Transcript Bronsted-Lowry Acids & Bases

Modern Theories
of
Acids & Bases
The Arrhenius
and
Bronsted-Lowry Theories
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.
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+ + ClAny substance that ionizes in water to
produce hydronium ion can be classified as
an acid.
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+ + Cland
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.
Arrhenius Base
Svente Arrhenius also identified bases as
substances that ionize in water to produce
hydroxide ion.
For example:
NaOH  Na+ + OHAny substance that ionizes in water to produce
hydroxide ion can be classified as a base.
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 metals to produce a salt plus
hydrogen gas.
Have pH’s less than 7 / Affect indicators
Why?
Because all acids have H3O+ ions present!
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 pH’s greater than 7 / Affect indicators
Why?
Because all bases have OH- ions present!
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 (nonmetallic or polyatomic) ions. The more familiar you
become with Table E, the easier it will be for you to identify
salts.
Practice
Identify each of the following as
acids/bases/salts:
1.
2.
3.
4.
5.
HC2H3O2
K2SO4
KOH
LiOH
HNO3
Acid HC2H3O2 H+ + C2H3O2Salt K2SO4  2K+ + SO42Base KOH  K+ + OHBase LiOH  Li+ + OHAcid HNO3  H+ + NO3-
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 don’t completely break up because they go to
equilibrium!
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 don’t completely break up because they go to
equilibrium!
Bronsted-Lowry Acids &
Bases
Another theory of acids & bases
Not everyone was happy with
Arrhenius’s 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.
Relationship between the two
models
Arrhenius
Acids/Bases only
exist in water
solutions.
BronstedLowry
Acids/Bases
can exist
when no
water is
present
All Arrhenius acids and bases can also be classified as
Bronsted-Lowry acids and bases.
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
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 BronstedLowry acid because it is giving a H+ ion to
the NH3 (which is acting as a H+ ion
acceptor - a base)
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
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.
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.
Ammonia as a base
• Let’s 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.
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.
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 can’t switch
back and forth. However, acids and bases are NOT
like this.
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.
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.
Reality check
For each of the following reactions identify any BronstedLowry acids and bases.
acid
•
•
•
•
•
base
+
HNO
+
H
O

H
O
+
NO
2
3
3
acid 3
base
+
HNO
+
NH

NH
+
NO
3
3
4
3
base
acid
2S
+ H2O  HS- + OHacid
base
HS
+ OH-  S2- + H-OH
base
acid
HS
+ HCl  H2S + Cl-
Are any of the substances above amphoteric?
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.
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.
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
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
Identify the
amphoteric
substances in
this chart.
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.
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.
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.
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.
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.)
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.)
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.)
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.
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.
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.
Answers