Transcript Chap 4

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 H12O 6 (s ) (glucose)  C6 H12O 6 (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)  SO 4 (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)  Na2CO 3 (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( NO 3 )2 (aq )  K 2CO 3 (aq )  CaCO3 (s )  2KNO 3 (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.
2HNO 3 (aq)  Mg(OH )2 (s )  2H 2O(l )  Mg( NO 3 )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 )  AgNO 3 (aq )  AgCl(s )   NaNO 3 (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 Na 3 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,


NH 3 (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+
Types of Chemical Reactions
Acid-Base Reactions
• Neutralization Reactions
In a reaction involving HCN(aq), a weak acid,
and KOH(aq), a strong base, the product is
KCN, a strong electrolyte
Referring to Tables 4.1, 4.2 and 4.3, we obtain
this net ionic equation:


HCN(aq)  OH (aq)  CN (aq)  H 2O(l )
H+
Note the proton transfer.
Types of Chemical Reactions
Acid-Base Reactions
• Acid-Base Reactions with Gas Formation
Carbonates react with acids to form CO2,
carbon dioxide gas.
Na2CO3  2HCl  2NaCl  H 2O  CO2 
Sulfites react with acids to form SO2, sulfur
dioxide gas.
Na2SO 3  2HCl  2NaCl  H 2O  SO 2 
Gas Production in
Neutralization Reactions
• The previous two reactions are overall
reactions of the actual molecular events.
Na2CO 3  HCl  NaCl  NaHCO 3
NaHCO 3  Na  HCO 3


HCO 3  OH   CO 2
OH   HCl  H 2O  Cl 
Cl   Na  NaCl
Na2CO3  2HCl  2NaCl  H 2O  CO2 
Types of Chemical Reactions
Acid-Base Reactions
• Acid-Base Reactions with Gas Formation
Sulfides react with acids to form H2S, hydrogen
sulfide gas.
Na 2S  2HCl  2NaCl  H 2S 
• The Driving Force of Neutralization
reactions like that in precipitation
reactions is the removal of ions from
solution in this case to form water.
Working with Solutions
• The majority of chemical reactions
discussed so far occur in aqueous
solution.
When you run reactions in liquid solutions, it is
convenient to dispense the amounts of reactants
by measuring out volumes of reactant solutions
and not mass.
Solution Stoichiometry
• Molarity is the measurement of the
concentration of a chemical in solution.
– The unit of molarity is the Molar (M).
moles of solute
Molarity 
L of solution
Example: Calculate the molarity of a solution made by dissolving
12.94g of Ca(OH)2 in enough water to make 1.23L of solution.
12.94g Ca(OH) 2 x
Molarity 
1mol Ca(OH) 2
 0.175mol Ca(OH) 2
74.1g Ca(OH) 2
0.175mol Ca(OH) 2
 0.142M Ca(OH) 2
1.23L of solution
(see Figure 4.19)
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Types of Oxidation-Reduction Reactions
Most of the oxidation-reduction reactions fall into
one of the following simple categories:
Combination Reactions
Decomposition Reactions
Displacement Reactions
Combustion Reactions
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Combination Reactions
A combination reaction is a reaction in which
two substances, usually two elements, combine
to form a third substance.
2 Na( s)  Cl2 ( g )  2 NaCl( s)
Sodium and chlorine combine in a fiery
reaction. (see Figure)
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Combination Reactions
Other combination reactions involve
compounds as reactants.
CaO(s )  SO 2 (g )  CaSO3 (s )
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Decomposition Reactions
A decomposition reaction is a reaction in which
a single compound reacts to give two or more
substances.
( NH 4 )2 Cr2O 7 (s )  Cr2O 3 (s )  4H 2O(g )  N 2 (g )
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Displacement Reactions
A displacement reaction (also called a singlereplacement reaction) is a reaction in which an
element reacts with a compound, displacing an
element from it.
Zn(s )  2HCl(aq)  ZnCl 2 (aq)  H 2 (g )
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Combustion Reactions
A combustion reaction is a reaction in which a
substance reacts with oxygen, usually with the
rapid release of heat to produce a flame.
2 C4 H10 (g )  13 O 2 (g )  8 CO 2 (g )  10 H 2O(g )
Molarity
Example: how many grams of ammonium nitrate
are in a 172.7mL sample of 1.21M NH4NO3
solution?
moles NH4 MNO3
0.1727L of solution
moles NH 4 NO3  1.21M NH4 NO3 x 0.1727L of solution 
1.21M NH4 NO3 
0.209moles NH4 NO3
0.209moles NH4 NO3 
mass NH4 NO3
80.0g NH4 NO3
mass NH4 NO3  0.209moles NH4 NO3 x 80.0g NH 4 NO3 
16.7g NH4 NO3
Diluting Solutions
• When diluting a solution the number of
moles is constant.
(Molarity)(Volume) =moles
M1xV1 = n = M2xV2
• So, as water is added, increasing the
final volume, V2, the final molarity, M2,
decreases.
M2=M1xV1/V2
Acid Base Titrations
• A titration is a laboratory technique used
to determine the concentration of a
solution sample from the volume of a
known concentration solution required to
complete a given reaction.
• Titrations are usually used to determine
the concentration of acids or bases.
Acid/Base Indicators
• Most acids and bases as well as the
resulting salt solution are colorless.
– In order to determine when the reaction is
complete, we must use chemical indicators.
• Chemical Indicators in the case of acid/base
reactions are weak acids that have the property of
changing colors when going from basic to acidic
solutions or vice-versa.
• The most used acid/base indicator is
phenolphthalein.
Indicators
• The job of the indicator is to signal to you the
•
point when you are done with the experiment.
The point when the color changes is defined as
the end point.
The equivalence point is not the same as the
end point, ideally it should be but those
occasions are rare. The equivalence point is
when the amount of titrant added is exactly the
amount needed to “neutralize” the analyte in
the flask.
Types of Chemical Reactions
• Oxidation-Reduction (RedOx) Reactions
RedOx reactions are by far the most
important type of reactions.
RedOx reactions involve the transfer of
electrons from one species to another.
Oxidation is defined as the loss of electrons.
Reduction is defined as the gain of electrons.
Oxidation and reduction always occur
simultaneously, since the electrons lost in the
Oxidation must go somewhere.
Types of Chemical Reactions
• Oxidation-Reduction Reactions
The reaction of an iron nail with a solution of
copper(II) sulfate, CuSO4, is an oxidationreduction reaction. (see Figure 4.11)
The molecular equation for this reaction is:
Fe(s )  CuSO4 (aq)  FeSO 4 (aq)  Cu(s )
Types of Chemical Reactions
• Oxidation-Reduction Reactions
The net ionic equation shows the reaction of
iron metal with Cu2+(aq) to produce iron(II) ion
and copper metal.
Loss of 2 e-1 oxidation
2
2
Fe(s )  Cu (aq)  Fe (aq)  Cu(s )
Gain of 2 e-1 reduction
RedOx Reactions
• The species that is reduced itself causes
another species to be oxidized and is
therefore known as the oxidizing agent.
• Similarly, the species that are oxidized
causes another to be reduced and is
therefore known as the reducing agent.
– Note: An element is reduced/oxidized
the compound containing that element
is the oxidizing/reducing agent.
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Oxidation Numbers
The concept of oxidation numbers is a simple
way of keeping track of electrons in a reaction.
The oxidation number (or oxidation state) of
an atom in a substance is the actual charge of
the atom if it exists as a monatomic ion.
Alternatively, it is hypothetical charge assigned
to the atom in the substance by simple rules.
Oxidation Number Rules
Oxidation Numbers
• It is possible to predict the upper and
lower limits of main group elements…
– The upper limit is equal to the group number
– The lower limit is the group number-8
• Keep in mind couple things like… Oxygen will
never have an ON=+6 and Fluorine will never
have an ON=+7
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Describing Oxidation-Reduction Reactions
Look again at the reaction of iron with
copper(II) sulfate.
2
2
Fe(s )  Cu (aq)  Fe (aq)  Cu(s )
The Iron losses 2 electrons so it is oxidized and at
the same time it is the reducing agent.
The Copper gains 2 electrons and so it is reduced
and at the same time it is the oxidizing agent.
Writing RedOx reactions
• There are two ways to deal with RedOx
reactions (balancing purposes):
– Treat them as any other reaction
– We can write this reaction in terms of two
half-reactions.
• Driving force for these type of reactions is
the exchange of electrons.
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Describing Oxidation-Reduction Reactions
A half-reaction is one of the two parts of an
oxidation-reduction reaction. One involves the
loss of electrons (oxidation) and the other
involves the gain of electrons (reduction).
2

Fe(s )  Fe (aq)  2e
2

Cu (aq)  2e  Cu(s )
oxidation half-reaction
reduction half-reaction
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Balancing Simple Oxidation-Reduction Reactions
At first glance, the equation representing the
reaction of zinc metal with silver(I) ions might
appear to be balanced.

2
Zn(s )  Ag (aq)  Zn (aq)  Ag(s )
However, a balanced equation must have a
charge balance as well as a mass balance.
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Balancing Simple RedOx Reactions
As mentioned before we can split the reaction
into two half-cells before balancing. You will
learn this method in Chem 102 when you cover
chapter 20.
We will balance RedOx reactions using the
Oxidation Number method.
Types of Chemical Reactions
Oxidation-Reduction Reactions
• Balancing Simple RedOx Reactions
– There are some steps to follow to balance these
reactions.
• Assign ON to ALL elements in the reaction
• Identify the species that are oxidized/reduced
• Compute the number of e-s lost in OX and gained in RED
draw lines between the two pairs including the number of
e-s lost/gained
• Multiply one or both reactions so that both numbers
match, use these factors as balancing coefficients
• Balance any other species that were not involved in the
electron exchange.
Quantitative Analysis
• Analytical chemistry deals with the
determination of composition of materialsthat is, the analysis of materials.
Quantitative analysis involves the
determination of the amount of a substance or
species present in a sample of material.
Quantitative Analysis
Gravimetric Analysis
• Gravimetric analysis is a type of
quantitative analysis in which the amount
of a species in a material is determined by
converting the species into a product that
can be isolated and weighed.
Precipitation reactions are often used in
gravimetric analysis.
The precipitate from these reactions is then
filtered, dried, and weighed.
Quantitative Analysis
Gravimetric Analysis
• Consider the problem of determining the
amount of lead in a sample of drinking
water.
Adding sodium sulfate (Na2SO4) to the sample
will precipitate lead(II) sulfate.
2

Na2SO 4 (aq)  Pb (aq)  2Na (aq)  PbSO 4 (s )
The PbSO4 can then be filtered, dried, and
weighed. This figure shows a similar
procedure.
Quantitative Analysis
Gravimetric Analysis
• Suppose a 1.00 L sample of polluted water was
analyzed for lead(II) ion, Pb2+, by adding an
excess of sodium sulfate to it. The mass of
lead(II) sulfate that precipitated was 229.8 mg.
What is the mass of lead in a liter of the
water? Express the answer as mg of lead per
liter of solution.
2

Na2SO 4 (aq)  Pb (aq)  2Na (aq)  PbSO 4 (s )
Quantitative Analysis
Gravimetric Analysis
• First we must obtain the mass percentage of
lead in lead(II) sulfate, by dividing the molar
mass of lead by the molar mass of PbSO4, then
multiplying by 100.
207.2 g/mol
%Pb 
 100  68.32%
303.3 g/mol
Then, calculate the amount of lead in the PbSO4
precipitated.
Amount Pb in sample  229.8 mg PbSO 4  0.6832  157.0 mg Pb
Quantitative Analysis
Volumetric Analysis
• An important method for determining the
amount of a particular substance is based on
measuring the volume of the reactant solution.
Titration is a procedure for determining the
amount of substance A by adding a carefully
measured volume of a solution with known
concentration of B until the reaction of A and B
is just complete.
Volumetric analysis is a method of analysis
based on titration.
Chemical Reactions
Summary
• Reactions often involve ions in aqueous solution.
Many of these compounds are electrolytes.
We can represent these reactions as molecular
equations, complete ionic equations (with strong
electrolytes represented as ions), or net ionic equations
(where spectator ions have been canceled).
Most reactions are either precipitation reactions, acidbase reactions, or oxidation-reduction reactions.
Acid-base reactions are proton-transfer reactions.
Oxidation-reduction reactions involve a transfer of
electrons from one species to another.
Chemical Reactions
Summary
• Oxidation-reduction reactions are the most
•
important type of reactions.
Oxidation-reduction reactions usually fall into the
following categories: combination reactions,
decomposition reactions, displacement reactions,
and combustion reactions.
Molarity is defined as the number of moles of solute per liter
of solution. Knowing the molarity allows you to calculate the
amount of solute in a given volume of solution.
Quantitative analysis involves the determination of the
amount of a species in a material.
Chemical Reactions
Summary
In gravimetric analysis, you determine the
amount of a species by converting it to a product
you can weigh.
In volumetric analysis, you determine the amount of a
species by titration.
Operational Skills
Using solubility rules.
Writing net ionic equations.
Deciding whether precipitation occurs.
Classifying acids and bases as weak or strong.
Writing an equation for a neutralization.
Writing an equation for a reaction with gas formation.
Assigning oxidation numbers.
Balancing simple oxidation-reduction reactions.
Calculating molarity from mass and volume.
Using molarity as a conversion factor.
Diluting a solution.
Determining the amount of a substance by gravimetric
analysis.
Calculating the volume of reactant solution needed.
Calculating the quantity of a substance by titration.
Dissociation
Return to Lecture
Electrolytic Solutions
Negatively charged
ions move towards
the positive electrode
and positively charged
ions moves towards
the negative electrode
Return to Lecture
Conductivity
Since there are no “free” ions on non-electrolytic solutions there is no flow of
energy and the bulb does not lit. The “free” ions on electrolytic solutions
completer the circuit and allow energy to flow lighting the bulb.
Return to Lecture
Weak vs Strong Electrolytes
While both types of electrolytes conduct electricity the amount of “free”
ions manifests itself by the brightness of the light emitted by the
bulb.
Return to Lecture
Water Solvation of Ions
Return to Lecture
Precipitation
Mixing KI (aq) and
Pb(NO3)2 (aq)
leading to
precipitation of
PbI2
Return to Lecture
Figure 4.5: Limestone Formations.Photo ©Corbis.
Return to
Lecture
Return to Lecture
Return to Lecture
Figure 4.7: Household acids and bases.
Return to
Lecture
Photo courtesy of American Color.
Figure 4.8:
Preparation of red
cabbage juice as an
acid-base
indicator.Photo courtesy of
James Scherer.
Return to Lecture
Neutralization Reaction
Neutralization reaction between Acetic Acid (Vinegar)
and Baking Soda (Sodium Bicarbonate).
Making a Solution
a)
b)
c)
a) Measure the mass of the compound to dissolve, b)make sure all
solid makes it into the volumetric flask using the solvent, then dilute
to the bottom of the neck, and c) add the last amount of solvent
carefully making sure not to go past the volumetric mark.
Return to Lecture
Titration
a)
b)
c)
Titration of an acid with a base. a) the flask
contains acid and a few drops of
phenolphthalein, which is colorless in acidic
conditions, b) the endpoint has been reached
(notice the faint pink color of the solution),
finally in c), the solution is well beyond the end
point since more base was added.
Return to Lecture
Electron Exchange
Electrons from the iron nail are transferred to the copper in
solution, the solid copper plates the nail. Notice the color
change in the solution, this is due to the lower amount of
copper ions in solution.
Return to Lecture
RedOx Summary
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Figure 4.13: A representation of an oxidation reduction reaction.
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Figure 4.14: Oxidation reduction reaction of
mercury (III) oxide into its elements. Photo courtesy
of James Scherer.
Return to Lecture
Figure 4.15:
Oxidation reduction
reaction of zinc
metal and
hydrochloric acid.
Photo courtesy of American
Color.
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Figure 4.16: Oxidation reduction
reaction of iron wool and
oxygen. Photo courtesy of James Scherer.
Return to Lecture
Calcium/Chlorine Reaction
Return to Lecture
Gravimetric Analysis
A solution of potassium chromate is poured down a stirring rod
into a solution containing an unknown amount of barium ion to
precipitate Barium Chromate which is then filtered dried and
weighed.
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