Acids

1
Acids BasesPart I Some Definitions

• Dr. C. Yau
• Fall 2009

Based on Brady Senese 5/eChap. 15 Sec 1 2
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Different Definitions of Acid/Base
Arrhenius Definition An acid is a substance that
increases the H3O concentration when placed in
water. A base is a substance that increases the
OH- concentration when placed in water.
NaOH (s) Na(aq) OH-(aq)
HCl (g) H2O (l) H3O(aq) Cl-(aq)
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Different Definitions of Acid/Base
Brønsted-Lowry Definition An acid is a proton
donor. A base is a proton acceptor.
proton donor
proton acceptor
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Brønsted-Lowry Definitions

• Why do we need new definitions?
• The Arrhenius definitions were based on placing
  substances in water, but not all reactions are in
  water.
• e.g. NH3 (g) HCl (g) NH4Cl (s)
• This is an acid-base reaction and yet no H3O nor
  OH- ions are involved.
• However, proton transfer is involved.
• Write the Lewis structures for the reaction above
  and draw in curved arrows to show the flow of
  electrons.

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Brønsted-Lowry Acids/Bases

• Organic carboxylic acids have COOH group
  attached. Acetic acid has the structure
• Recognize all the different ways of writing its
  formula
• HC2H3O2 CH3COOH CH3CO2H
• Remember -COOH does NOT mean C-O-O-H

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• Write the equation using Lewis structures and
  curved arrows to show the reaction of acetic acid
  with ammonia.
• Which is the proton donor, which is the proton
  acceptor? Which is the proton being transferred?

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Conjugate Acids Conjugate Bases

• We will abbreviate "Brønsted-Lowry" to just
  "Brønsted."
• Write the equation for the reaction of acetic
  acid and ammonia using molecular formulas
  HC2H3O2 NH3
• HC2H3O2 NH3 C2H3O2- NH4
• acid base
• How is the acetate a base? and NH4 an acid?

conjugate conjugate base acid
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• Identify the Conjugate Partner for Each

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Conjugate Acid-Base Pairs

• Write the equation for the reaction of formic
  acid with water using molecular formulas
• HCO2H H2O
• Identify which is the acid, base, conjugate acid,
  conjugate base.
• Give the conjugate acid-base pairs

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Conjugate Acid-Base Pairs

• Example 15.1 p.608
• What is the conjugate base of nitric acid and
  what is the conjugate acid of the hydrogen
  sulfate ion?
• Do Pract Exer 1, 2, 3 on p.609

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Conjugate Acid-Base Pairs

• Example 15.2 p.609
• The anion of sodium hydrogen sulfate reacts as
  follows with the phosphate ion. Identify the 2
  conjugate acid-base pairs.
• HSO4? (aq) PO43? (aq) ?? SO42? (aq) HPO42?
  (aq)
• Do Pract Exer 4, 5, 6 p.610.

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Amphoteric Substances

• An amphoteric substance is one that can act
  either as an acid or a base.
• Water is such a substance.
• Although it is exists primarily as a molecule,
  there are some ions present due to the
  self-ionization of water
• HOH HOH H3O OH-
• Write the equation for water reacting with
  ammonia.
• Write the equation for water reacting with
  hydrochloric acid.

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Amphoteric Substances

• Write the equation for the reaction of the
  bicarbonate ion with hydroxide.
• Write the equation for the reaction of the
  bicarbonate ion with hydronium ion.
• Do Pract Exer 7, 8 p.611

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Acid and Base Strengths

• First recall what an acid is supposed to do.
  proton donor gives away a proton.
• A strong acid is one that can willingly "gives
  away" its proton.
• HCl H2O H3O Cl-
• A weak acid is one that "hangs on" to its proton.
• HC2H3O H2O H3O C2H3O2-

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Acid and Base Strengths

• Recall what a base is supposed to do
• proton acceptor. grabs up protons.
• A strong base is one that "greedily grabs"
  protons.
• NH2- H2O NH3 OH-
• A weak base is one that does not particularly
  want to grab a proton.
• Cl- H2O HCl OH-

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H3O and OH-

• H3O is the strongest acid that can exist in
  water.
• Note that mineral acids (nonorganic acids) such
  as HCl, HNO3, H2SO4, are stronger acids, but as
  soon as they are placed in water, they dissociate
  100 and no longer exist in water as such.
• They become H3O and the corresponding anions.

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H3O and OH-

• OH- is the strongest base that can exist in
  water.
• O2- is a stronger base but as soon as it goes
  into the water, it is converted to OH- and does
  not exist as O2-.

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Relationship between strengths of acid and
conjugate base
HC2H3O H2O H3O
C2H3O2- Identify the acid, base, conjugate
acid, conjugate base. Is the acid strong or
weak? Is its conjugate base strong or weak? Is
the base strong or weak? Is the conjugate acid
strong or weak?
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Relationship between strengths of acid and
conjugate base
HC2H3O H2O H3O C2H3O2- The
pattern we see is A weak acid produces a strong
conjugate base. A strong acid produces a weak
conjugate acid. A weak base produces a strong
conjugate acid, a strong base produces a weak
conjugate base.
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Relationship between strengths of acid and
conjugate base
HC2H3O H2O H3O
C2H3O2- Another pattern we see is that the
position of the equilibrium lies in favor of the
weaker acid and base. REMEMBER Equilibrium
goes from Strong acid to Weaker acid.
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Example 15.3 p.613

• In this rxn, will the position of equilibrium lie
  to the left or the right, given the fact that
  acetic acid is known to be a stronger acid than
  the hydrogen sulfite ion?
• HSO3?(aq) C2H3O2? (aq) HC2H3O2 (aq)
  SO32? (aq)
• Do Pract Exer 9, 10 p.614