Semester 2 Unit 4

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Acids and Bases

• Semester 2 Unit 4

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Properties of Acids and Bases
ACIDS NEUTRAL SOLUTIONS BASES
SOUR TASTE BITTER TAST, SLIPERY FEEL
REACT WITH METALS NO REACTION WITH METALS NO REACTION WITH METALS
TURNS BLUE LITMUS PINK TURNS RED LITMUS BLUE
PHENOLPHTHALEN STAYS CLEAR PHENOLPHTHALEN STAYS CLEAR PHENOLPHTHALEN TURNS PINK
pHlt 7 pH 7 pHgt 7
CONDUCTIVE NONCONDUCTIVE CONDUCTIVE
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Defining Acids and Bases
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Ions in Solution

• Neutral Solutions- contain equal amounts of
  hydrogen ion (H) and hydroxide ions (OH-)
• Acidic Solutions- contain more hydrogen ions than
  hydroxide ions
• Basic Solutions- contain more hydroxide ions than
  hydrogen ions

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Self-Ionization of Water

• H2O H2O? H3O OH-
• H2O ? H OH-
• Hydrogen ions and hydronium ions are
  interchangeable, both indicate the presence of an
  acid

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The Arrhenius Model of Acids and Bases

• Acid- any substance that contains hydrogen ions
  and ionizes to produce hydroxide ions in aqueous
  solutions
• HCl? H Cl_
• Bases- any substance that contains a hydroxide
  group and dissociated to produce hydroxide ions
  in aqueous solutions
• NaOH? Na OH-
• Not a good definition because it does not
  incorporate all bases
• NH3 H2O ? NH4 OH-

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The Bronsted-Lowry Model of Acids and Bases

• Acid- hydrogen ion donor
• HF H2O ? H3O F-
• Base- hydrogen ion acceptor
• NH3 H2O ? NH4 OH-
• Conjugate Acid- the new acid formed when a base
  accepts an H ion
• Conjugate Base- the new base formed when an acid
  donates an H ion

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The Bronsted-Lowry Model of Acids and Bases

• Amphoteric Substances- substances that can act as
  either an acid or a base

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Strength vs. Concentration
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Acid Base Strength

• Strong Acids- any acids that dissociate
  completely in an aqueous solution
• Strong Bases- any base that dissociates
  completely in an aqueous solution
• Reaction equations will show a one way arrow and
  their will be no equilibrium
• Good conductors of electricity

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Acid Base Strength

• Weak Acids-acids that do not ionize completely in
  aqueous solutions
• Weak Bases- bases that do not ionize completely
  in aqueous solutions
• Reactions will show a two way arrow and
  equilibrium will be established
• Not good conductors of electricity

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Acid Base Strength

• Strong Acid Weak Acid
• HCl? H Cl- HF ? H F-

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Acid Base Concentration

• Concentration- refers to the amount of solute in
  a given amount of solution
• Concentrated Acid- more solute in a given amount
  of solution than another solution of the same
  volume
• Dilute Acid- less solute in a given amount of
  solution than another solution of the same volume

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Acid Base Concentration

• Concentrated Acid Dilute Acid
• HCl? H Cl- HCl? H Cl-

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Equilibrium Expressions and Constants
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Equilibrium Constants

• Law of Chemical Equilibrium- at a given
  temperature, at equilibrium, a ratio of reactant
  to product concentrations has a constant value
• Equilibrium constants (Keq)- the numeric value
  that compares reactant and product concentrations
• Equilibrium constant expressions can only be
  written for reactions at equilibrium and solids
  and liquids are always excluded from the
  equilibrium constant expression

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Equilibrium Constants

• If the value of Keq lt 1- there is more reactant
  at equilibrium
• If the value of Keq gt 1- there is more product at
  equilibrium
• Equilibrium Constant Expression aA bB ? cC
  dD
• A, B- reactants
• C, D- prodcuts
• a, b, c, d- coefficients of the balanced equation
• - concentration

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Example 2H2S(g) ? 2H2(g) S2 (g)

• Write the equilibrium constant expression.
• If H2S 0.184M, H20.0377M and S20.0540M,
  calculate the value of Keq.
• Does the reaction favor the production of
  reactants or products? How do you know?

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Acid Ionization Constant

• The acid ionization constant (Ka) is the value of
  the equilibrium constant expression for the
  ionization of a weak acid
• The base ionization constant is the value of the
  equilibrium constant expression for a weak base
• Large value of Ka or Kb stronger acid or base
• Small value of Ka or Kb weaker acid or base

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pH and pOH
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Ion Product Constant for Water (Kw)

• The ion product constant for water is the value
  of the equilibrium constant expression for the
  self-ionization of water
• Kw KeqHOH-
• Kw 1.0x10-14
• Kw can be used to calculate H or OH-

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Ion Product Constant for Water (Kw)

• Example At 298K the H ion concentration of an
  aqueous solution is 1.0 x 10-5M. What is the OH-
  concentration of the solution?

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pH and pOH

• pH -logH (the negative log of the hydrogen
  ion concentration)
• pOH -logOH- (the negative log of the hydroxide
  ion concentration)
• pH pOH 14

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Example- Calculating pH and pOH

• Ammonia cleaner is an aqueous solution of ammonia
  gas with a hydroxide ion concentration of 4.0 x
  10-3M. Calculate the pOH and pH of the ammonia
  cleaner.

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Example- Calculating H and OH-

• What are the hydrogen and hydroxide ion
  concentrations in a healthy persons blood if the
  blood has a pH of 7.40?

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Neutralization Reactions
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Neutralization Reactions

• Neutralization reactions are reactions in which
  an acid and a base react to form a salt and water
• Mg(OH)2 (aq) 2HCl(aq) ? MgCl2 2H2O

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Acid Base Titrations

• Method of determining the concentration of a
  solution by reacting a known volume of the
  solution with a solution of known concentration
• M1V1M2V2
• Equivalence Point- point at which the moles of H
  ion from the acid moles of OH- ion from the
  base
• Not the same as being reacted to completion

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Acid Base Titrations

• Indicators for a titration should be chosen to
  change color over the end point
• The end point of a titration is when the reaction
  reaches the equivalence point
• Examples
• What indicator would you choose for a titration
  with an equivalence point of 9.8?
• What indicator would you choose for a titration
  with an equivalence point of 10.7?

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Titration Problems

• What is the molarity of CsOH solution if 30.0mL
  of the solution is neutralized by 26.4mL of a
  0.250M HBr solution?
• What is the molarity of a nitric acid solution if
  43.44mL of 0.1M KOH solution is needed to
  neutralize 20.00mL?

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Buffers
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Buffers

• Solutions that resist changes in pH when limited
  amounts of acid or base are added.
• Mixture of a weak acid and its conjugate base or
  a weak base and its conjugate acid
• Buffers help to resist changes in pH by reacting
  with excess H or OH- ions that are placed in the
  solution

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Buffers

• HF(aq) H2O ? H(aq) F-(aq)