Modern Theories of Acids

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Modern Theories of Acids Bases

• The Arrhenius
• and
• Bronsted-Lowry Theories

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Acids Bases

• Acids and bases are special kinds of
  electrolytes. Like all electrolytes they break
  up into charged particles.
• What sets them apart from each other, and other
  electrolytes is the way that they break up.

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Arrhenius Acids

• Svente Arrhenius (who first proposed the theory
  of ionization) identified acids as substances
  that ionize in water to produce hydronium ion.
• For example
• HCl H2O ? H3O Cl-

Any substance that ionizes in water to produce
hydronium ion can be classified as an acid.
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H3O (aka H)

• The hydronium ion is also known as a hydrogen
  ion.
• This allows us to shorten the ionization
  reactions for acids. The following equations
  represent the same chemical change
• HCl H2O ? H3O
  Cl-
• 
  and
• HCl ? H
  Cl-

However, it should be understood that H is an
abbreviation for the hydronium ion. H ions DO
NOT exist in water solution but are snatched up
by water molecules to form hydronium ions.
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Arrhenius Base

• Svente Arrhenius also identified bases as
  substances that ionize in water to produce
  hydroxide ion.
• For example
• NaOH ? Na OH-

Any substance that ionizes in water to produce
hydroxide ion can be classified as a base.
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Salts

• Ionic substances that break up in solution to
  produce ions other than hydronium and hydroxide
  ions.
• NaCl (s) ? Na(aq) Cl- (aq)
• KNO3(s) ? K (aq) NO3- (aq)
• Li2SO4 (s) ? 2 Li (aq) SO42-(aq)

Salts are made up of positive (metal) and
negative (non-metallic or polyatomic) ions. The
more familiar you become with Table E, the easier
it will be for you to identify salts.
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Salts

• Salts are generally defined as ionic substances
  that PRIMARILY produce positive and negative ions
  other than hydronium or hydroxide when they
  dissolve in water.

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Practice

• Identify each of the following as
  acids/bases/salts and show how they ionize
• HC2H3O2
• K2SO4
• KOH
• LiOH
• HNO3

Acid HC2H3O2 ?H C2H3O2-
Salt K2SO4 ? 2K SO42-
Base KOH ? K OH-
Base LiOH ? Li OH-
Acid HNO3 ? H NO3-
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Properties of Acids
All acids have the following properties
Neutralize bases to form a salt and water
Have a sour taste (example citric acid, vinegar)
React with active metals to produce a salt plus
hydrogen gas.
Have pHs less than 7 Affect indicators
Why?
Because all acids have H3O ions present!
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Properties of Bases
All bases have the following properties
Neutralize acids to form a salt and water
Have a bitter taste (example unsweetened
chocolate, heroin)
Are slippery. React with fats/oils to form soap
(saponification)
Have pHs greater than 7 / Affect indicators
Why?
Because all bases have OH- ions present!
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Bronsted-Lowry Acids Bases

• Another theory of acids bases

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Not everyone was happy with Arrheniuss definition

• A major problem with the Arrhenius definition of
  acids and bases is that it limits acids and bases
  to water (aqueous) solutions.
• Since an acid requires water to ionize and form
  hydronium ions, there can be no Arrhenius acids
  unless water is involved as the solvent.

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Relationship between the two models
Bronsted-Lowry Acids/Bases can exist when no
water is present
Arrhenius Acids/Bases only exist in water
solutions.
All Arrhenius acids and bases can also be
classified as Bronsted-Lowry acids and bases.
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Bronsted-Lowry Definitions

• Bronsted and Lowry felt that this was too
  limiting, since there are many non-aqueous
  systems (no water is present).
• They came up with the following definitions for
  acids and bases.
• An acid is a proton (H ion) donor
• A base is a proton acceptor

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An example
In the reaction below there are no Arrhenius
acids or bases present (because no hydronium ions
or hydroxide ions are formed).
However, the HCl is acting as a Bronsted-Lowry
acid because it is giving a H ion to the NH3
(which is acting as a H ion acceptor - a base)
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Acid Bases are Roles

• In the Bronsted-Lowry definition, substances are
  classified as acids or bases depending on how
  they behave in a given situation.
• This means that the same substance can act as a
  acid in one reaction (by donating a proton) while
  acting as a base in another reaction

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HCl as a proton donor

• Consider the following reaction
• Since the HCl gives up a H ion to the water it
  is acting as a Bronsted-Lowry acid. In the
  process of donating the proton it also forms a
  hydronium ion, and that makes it an Arrhenius
  acid as well.

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Water as a base

• But what does that make the water molecule?
• Since the water molecule is accepting the H ion,
  it is acting as a Bronsted-Lowry base. Since
  there is no hydroxide ion (OH-) formed, the water
  is not acting as an Arrhenius base in this
  reaction.

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Ammonia as a base

• Lets look at another example
• Here the ammonia molecule is accepting a H ion
  and therefore is acting as an Bronsted-Lowry
  base. However, in the process of reacting with
  the water it is also forming a hydroxide ion.
  That makes the ammonia an Arrhenius base as well.

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But what about the water?
Since the water is giving up a H ion, it is
acting as a Bronsted-Lowry acid. Since it does
not form hydronium ions, it is NOT acting an
Arrhenius acid.
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So is water an acid or a base?

• In one example, we said that water was acting as
  a base, and in another example we said that it
  was acting as an acid.
• Some of you may be confused by this because you
  are thinking of acids and bases as being like
  boys and girls. Boys are boys and girls are
  girls, and they cant switch back and forth.
  However, acids and bases are NOT like this.

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Teacher-student model

• Teachers give off information (like acids give
  off protons)
• Students accept information (like bases accept
  protons)
• Sometimes teachers are students, and sometimes
  students are teachers
• Teacher and students are roles that individuals
  play depending on the situation.
• Acid and base are roles that molecules play in a
  particular chemical reaction. In different
  reactions they may play different roles.

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Amphoteric/Amphiprotic

• Sometimes a molecule can donate a proton (act as
  an acid) and sometimes it can accept a proton
  (act as a base).
• Molecules that have this ability to act as both
  an acid and a base are called amphoteric or
  amphiprotic.
• Water is the most common example of an amphoteric
  substance.

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Reality check

• For each of the following reactions identify any
  Bronsted-Lowry acids and bases.
• HNO3 H2O ? H3O NO3-
• HNO3 NH3 ? NH4 NO3-
• S2- H2O ? HS- OH-
• HS- OH- ? S2- H-OH
• HS- HCl ? H2S Cl-
• Are any of the substances above amphoteric?

acid
base
acid
base
base
acid
acid
base
acid
base
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• Go to pH- Indicator PowerPoint

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Strong/Weak Acids

• Acids can be either strong electrolytes or weak
  electrolytes.
• Strong acids (such as HCl) completely break up
  into their ions
• HCl (aq) ? H(aq) Cl-(aq)
• Weak acids (such as HC2H3O2) only partially break
  up into their ions
• HC2H3O2 ? H (aq) C2H3O2-(aq)
• Weak acids dont completely break up because
  they go to equilibrium!

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Acid-base equilibrium

• Many acid base reactions go to equilibrium, that
  is they have both a forward and reverse reactions
• For instance, acetic acid (HC2H3O2) reacts with
  water to form hydronium ion and acetate ion.

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The reverse reaction

• However, the acetic acid only partially ionizes
  because a reverse reaction takes place preventing
  the forward reaction from reaching completion.
• In the reverse reaction, the Hydronium ion acts
  as an acid (a proton donor) while the acetate ion
  acts as the base.

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Conjugate Acid-Bases

• When a substance donates a proton, the substance
  that is left is its conjugative base
• Notice that the conjugative base is accepting a
  proton in the reverse reaction.
• Every acid has a conjugative base

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Conjugative Acid-Base Pairs 2

• When a substance accepts a proton, the substance
  that is formed is its conjugative acid
• Notice that the conjugative acid is donating a
  proton in the reverse reaction.
• Every base has a conjugative acid

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Strong/Weak Bases

• Bases can be either strong electrolytes or weak
  electrolytes.
• Strong bases (such as NaOH) completely break up
  into their ions
• NaOH (aq) ? Na(aq) OH-(aq)
• Weak bases (such as NH3) only partially break up
  into their ions
• NH3 (aq) H2 O ? NH4 (aq) OH-(aq)
• Weak bases dont completely break up because
  they go to equilibrium!

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Identify the amphoteric substances in this chart.
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Types of Salts

• Salts can be classified as being
• neutral
• acidic
• Basic
• How a salt is classified depends upon whether
  secondary reactions between the ions making up
  the salt and water form either hydronium or
  hydroxide ions.

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Determining the type of salt

• The type of salt for a particular salt can be
  determined experimentally by testing the salt
  solution with universal indicator paper.
• neutral salts will have a pH of 7
• acidic salts will have a pH of less than 7
• basic salts will have a pH of greater than 7
• Typically, the pH values of salt solutions will
  be closer to 7 than that of acids or bases.

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Predicting the type of salt

• All salts can be considered to be formed from
  their parent acid and base by means of the
  neutralization reaction.
• Acid Base ? Salt Water
• The type of salt can be theoretically predicted
  based on the properties of the parent acid and
  the parent base.

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Neutral Salts

• Neutral salts are formed from a reaction between
  a strong acid and a strong base.
• (Remember neutralization reactions are double
  replacement reactions.)
• For instance,
• HCl NaOH ? NaCl HOH
• (since HCl is a strong acid and NaOH is a strong
  base, NaCl is a neutral salt.)

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Acidic Salts

• Acidic salts are formed from a strong acid and a
  weak base.
• For instance,
• HCl NH4OH ? NH4Cl HOH
• (since HCl is a strong acid and NH4OH is a weak
  base, NH4Cl is an acidic salt.)

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Basic Salts

• Basic salts are formed from a weak acid and a
  strong base.
• For instance,
• HC2H3O2 NaOH ? NaC2H3O2 HOH
• (since HC2H3O2 is a weak acid and NaOH is a
  strong base, NaC2H3O2 is a basic salt.)

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Strong and weak acids

• In predicting the type of salt it is helpful to
  remember the three strong acids
• HCl hydrochloric acid
• HNO3 nitric acid
• H2SO4 sulfuric acid
• Most other acids are weak. Acetic acid (HC2H3O2)
  is the most common weak acid discussed in Regents
  chemistry.

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Strong and Weak Bases

• For bases, the Group I hydroxides are all strong
  bases.
• LiOH lithium hydroxide
• NaOH sodium hydroxide
• KOH potassium hydroxide
• Most other hydroxides can be considered as weak
  bases. Ammonia or Ammonium hydroxide (NH3 or
  NH4OH) is the most common weak base discussed in
  Regents chemistry.

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Try these

• Identify the parent acid and base for the
  following salts, identify them as strong or weak
  and predict the type of salt formed.

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Answers