Chapter 4 : Chemical Quantities and Aqueous Reactions

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Chapter 4 Chemical Quantities and Aqueous
Reactions

• Outline
• Reaction Stoichiometry
• Limiting Reagent
• Solution Concentration and Solution Stoichiometry
• Acid-Base Reactions
• Precipitation Reactions
• Oxidation-Reduction (REDOX) Reactions

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Global Warming
Overwhelming evidence that the Greenhouse effect
is caused by CO2 realized from chemical reactions
(combustion) involving fossil fuels.
How much carbon dioxide is realized in a given
processes ?
Fossil Fuels
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Stoichiometry

• the numerical relationships between chemical
  amounts in a reaction is called stoichiometry

• the coefficients in a balanced chemical equation
  specify the relative amounts in moles of each of
  the substances involved in the reaction

2 C8H18(l) 25 O2(g) ? 16 CO2(g) 18 H2O(g)
Stoichiometry
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A balanced chemical equation allows us to relate
quantities of substances involved in the
corresponding reaction. VERY IMPT !
Stoichiometry
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How many moles of O2 are produced from the
decomposition of 3.21 moles of potassium chlorate
? Reaction KClO3 (s) 2KCl (s)
3O2 (g)
Stoichiometry
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Example
Estimate the mass of CO2 produced in 2004 by the
combustion of 3.4 x 1015 g gasoline.
Assume that gasoline is octane, C8H18.
Stoichiometry
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Limiting Reagents
Limiting Reagent
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For reactions with multiple reactants, it is
likely that one of the reactants will be
completely used before the others.
When this reactant is used up, the reaction stops
and no more product is made.
The reactant that limits the amount of product is
called the limiting reactant. sometimes called
the limiting reagent. the limiting reactant gets
completely consumed.
Reactants not completely consumed are called
excess reactants.
The amount of product that can be made from the
limiting reactant is called the theoretical yield
Limiting Reagent
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Yields
The calculated quantity of product that one
expects from given quantities of reactants is
called the theoretical yield.
The measured quantity of product that one
actually obtains is called the actual yield.
The quotient of the actual yield and the
theoretical yield (x100) defines the percent
yield.
Limiting Reagent
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Whats the limiting reagent when hydrogen gas is
produced by the reaction of 1.84 g Al with 0.221
moles of HCl ?
Limiting Reagent
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What is the percent yield if the reaction of
25.0g of P4 with 91.5 g Cl2 is found to produce
104 g PCl3 ? P4 (s) 6Cl2 (g)
4PCl3 (l)
1st Determine the limiting reagent 2nd Use
the L.R. to determine the theoretical yield 3rd
Determine percent yield
Limiting Reagent
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Practice How many grams of N2(g) can be made
from 9.05 g of NH3 reacting with 45.2 g of
CuO?2 NH3(g) 3 CuO(s) ? N2(g) 3 Cu(s) 3
H2O(l)
Limiting Reagent
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The foolproof method can be slow (especially if
you have lots of reactants !)
A faster method.
Limiting Reagent
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Solutions
Many chemical reactions are carried out in
solution because mixing facilitates the close
contact between atoms or molecules required for a
reaction to occur.
A solution is a homogeneous mixture of two or
more components (usually liquids but not always)
The solute is the matter which is dissolved in
the solvent.
Dilute solutions have a small amount of solute
compared to solvent whereas concentrated
solutions have a large amount of solute compared
to solvent.
Solutions
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Different kinds of solutions
State of solute State of solvent State of resulting solution Example
Gas Gas Gas Air
Gas Liquid Liquid O2 dissolved in water
Gas Solid Solid H2 gas in Palladium
Liquid Liquid Liquid Beer ethanol in water
Liquid Solid Solid Mercury in gold
Solid Liquid Liquid NaCl in water
Solid Solid Solid Wedding rings Ni in Au (alloys)
Solutions
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Molarity
The concentration of a solution is given by its
molarity (M)
Solutions
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Examples
1) What is the molarity of an ammonium chloride
solution made by dissolving 0.2513 moles of
ammonium chloride in sufficient water to give
0.5000 L of solution ?
Bonus question What is the molarity of an
ammonium chloride solution made by dissolving
0.2513 grams of ammonium chloride in sufficient
water to give 0.5000 L of solution ?
Solutions
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Sometimes you need to work a bit to get the moles
of solute
Solute Mass (g)
Solution Molarity
Use unit conversion
Use solutes molar mass
Use molarity equation
Solute mass (kg, g, mg, etc)
Moles of solute
Use solutes density
Solute volume (L, mL, cm3, etc)
Solutions
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2) A solution is prepared by dissolving 25.0 mL
of C2H5OH (d 0.789 g/mL) in enough water to give
250.0 mL of solution. What is the molarity of
ethanol in this solution ?
Solutions
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Dilutions
Solutions
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Example 4.7 To what volume should you dilute
0.200 L of 15.0 M NaOH to make 3.00 M NaOH?
25.00 mL of a 0.1150 M stock solution of KMnO4
(potassium permanganate) is transferred to a
250.0 mL volumetric flask. The flask is made up
to volume with water. What is the final
concentration of KMnO4 in the flask ?
Solutions
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Stoichiometry of Reactions in Solution
Pb(NO3)2 (aq) 2KCl (aq)
2KNO3 (aq) PbCl2 (s)
What mass of lead (II) chloride will form from
the addition of 25.00 mL of 0.100M KCl to an
excess of lead (II) nitrate ?
Reaction in Solutions
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Solubility

• There are attractive forces between the solute
  particles holding them together.

• There are also attractive forces between the
  solvent molecules.

• When we mix the solute with the solvent, there
  are attractive forces between the solute
  particles and the solvent molecules.

• If the attractions between solute and solvent are
  strong enough, the solute will dissolve.

Solubility
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Each ion is attracted to the surrounding water
molecules and pulled off and away from the crystal
When it enters the solution, the ion is
surrounded by water molecules, insulating it from
other ions
The result is a solution with free moving charged
particles able to conduct electricity
Solubility
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Electrolytes and Nonelectrolytes

• materials that dissolve in water to form a
  solution that will conduct electricity are called
  electrolytes

• materials that dissolve in water to form a
  solution that will not conduct electricity are
  called nonelectrolytes

Solubility
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Molecular View of Electrolytes and Nonelectrolytes
In order to conduct electricity, a material must
have charged particles that are able to flow.
Electrolyte solutions all contain ions dissolved
in the water.
Nonelectrolyte solutions contain whole molecules
dissolved in the water.
Solubility
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Solubility Rules
Some metal salts are soluble in water and some
arent.

• Solubility Rules
• Na, K, and NH4 salts are soluble.
• Nitrates, acetates, and perchlorates are soluble.
• Ag, Pb, and Hg(I) salts are insoluble.
• Chlorides, bromides, and iodides are soluble.
• Carbonates, sulfides, oxides and hydroxides are
  insoluble.
• Sulfates are soluble except for Ca(SO4) and
  Ba(SO4).

These rules must be applied in order ! The rule
with the smaller number takes precedence.
Solubility
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Salt vs. Sugar Dissolved in Water
Solubility
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Solubility
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Ion Concentrations of Strong Electrolytes
Consider what happens when 0.100 mol of Al(NO3)3
(a strong electrolyte) is added to 500 mL of water
Solubility
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Reactions in Solutions
We will now begin to look at some specific types
of reactions in solution.
1. Precipitation Reactions
2. Acid-Base Reactions
3. REDOX Reactions
Acids and Bases
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Precipitation Reactions
Some salts are quite soluble in water while
others are not. A precipitation reaction occurs
when an anion and a cation combine, in solution,
to form an insoluble solid called a precipitate.
Precipitation Reactions
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How do you make a precipitate ?
You need a cation that makes a soluble salt with
some anions but an insoluble salt with other ions.
Example Pb(NO3)2 is soluble but Pbl2 is not
The reaction between two soluble salts is
sometimes called a double displacement reaction
Precipitation Reactions
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KI is a strong electrolyte. It fully dissolves in
water.
Pb(NO3)2 is a strong electrolyte. It also fully
dissolves in water.
KNO3 is a strong electrolyte. It too fully
dissolves in water.
Precipitation Reactions
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Just mixing two solutions of two different salts
does not guarantee a precipitation reaction.
Example Addition of KI solution to a NaCl
solution
Precipitation Reactions
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KI is a strong electrolyte. It fully dissolves in
water.
NaCl is a strong electrolyte. It also fully
dissolves in water.
KCl is a strong electrolyte. It too fully
dissolves in water.
NaI is a strong electrolyte. It too fully
dissolves in water.
Precipitation Reactions
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Writing Equations for Precipitation Reactions

• Determine what ions each aqueous reactant has
• Determine formulas of possible products
• Exchange ions
• () ion from one reactant with (-) ion from other
• Balance charges of combined ions to get formula
  of each product
• Determine Solubility of Each Product in Water
• Use the solubility rules
• If product is insoluble or slightly soluble, it
  will precipitate
• If neither product will precipitate, write no
  reaction after the arrow

Precipitation Reactions
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Examples
Write equations for the precipitation reactions
that might occur by mixing the following
compounds.
1. Potassium carbonate nickel (II) chloride
2. Sodium nitrate lithium sulfate
Precipitation Reactions
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Net Ionic Equations
Add NaCl to Ag(NO3)
Na(aq) Cl-(aq) Ag(aq) NO3-(aq)
AgCl(s) Na(aq) NO3-(aq)
Note that certain ions appear on both sides of
the reaction. These ions DO NOT PARTICIPATE in
the reaction. They are called spectator ions.
The reaction written without spectator ions is
called the net ionic equation (NIE).
Precipitation Reactions
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Examples of Precipitation Reactions
For each of the following cases, predict whether
a precipitate forms. If so write a net ionic
equation if not state no reaction.
a) AlCl3 (aq) KOH (aq)
b) K2SO4 (aq) FeBr3 (aq)
c) aluminum sulfate barium chloride
d) ammonium carbonate lead (II) nitrate
Precipitation Reactions
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Acids and Bases
Arrhenius Definitions
Acids provide H ions (protons) in water.
Bases provide OH- ions in water.
HCl(aq) ? H(aq) Cl-(aq)
HCl(aq) H2O ? H3O(aq) Cl-(aq)
KOH(aq) ? K(aq) OH-(aq)
Acids and Bases
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an acid and base in the same solution will
always react with each other
H (aq) OH- (aq) ? H2O (l)
Acids and Bases
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Strong acids and bases react with each other to
cancel (neutralize) the properties of the other.
The reaction products are water and a salt.
Example HCl (aq) KOH (aq)
H (aq) Cl- (aq) K (aq) OH- (aq)
H2O (l) K (aq) Cl- (aq)
Net Ionic Reaction (remove spectator ions)
Acid-Base Reactions
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In the previous example, both the acid and the
base were strong. Can weak acids and weak bases
also participate in neutralization reactions ?
Strong acid (HCl) Weak base (NH3)
H (aq) Cl- (aq) NH3 (aq) NH4 (aq)
Cl- (aq)
Weak acid (HF) Strong base (NaOH)
Acid-Base Reactions
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Titrations
In many reactions, it is desirable to have as
high a yield as possible of the products. This
generally requires an excess of at least one of
the reactants (usually the cheapest)
However, if we wish to quantitate how much of a
reactant we have we need to carry out the
reaction such that neither reactant is in excess.
One means to do this is known as titration.
Acid-Base Reactions
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Example 4.14The titration of 10.00 mL of HCl
solution of unknown concentration requires 12.54
mL of 0.100 M NaOH solution to reach the end
point. What is the concentration of the unknown
HCl solution?
Acid-Base Reactions
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How are titrations performed in practice ?
Titrations require some special equipment.
Acid-Base Reactions
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The base solution is the titrant in the burette.
As the base is added to the acid, the H reacts
with the OH to form water. But there is still
excess acid present so the color does not change.
At the titrations endpoint, just enough base has
been added to neutralize all the acid. At this
point the indicator changes color.
Acid-Base Reactions
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More Acid-Base Reactions

• Some reactions form a gas directly from the ion
  exchange
• K2S(aq) H2SO4(aq) ? K2SO4(aq) H2S(g)

• Other reactions form a gas by the decomposition
  of one of the ion exchange products into a gas
  and water
• K2SO3(aq) H2SO4(aq) ? K2SO4(aq) H2SO3(aq)
• H2SO3 ? H2O(l) SO2(g)

Acid-Base Reactions
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NaHCO3(aq) HCl(aq) ? NaCl(aq) CO2(g) H2O(l)
Acid-Base Reactions
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Compounds that UndergoGas Evolving Reactions
Reactant Type Reacting With Ion Exchange Product Decom-pose? Gas Formed Example
metalnS, metal HS acid H2S no H2S K2S(aq) 2HCl(aq) ? 2KCl(aq) H2S(g)
metalnCO3, metal HCO3 acid H2CO3 yes CO2 K2CO3(aq) 2HCl(aq) ? 2KCl(aq) CO2(g) H2O(l)
metalnSO3 metal HSO3 acid H2SO3 yes SO2 K2SO3(aq) 2HCl(aq) ? 2KCl(aq) SO2(g) H2O(l)
(NH4)nanion base NH4OH yes NH3 KOH(aq) NH4Cl(aq) ? KCl(aq) NH3(g) H2O(l)
Acid-Base Reactions
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REDOX Reactions
In some reactions, electrons are actually
transferred from one reactant to another. The
rusting of iron, the bleaching of hair, and the
production of electricity in batteries are all
examples of these reactions which are called
REDOX reactions.
Redox Reactions
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Oxidation States (O.S.)
To identify REDOX reactions we first must learn
how to identify oxidation states of atoms in
molecules, ions and salts.
Oxidation states are a convenient tool to help
describe chemical compounds. They are used to
describe the number of electrons an atom loses or
gains when it joins other atoms to make a
compound.
Redox Reactions
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Imperfect Rules for O.S.s
Rules are in order of priority

• 1. free elements have an oxidation state 0
• 2. monatomic ions have an oxidation state equal
  to their charge
• 3. (a) the sum of the oxidation states of all
  the atoms in a neutral compound is 0
• (b) the sum of the oxidation states of all the
  atoms in a polyatomic ion equals the charge on
  the ion

Redox Reactions
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• 4. (a) Group I metals have an oxidation state of
  1 in all their compounds
• (b) Group II metals have an oxidation state of
  2 in all their compounds

• 5. in their compounds, nonmetals have oxidation
  states according to the table below
• nonmetals higher on the table take priority

Nonmetal Oxidation State Example
F -1 CF4
H 1 CH4
O -2 CO2
Group 7A -1 CCl4
Group 6A -2 CS2
Group 5A -3 NH3
Redox Reactions
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A little help from the periodic table (typical
o.s. in compounds)
Redox Reactions
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Practice

• 1. Assign an oxidation state to each atom in the
  following compounds
• (a) HClO4 (b) KNO3 (c) Mn2O7 (d) CrO4-2 (e) NH4
• Assign an oxidation state to antimony in each of
  the following
• (a) SbCl3 (b) Sb4O6 (c) SbF52- (d) SbCl63-

Redox Reactions
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More Practice Assign an Oxidation State to Each
Element in the following

• Br2
• K
• LiF
• CO2
• SO42-
• Na2O2

Redox Reactions
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Identifying Redox Reactions
In a REDOX reaction there is an exchange of
electrons between the reactants. This means the
oxidation state of at least one of the atoms in
the reacting species must change.
In a REDuction reaction the O.S. of some element
gets reduced (it decreases)..
In an OXidation reaction the O.S. of some element
gets increased.
Redox Reactions
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Reduction - Oxidation (REDOX) Reactions
Consider the oxidation states of all the atoms in
the following rxn
Pb (s) 2 H (aq) H2(g) Pb2 (aq)
A REDOX reaction involves electron transfer among
reacting species and results in changes in the
o.s. of atoms.
Redox Reactions
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Reducing and Oxidizing Agents

• An oxidizing agent (oxidant)
• Contains an element whose oxidation state
  decreases in a redox reaction.
• Gains electrons in a redox reaction

• A reducing agent (reductant)
• Contains an element whose oxidation state
  increases in a redox reaction.
• Loses electrons in a redox reaction.

Redox Reactions
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Half Reactions
Reconsider this REDOX reaction
Pb (s) 2 H (aq) H2(g) Pb2 (aq)
Redox Reactions
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Identifying Half Reactions
Usually, identifying the half reactions from the
skeleton reaction is straightforward.
Redox Reactions
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Example Question
1) Identify the oxidation states of the elements
in the following reaction.
5VO2(aq) MnO4-(aq) H2O(l) 5VO2(aq)
Mn2(aq) 2H(aq)
2) Which species is reduced ? Which species is
oxidized ?
3) Which species is the reducing agent ? Which
species is the oxidizing agent ?
Redox Reactions
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Common Reducing and Oxidizing Agents
Common Reducing Agents O.S. of atom Common Oxidizing Agents O.S. of atom
BH4- MnO4- (aq)
N2H4 (aq) CrO4- 2 (aq)
Na (s) O2 (g)
H2 (g) O3 (g)
ClO- (aq)
Redox Reactions
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More examples to try from Chapter 4

1. Determine the stoichiometric coefficients (i.e.
   balance) in the following chemical equation
2. How many moles of Ag are produced in the
   decomposition of 1.00 kg of silver(I) oxide?
3. 25.00 mL of a 0.1150 M stock solution of KMnO4
   (potassium permanganate) is transferred to a
   250.0 mL volumetric flask. The flask is made up
   to volume with water. What is the final
   concentration of KMnO4 in the flask ?

Redox Reactions
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