Download 29 Sept 08 - Seattle Central

Plan for Mon, 29 Sept 08
• Exam I this Fri, 3 Oct 08
– Bring scantron form, scientific calculator, No. 2 pencils
– Oct 3 is the add/change deadline
• Exp. 1 this Wed, 1 Oct
– Buy Lab Manual
– Make sure to complete pre-lab assignment before lab.
– Don’t forget to bring your safety quiz!!
• Lecture
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Using the mole to count particles (3.3)
Using molar mass as a conversion factor (3.4)
Chemical equations: balancing, interpreting (3.7-8)
Mole ratios and stoichiometry calculations (3.9)
Calculations involving a limiting reactant (3.10)
• Quiz 1 returned
If we had 1 mol of bicycles…
– How many mol bike saddles
would we have? 1 mol saddles
– Tires? 2 mol tires
– Spokes? (assuming 36
spokes/wheel) 72 mol spokes
If we had 1 mol of isopropyl
alcohol (C3H8O)…
CH3
C
H
CH3
HO
– How many mol O atoms
would we have? 1 mol O atoms
– Carbon atoms? 3 mol C atoms
– Hydrogen atoms? 8 mol H atoms
“Massive” Vocabulary
atomic mass – average mass of an atom,
given in amu
molecular mass – mass of a molecule,
given in amu
formula mass – mass of the formula unit of
an ionic compound, given in amu. (e.g.,
NaCl, Ca2Br, BaSO4 are all formula units.)
molar mass – mass of one mole of a
substance, given in g/mol
Examples: Finding # particles
1. How many water molecules are in a
10.0-g sample of water?
2. What is the total number of ions present
in a 1.0-g sample of zinc oxide (ZnO)?
Molar Mass as a Conversion Factor
Molar mass (the mass of one mole of a substance),
allows us to convert between macroscopic and
particulate points of view.
How many grams of oxygen corresponds to 0.50 moles?
? g O2 = 0.5 mol O2
32 g O2
1 mol O2
= 16 g O2
How many moles of oxygen are there in 1.2 grams?
? mol O2 = 1.2 g O2
1 mol O2
32 g O2
= 0.038 mol O2
Example: mass from moles
1. What is the mass of 7.65 x 1022 sulfur
atoms?
2. Calculate the mass of 4.85 mol of acetic
acid, CH3COOH.
Chemical Equations
• Chemistry is the study of the rearrangement of matter
due to the flow of energy.
• In a chemical reaction, some bonds are broken and
others are formed, resulting in a reorganization of the
atoms.
• Atoms are neither created or destroyed in a chemical
reaction!
When methane (CH4) reacts with oxygen,
carbon dioxide and water are formed…
+
+
Reactants and products must occur in numbers that give the
same number of each type of atom on both sides of the arrow.
Balancing Chemical Equations
1. Determine what reaction is occuring.
What are the reactants, products, and
physical states involved?
2. Write the unbalanced equation that
summarizes the reaction in step 1.
3. Balance the equation by inspection. Do
not change the identities (formulas) of
any of the reactants or products.
Example
•
Write balanced chemical equations for the
following:
1. Aqueous solutions of lead(II) nitrate and
sodium phosphate are mixed, resulting in the
precipitate formation of lead(II) phosphate, with
aqueous sodium nitrate as the other product.
2. The combustion of liquid ethanol (C2H5OH)
forms carbon dioxide and water vapor.
Combustion refers to the reaction of a
substance with oxygen gas.
Interpreting Chemical Equations
• A reaction results in the rearrangement of
atoms in one or more reactants to produce
one or more products.
CO(g) +
2 H2(g) 
CH3OH(l)
• What is this chemical equation telling us?
Interpreting (cont.)
+
2H2(g)

CH3OH(l)
1 CO molecule
+
2 H2 molecules

1 CH3OH molecule
1 dozen CO
molecules
+
2 dozen H2
molecules

1 dozen CH3OH
molecules
6.022 x 1023
CO molecules
+
2(6.022 x 1023)
H2 molecules

6.022 x 1023 CH3OH
molecules
1 mol CO
molecules
+
2 mol H2
molecules

1 mol CH3OH
molecules
CO(g)
Stoichiometry
• The relative quantities of reactants and products
involved in a chemical reaction.
1 ¾ cup cake flour
3 tsp baking powder
2 oz. baking chocolate
1 ½ cup sugar
½ cup butter
4 eggs
½ cup milk
one chocolate cake
3 1/2 cup cake flour
6 tsp baking powder
4 oz. baking chocolate
3 cup sugar
1 cup butter
8 eggs
1 cup milk
two chocolate cakes
Mole Ratios
• We can use a balanced chemical equation to predict the
number of moles of products that a given number of
moles of reactants will produce.
2H2O(l) 
2H2O(l)
2H2(g) + O2(g)
2H2(g) + O2(g)
4H2O(l)
4H2(g) + 2O2(g)
Mole Ratios (cont)
• What if we wanted to know the number of moles of H2 and O2
produced from the decomposition of 5.8 mol of H2O?
2H2O(l)  2H2(g) + O2(g)
• We know the following:
2 mol H2O
2 mol H2
2 mol H2O
1 mol O2
• We can represent these molar equivalencies as conversion
factors:
2 mol H2O
~
2 mol H2
2 mol H2
2 mol H2O
2 mol H2O
~
1 mol O2
1 mol O2
2 mol H2O
=
1 mol H2
1 mol H2O
Mole Ratios (cont)
• Now we can answer the question…How many moles of H2
and O2 are produced from the decomposition of 5.8 mol
of H2O?
2H2O(l)  2H2(g) + O2(g)
? mol H2
= 5.8 mol H2O
1 mol H2
=
5.8 mol H2
=
2.9 mol O2
1 mol H2O
? mol O2
= 5.8 mol H2O
1 mol O2
2 mol H2O
Example
• Calculate the number of moles of oxygen
required to react exactly with 4.30 mol of
propane, C3H8.
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)
Stoichiometry and Mass
• Recall: the number of moles of products
produced or reactants consumed
represents numbers of particles, and we
can’t count numbers of particles directly.
• In chemistry, we have to count by
weighing.
• So, we need to use molar masses along
with our mole ratios to convert between
moles and grams.
Stoichiometry and Mass (cont)
• Consider the reaction in which powdered aluminum
and finely ground iodine react violently to produce
aluminum iodide:
2 Al(s) + 3 I2(s)  2 AlI3(s)
• Let’s say we have 35.0 g of aluminum powder.
• How many grams of ground iodine do we need to
react completely with this amount of aluminum?
Stoichiometry and Mass (cont)
• We need to construct the mole ratios from our
balanced chemical equation.
2Al(s) + 3I2(s)  2AlI3(s)
~
2 mol Al
3 mol I2
3 mol I2
2 mol Al
• We want to do the following calculation:
? g I2 required
=
mol Al present
3 mol I2
2 mol Al
But we have GRAMS
of Al, not moles.
Stoichiometry and Mass (cont)
• We need to convert our grams of Al into moles of Al:
35.0 g Al present
1 mol Al
26.98 g Al
? g I2 required
? g I2 required
=
=
mol Al present
3 mol I2
2 mol Al
35.0 g Al present
But we want GRAMS
of I2, not moles.
1 mol Al
3 mol I2
26.98 g Al
2 mol Al
Stoichiometry and Mass (cont)
• We need to convert our moles of I2 into grams of I2:
mol I2 calculated
253.8 g I2
1 mol I2
? g I2 required
? g I2 = 35.0 g Al
=
35.0 g Al present
1 mol Al
3 mol I2
26.98 g Al
2 mol Al
1 mol Al
3 mol I2
253.8 g I2
26.98 g Al
2 mol Al
1 mol I2
=
495 g I2