩa này chỉ áp dụng được trong dung dịch nước
Hoa Dai Cuong A2
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Brønsted-Lowry Acids and Bases
The H+ Ion in Water
The H+(aq) ion is simply a proton with no electrons. (H has one proton, one electron, and no neutrons.)
In water, the H+(aq) form clusters.
The simplest cluster is H3O+(aq). Larger clusters are H5O2+ and H9O4+.
Generally we use H+(aq) and H3O+(aq) interchangeably.
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Brønsted-Lowry Acids and Bases
The H+ Ion in Water
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Brønsted-Lowry Acids and Bases
Proton Transfer Reactions
Focus on the H+(aq).
Brønsted-Lowry: acid donates H+ and base accepts H+.
Brønsted-Lowry base does not need to contain OH-.
Consider HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq):
HCl donates a proton to water. Therefore, HCl is an acid.
H2O accepts a proton from HCl. Therefore, H2O is a base.
Water can behave as either an acid or a base.
Amphoteric substances can behave as acids and bases.
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Brønsted-Lowry Acids and Bases
Proton Transfer Reactions
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Brønsted-Lowry Acids and Bases
Conjugate Acid-Base Pairs
Whatever is left of the acid after the proton is donated is called its conjugate base.
Similarly, whatever remains of the base after it accepts a proton is called a conjugate acid.
Consider
After HA (acid) loses its proton it is converted into A- (base). Therefore HA and A- are conjugate acid-base pairs.
After H2O (base) gains a proton it is converted into H3O+ (acid). Therefore, H2O and H3O+ are conjugate acid-base pairs.
Conjugate acid-base pairs differ by only one proton.
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Chapter 3
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Brønsted-Lowry Acids and Bases
Relative Strengths of Acids and Bases
The stronger the acid, the weaker the conjugate base.
H+ is the strongest acid that can exist in equilibrium in aqueous solution.
OH- is the strongest base that can exist in equilibrium in aqueous solution.
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Chapter 3
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Brønsted-Lowry Acids and Bases
Relative Strengths of Acids and Bases
Any acid or base that is stronger than H+ or OH- simply reacts stoichiometrically to produce H+ and OH-.
The conjugate base of a strong acid (e.g. Cl-) has negligible acid-base properties.
Similarly, the conjugate acid of a strong base has negligible acid-base properties.
Hoa Dai Cuong A2
Chapter 3
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The Autoionization of Water
The Ion Product of Water
In pure water the following equilibrium is established

at 25 C





The above is called the autoionization of water.
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The pH Scale
In most solutions [H+(aq)] is quite small.
We define pH = -log[H+] = -log[H3O+]. Similarly pOH = -log[OH-].
In neutral water at 25 C, pH = pOH = 7.00.
In acidic solutions, [H+] > 1.0  10-7, so pH < 7.00.
In basic solutions, [H+] < 1.0  10-7, so pH > 7.00.
The higher the pH, the lower the pOH, the more basic the solution.
Most pH and pOH values fall between 0 and 14.
There are no theoretical limits on the values of pH or pOH. (e.g. pH of 2.0 M HCl is -0.301)
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Chapter 3
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The pH Scale
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The pH Scale
Other “p” Scales
In general for a number X,

For example, pKw = -log Kw.
Since
Hoa Dai Cuong A2
Chapter 3
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The pH Scale
Measuring pH
Most accurate method to measure pH is to use a pH meter.
However, certain dyes change color as pH changes. These are indicators.
Indicators are less precise than pH meters.
Many indicators do not have a sharp color change as a function of pH.
Most indicators tend to be red in more acidic solutions (two exceptions: phenolphthalein and alizarin yellow R are both red in base).
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Chapter 3
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The pH Scale
Measuring pH
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Chapter 3
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Strong Acids and Bases
Strong Acids
The strongest common acids are HCl, HBr, HI, HNO3, HClO3, HClO4, and H2SO4.
Strong acids are strong electrolytes.
All strong acids ionize completely in solution:
HNO3(aq) + H2O(l)  H3O+(aq) + NO3-(aq)
Since H+ and H3O+ are used interchangeably, we write
HNO3(aq)  H+(aq) + NO3-(aq)
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Strong Acids and Bases
Strong Acids
In solutions the strong acid is usually the only source of H+. (If the molarity of the acid is less than 10-6 M then the autoionization of water needs to be taken into account.)
Therefore, the pH of the solution is the initial molarity of the acid.
Strong Bases
Most ionic hydroxides are strong bases (e.g. NaOH, KOH, and Ca(OH)2).
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Chapter 3
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Strong Acids and Bases
Strong Bases
Strong bases are strong electrolytes and dissociate completely in solution.
The pOH (and hence pH) of a strong base is given by the initial molarity of the base. Be careful of stoichiometry.
In order for a hydroxide to be a base, it must be soluble.
Bases do not have to contain the OH- ion:
O2-(aq) + H2O(l)  2OH-(aq)
H-(aq) + H2O(l)  H2(g) + OH-(aq)
N3-(aq) + H2O(l)  NH3(aq) + 3OH-(aq)
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Weak Acids
Weak acids are only partially ionized in solution.
There is a mixture of ions and unionized acid in solution.
Therefore, weak acids are in equilibrium:



or

Ka is the acid dissociation constant.
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Weak Acids
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Weak Acids
Note [H2O] is omitted from the Ka expression. (H2O is a pure liquid.)
The larger the Ka the stronger the acid (i.e. the more ions are present at equilibrium relative to unionized molecules).
If Ka >> 1, then the acid is completely ionized and the acid is a strong acid.
Using Ka to Calculate pH
Weak acids are simply equilibrium calculations.
The pH gives the equilibrium concentration of H+.
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Weak Acids
Using Ka to Calculate pH
Using Ka, the concentration of H+ (and hence the pH) can be calculated.
Write the balanced chemical equation clearly showing the equilibrium.
Write the equilibrium expression. Find the value for Ka.
Write down the initial and equilibrium concentrations for everything except pure water. We usually assume that the change in concentration of H+ is x.
Substitute into the equilibrium constant expression and solve. Remember to turn x into pH if necessary.
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Weak Acids
Using Ka to Calculate pH
Percent ionization is another method to assess acid strength.
For the reaction



Percent ionization relates the equilibrium H+ concentration, [H+]eqm, to the initial HA concentration, [HA]0.
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Weak Acids
Using Ka to Calculate pH
The higher percent ionization, the stronger the acid.
Percent ionization of a weak acid decreases as the molarity of the solution increases.
For acetic acid, 0.05 M solution is 2.0 % ionized whereas a 0.15 M solution is 1.0 % ionized.
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Weak Acids
Using Ka to Calculate pH
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Weak Acids
Polyprotic Acids
Polyprotic acids have more than one ionizable proton.
The protons are removed in steps not all at once:



It is always easier to remove the first proton in a polyprotic acid than the second.
Therefore, Ka1 > Ka2 > Ka3 etc.
Most H+(aq) at equilibrium usually comes from the first ionization (i.e. the Ka1 equilibrium).
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Weak Acids
Polyprotic Acids
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Weak Bases
Weak bases remove protons from substances.
There is an equilibrium between the base and the resulting ions:

Example:

The base dissociation constant, Kb, is defined as



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Weak Bases
The larger Kb the stronger the base.

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Weak Bases
Types of Weak Bases
Bases generally have lone pairs or negative charges in order to attack protons.
Most neutral weak bases contain nitrogen.
Amines are related to ammonia and have one or more N-H bonds replaced with N-C bonds (e.g., CH3NH2 is methylamine).
Anions of weak acids are also weak bases. Example: OCl- is the conjugate base of HOCl (weak acid):


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Chapter 3
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Relationship Between Ka and Kb
We need to quantify the relationship between strength of acid and conjugate base.
When two reactions are added to give a third, the equilibrium constant for the third reaction is the product of the equilibrium constants for the first two:
Reaction 1 + reaction 2 = reaction 3
has
K1  K2 = K3.
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Relationship Between Ka and Kb
For a conjugate acid-base pair
Ka  Kb = Kw
Therefore, the larger the Ka, the smaller the Kb. That is, the stronger the acid, the weaker the conjugate base.
Taking negative logarithms:
pKa + pKb = pKw
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Chapter 3
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Acid-Base Properties of Salt Solutions
Nearly all salts are strong electrolytes.
Therefore, salts exist entirely of ions in solution.
Acid-base properties of salts are a consequence of the reaction of their ions in solution.
The reaction in which ions produce H+ or OH- in water is called hydrolysis.
Anions from weak acids are basic.
Anions from strong acids are neutral.
Anions with ionizable protons (e.g. HSO4-) are amphoteric.
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Acid-Base Properties of Salt Solutions
To determine whether a salt has acid-base properties we use:
Salts derived from a strong acid and strong base are neutral (e.g. NaCl, Ca(NO3)2).
Salts derived from a strong base and weak acid are basic (e.g. NaOCl, Ba(C2H3O2)2).
Salts derived from a weak base and strong base are acidic (e.g. NH4Cl, Al(NO3)3).
Salts derived from a weak acid and weak base can be either acidic or basic. Equilibrium rules apply!
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Chapter 3
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Acid-Base Behavior and Chemical Structure
Factors That Affect Acid Strength
Consider H-X. For this substance to be an acid we need:
H-X bond to be polar with H+ and X- (if X is a metal then the bond polarity is H-, X+ and the substance is a base),
the H-X bond must be weak enough to be broken,
the conjugate base, X-, must be stable.
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Acid-Base Behavior and Chemical Structure
Binary Acids
Acid strength increases across a period and down a group.
Conversely, base strength decreases across a period and down a group.
HF is a weak acid because the bond energy is high.
The electronegativity difference between C and H is so small that the C-H bond is non-polar and CH4 is neither an acid nor a base.
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Chapter 3
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Acid-Base Behavior and Chemical Structure
Binary Acids
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Acid-Base Behavior and Chemical Structure
Oxyacids
Oxyacids contain O-H bonds.
All oxyacids have the general structure Y-O-H.
The strength of the acid depends on Y and the atoms attached to Y.
If Y is a metal (low electronegativity), then the substances are bases.
If Y has intermediate electronegativity (e.g. I, EN = 2.5), the electrons are between Y and O and the substance is a weak oxyacid.
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Acid-Base Behavior and Chemical Structure
Oxyacids
If Y has a large electronegativity (e.g. Cl, EN = 3.0), the electrons are located closer to Y than O and the O-H bond is polarized to lose H+.
The number of O atoms attached to Y increase the O-H bond polarity and the strength of the acid increases (e.g. HOCl is a weaker acid than HClO2 which is weaker than HClO3 which is weaker than HClO4 which is a strong acid).
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Acid-Base Behavior and Chemical Structure
Oxyacids
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Acid-Base Behavior and Chemical Structure
Carboxylic Acids
These are organic acids which contain a COOH group (R is some carbon containing unit):
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Acid-Base Behavior and Chemical Structure
Carboxylic Acids
When the proton is removed, the negative charge is delocalized over the carboxylate anion:




The acid strength increases as the number of electronegative groups on R increases.
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Lewis Acids and Bases
Brønsted-Lowry acid is a proton donor.
Focusing on electrons: a Brønsted-Lowry acid can be considered as an electron pair acceptor.
Lewis acid: electron pair acceptor.
Lewis base: electron pair donor.
Note: Lewis acids and bases do not need to contain protons.
Therefore, the Lewis definition is the most general definition of acids and bases.
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Lewis Acids and Bases
Lewis acids generally have an incomplete octet (e.g. BF3).
Transition metal ions are generally Lewis acids.
Lewis acids must have a vacant orbital (into which the electron pairs can be donated).
Compounds with p-bonds can act as Lewis acids:
H2O(l) + CO2(g)  H2CO3(aq)
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Lewis Acids and Bases
Hydrolysis of Metal Ions
Metal ions are positively charged and attract water molecules (via the lone pairs on O).
The higher the charge, the smaller the metal ion and the stronger the M-OH2 interaction.
Hydrated metal ions act as acids:

The pH increases as the size of the ion increases (e.g. Ca2+ vs. Zn2+) and as the charge increases (Na+ vs. Ca2+ and Zn2+ vs. Al3+).