Chapter 10: Acids and Bases

Chapter 10: Acids and Bases • When we mix aqueous solutions of ionic salts, we are not mixing single components, but rather a mixture of the ions in the solid • The ionic solid dissolves in the water • We call a compound that dissolves in water soluble and if it doesn’t, it is insoluble

Electrolytes • When an ionic compound dissolves in water, it forms an electrolyte solution • The compound may be a strong electrolyte if it dissolves completely or a weak electrolyte if it only partially dissolves (doesn’t exist entirely as ions in solution)

Precipitation Reactions • A precipitation reaction takes place when solutions of 2 strong electrolyte solutions are mixed and react to form an insoluble solid

Complete and Net Ionic Equations AgNO3(aq) + NaCl (aq) --> AgCl (s) + NaNO3(aq) • A Complete Ionic Equation shows all of the ions and solids in a precipitation reaction Complete Ionic Equation: Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) --> AgCl(s) + Na+(aq) + NO-3(aq)

Complete and Net Ionic Equations AgNO3(aq) + NaCl (aq) --> AgCl (s) + NaNO3(aq) • A Net Ionic Equation removes the spectator ions from the complete ionic equation • Spectator Ions don’t do anything in the reaction and are found on both sides of the arrow. Complete Ionic Equation: Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) --> AgCl(s) + Na+(aq) + NO-3(aq) Net Ionic Equation: Ag+(aq) + Cl-(aq) --> AgCl(s) 

Acids and Bases • There are several possible definitions of acids and bases, but we’ll start with the Bronsted definition initially A Bronsted Acid is a Proton Donor A Bronsted Base is a Proton Acceptor • Acids are only acids once they donate their proton to an accepting base • Bases are only bases once they accept a proton from a donor

HCl and Phase • In the gas phase, HCl is just another molecule with 2 atoms • Once we add the molecule to water however…

Strong and Weak Acids HCl (aq) + H2O (l) --> H3O+(aq) + Cl-(aq) • The reaction goes almost to completion (K is very ____), so we only draw a single arrow. • HCl is a strong acid HCN (aq) + H2O (l) --> H3O+(aq) + CN-(aq) • The K value for this reaction is low, so the reaction favors the _______ • HCN is a weak acid A Strong Acid is fully deprotonated in solution A Weak Acid is only partially deprotonated in solution

Strong and Weak Bases • A Bronsted base is a proton acceptor • This means it has a lone pair to accept the proton (more on this in a little bit…) • Let’s look at CaO: CaO (aq) + H2O (l) --> Ca(OH)2(aq) Ca2+(aq) + O2-(aq) + H2O(l) --> Ca2+(aq) + 2OH-(aq) O2-(aq) + H2O(l) --> 2OH-(aq) The K value for this reaction is very high and oxide ions are strong bases in water

Strong and Weak Bases NH3(aq) + H2O (l) --> NH4+(aq) + OH-(aq) • NH3 is electrically neutral, and it has a lone pair to accept the proton, but the K value for the reaction is very low • Ammonia is a weak base • All amines, organic derivatives of ammonia, are weak bases

Conjugate Acids and Bases • The products of proton transfer may also react with water HCN (aq) + H2O (l) CN-(aq) +H3O+ (aq) • The cyanide ion may take/accept a proton to reform HCN • This is called a Conjugate Base • The HCN formed when CN- accepted a proton is called the Conjugate Acid of CN-

The Conjugate Base of an acid is the species left when the acid donates a proton The Conjugate Acid is the species formed when the base accepts a proton

Lewis Acids and Lewis Bases • Because of the sheer possibilities that exist in the chemical world, we need to expand our definition of acids and bases to include more than just protons. • A Lewis Acid is an electron pair acceptor • A Lewis Baseis an electron pair donor

Lewis Acids and Bases We’ll use Lewis structures to show how electron pairs move in the reactions of Lewis acids and bases. • Oxide anion reacting with water • The oxide anion is a Lewis base (electron pair donor) • Ammonia reacting with water • The lone pair in Nitrogen grabs a water proton Carbon dioxide accepts an electron pair from the oxygen of water

Acidic, Basic and Amphoteric Oxides • Acidic oxides react with water to form a Bronsted acid CO2(g) + H2O(l) H2CO3(aq) • Acidic oxides are molecular compounds of nonmetal oxides • Basic oxides react with water to form a Bronsted base CaO(s) + H2O(l) --> Ca(OH)2(aq) • Basic oxides are ionic compounds of metals • Oxides of the metalloids are amphoteric meaning that they react with both acids and bases Al2O3(s) + 6HCl(aq) --> 2AlCl3 + 3H2O(l) Al2O3(s) + 2NaOH(aq) --> 2Na[Al(OH)4](aq)

Autoprotolysis Water is both an acid and a base H2O(l) + O2-(g) --> 2OH- (water as an acid) H2O(l) + HCl(aq) --> H3O+ + OH- (water as a base) Water is Amphiprotic meaning that it can act as a proton donor or proton acceptor

Autoprotolysis • Because water is amphiprotic, proton transfer between water molecules spontaneously happens • In fact, water is never just H2O 2H2O(l) H3O+ + OH- This is autoprotolysis We can describe K as:

Autoprotolysis Kw = [H3O+][OH-] • From experiments, we can measure the concentrations of H3O+ and OH- and find them to be equal and 1.0x10-7 M Kw = [H3O+][OH-]=(1.0x10-7)(1.0x10-7) = 1.0x10-14 Kw is still an equilibrium constant, so whatever we do to one product, the other will compensate to maintain Kw = 1.0x10-14 

The pH Scale pH = -log[H3O+] • In a pure water sample, the [H3O+] = 1.0x10-7M and the pH is 7.00 • At values lower than 7, the [H3O+] is increasing • At values higher than 7, the [H3O+] is decreasing (and the pOH is increasing)

Chapter 10: Acids and Bases