CHE 141 1st Edition Lecture 10 Current LectureAqueous EquilibriumDefinitions of Acids and Bases- 3 ways to define an acid:o Arrhenius (specific to water) Acid: a substance which increases [H+] Base: a substance which increases [OH-]o Bronsted-Lowry (most useful/clear definition) Acid: a substance which donates a proton [H+] Base: a substance which accepts a proton [H+]o Lewis (most general definition) Acid: a substance which accepts an electron pair Base: a substance which donates an electron pairConjugate Acid-Base Pairs- A Bronsted-Lowry acid and base differing from each other only by the presence or absence of an H+ ion is known as a conjugate acid-base pair- A conjugate base is formed when a Bronsted-Lowry acid donates a proton: acid reversible conjugate base + H+- A conjugate acid is formed when a Bronsted-Lowry base accepts a proton: base + H+ reversible conjugate acidLewis Acids and Bases- For a substance to be a proton acceptor (a Bronsted-Lowry base), it must possess an unshared pair of electrons for binding the proton- Consider the reaction between H+ and NH3, drawn using Lewis structures- Gilbert Lewis defined an electron theory of acid-base reactionso A Lewis acid is an electron-pair acceptoro A Lewis base is an electron-pair donor- Every Bronsted-Lowry base is also a Lewis base- Any molecule that can accept electrons acts as a Lewis acid, thus there are more Lewis acids than Bronsted-Lowry acids- For aqueous solutions, the Bronsted Lowry (protonic) theory is more usefulThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.The H+ Ion in Water - An H+ ion is imply a proton with no surrounding valence electron- Small positively charged particle, so interacts strongly with nonbonding electron pairs in H2O- Forms hydrated hydrogen ions called hydronium H3O+ (aq) (chemists use H+(aq) and H3O+(aq) interchangeably)- Consider this equilibrium: NH3(aq)(base)+H2O(l)(acid) reversible NH4+(aq)(conjugate acid)+OH-(aq)(conjugate base)o H2O donates a proton (Bronsted-Lowry acid)o H2O acts as a Bronsted Lowry base in its reaction with HCl and a Bronsted Lowry acid in its reaction with NH3Amphoteric and Amphiprotic- For the Bronsted-Lowry theory of acid-base reactions, a substance that can behave either as a proton acceptor or a proton donor is amphiprotic- A substance that is cable of acting as either an acid or base is known as an amphoteric substance- Amphiprotic molecules are a type of amphoeteric substance-not all amphoeteric substances are amphiprotic Conjugate Acid-Base Pairs- Water is amphiprotic and can act as a Bronsted-Lowry acid or a Bronsted-Lowry base- Acid(aq)+H2O(l)(base) reversible conjugate base(aq)+H3O+(aq)conjugate acid- Base(aq)+H2O(l)(acid) reversible conjugate acid(aq) +OH-(aq) conjugate baseStrengths of Acids and Bases- Electrolyte: a substance that dissociates into ions when it dissolves- Strong electrolyte: a substance that dissolves in water- Weak electrolyte: a substance that only partly dissociates into ions when it dissolves in water- Some acids are better proton donors than others- Strong acids: are strong electrolytes (dissociates completely into ions)- Weak acids: are weak electrolytes (partially dissociates into ions)- Some bases are better proton acceptors than others- Strong bases are strong electrolytes- Weak bases are weak electrolytes- Nitric acid is a strong acid as it ionizes completely in water- Nitrous acid is a weak acid which only partially ionizes in water- Ka is called the acid dissociation constanto The magnitude of Ka indicates the tendency of the hydrogen atom to ionize- Kb is called the base protonation constant (the magnitude of Kb indicates the tendency ofthe base to accept H+)Strengths of Acids- Characteristics of a strong acid:o Completely ionizes in water (strong electrolyte)o Even though the reaction is reversible, the equilibrium lies vary far to the right Ka>>1 thus the reaction essentially goes to completion- Characteristics of a weak acid:o Partially dissociates in aqueous solution(weak electrolyte)o Weak acids have Ka<1Strengths of Bases- Characteristics of a strong base:o Completely ionizes in water (strong electrolyte)o Even though the reaction is reversible, the equilibrium lies vary to the right, Kb>>1, thus the reaction essentially goes to completion- Characteristics of a weak base:o Partially dissociate in aqueous solution (weak electrolyte)o Weak bases have Kb<1Base Protonation Constant Kb- Base(aq)+H2O(l)(acid) reversible conjugate acid (aq)+OH-(aq)(conjugate base)- Generic equilibrium using B to represent a weak base: B(aq)+H2O(l) reversible HB+(aq)+OH-(aq) or BOH(aq) reversible B+(aq) +OH-(aq)- Heterogeneous equilibria o We do not include [H2O] in the equilibrium constant expression: Kb=[HB+][OH-]/[B] or Kb=[B+][OH-]/[BOH]Strengths of Acids and Bases- The conjugate of a strong acid is a weak base- If HCl is a good proton donor then Cl- is a poor proton acceptor- The conjugate of a strong base is a weak acid- If O2- is a good proton acceptor then OH- is a poor proton
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