Chemistry

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Chemistry

• Grade 12

Based on the Nelson Chemistry 12 textbook
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Product Constant for Water, Kw

• We can use the product constant for water, Kw,
  to calculate the hydrogen ion concentration or
  the hydroxide ion concentration in an aqueous
  solution of a strong or weak acid or base at
  SATP, if the other concentration is known.
• Since Kw H(aq)OH-(aq)
• Then Kw H(aq)
• OH-(aq)
• And Kw OH-(aq)
• H(aq)
• In neutral solutions H(aq) OH-(aq)
• In acidic solutions H(aq) gt OH-(aq)
• In basic solutions H(aq) lt OH-(aq)

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Strong Acid

• For strong acids we can use the concepts that
  strong acids ionize quantitatively in solution
  and use the value of Kw to calculate H(aq)and
  OH-(aq) in acidic solutions.
• EXAMPLE
• A 0.20 mol/L solution of hydrobromic acid,
  HBr(aq), at SATP is found to have a hydrogen ion
  concentration of 0.20 mol/L. Calculate the
  OH-(aq).

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• 0.20 mol/L
• HBr (aq) H(aq) Br -(aq)
• 0.20 mol/L 0.20 mol/L
• H2O(l) ? H(aq) OH-(aq)
• We can ignore the contribution of H(aq) from
  autoionization
• The OH-(aq) can also be ignore.
• Therefore the major entities in the solution are
  
• (which are the major entities affecting
  acid-base characteristics)
• H(aq) and Br -(aq)
• but the Br -(aq) is the conjugate base of a
  strong acid and therefore can be ignored as weak

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• So since HBr(aq) ionizes quantitatively (100),
  H(aq) 0.20 mol/L
• Now we can use the Kw expression to calculate the
  concentration of OH-(aq).
• OH-(aq) __Kw____
• H(aq)
• OH-(aq) 1.0 x 10-14/0.20 5.0 x 10-14 mol/L

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Strong Base

• Just as with acids, we can use the two concepts
  for solutions of strong bases that they
  dissociate quantitatively in solution and the
  value of Kw to calculate the hydrogen ion
  concentration or hydroxide ion concentration.

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• EXAMPLE
• Calculate hydrogen ion concentration in a 0.20
  mol/L solution of magnesium hydroxide, a strong
  base.
• Mg(OH)2(aq) Mg2(aq) 2OH-(aq)
• 0.20 mol/L 0.20 mol/L 2(0.20
  mol/L) 0.40 mol/L
• Major entities Mg2(aq), 2OH-(aq), H2O(l)

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pH measure of the hydrogen ion concentration

• pH -logH(aq)
• if H(aq) 2.5 x 10-14
• pH -log H(aq)
• -log (2.5 x 10 -14)
• pH 13.60
• In water and any neutral solution, the pH is 7.00
  (show calculations, as on p. 541).

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• To build on the previous chart
• In neutral solutions H(aq) OH-(aq) pH
  7.00
• In acidic solutions H(aq) gt OH-(aq) pH
  lt 7.00
• In basic solutions H(aq) lt OH-(aq) pH
  gt 7.00

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pOH measure of the hydroxide ion concentration

• pOH -logOH-(aq)
• OH-(aq) 10-pOH
• pH pOH pKw 14.00
• EXAMPLE
• What is the pH of a solution whose pOH is 2.3?
• OH-(aq) 10-pOH 10-2.3
• OH-(aq) 5.0 x 10-3

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The pH of Strong Acids and Bases

• The pH of Strong Acids
• The pH of solutions of strong monoprotic acids is
  calculated from the concentration of H(aq) ions,
  which is assumed to be the molar concentration of
  the solute molecule before ionization.
• The pH of Strong Bases
• As with strong acids, the pOH and the pH of
  strong bases are determined entirely by the
  OH-(aq) ion contributed by the dissociation of
  one of the ionic hydroxide solution.

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Weak Acids

• weak electrolyte
• does not ionize completely in water to form
  hydrogen ions
• most common acids are weak acids HF(aq),,
  H2CO3(aq), H2S(aq), H3BO3(aq)
• A weak acid is an acid that partially ionizes in
  solution but exists primarily in the form of
  molecules.

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Weak Bases

• Arrhenius theory of bases states that bases are
  soluble ionic hydroxides that dissociate in water
  into positive metal ions and negative hydroxide
  ions.
• There are some molecular and ionic compounds,
  other than hydroxides, which also dissolve in
  water to produce basic solutions that are called
  weak bases as they are not as basic as ionic
  hydroxide solutions of the same concentration.

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• The Brønsted-Lowry definition of a weak base is a
  compound that reacts non-quantitatively
  (incompletely) with water to form an equilibrium
  that includes hydroxide ions according to the
  following general equation
• B(aq) H2O(l) ? OH-(aq) HB(aq)

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Percent Ionization of Weak Acids

• most weak acids ionize less than 50, unlike
  strong acids that ionize close to 100
• Percent Ionization (p) is defined as follows
• p conc. of acid ionized x 100
• conc. of acid solute
• For a general acid ionization reaction HA(aq) ?
  H(aq) A-(aq)
• p H(aq) x 100
• HA(aq)
• H(aq) p x HA(aq)
• 100
• If we know the pH of a weak solution, we can
  calculate the percent ionization of the acids.

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Ionization Constants for Weak Acids

• We can consider equilibrium solutions of a weak
  acid dissolved in water to be just like the
  equilibrium systems we looked at in Chapter 7
• we can use the equilibrium law expression and
  calculate equilibrium constants
• acid ionization constant Ka
• Ka is usually determined experimentally
• Percent ionization can be used to calculate the
  Ka value (using an ICE table)

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EXAMPLE

• Calculate the acid ionization constant, Ka, of
  acetic acid if 0.1000 mol/L solution at
  equilibrium at SATP has a percent ionization of
  1.3.
• HC2H3O2 (aq) ? H (aq) C2H3O2-(aq)
• Ka H(aq)C2H3O2-(aq)
• HC2H3O2 (aq)
• Create ice table and substitute the values back
  into the Ka expression to solve. Ka 1.7 x 10-5

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Ionization Constants for Weak Bases

• Weak bases from dynamic equillibria in aqueous
  solutions
• The reaction of weak bases with water may be
  defined by the equilibria law)
• OH-(aq) ions are produced and affect the
  acid-base characteristics of solutions
• Calculated the same way as weak acids

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Relationship between Ka and Kb

• Kw KaKb
• This equation allows us to convert the Ka values
  of acids into the Kb values of their conjugate
  bases, and vice versa, given the value of Kw,
  which is a constant.

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Polyprotic Acids

• Can anyone remember acids with more than one
  ionizable proton (more than one proton to give
  away)?
• Exs sulfuric acid, phosphoric acid, and boric
  acid
• Donate one proton at a time in a stepwise fashion
• Each ionization reaction has its own acid
  ionization constant, Ka1, Ka2
• The acid in each step is weaker, generally
• Ka1 gt Ka2 gt Ka3

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Acid-Base Properties of Salt Solutions
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Salts

• Solids
• Composed of cations and anions
• Dissolve in water
• May or may not alter the pH of a solution, due to
  the cation, anion, or both

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Salts that form Neutral Solutions

• Salts of cations from strong bases and anions of
  strong acids have no effect of the pH of an
  aqueous solutionExs NaCl(aq), KCl(aq), NaI(aq),
  NaNO3(aq)

Cation from Strong Base Anion from Strong Acid Salts
NaOH HCl NaCl
KOH HCl KCl
NaOH HI NaI
NaOH HNO3 NaNO3
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Salts that form acidic solutions

• The salt of a weak base (cation) and strong acid
  (anion) dissolves in water to form acidic
  solutions.
• The cation reacts with water to liberate H
• The solution has a pH less than 7
• NH4Cl ? dissociates ? NH4 Cl-
• NH4 H2O ? H3O(aq) NH3(aq)

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Salts that form basic solutions

• The salt of a strong base (cation) and weak acid
  (anion) dissolves in water to form basic
  solutions.
• The anion reacts with water to liberate OH-
• The solution has a pH greater than 7
• NaC2H3O2 ?dissociates ? Na C2H3O2-
• C2H3O2- (aq) H2O ? HC2H3O2(aq) OH-(aq)

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Salts that act as acids and bases

• Some salts
• contain both the cation of a weak base and the
  anion of a weak acid
• both ions can hydrolyze
• we can predict whether the solution is acidic,
  basic or neutral
• Ka gt Kb then the solution is acidic
• Kb gt Ka then the solution is basic

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Acid-Base Titration
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• Start by matching titration terms

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Totally Terrific Titration Terms

• Titration a chemical analysis involving the
  progressive addition of a solution of known
  solute concentration into a solution of unknown
  concentration to determine the amount of a
  specified chemical
• Titrant solution of known concentration, usually
  found in a buret during titration

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Terms

• sample the solution of unknown concentration
  being analyzed in a titration
• primary standard chemical that is available in a
  pure and stable form that can be used to produce
  an accurate concentration
• standard solution a stock solution that is of
  known concentration that is used to create the
  titrant

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Terms

• endpoint point in titration when the pH
  indicator changes colour
• equivalence point point in titration when
  chemically equivalent amounts of reactants have
  reacted (point at which equal amounts of H3O(aq)
  and OH-(aq) have been added) generally, an
  equilibrium is established at this point

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Terms

• Standardization a titration used to find the
  concentration of the titrant using a primary
  standard
• indicator an acid-base indicator that will
  change colour at a known pH to signify to signify
  a specific pH in the neutralization reaction

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Titration of a strong acid with a strong base

• At the equivalence point HOH- or pH7.
• Phenolphthalein is a popular indicator because it
  is colourless in acidic solutions and pink in
  basic.
• Remember to consider the reaction at the molar
  level. (convert to moles!)
• Cn/V and C1V1 C2V2

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Titration of a strong acid with a strong base

• Titration curve a plot of the pH vs. Volume of
  titrant added.

pH
Vol. of titrant
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Titration Websites

• Titrations
• http//www.chemguide.co.uk/physical/acidbaseeqia/p
  hcurves.html (curves notes)
• http//www.chem.uoa.gr/applets/AppletTitration/App
  l_Titration2.html (show different graphs)
• http//www.vias.org/simulations/simusoft_titration
  .html (simulation)

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Buffers

• Website
• http//www.chemguide.co.uk/physical/acidbaseeqia/b
  uffers.htmltop