ACIDS AND BASES

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ACIDS AND BASES

• for it cannot be
• But I am pigeon-liverd and lack gall
• To make oppression bitter
• Hamlet

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Learning objectives

• Describe properties of acids and bases
• Define acid and base using Arrhenius and Bronsted
  definitions
• Identify Bronsted acids and bases in solution
  equilibria
• Distinguish between strength and concentration of
  acids and bases
• Estimate pH of common substances
• Describe phenomenon of acid rain

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Properties of acids

• Acids are sour
• Acids attack metals
• Acids react with bases and form salts
• Acids turn litmus red

4
Properties of bases

• Bases taste bitter
• Bases are slippery
• Bases react with acids to form salts
• Bases turn litmus blue

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Neutralization

• The mixing of an acid with a base
• ACID BASE SALT WATER
• The reaction of carbonic acid (CO2 in H2O) to
  give limestone
• H2CO3 Ca(OH)2 CaCO3 2H2O

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Arrhenius its about water

• The meaning of acid and base has changed over the
  years
• Arrhenius acid is one that generates protons when
  dissolved in water
• Arrhenius base is one that generates hydroxide
  ions when dissolved in water

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Hydronium ion is the active ingredient of an acid
in water

• Protons do not exist in solution
• CH3CO2H H2O H3O CH3CO2-
• Vinegar in water produces hydronium ions

8
Hydroxide ion is the active ingredient of a base
in water

• NH3 H2O NH4 OH-
• Ammonia, a base, dissolves in water and produces
  hydroxide ions

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The essence of neutralization

• Elimination of the components of acid and base by
  combination to give H2O
• H OH- ? H2O

BASE
ACID
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Brønsted and Lowry All about protons

• Broader definition of acids and bases
• Reaction NH3 HCl NH4Cl has all elements of
  acid-base neutralization but no H2O
• Brønsted acid donates a proton
• Brønsted base accepts a proton

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Brønsted acid

• HCl H2O H3O Cl-

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Brønsted base

• NH3 H2O NH4 OH-
• water
• NH3 HCl NH4Cl-
• No water

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Substances can be both acids and bases depends
on environment

• Note that in one instance H2O behaves like a base
  accepting protons, and in another, behaves like
  an acid donating protons
• HCl H2O H3O Cl-
• In presence of an acid H2O is a base
• NH3 H2O NH4 OH-
• In presence of a base H2O is an acid

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Salts

• Products of acid-base neutralization
• Contain metal cation and nonmetal anion
• Acid base salt water
• HCl NaOH NaCl H2O
• HCl KOH KCl H2O
• HNO3 KOH KNO3 H2O
• 2HCl Ca(OH)2 CaCl2 2H2O
• HCN NaOH NaCN H2O

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Strong coffee (or concentrated?)

• Not all acids completely donate protons to water
  molecules
• HA H2O ? A- H3O
• Strength Degree of ionization
• Concentration Number of moles per unit volume

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Strong and weak

• Strong acid (HCl)
• Fully ionized
• All H and Cl-
• Corrosive
• Weak acid (Acetic)
• Weakly ionized
• Mostly CH3COOH
• Edible

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Changing concentration does not change strength

• Strength refers to degree of ionization
• Strong is completely ionized (100 )
• Weak is partly ionized (1 - 1106)
• Concentration refers to number of moles per unit
  volume
• An acid (or base) can be strong and concentrated,
  weak and concentrated, strong and dilute, weak
  and dilute

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Ionization of water

• Even in pure water, molecules are ionized and
  concentrations of OH- and H3O are equal
• H2O H2O H3O OH-
• H3O OH-

Concentration
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In all aqueous solutions, product of
concentrations is a constant

• H3OOH- constant
• Increasing H3O decreases OH-
• (acidic conditions)
• Increasing OH- decreases H3O
• (basic conditions)

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The pH scale reduces large range of numbers to
small

• In water H3OOH- 10-14
• pH - log10H3O
• Range of H3O 10 M 10-14 M
• Range of pH -1 to 14
• Low pH acid high pH basic
• pH 7 neutral

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Relating pH to H3O

• For pH, take exponent of H3O, change sign
• 10-1 M (0.1 M) HCl has pH 1
• Pure water has H3O 10-7 M, pH 7
• Ammonia has H3O 10-11 M, pH 11
• Note change of 1 unit in pH is factor of ten

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Estimating pH

• Estimating pH is often more useful than doing
  exact calculations
• Smaller pH value means larger H concentration
• Estimating pH

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Acidity and the environment

• Rain is naturally weakly acidic because of CO2
• Alkaline rocks limestone neutralize the acid

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Acid Rain

• Acid rain is polluted by acid in the atmosphere.
  Two common pollutants acidify rain sulphur
  dioxide (SO2) and nitrogen oxides (NOX)
• Following information from The Green LaneTM,
  Environment Canada's World Wide Web site -
  www.ec.gc.ca/regeng.html

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Whats the big deal?

• Damage to aquatic life
• Damage to buildings
• Damage to forests
• Damage to air quality

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Source of the problem

• Sulphur dioxide (SO2) byproduct of industrial
  processes and burning fossil fuels.
• Ore smelting
• coal-fired power generators
• natural gas processing

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Where do NOX emissions come from?

• Main source of NOX is combustion of fuels in
  motor vehicles, residential and commercial
  furnaces, industrial and electrical-utility
  boilers and engines.
• NOX emissions were 2.5 million tonnes in 2000.
  U.S. NOX emissions for 2000 were 21 million
  tonnes.

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Legislative success with acid rain

• Eastern Canada Acid Rain program committed Canada
  to cap SO2 emissions at 2.3 million tonnes by
  1994 - 40 reduction from 1980 levels
• Targets achieved or exceeded
• By 2001, emissions were 63 reduction from 1980
  levels.

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Would acid rain remain a problem without further
controls?

• Yes. That is why The Canada-Wide Acid Rain
  Strategy for Post-2000 calls for further emission
  reductions in both Canada and the United States.
• In total, without further controls, almost
  800,000 km2 in southeastern Canada-an area the
  size of France and the United Kingdom
  combined-would receive harmful levels of acid
  rain that is, levels well above critical load
  limits for aquatic systems.