Aqueous Equilibria: Acids & Bases McMurry and Fay ch. 14

Acid-base definition (Arrhenius) Acid: increases H+ ions in solution Base: increases OH- ions in solution

Arrhenius Acids & Bases Acids: HCl  H+ + Cl- H2SO4  2 H+ + SO42- Bases: KOH  K+ + OH- NaOH  Na+ + OH-

Ammonia as a base Ammonia (NH3) neutralizes acids. How can it make OH- in solution?

An important equilibrium constant H2O H+ + OH- (self-ionization of water) [H+][OH-] = Kw

Ammonia as a base NH3 + H2O  NH3 + H+ + OH-  NH4+ + OH-

Acid-base definition (Brønsted-Lowry) Acid: proton donor Base: proton acceptor (the word proton refers to H+)

Brønsted-Lowry acid-base reactions are reversible NH3 + H2O NH4+ + OH- accepts proton BASE donates proton ACID donates proton ACID accepts proton BASE

Brønsted-Lowry acid-base reactions are reversible NH3 + H2O NH4+ + OH- CONJUGATE ACID CONJUGATE BASE accepts proton BASE donates proton ACID donates proton ACID accepts proton BASE

Conjugate acid-base pairs Two substances that differ by only one proton The acid has one more proton than the base The base has one less proton than the acid

In the following reaction, which is the conjugate base of CH3COOH In the following reaction, which is the conjugate base of CH3COOH? CH3COOH + H2O CH3COO- + H3O+ CH3COOH H2O CH3COO- H3O+

Hydronium ion H+ is not very stable by itself Charge on proton in solution is stabilized by IMFs H+ ion is frequently written as H3O+ H+ O H In this class we will use H+ and H3O+ interchangeably

Acid strength: old version The stronger the acid, the more H+ it generates Acid strength: old version HCl (aq) H+ (aq) + Cl- (aq) (strong acid because no HCl remains; all turns into H+) CH3COOH (aq) CH3COO-(aq) + H+ (aq) (weak acid because little H+ is made; most remains as CH3COOH)

Acid strength: Equilibrium Version Strength depends on equilibrium positions HCl + H2O H3O+ + Cl- CH3COOH + H2O CH3COO- + H3O+ In strong acids, the conjugate base form is favored at equilibrium In weak acids, the acid form is favored at equilibrium

Characteristics of a Strong Acid Weak bond (easy to break) between H & anion is weak Anion is stable in solution

Acid strength: Binary acids in the same group of the periodic table As valence shell increases, bond strength decreases Easier to break bonds between H & larger atoms H F Cl H Acid strength increases Br H I H

Acid strength: Binary acids in the same row of the periodic table - H H H H N N - As you move to the right, electronegativity increases More electronegative atoms are more happy as ions More electronegative elements are stronger acids H H H H H H H H O - O - F H F

Acid strength: Oxoacids Oxoacids are acids composed of hydrogen, oxygen, and some other element (examples: H2SO4, H3PO4, HNO3) General rule: as central atom electronegativity increases, strength of the O-H bond decreases  Higher electronegativity = stronger acid strength

Acid strength: Oxoacids For a given element, there can be oxoacids with different numbers of oxygen atoms bound to the central atom H2SO4 H2SO3 O O O S O O S O O

Acid strength: Oxoacids More resonance structures = more stable conjugate base General rule: The larger the number of lone oxygen atoms, the stronger the acid

Lewis Acids and Bases Explain acid-base chemistry that occurs outside of aqueous solutions Lewis acid: electron pair acceptor Lewis base: electron pair donor

Lewis Base examples: H N H H O H H .. .. ..

Lewis acid-base example NH3 + BF3