Chapter 2Acids & Bases

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Arrhenius acids and bases

Bronstead-Lowry acids and bases

Acids and Bases

Acid-base systems:

2.1 Arrhenius acids and bases

• An Arhennius acid yields a proton in solution.• An Arhennius base yields a hydroxide ion in

solution.

I. Definitions Acids & Bases

A. Arrhenius Acids and BasesH+ + H2O H3O+ Hydronium Ion

Note: 1) I will use H+ in place of H3O+; 2) [ ] = m/L = M

Acid: A substance which increases [H+] when dissolved in water.Base: A substance which increases [OH-] when dissolved in water.

-The above concepts were introduced by Svante Arrhenius in his doctoral thesis in 1884 in Sweden. His thesis was assumed to be wrong, and he was given the lowest possible passing grade; awarded the Nobel Prize in 1903for his work!

Arrhenius Acids

- Strong acids completely ionize in water to release H+ (H3O+) and an anion; know the names & formulas of the strong & weak acids & bases.

Strong Acids: HClO4 H2SO4 HI HBr HCl HNO3

HI + H2O H3O+ + I- or HI H+ + I- (100%)

Notes:

1) Caution: Strong acids react with proteins, carbohydrates and fats.

2) Caution: HClO4 and HNO3 can react explosively with organics.

3) Only the first ionization is 100% with H2SO4

4) Important: Know the Strong & Weak acids/Bases and their Names.

Arrhenius Acids

- Weak acids only partially ionize in water; < 5% ionization.

Weak Acids: HC2H3O2 HF H3PO4 HCN H2S HNO2 NH4+

HC2H3O2 H+ + C2H3O2- (~ 1% ionized)

Note: - only the H attached to the O in acetic acid is released; H’s attached to C are NOT acidic; most organic acids are weak acids.

- Know the names & formulas of the above weak acids.

C CHH

HO

OH

Arrhenius BasesStrong bases ionize 100 % in water and weak bases only partially ionize.

Examples of Strong Bases: LiOH NaOH KOH Ca(OH)2 Sr(OH)2 Ba(OH)2 Na3PO4

Examples of Weak Bases: NH3 CO3-2 HCO3

- CN- C2H3O2- F-

NaOH Na+ + OH- (strong, 100 % Ionized)

NH3 + H2O NH4+ + OH- (weak, ≤1 % Ionized)

F- + H2O HF + OH- (weak, ≤1 % Ionized)

Notes: 1) Salts of weak acids are generally weak bases

2) Bases are much more damaging to proteins and triglyceride fats than acids. Eye damage can occur on contact; even with dilute bases.

3) Important; know the strong & weak bases/acids & their names.

Arrhenius Acids & Bases- H+ and OH- are in dynamic equilibria in water; changing the M of one will change the M of the other. - The following equilibria always takes place in water regardless of any other chemical present.

H2O H+ + OH-

- [H+] & [OH-] = 1.00x10-7 M for pure water

- Notes: 1) Le Chatelier’s Principle predicts effect of adding additional H+ or OH- to H2O.

2) Acid + Base react completely to yield a salt plus water.Example: 2HCl + Ca(OH)2 ----) CaCl2 + 2H2O ; NIE: H+ + OH- ----) H2O

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2.2 Acids and Bases: The Brønsted–Lowry Definition

• The terms “acid” and “base” can have different meanings in different contexts

• For that reason, we specify the usage with more complete terminology

• The idea that acids are solutions containing a lot of “H+” and bases are solutions containing a lot of “OH-” is not very useful in organic chemistry

• Instead, Brønsted–Lowry theory defines acids and bases by their role in reactions that transfer protons (H+) between donors and acceptors

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Brønsted Acids and Bases

• “Brønsted-Lowry” is usually shortened to “Brønsted”

• A Brønsted acid is a substance that donates a hydrogen ion (H+)

• A Brønsted base is a substance that accepts the H+

– “proton” is a synonym for H+ - loss of an electron from H leaving the bare nucleus—a proton

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Brønsted-Lowry Acids and Bases:• Proton donors and acceptors.• H+ or H3O+ = a proton

Acids and Bases

Figure 2.1 Examples of Brønsted - Lowry acids and bases

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The Reaction of Acid with Base

• Hydronium ion, product when base H2O gains a proton• HCl donates a proton to water molecule, yielding hydronium ion

(H3O+) [conjugate acid] and Cl [conjugate base]• The reverse is also a Brønsted acid–base reaction of the conjugate

acid and conjugate base

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Factors that Determine Acid Strength:

• No matter which of these factors is discussed, to compare the acidity of any two acids:o Always look at the conjugate bases.o Determine which conjugate base is more stable.o The more stable the conjugate base, the more acidic the acid.

• The strengths of a conjugate acid and its conjugate base are inversely related.• A strong conjugate base has a weak conjugate acid.• A weak conjugate base has a strong conjugate acid.

Acids and Bases

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Acid and Base Strength

• The equilibrium constant (Keq) for the reaction of an acid (HA) with water to form hydronium ion and the conjugate base (A-) is a measure related to the strength of the acid

• Stronger acids have larger Keq

• Note that brackets [ ] indicate concentration, moles per liter, M.

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Acid Strength and pKa :

Because the concentration of the solvent H2O is essentially constant, the equation can be rearranged and a new equilibrium constant, called the acidity constant, Ka, can be defined.

It is generally more convenient when describing acid strength to use “pKa” values than Ka values.

Acids and Bases

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Position of the equilibrium in Acid-Base

• In an acid-base reaction, the position of equilibrium always favors reaction of the stronger acid and stronger base to form the weaker acid and weaker base.

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Factors that Determine Acid Strength:Element Effects—Trends in the Periodic Table.

Across a row of the periodic table, the acidity of H—A increases as the electronegativity of A increases.

Positive or negative charge is stabilized when it is spread over a larger volume.

Acids and Bases

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Factors that Determine Acid Strength: Resonance Effects

• Resonance is a second factor that influences acidity.

• In the example below, when we compare the acidities of ethanol and acetic acid, we note that the latter is more acidic than the former.

Acids and Bases

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Factors that Determine Acid Strength: Resonance Effects

• When the conjugate bases of the two species are compared, it is evident that the conjugate base of acetic acid enjoys resonance stabilization, whereas that of ethanol does not.

Acids and Bases

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• Resonance delocalization makes CH3COO¯ more stable than CH3CH2O¯, so CH3COOH is a stronger acid than CH3CH2OH.

• The acidity of H—A increases when the conjugate base A:¯ is resonance stabilized.

Acids and Bases

Factors that Determine Acid Strength: Resonance Effects

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Factors that Determine Acid Strength: Inductive Effects

• An inductive effect is the pull of electron density through bonds caused by electronegativity differences between atoms.

• Inductive effect is the polarization of electron density transmitted through covalent bonds by a nearby atom of higher electronegativity.

• In the example below, when we compare the acidities of ethanol and 2,2,2-trifluoroethanol, we note that the latter is more acidic than the former.

Acids and Bases

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• The reason for the increased acidity of 2,2,2-trifluoroethanol is that the three electronegative fluorine atoms stabilize the negatively charged conjugate base.

• This effect is limited to a three bond distance.

Factors that Determine Acid Strength: Inductive Effects

Acids and Bases

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Factors that Determine Acid Strength:Element Effects—Trends in the Periodic Table.

• Down a column of the periodic table, the acidity of H—A increases as the size of A increases.

• Size determines acidity down a column.• The acidity of H—A increases both left-to-right across a row and

down a column of the periodic table.• Although four factors determine the overall acidity of a particular

hydrogen atom, element effects—the identity of A—is the single most important factor in determining the acidity of the H—A bond.

Acids and Bases