Strong Acids and Bases

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Strong Acids and Bases
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Note

• It is important that you don't confuse the words
  strong and weak with the terms concentrated and
  dilute.
• As you will see, the strength of an acid is
  related to the proportion of it that has reacted
  with water to produce ions.
• The concentration tells you about how much of the
  original acid is dissolved in the solution.
• It is possible to have a concentrated solution of
  a weak acid, or a dilute solution of a strong
  acid.

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Acids

• When an acid dissolves in water, a proton
  (hydrogen ion) is transferred to a water molecule
  to produce a hydroxonium ion and a negative ion
  depending on what acid you are starting from.
• In the general case . . .
• HA H2O H3O A-
• The strength of an acid is defined by the
  equilibrium position of the dissociation reaction
  shown above

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Strong Acids

• The reaction is reversible, but with a strong
  acid the equilibrium lies far to the right. This
  means the original acid is virtually 100
  dissociated(ionized)
• For example, when hydrogen chloride dissolves in
  water to make hydrochloric acid, so little of the
  reverse reaction happens that we can write
• HCl H2O H3O Cl-

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Strong Acids

• At any one time, virtually 100 of the hydrogen
  chloride will have reacted to produce hydroxonium
  ions and chloride ions. Hydrogen chloride is
  described as a strong acid.
• A strong acid is one which is virtually 100
  ionized in solution.
• Other common strong acids include HCl -
  hydrochloric acid, HNO3- nitric acid H2SO4-
  sulfuric acid, HBr- hydrobromic acid, HI-
  hydroiodic acid, and HClO4- perchloric acid

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Strong Acid

• There are four ways to describe a strong acid
• Ka is large
• Position of the dissociation equilibrium lies far
  to the right
• The equilibrium concentration of H approximately
  equal to the original concentration of HA
  (HHA)
• The Strength of conjugate base of a strong acid
  is a much weaker base than H2O

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Working out the pH of a strong acid

• With strong acids this is easy.
• Hydrochloric acid is a strong acid - virtually
  100 ionized. Each mole of HCl reacts with the
  water to give 1 mole of hydrogen ions and 1 mole
  of chloride ions
• That means that if the concentration of the acid
  is 0.001M then the concentration of hydrogen ions
  is also 0.001M
• So finding the pH is easy, just use your
  equation pH-logH

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Example-Strong Acids

• Calculate the pH of an aqueous solution
  containing 2.5x10-5M of HCl.

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Solution

• So first write the dissociation expression with
  HCl and water
• HCl H2O H3O Cl-
• Second, since we know that all the HCl will be
  converted to products, because HCl is a strong
  acid, we know that the concentrations of H3O and
  Cl- will be the same as the original acid.
• All we need to do now is plug the value into the
  pH equation
• Recall from yesterday pH-logH3O
• Doing so we get
• pH-log(2.5x10-5)4.6

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Strong Bases

• Much the same as strong acids, strong bases
  dissociate 100 into the cation and OH-
  (hydroxide ion). The hydroxides of the Group IA
  and Group IIA metals usually are considered to be
  strong bases, they include
• Group IA-
• LiOH, NaOH,KOH, RbOH and CsOH(only NaOH and KOH
  are common because the others are expensive to
  work with)
• Group IIA- Ca(OH)2, Ba(OH)2, Sr(OH)2
• For these soultions 2 moles of hydroxide ion is
  produced for every 1 mole of metal hydroxide
  dissolved in aqueous solution.

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Strong Bases

• Much the same as acids, there are
• four ways to describe a strong base
• Kb is very large
• Position of the dissociation equilibrium lies far
  to the right
• Equilibrium concentration of OH- compared t
  original strong base is approximately equal
• Strength of conjugate acid is much weaker acid
  than H2O

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Working out the pH of a strong base

• With strong bases this is easy as well.
• Sodium hydroxide is a strong base - virtually
  100 ionized. Each mole of NaOH reacts with the
  water to give 1 mole of hydroxide ions and 1 mole
  of sodium ions
• That means that if the concentration of the base
  is 0.001M then the concentration of hydroxide
  ions is also 0.001M
• So finding the pH is easy, use the equation
  pHpOH14, find the pOH and subtract from 14 to
  get the pH.(You could also use the Kw expression
  to find the H then find the pH, either way is
  acceptable)

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Example

• Calculate the pH of a 5.0x10-2M KOH

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Solution

• So first write the dissociation expression with
  KOH and water
• KOH K OH-
• Second, since we know that all the KOH will be
  converted to products, because KOH is a strong
  base, we know that the concentrations of K and
  OH- will be the same as the original base
• All we need to do now is find pOH and use the
  realtionship between pOH and pH
• Recall from yesterday pOH-logOH-
• Doing so we get
• pOH-log(5.0x10-2)1.30
• Next subtract from 14 to get pH
• pH14-pOH12.7

H2O
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Quizes

• Go to Mr. Richards home page on Horton website
• Go to AP page and choose from units tab, acids
  and bases
• Go to pH tutorial
• Read over sections 4 and 6 and do quizzes