Transcript No Slide Title

CHE 111 - Module 4
CHAPTER 4 & 5
LECTURE NOTES
Stoichiometry & Balancing
Equations
• Remember we stated in the previous chapter
that stoichiometry is the study of the
quantitative relationships between the
amounts of reactants and products in
chemical reactions.
• We use BALANCED equations to
understand stoichiometric relationships of
the elements and compounds within a
chemical reaction.
The Balanced Equation
2Al(s) + 3Br2(l)  Al2Br6(s)
2mol of Al : 3mol of Br2 : 1mol of Al2Br6
2 atoms of Al
6 atoms of Br
=
=
2 atoms of Al
6 atoms of Br
The number of the same atom of each element
must be equal on each side of the equation.
A Closer Look at the Equation
2Al(s) + 3Br2(l)  Al2Br6(s)
• The chemicals on the left are the reactants and the
right are the products.
• The coefficient in front of the chemical denotes
the stoichiometric relationship.
• The numerical subscript represents the number of
atoms present in the molecule.
• The letter subscripted denotes the phase of matter.
Balancing Equations
For example the following is balanced.
CH4 + 2O2  CO2 + 2H2O
Try to balance the following:
• Fe2S3 + O2  Fe2O3 + S
• Al + H2SO4  Al2(SO4)3 + H2
• Ca + Al2Br6  CaBr2 + Al
Balanced Equations
Check your answer from the previous slide:
• 2Fe2S3 + 3O2  2Fe2O3 + 6S
• 2Al + 3H2SO4  Al2(SO4)3 + 3H2
• 3Ca + Al2Br6  3CaBr2 + 2Al
Types of Reactions
• Combination Reactions
• Decomposition Reactions
• Displacement (Single-Replacement)
Reactions
• Metathesis (Double-Replacement)
Reactions
• Combustion Reactions
Combination Reactions
• A combination reaction is a reaction
where two substances chemically
combine to form another substance.
A + B  AB
2Na(s) + Cl2(g)  2NaCl(s)
P4(s) + 6Cl2(g)  4PCl3(s)
Decomposition Reaction
• A decomposition reaction is when a
single compound decomposes into two
or more other substances.
AB  A + B
2KClO3(s)  2KCl(s) + 3O2(g)
2NO2(g)  2NO(g) + O2(g)
Displacement Reaction
• A displacement (single replacement)
reaction is a reaction where one
element displaces another element.
A + BC  B + AC
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Metathesis Reaction
•
A metathesis (double replacement)
reaction is a reaction where two
compounds switch cations to form two
new compounds.
A+B- + C+D-  A+D- + C+B-
CaCl2(aq) + Na2CO3(aq)  CaCO3(s) + 2NaCl(aq)
AgNO3(aq) + KCl(aq)  ???
SOLUBILITY
• Solubility – the amount of a substance that can be
dissolved in a given quantity of solvent (like
water) at a specific temperature
• Unsaturated – amount of substance less than
saturated
• Saturated – the exact amount at solubility
• Supersaturated – excess amount of substance
How Solubility Influences Rxn
• When a substance is soluble in water, it will
appear with a subscript of (aq) meaning that
the substance is broken up into it’s ions
incorporated into the water lattice.
• When a substance is insoluble in water, it
will be written with a subscript of (s), (l), or
(g) and will precipitate out of solution.
Solubility of Ionic Compounds in Water
Soluble Compounds
Exception
s
Sodium, potassium, and
ammonium compounds
Acetates and nitrates
Halides (chlorides, bromides,
and iodides)
Lead(II), silver, and mercury (I)
halides are insoluble
Sulfates
Calcium, strontium, barium, and
Lead(II) sulfates are insoluble
Insolubility of Ionic Compounds in Water
Insoluble Compounds
Exceptions
Carbonates and phosphates
Sodium, potassium, and
ammonium compounds
are soluble.
Hydroxides
Sodium, potassium, calcium,
strontium, and barium compounds
are soluble
Sulfide
Sodium, potassium, calcium,
and ammonium compounds
are soluble
A Look at Metathesis Again
• Looking back at slide 10 to the first
reaction: when the cations rearranged, the
CaCO3 being insoluble by our definition is
recorded as CaCO3 (s). The CaCO3 would
precipitate out of solution as a solid.
• Looking at AgNO3(aq) + KCl(aq)  ?, we can
rearrange the cations and conclude that the
AgCl is a solid and will precipitate out of
solution
Types of Metathesis Reactions
• Three classifications of metathesis reactions
– Precipitation reaction - formation of a solid
Pb(NO3)2(aq) + Na2CO3(aq)  PbCO3(s) + 2NaNO3(aq)
– Neutralization reaction - formation of water
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
– Gas formation reaction - CO2, H2S, SOx, & NOx
are typically formed
Precipitation Reactions
• A solid precipitate is produced in the
rearrangement of cations as follows:
Pb(NO3)2(aq) + Na2CO3(aq)  PbCO3(s) + 2NaNO3(aq)
• The Ionic Equation is expressed as:
Pb+2 + 2NO3- + 2Na+ + CO3-2  PbCO3(s) + 2Na+ + 2NO3-
• After neglecting the spectator ions, the net
ionic equation will look like:
Pb+2(aq) + CO3-2(aq)  PbCO3(s)
Reviewing Ionic Compounds
Ca+2 + 2Cl-  CaCl2
Each ion comes together based on charge
to form an overall neutral ionic compound.
3Ca+2 + 2PO4-3  Ca3(PO4)2
The cation and the polyatomic ion come
together based on charge to form an overall
neutral ionic compound.
Net Ionic Equations (NIE)
• If you were given the reactants Ca(NO3)2
and Na3PO4 you should be able to predict
the precipitate and write a balance equation,
the ionic equation, and the net ionic
equation (NIE) for this reaction.
• The NIE for these reactants is as follows:
3Ca+2(aq) + 2PO4-3(aq)  Ca3(PO4)2(s)
Common Polyatomic Ions
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carbonate ion
sulfate ion
sulfite ion
hydroxide
phosphate
permanganate
chromate
dichromate
CO3-2
SO4-2
SO3-2
OHPO4-3
MnO4CrO4-2
Cr2O7-2
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•
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ammonium NH4+
oxalate
C2O4-2
bicarbonate HCO3cyanide ion CNacetate
C2H3O3-
Neutralization Reaction
• A neutralization reaction is a reaction that
occurs between an acid and a base with the
production of a salt and water.
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
acid
base
salt
water
Gas Formation Reaction
• A gas formation reaction is a metathesis
reaction that generates a gas as a product.
–
–
–
–
Metal carbonates or bicarbonates + acid
Metal sulfides + acid
Metal sulfites + acid
Ammonium salts and strong base
Metal Carbonates
• Metal carbonates or bicarbonates when
combined with an acid form salt, water and
carbon dioxide gas.
Na2CO3(aq)+ 2HCl(aq)  2NaCl(aq)+ H2O(l)+ CO2(g)
– Where CO2 gas is given off
Metal Sulfides
• Metal sulfides when combined with an acid
form salt and hydrogen sulfide gas.
Na2S(aq)+ 2HCl(aq)  2NaCl(aq)+ H2S(g)
– Where H2S gas is given off
Metal Sulfites
• Metal sulfites when combined with an acid
form salt, water, and sulfur dioxide gas.
Na2SO3(aq)+ 2HCl(aq)  2NaCl(aq)+ H2O(l)+ SO2(g)
– Where SO2 gas is given off
Ammonium Salts
• Ammonium salts when combined with a
base produce salt, water and ammonia.
NH4Cl (aq)+ NaOH(aq)  NaCl(aq)+ H2O(l)+ NH3(g)
– Where ammonia gas is given off
Combustion Reactions
•
A combustion reaction is a reaction with
molecular oxygen to form products in
which all elements are combined with
oxygen.
CH4 + 2O2  CO2 + 2H2O
Limiting Reactants
• One of the reactants is in limited supply and
thus restricts the amount of product formed.
• Think of it as: If you wanted to bake a batch
of peanut butter cookies and the recipe calls
for 1 cup of peanut butter and all you have
is ½ a cup, even though you have all the
other ingredients, you can at most make ½ a
batch of cookies.
Limiting Reactants (cont.)
• Consider the combustion reaction:
CH4 + 2O2  CO2 + 2H2O
• How much CO2 can be produced if you
have 0.13g of methane and 0.45g of O2?
Percent Yield
• The maximum amount of product that
can be obtained from a chemical reaction
is the theoretical yield.
• The actual amount produced in a
chemical process is the actual yield.
• The percent yield is equal to the actual
yield divided by the theoretical yield times
100%.
Redox Reactions
• Oxidation of an element takes place when
electrons are lost from the valence shell of
the element.
• Reduction of an element takes place when
electrons are added to the valence shell of
the element.
• Redox reactions show the transfer of
electrons that takes place during oxidation
and reduction.
Redox Reactions (cont.)
• All oxidation and reduction reactions involve
transfer of electrons between substances.
• View CD-ROM screen 5.12
• Ag+ accepts electrons for Cu and is reduced
to Ag and Cu loses electrons to Ag+ and is
oxidized to Cu+2 in the following redox rxn:
2Ag+(aq) + Cu(s)  2Ag(s) + Cu+2(aq)
Redox Reactions (cont.)
• The oxidation half reaction is :
Cu(s) – 2e-  Cu+2(aq)
• The reduction half reaction is:
2Ag+(aq) + 2e-  2Ag(s)
• Cu is called the reducing agent because it caused
Ag+ to be reduced; and Ag+ is called the oxidizing
agent because it caused Cu to be oxidized.
Determining Oxidation Numbers
• Each atom in a pure element has an oxidation
number of zero.
• For monoatomic ions, the ox. number is equal to
it’s ionic charge.
• F is always –1, other halogens are –1 as well
except with oxygen or fluorine.
• The ox. number for H is +1 except with hydrides
(CaH2).and O is –2 except with peroxides (H2O2).
• The  ox.# must = 0 for a compound or = to the
overall charge of polyatomic ion being considered.
Balancing Redox Reactions
• We can use the balance of electrons transferred in
a redox reaction to help us balance the overall
equation.
• Consider the unbalanced equation:
Zn(s) + HCl(aq)  ZnCl2(aq) + H2(g)
• The balanced equation takes into consideration the
oxidation of the Zn and the reduction of the H+.
Zn(s) + 2HCl(aq)  ZnCl2(aq) +
H2(g)
Molarity
Molarity =
Moles of Solute
Liters of Total Solution

Symbol for molarity is M

Units are moles/Liter
Solution Preparation
• To prepare a 1.0M solution of NaCl, you would
determine how many grams of NaCl is contained in
1.0 moles of NaCl and then dissolve that amount in
a 1.0L volumetric flask. You would then qs with
distilled H2O.
• 1.0M NaCl = 1 mole (or 58.44g) NaCl
1.0L of solution
• How much NaCl would you use to make a 0.1M
solution of NaCl?
As 1/10 of a mole = 5.844g NaCl, you would dissolve
0.1mole (5.844g) of NaCl in 1.0L of solution.
Acids
An acid is defined as follows:
• Arrhenius – releases H+ when dissolved in
H2O
• Bronsted-Lowrey – a substance that can
donate a proton to another substance
• Lewis – a substance that can accept a pair of
electrons from another atom to form a new
bond
Bases
A base is defined as follows:
• Arrhenius – releases OH- when dissolved in
H2O
• Bronsted-Lowrey – a substance that can
accept a proton from another substance
• Lewis – a substance that can donate a pair
of electrons to another atom to form a new
bond
pH and Concentrations of
Acids and Bases
pH = -log [H+]
1 – acidic – 7 – basic – 14
When dealing with [H+] less than 0.1M
(pH=1), we use activity coefficients instead
of pH.
pH of Household Items
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pH of vinegar = 2.80
pH of soda = 2.90
pH of orange juice = 3.80
pH of pure water = 7.00
pH of blood = 7.40
pH of ammonia = 11.00
pH of oven cleaner = 11.7
Titration
• A method for quantitative analysis of a substance
by essentially complete reaction in solution with a
measured quantity of a reagent of known
concentration.
• Often used in redox reactions
– Many redox reactions go rapidly to completion in
aqueous media to determine the equivalency point.
• Typically used for neutralization reactions.
– Acid is titrated with a base using an indicator to
determine the equivalency point of the neutralization
reactions.