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Chemical
Reactions
Chapter 5
Dr. Victor Vilchiz
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)
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 NH4 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 NH4 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)  FeSO4 (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
• 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.
( NH4 )2 Cr2O7 (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 CO2 (g )  10 H 2O(g )
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

Na2SO4 (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

Na2SO4 (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
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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
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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.
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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.
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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.
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Calcium/Chlorine Reaction
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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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