Acids and Bases

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Acids and Bases

• Chapter 20

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Properties of Acids Bases

• Acids
• Tart or sour taste
• Are electrolytes (a substance that conducts
  electricity when present in solution)
• Causes blue litmus paper to turn red
• React with active metals to produce hydrogen
• React with bases to form water and salts
• Bases
• Taste bitter and feel slippery
• Causes red litmus paper to turn blue
• Electrolytes
• React with acids to form water and salts

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Hydronium Hydroxide Ions

• All water solutions contain hydrogen ions (H)
  and hydroxide ions (OH-).
• The relative amounts of the two ions determine
  whether an aqueous solution is acidic, basic, or
  neutral.
• Neutral solutions are neither acidic or basic.
• An acidic solution contains more hydrogen ions
  than hydroxide ions.
• A basic solution contains more hydroxide ions
  than hydrogen ions.
• A neutral solution contains equal amounts of each.

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The Arrhenius Model

• The Arrhenius model states that an acid is a
  substance that contains hydrogen and ionizes to
  produce hydrogen ions in solution.
• HCl(g) H(aq) Cl-(aq)
• A base is a substance that contains a hydroxide
  group and dissociates to produce a hydroxide ion
  in aqueous solution.
• NaOH(s) Na(aq) OH-(aq)
• These definitions are correct but not general
  enough to include the wide range of acid and base
  substances which are known to exist. In addition,
  they rely on the use of water as a solvent, which
  is also too narrow.

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The Bronsted Lowry Model

• In the Bronsted Lowry model of acids and bases,
  an acid is a hydrogen ion (proton) donor and a
  base is a hydrogen ion (proton) acceptor.
• HX(aq) H2O (l) H3O(aq) X-(aq)
• An acid-base reaction can occur in the reverse
  direction

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Conjugate Acids Bases

• A conjugate acid is an acid that forms when a
  base gains a proton
• A conjugate base is a base that forms when an
  acid loses a proton
• A conjugate acid-base pair consists of two
  substances related to each other by the donating
  and accepting of a single hydrogen ion.

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Amphoteric Substances

• Bronsted-Lowery Acid-Base Chemistry
• Now look again at HCl dissolving in water
• or ammonia dissolving in water
• We see water acting in one case as a base and in
  the other as an acid. Water is amphoteric, it can
  act as both an acid or a base.

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Strengths of Acids Bases

• Strong Acids
• Completely ionize
• Good conductors of electricity
• Weak Acids
• Partially ionizes
• Cannot conduct electricity as well
• Strong Bases
• Dissociates entirely into metal ions and
  hydroxide ions
• Weak Bases
• Partially ionizes

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pH pOH

• pH and pOH are logarithmic scales that express
  the concentrations of hydrogen ions hydroxide
  ions in aqueous solutions
• pH -log H
• pOH -log OH-
• pH pOH 14.00

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Sample Problems

• Calculate pH of solutions having the following
  ion concentrations
• H 1.0 x 10-2 M
• H 3.0 x 10-6 M
• Calculate pOH of solutions having the following
  ion concentrations
• H 8.2 x 10-6 M
• H 6.5 x 10-4 M

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Neutralization

• A neutralization reaction is a reaction in which
  an acid and a base in an aqueous solution react
  to produce a salt and water.
• Its a double replacement reaction
• Mg(OH)2(aq) 2HCl(aq) MgCl2(aq) 2H2O(l)

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Acid Base Titration

• Titration is a method for determining the
  concentration of a solution by reacting a known
  volume of that solution with a solution of known
  concentration.
• You can use it to find the concentration of an
  acid or base.

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Titration Procedure

• A measured volume of an acidic or basic solution
  of unknown concentration is placed in a beaker.
  The electrodes of a pH meter are immersed in this
  solution, and the initial pH of the solution is
  read and recorded.
• A buret is filled with the titrating solution of
  known concentration. This is called the standard
  solution, or titrant.
• Measured volumes of the standard solution are
  added slowly and mixed into the solution in the
  beaker. The pH is read and recorded after each
  addition. This process continues until the
  reaction reaches the equivalence point, which is
  the point at which moles of H ion from the acid
  equal moles of OH- ion from the base.

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Acid-Base Indicators

• Chemical dyes whose colors are affected by acidic
  and basic solutions are called acid-base
  indicators.
• The point at which the indicator used in
  titration changes color is called the end point
  of the titration.
• It is important to choose the right indicator for
  the titration.
• Bromthymol blue is a good choice for a titration
  of a strong acid with a strong base.
• Phenophthalein is good for titration of a weak
  acid with a strong base.

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Describing Acids Bases

• Names Formulas of Acids Bases
• An acid is a compound that produces hydrogen ions
  (H) when dissolved in water.
• The chemical formula of acids in general is HX,
  where X is a monatomic or polyatomic anion.
• Copy the 3 rules for naming acids p. 578
• See table 20.1 p. 578
• A base is a compound that produces hydroxide ions
  (OH-) when dissolved in water.
• Name the cation first and then the anion
• Balance the ionic charges

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Some common acids