Chapter 3 Notes

Report
Chapter 3
• OWL Deadline 27-September
– All material through Chapter 3 is fair game for
Exam #1 on 24-September.
• Lab the week of 10-September: Synthesis of
Alum
• Stoichiometry and Equations
–
–
–
–
–
The Mole
Stoichiometry and Compounds
Chemical Reactions
Limiting Reactants
% and Theoretical Yield
What is a mole?
• A standard unit of
quantity.
• The number of atoms
present in exactly 12
grams of 12C.
• Avogadro’s number is
the constant for 1 mole
• NA = 6.02 x 1023
Stoichiometry & Compounds
• In terms of compounds,
stoichiometry refers to the
ratio of elements within a
molecule or substance
(including polyatomic ions
and ionic compounds)
• Any unique compound will
have its formula
• The formula will have a
specific ratio of atoms
• These ratios can be
expressed in weight (grams
for example) or quantity
(moles usually)
• H2O
• C3H8
• PO4-3
• H2C2O42 H2O
• Na2CO3
• Formula Mass (weight)
• The sum of the atomic masses of all atoms in a formula unit of the
compound, whether molecular or not.
– Empirical Formula Mass
– Molecular Formula Mass (another term for molecular mass)
– The mass of some other unit containing multiple atoms
• Molar Mass (weight)
– The sum of the masses of all of the atoms or molecules in a mole of a
compound, ordinarily expressed in grams/mole.
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All rights reserved.
3|4
Ca(OH)2
1 Ca
1(40.08) =
2O
2(16.00) =
2H
2(1.008) =
Total
40.08 amu
32.00 amu
2.016 amu
3 significant figures
74.095
74.1 amu
Calculate the formula weight of the following compound from its formula.
Report your answers to three significant figures.
Calcium hydroxide, Ca(OH)2
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3|5
• What is the mass in amu of the nitrous
acid molecule, HNO2?
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3|6
• A sample of nitric acid, HNO3, contains 0.253
mol HNO3. How many grams is this?
First, find the molar mass of HNO3:
1H
1N
3O
1(1.008) =
1(14.01) =
3(16.00) =
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All rights reserved.
1.008
14.01
48.00
63.018
3|7
(2 decimal places)
63.02 g/mol
Next, using the molar mass, find the mass of 0.253
mole:
63.02 g
0.253 mole x
1 mole
= 15.94406 g
 15.9 g
(3 significant figures)
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3|8
Remember, moles (mol) are really used for counting
and deriving data from those counts.
How many grams does 0.45 mol of sulfur trioxide weigh?
Percent Composition
• Often used in chemical
analysis and identifying
empirical or molecular
formulas
• A weight ratio of the
elements in a
compound
• Write the name of,
identify the
stoichiometry involved
in, and determine the %
composition of each
element in
• MgCO3
Empirical/Molecular Formulas and %
Composition
• Chemists may discover a
compound and, if it is new to
them, have no idea what the
chemical formulas are
– Empirical
– Molecular
• You can use a step-by-step
approach to determine the
Empirical and Molecular
formulas even if you have
limited data.
• Determine % composition (wt
%) of each element.
• Calculate the mole ratio for
each element.
– Round up to whole numbers
• Use the molecular weight and
the empirical formula weight to
calculate the molecular formula
– We estimate, pretty closely,
the molecular weight from
techniques such as mass
spectrometry
• You’ve discovered what you think is a new
compound:
– 2.00 grams of it were found
– Chemical analysis shows that it has Carbon and
Hydrogen
• What kind of compound is it?
– Chemical analysis shows that of the sample that
you have, 0.151 grams of Hydrogen.
– Mass spectrometry shows that the molecular
weight is 26.02 grams/mole
– What compound is it?
Figure 3.6: Combustion method for determining the percentages of
carbon and hydrogen in a compound
1. Calculate % Composition
2. Calculate Stoichiometry (Mole Ratios)
1. Empirical Formula
3. Calculate Ratio of Molecular Weight to
Empirical Formula Weight
4. Use ratio from #3 to decipher molecular
formula from empirical formula
Stoichiometry and Reactions
• Chemical reactions take place with defined
ratios of reactants and products
• These defined ratios are based on the
stoichiometry of the compounds or atoms in
the reaction, and the stoichiometry related to
the reaction itself.
– Stoichiometric coefficients
• Let’s identify
–
–
–
–
–
–
Products (and name them)
Reactants (and name them)
Stoichiometry of compounds in this reaction
Stoichiometric coefficients
Phases
Reaction Direction (arrow)
• What important law is adhered to by this reaction
(and all the reactions we will study in CHEM 111)
BaCl2 (aq) + Na2SO4(aq)  BaSO4(s) + 2NaCl(aq)
• Let’s identify
–
–
–
–
–
–
Products (and name them)
Reactants (and name them)
Stoichiometry of compounds in this reaction
Stoichiometric coefficients
Phases
Reaction Direction (arrow)
• What important law is adhered to by this reaction
(and all the reactions we will study in CHEM 111)
Balancing Equations
• Remember, mass MUST be conserved!
– All of the atoms of reactants must appear, in the same
mole quantities, in the products
– This can be extrapolated to weights later on with reaction
stoichiometry
Practice Balancing Equations
• Methane gas reacts with oxygen gas to produce
carbon dioxide gas and liquid water.
– Write the reaction
– Balance elements that appear in only one compound
first
• Remember that compound stoichiometry (subscripts) can
not change
– Balance elements that appear in other compounds
second
– Round coefficients up to the lowest whole numbers
– Check answer
Aqueous acetic acid reacts with aqueous
sodium hydroxide to produce aqueous
sodium acetate and water. This reaction
is in “equilibrium”.
Reaction Stoichiometry
• Once we have a balanced reaction, we can
evaluate how much of one substance can form
(or cause to be formed) another substance.
– Reaction stoichiometry is almost ALWAYS studied
using moles (stoichiometric coefficients and
subscripts)
Propane is combusted in a cooking grill using
atmospheric oxygen. The products are carbon dioxide
and water. How much water (mol and grams) will form
if 10 grams of propane are consumed?
In the Haber Process, 2.00 grams of
hydrogen are consumed. How much
nitrogen is consumed in with it?
Limiting Reactants
• Often, we have reactants where:
– There is an excess of one (compared to the others)
– There is a limiting amount of another (compared
to the others.
– The Limiting reactant controls how much product
can be produced
– If in the reaction above, we have 4 moles of
butane and 20 moles of oxygen (O2), which is
limiting and which is excess?
Molecular view of 2H2 + O2  2H2O reaction with H2 as the
limiting reactant
If we have one gram of hydrogen gas (H2), how many moles of O2 will be consumed and
how many grams of water will be formed?
Theoretical and % Yield
• Rarely do reactions produce 100% of the product
they should
– Impurities
– Some reactants/products produce other things
– Spillage
• The theoretical yield is what you would get,
ideally.
– Calculated from the balanced equation and limiting
reactant
• The % yield (percent yield) is what you actually
produce, relative to the theoretical yield
If we have 1.00 grams of hydrogen gas (H2), and 2.00 grams of O2
reacting to form hydrogen peroxide (H2O2 (g)), answer the
following:
-Write
-Identify
-Calculate
the
balanced
the
the
chemical
limiting
theoretical
reaction
reactant
yield
-Calculate the percent yield if 1.5 grams of hydrogen
peroxide
is
formed.

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