Transcript Chemical Reactions - Virginia State University

Chemical
Reactions
Chapter 5
Dr. Victor Vilchiz
Solution Components
• In order to have a solution we must have
at least “TWO” components.
– The Solvent which is the compound present
in biggest abundance, water in most cases
presented in this chapter.
– The Solute which is the “impurity” or
compound present in the smallest amount.
– The resulting mixture is the solution.
Dissolving Ionic Compounds
• Some compounds dissociate when placed
in water.
– This does not mean we will get the
constituent elements once in water.
– For example: when table salt (NaCl) is added
to a container with water as soon as the
grains of salt touch the water NaCl ceases to
exist. In solution, we have the component
ions Na+ and Cl-
Ions in Aqueous Solution
Ionic Theory of Solutions
• Many ionic compounds dissociate into
independent ions when dissolved in water
H 2O


NaCl(s )  Na (aq)  Cl (aq)
These compounds that “freely” dissociate into
independent ions in aqueous solution are called
electrolytes.
Their aqueous solutions are capable of conducting
an electric current. Figure 4.2 illustrates this.
Ions in Aqueous Solution
Ionic Theory of Solutions
• Electrolytes are substances that dissolve in
water to give an electrically conducting
solution.
Thus, in general, ionic solids that dissolve in water
are electrolytes.
Some molecular compounds, such as acids, also
dissociate in aqueous solution and are considered
electrolytes.
Ions in Aqueous Solution
Ionic Theory of Solutions
• Molecular compounds that dissolve usually
do not dissociate into ions.
C6 H12O6 (s) (glucose)  C6 H12O6 (aq)
H 2O
These compounds are referred to as
nonelectrolytes. They dissolve in water to give a
nonconducting solution.
Conductivity Test
Ions in Aqueous Solution
Ionic Theory of Solutions
• There are few molecular compounds
(acids & alcohols) that upon solvation
dissociate into ions, this is due to the weak
interaction between the atoms.


HCl(aq)  H (aq)  Cl (aq)
The resulting solution is electrically conducting,
and so we say that the molecular substance is an
electrolyte.
Does water conduct?
• As we have seen current/energy does not
flow in a circuit unless there are “free”
ions in solution.
– This means that a sample or “pure” water will
not conduct electricity or current.
– Tap water conducts electricity and current
because there are dissolved ions present put
there on purpose and some from pipes
dissolving.
Strong Electrolytes
Ionic Theory of Solutions
• Electrolytes can be separated into two
different categories
– Strong electrolytes.
A strong electrolyte is an electrolyte that exists
in solution almost entirely as ions.


NaCl(s)  Na (aq)  Cl (aq)
H 2O
The solvation process is represented by a one-way
arrow in the chemical reaction implying a path of
no return.
Weak Electrolytes
Ionic Theory of Solutions
– Weak electrolytes.
A weak electrolyte is an electrolyte that
dissolves in water to give a relatively small
percentage of ions.
MgSO4 (aq)

2
2
Mg (aq)  SO4 (aq)
The solvation process is presented by a double
sided arrow implying an equilibrium between
reactants and products.
Most soluble molecular compounds are either
nonelectrolytes or weak electrolytes.
Strength Test
Why dissociation?
• Dissociation takes place because the
attractive force between the ions can be
overcome by other forces.
– The solvent is able to surround ions and
provide stronger forces of attraction. Why?
• If water has no charge how can it create this
attractive forces that compete with coulombic
interaction?
– This can be explained if we look at the electron
distribution in water.
Polarity
• Electron distribution
– If we draw a water molecule representing its
true shape we will see that the electrons are
not evenly distributed.
The Red represents a
high density of
electrons (-); the
blue represents a low
density of electrons
(+).
While there are no
real charges the
difference in electron
density acts as a
separation of charges
which leads to a
pseudo ionic
behavior.
The separation of charge we see in water is labeled as the polarity
of the molecule; the higher the difference in electron density the
higher the polarity of the molecule.
Polarity
• The polarity of a molecule depends mainly on two
factors
– Shape of the molecule
– Composition
• It is represented by an arrow with its head pointing
towards the negative charge side and a crossed tail on
the positive side of the molecule
Polarity and Solvation
• Molecules that have a separation of
charges are called polar molecules
• When ionic compounds are added to
water, the ions break apart and the water
molecules arrange themselves so the
negative end (O) points towards the
cations and the positive end (H’s) point
towards the anions.
Representation
Insoluble Compounds
• No compound is really insoluble.
– However, if the amount that dissolves is
compared to the starting amount it is found to
be insignificant that they are said to be
insoluble.
• Example
– NaCl in H2O @20°C =365g/L
– AgCl in H2O @20°C =0.009g/L
Chemical Equations
Molecular and Ionic Equations
• A molecular equation is one in which
the reactants and products are written as
if they were molecules, even though
they may actually exist in solution as
ions.
Ca(OH )2 (aq)  Na2CO3 (aq)  CaCO3 (s)  2NaOH(aq)
Note that Ca(OH)2, Na2CO3, and NaOH are all
soluble compounds but CaCO3 is not.
Chemical Equations
Molecular and Ionic Equations
• An ionic equation, however, represents
strong electrolytes as separate
independent ions. This is a more accurate
representation of the way electrolytes
behave in solution.
2
2


Ca (aq )  2OH (aq )  2Na (aq )  CO 3 (aq ) 


CaCO 3 (s )  2Na (aq )  2OH (aq )
The downward arrow represents a precipitate
which will fall to the bottom of the container
Chemical Equations
Molecular and Ionic Equations
• Complete and net ionic equations
A complete ionic equation is a chemical
equation in which strong electrolytes (such as
soluble ionic compounds) are written as
separate ions in solution.
Ca( NO3 )2 (aq)  K 2CO3 (aq)  CaCO3 (s )  2KNO3 (aq)
(strong)
(strong)

(insoluble)
(strong)
2
Ca 2 (aq )  2NO 3 (aq )  2K  (aq )  CO 3 (aq ) 

CaCO 3 (s )  2K  (aq )  2NO 3 (aq )
Chemical Equations
Molecular and Ionic Equations
• Complete and net ionic equations.
A net ionic equation is a chemical equation
from which the spectator ions have been
removed.
A spectator ion is an ion in an ionic equation
that does not take part in the reaction (present
on both sides of the arrow in the same state.

2
2

Ca (aq )  2NO 3 (aq )  2K (aq )  CO 3 (aq ) 

CaCO 3 (s )  2K  (aq )  2NO 3 (aq )
Chemical Equations
Molecular and Ionic Equations
• Complete and net ionic equations
Let’s try an example. First, we start with a
molecular equation.
2HNO3 (aq)  Mg(OH )2 (s)  2H 2O(l )  Mg( NO3 )2 (aq)
Nitric acid, HNO3, and magnesium nitrate,
Mg(NO3)2, are both strong electrolytes.
Chemical Equations
Molecular and Ionic Equations
• Complete and net ionic equations

Separating the strong electrolytes into
separate ions, we obtain the complete ionic
equation.

2H (aq )  2NO 3 (aq )  Mg(OH )2 (s ) 
2

2H 2O(l )  Mg (aq )  2NO 3 (aq )
Note that the nitrate ions did not participate in
the reaction. These are spectator ions.
Chemical Equations
Molecular and Ionic Equations
• Complete and net ionic equations
Eliminating the spectator ions results in the
net ionic equation.


2H (aq )  2NO 3 (aq )  Mg(OH )2 (s ) 

2
2H 2O(l )  Mg (aq )  2NO 3 (aq )

2
2H (aq)  Mg(OH )2 (s)  2H 2O(l )  Mg (aq)
This equation represents the “essential” reaction.
Types of Chemical Reactions
Most of the reactions we will study fall into
one of the following categories
Precipitation Reactions
Acid-Base Reactions
Oxidation-Reduction Reactions
Types of Chemical Reactions
Precipitation Reactions
• In a precipitation reaction we start with 2
soluble compounds dissolved in water and when
mixed they produce at least one insoluble
compound, which precipitates (falls to the
bottom).
For example, the reaction of sodium chloride
with silver nitrate forms AgCl(s), an insoluble
precipitate.
NaCl(aq )  AgNO3 (aq )  AgCl(s)   NaNO3 (aq )
Precipitation Reactions
• Does this mean that if we mixed two soluble
ionic compounds we will always form a
precipitate?
NO
• What is the driving force for precipitation
reactions?
– While the formation of the solid can be viewed as
the driving force, it is the removal of ions from
solution that is the true driving force.
Types of Chemical Reactions
Precipitation Reactions
• Solubility rules
Substances vary widely in their solubility, or
ability to dissolve, in water.
For example, NaCl is very soluble in water
whereas calcium carbonate, CaCO3, is insoluble
in water. (see Figure 4.5)
Types of Chemical Reactions
Precipitation Reactions
• Predicting Precipitation Reactions.
To predict whether a precipitate will form, we
need to look at potential insoluble products.
Table 4.1 lists eight solubility rules for ionic
compounds. These rules apply to the most
common ionic compounds.
Types of Chemical Reactions
Precipitation Reactions
• Predicting Precipitation Reactions.
Suppose you mix together solutions of
nickel(II) chloride, NiCl2, and sodium
phosphate, Na3PO4.
NiCl 2  Na3 PO 4 
How can you tell if a reaction will occur, and if
it does, what products to expect?
Types of Chemical Reactions
Precipitation Reactions
• Predicting Precipitation Reactions.
Precipitation reactions have the form of an
“exchange reaction.”
NiCl 2  Na3 PO 4  Ni 3 ( PO 4 )2  NaCl
An exchange (or metathesis) reaction is a
reaction between compounds that, when written
as a molecular equation, appears to involve an
exchange of cations and anions.
Types of Chemical Reactions
Precipitation Reactions
• Predicting Precipitation Reactions.
Now that we have predicted potential products,
we must balance the equation.
3 NiCl 2 2 Na3 PO 4  Ni 3 ( PO 4 )2  6 NaCl
We must verify that NiCl2 and Na3PO4 are
soluble and then check the solubilities of the
products.
Types of Chemical Reactions
Precipitation Reactions
• Predicting Precipitation Reactions.
Table 4.1 indicates that our reactants, nickel(II)
chloride and sodium phosphate are both
soluble.
3NiCl 2(aq)  2Na3 PO 4(aq) 
Ni 3 ( PO 4 )2 (s)  6NaCl (aq)
Looking at the potential products we find that
nickel(II) phosphate is not soluble although
sodium chloride is.
Types of Chemical Reactions
Precipitation Reactions
• Predicting Precipitation Reactions.
We predict that a reaction occurs because
nickel(II) phosphate is insoluble and precipitates
from the reaction mixture.
To see the reaction that occurs on the ionic
level, we must rewrite the molecular equation
as an ionic equation.
Types of Chemical Reactions
Precipitation Reactions
• Predicting Precipitation Reactions.
First write strong electrolytes (the soluble ionic
compounds) in the form of ions to obtain the
complete ionic equation
2


3
3Ni (aq)  6Cl (aq)  6Na (aq)  2PO 4 (aq) 
Ni 3 ( PO 4 )2 (s )  6Na  (aq )  6Cl  (aq )
Types of Chemical Reactions
Precipitation Reactions
• Predicting Precipitation Reactions.
After canceling the spectator ions, you
obtain the net ionic equation.
2

3

3Ni (aq)  6Cl (aq)  6Na (aq)  2PO 4 (aq) 


Ni 3 ( PO 4 )2 (s )  6Na (aq )  6Cl (aq )
2
3
3Ni (aq )  2PO 4 (aq )  Ni 3 ( PO 4 )2 (s )
This equation represents the “essential” reaction.
Types of Chemical Reactions
• Acid-Base Reactions
Acids and bases are some of the most
important electrolytes. (see Table 4.2)
They can cause color changes in certain dyes
called acid-base indicators.
Household acids and bases. (see Figure 4.7)
Red cabbage juice as an acid-base indicator.
(see Figure 4.8)
Types of Chemical Reactions
Acid-Base Reactions
• The Ancient Concept
In ancient times acids and bases had a different
meaning.
An acid was defined as a sour substance.
A Base was defined as a substance that was both bitter
and slippery
Types of Chemical Reactions
Acid-Base Reactions
• The Arrhenius Concept
The Arrhenius concept defines acids as substances
that contain H and produce hydrogen ions, H+,
when dissolved in water.
An example is nitric acid, HNO3, a molecular substance
that dissolves in water to give H+ and NO3-.


HNO 3 (aq ) 
 H (aq )  NO 3 (aq )
H 2O
Types of Chemical Reactions
Acid-Base Reactions
• The Arrhenius Concept
The Arrhenius concept defines bases as substances
that contain OH and produces hydroxide ions,
OH-, when dissolved in water.
An example is sodium hydroxide, NaOH, an ionic
substance that dissolves in water to give sodium
ions and hydroxide ions.


NaOH(s) 
 Na (aq)  OH (aq)
H 2O
He really meant contain OH-
Types of Chemical Reactions
Acid-Base Reactions
• The Arrhenius Concept
However, there are substances that we now
classify as bases or acids but they do not follow the
Arrhenius definition.
For example ammonia, NH3, is a base but it
does not contain OH-,




NH 3 (aq )  H 2O(l )  NH 4 (aq )  OH (aq )
Therefore we need a second definition that can
take compounds like ammonia into account.
Types of Chemical Reactions
Acid-Base Reactions
• The Brønsted-Lowry Concept
The Brønsted-Lowry concept of acids and
bases avoids the problems of composition
inherent in the Arrhenius definitions by basing
the definitions on the transfer of protons (H+)
instead.
In this view, acid-base reactions are protontransfer reactions and there must be two
reactions taking place at once.
Types of Chemical Reactions
Acid-Base Reactions
• The Brønsted-Lowry Concept
The Brønsted-Lowry concept defines an acid as
the species (molecule or ion) that donates a
proton (H+) to another species in a protontransfer reaction.
A base is defined as the species (molecule or ion)
that accepts the proton (H+) in a proton-transfer
reaction.
Types of Chemical Reactions
Acid-Base Reactions
• The Brønsted-Lowry Concept
In the reaction of ammonia with water,


NH3 (aq)  H  O  H (l )  NH 4 (aq)  OH  (aq)
H+
– The H2O molecule is the acid because it donates a proton.
The NH3 molecule is a base, because it accepts a proton.
– Likewise NH4+ is an acid because it can donate one of the
protons, and OH- is a base since it can accept a proton.
Types of Chemical Reactions
Acid-Base Reactions
• The Brønsted-Lowry Concept
– The H+(aq) ion due to its small size has a very high
positive charge density.
– The polarity of the water molecules allowed for the
water molecules to be closely associated with the
proton making it appear as if the hydrogen ion was
bonded to water molecules as it moves.

•

H (aq )  H 2O(l )  H 3O (aq )
This “mode of transportation” for the H+ ion
is called the hydronium ion.
Types of Chemical Reactions
Acid-Base Reactions
• The Brønsted-Lowry Concept
The dissolution of nitric acid, HNO3, in water is
therefore a proton-transfer reaction

HNO 3 (aq )  H 2O(l )  NO 3 (aq )  H 3O  (aq )
H+
where HNO3 is an acid (proton donor) and H2O is a
base (proton acceptor).
Water and Acid/Base Rxns
Acid-Base Reactions
• As we have seen there are cases in which
water:
– Donates a Proton acting as an acid.
– Accepts a Proton acting as a base.
• Molecules can act both as acid or base
depending on the environment they are in
are called amphiprotic.
Types of Chemical Reactions
Acid-Base Reactions
• In summary, the Arrhenius concept is very
basic and the Brønsted-Lowry concept was
developed to cover cases left out;
however, in water they are almost the
same.
Arrhenius Concept
acid: proton (H+) donor to the water
base: hydroxide ion (OH-) donor to the water
Types of Chemical Reactions
Acid-Base Reactions
• In summary, any Arrhenius acid/base is
also a Brønsted-Lowry acid/base but not
the other way around.
The Brønsted-Lowry concept
acid: proton (H+) donor to anything
base: proton (H+) acceptor from anything
Acid/Base Concepts
Acid-Base Reactions
Lewis Acid/Base Concept
Bronsted-Lowry Acid/Base Concept
Arrhenius Acid/Base Concept
Types of Chemical Reactions
Acid-Base Reactions
• Arrhenius Acids/Bases
Acids can be separated into two subcategories
depending on the strength of the acid.
The Strength of the acid is determined by
how easily it releases the proton. The easier it
is to give the proton away the stronger the
acid.
A strong acid is an acid that ionizes
completely in water; it is a strong
electrolyte.
Strong Acids
Acid-Base Reactions
• Furthermore, if the proton comes off a
molecule easier than it comes off from the
solvent molecule then that acid is treated as
having the same strength as the solvent, this
is the leveling effect.
HNO3
HCl
HClO4
HBr
H2SO4
HI
These are 6
compounds that
give up their proton
readily, hence they
are strong acids in
water.
Weak Acids
Acid-Base Reactions
• Weak Acids
A weak acid is a molecule that holds on to its
proton tightly allowing for a very small
percentage of ionization, it is a weak
electrolyte.
HCN
HF
HC2H3O2
H2S
H3PO4
NH4+
These are 6
common weak acids
Strong Bases
Acid-Base Reactions
• Strong Bases
A strong base just like a strong acid is a
compound that dissociates completely.
Moreover, it readily accepts the proton given up
by an acid.
LiOH
Ca(OH)2
NaOH
Sr(OH)2
KOH
Ba(OH)2
These are 6
compounds that are
happy to accept any
proton given up by
an acid.
Weak Bases
Acid-Base Reactions
• Weak Bases
A weak base is a base that is only partially
ionized in water; it is a weak electrolyte.
It does not want to accept a proton and if it
does the new compound/ion is likely to give it
up the first chance it has.



NH 3 (aq )  H 2O(l )  NH 4 (aq )  OH (aq )
Types of Chemical Reactions
Acid-Base Reactions
• Strong and Weak Acids and Bases
You will find it important to be able to identify
an acid or base as strong or weak.
When you write an ionic equation, strong
acids and bases are represented as separate
ions.
Weak acids and bases are represented as
undissociated “molecules” in ionic equations
since they hardly dissociate.
Types of Chemical Reactions
Acid-Base Reactions
• Neutralization Reactions
One of the chemical properties of acids and bases
is that they neutralize one another.
A neutralization reaction is a reaction of an acid
and a base that results in an ionic compound and
water.
The ionic compound that is the product of a
neutralization reaction is called a salt.
HCN(aq)  KOH(aq)  KCN(aq)  H 2O(l )
acid
base
salt
Types of Chemical Reactions
Acid-Base Reactions
• Neutralization Reactions
The net ionic equation for each acid-base
neutralization reaction involves a transfer of a
proton.
Consider the reaction of the strong acid ,
HCl(aq) and a strong base, LiOH(aq).
HCl(aq)  KOH(aq)  KCl(aq)  H 2O(l )
Types of Chemical Reactions
Acid-Base Reactions
• Neutralization Reactions
Writing the strong electrolytes in the form of
ions (refer to Table 4.1 and 4.3) gives the
following complete ionic equation.




H (aq)  Cl (aq)  K (aq)  OH (aq) 


K (aq)  Cl (aq)  H 2O(l )
Types of Chemical Reactions
Acid-Base Reactions
• Neutralization Reactions
Canceling the spectator ions results in the net
ionic equation. Note the proton transfer.




H (aq)  Cl (aq)  K (aq)  OH (aq) 
K  (aq)  Cl  (aq)  H 2O(l )
H  (aq)  OH  (aq)  H 2O(l )
H+