Chapter 10: Acids and Bases

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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 doesnt, it is insoluble

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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 (doesnt exist entirely
  as ions in solution)

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Precipitation Reactions

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

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Complete and Net Ionic Equations

• AgNO3 (aq) NaCl (aq) --gt 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) --gt
  AgCl(s) Na(aq) NO-3 (aq)

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Complete and Net Ionic Equations
AgNO3 (aq) NaCl (aq) --gt AgCl (s) NaNO3 (aq)

• A Net Ionic Equation removes the spectator ions
  from the complete ionic equation
• Spectator Ions dont 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) --gt
  AgCl(s) Na(aq) NO-3 (aq)
• Net Ionic Equation
• Ag(aq) Cl-(aq) --gt AgCl(s)

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Acids and Bases

• There are several possible definitions of acids
  and bases, but well 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

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HCl and Phase

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

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Strong and Weak Acids

• HCl (aq) H2O (l) --gt 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) --gt 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

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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)
• Lets look at CaO

CaO (aq) H2O (l) --gt Ca(OH)2 (aq) Ca2(aq)
O2-(aq) H2O(l) --gt Ca2(aq) 2OH-(aq) O2-(aq)
H2O(l) --gt 2OH-(aq)
The K value for this reaction is very high and
oxide ions are strong bases in water
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Strong and Weak Bases

• NH3 (aq) H2O (l) --gt 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

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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-

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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
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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 Base is an electron pair donor

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Lewis Acids and Bases
Well 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
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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) --gt 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) --gt 2AlCl3 3H2O(l)
• Al2O3(s) 2NaOH(aq) --gt 2NaAl(OH)4(aq)

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

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Autoprotolysis

• Kw H3OOH-
• From experiments, we can measure the
  concentrations of H3O and OH- and find them to
  be equal and 1.0x10-7 M
• Kw H3OOH-(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

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The pH Scale

• pH -logH3O
• 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)