CHM 101 Chapter Three

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CHM 101 Chapter Three

• Chemical Equations
• Simple Patterns of Chemical Reactivity
• Formula Weights
• Avagadros Number the Mole
• Empirical Formula
• Quantitative Information from Chemical Reactions
• Limiting Reactants
• Yield

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CHM 101 Chemical Equations
Chemical reactions involve the rearrangement of
atoms in the reactants to form the products
When carbon burns in excess oxygen, the product
is carbon dioxide
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CHM 101 Chemical Equations
Chemical reactions involve the rearrangement of
atoms in the reactants to form the products
When carbon burns in limited oxygen, one of the
products is carbon monoxide
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CHM 101 Chemical Equations
Chemical reactions involve the rearrangement of
atoms in the reactants to form the products
When methane (CH4) or any other hydrocarbon burns
in excess oxygen, the products are carbon dioxide
and water.
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CHM 101 Representative Reactions
Two broad categories of chemical reactions
include
Combination reactions Two or more substances
react to form one compound. For example, when
iron reacts with oxygen, the product is an ionic
compound
Decomposition reactions A substance breaks down
into one or more simpler substances. The
reaction that is used in air bags is a good
example.
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CHM 101 Formula Weights
When atoms combine, the mass of the molecule or
compound they form, called the formula or
molecular weight, is the sum of their masses.
Molecular weight of fluorine gas F2
(NH4)2SO4
Formula Weight of ammonium sulfate
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CHM 101 The Mole
Consider the reciprocals of intensive quantities
The density of copper, 8.96 g/cm3, also provides
the volume of a gram of Cu
The mass of a piece of paper also provides the
number of pieces per gram. Suppose a piece of
filter paper was found to weigh 0.10 g
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CHM 101 The Mole
The atomic mass unit (amu) is approximately the
mass of a proton or neutron (nucleons)
1 amu mass of a nucleon 1.66x10-24 g
Recall that the amu is defined so that the mass
of the Carbon-12 nucleus is exactly 12 amu. How
many 12C atoms are contained in exactly 12 g of
12C?
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CHM 101 The Mole
In fact,12 grams of 12C contains exactly one mole
of 12C atoms. This is the formal definition of a
mole.
Thus, just as the atomic mass in amu is the mass
of one atom, molecule or compound, the atomic
mass in grams is the mass of one mole of the
atoms, molecules or compounds
Then for carbon dioxide (CO2 44.0 g/mol)
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CHM 101 The Mole
Then for carbon dioxide (CO2 44.0 g/mol)
44.0 g 1 mol CO2 6.02x1023 molecules
How many moles in 0.145 g of this CO2?
What is the mass of 300,000 molecules of CO2?
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CHM 101 Percent by Weight
Consider sodium carbonate
In one mole of Na2CO3 there are __ mole(s) of Na,
__ mole(s) of C and __ mole(s) of O atoms. Thus
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CHM 101 Empirical Formula
In determining wt , the count of atoms in the
molecule or compound, determined from its
formula, is converted to a mass relationship
using the atomic, molecular or formula weight.
The same proportional relationship (atomic,
molecular or formula weight) can be used to
determine relative count of atoms (empirical
formula) from its weight percent
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CHM 101 Empirical Formula
The hydrocarbon butane, used in disposable
lighters, is 82.6 C. To determine its empirical
formula
1) Determine the H in the compound
2) Then in 100 g of butane there are
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CHM 101 Empirical Formula
Thus, for every 1 mol C, there are 2.5 mol H. the
empirical formula is
The molecular formula, then, could be ____, or
____, or _____, or any other integer multiple of
the empirical formula. To determine it, we need
to know that the molecular weight of butane is 58
g/mol.
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CHM 101 Stoichiometry
Consider the high temperature reaction between
methane and water
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CHM 101 Stoichiometry
If 20.5 g of butane (C4H10) are combusted in
excess oxygen, what mass of carbon dioxide is
produced?
C4H10(g) O2 (g)
CO2(g) H2O(l)
How many grams of water would be produced in this
reaction?
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CHM 101 Stoichiometry
C4H10(g) O2 (g)
CO2(g) H2O(l)
5
6.5
13
10
4
2
8
How much oxygen is consumed when 20.5 g of C4H10
are burned?
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CHM 101 Stoichiometry

• Total mass of reactants EQUALS total mass of
  products.

• Total moles of product DOES NOT EQUAL total
  moles of product

• Moles of reactants consumed and products
  produced are related stoichiometrically

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CHM 101 Stoichiometry
Consider the reaction between gaseous hydrogen
and oxygen to form gaseous water.
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CHM 101 Stoichiometry
To make a Porsche, you need one body, two windows
and four wheels.
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CHM 101 Stoichiometry
How many Porsches can be manufactured from 250
bodies, 450 windows and 1024 tires?
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CHM 101 Stoichiometry
The limiting part also determines how many bodies
and tires will be used,
250
450
1024
0
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CHM 101 Stoichiometry
How many grams of carbon dioxide would be
produced by the complete reaction of 25.0 g of
methane with 1.75 moles of oxygen?
CH4(g) 2 O2(g)
CO2(g) 2 H2O(l)
1 CH4 2 O2 CO2 2H2O
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CHM 101 Stoichiometry
CH4(g) 2 O2(g)
CO2(g) 2 H2O(l)
1 CH4 2 O2 CO2 2H2O
How many grams of water would be produced by this
reaction?
How many grams of methane would be consumed?
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CHM 101 Stoichiometry
CH4(g) 2 O2(g)
CO2(g) 2 H2O(l)
Summary (moles)
1.56
Summary (grams)
25.0
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CHM 101 Stoichiometry
CH4(g) H2O(g)
CO(g) H2(g)
3
A student reacts 24 g of methane with excess
water and obtains 6.5 g of hydrogen. What is the
yield for this process?