Transcript CHM 103 Lecture 11 S07

Announcements & Agenda (02/02/07)
Quiz Today!!!
Exam NEXT FRIDAY!!!
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Covers Ch 1-5
Intermolecular Forces (Notes + bits & pieces of Ch 6)
Ch 7.1-7.3
CHM 103 Lab
Today
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The Mole (5.5-5.8)
Reactions Types (Especially Redox Rxns) (5.3, 5.4)
Energy of Chemical Reactions (5.9)
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Last Time: Molecular Shapes
Electron
Group
Bonded Lone Bond
Molecular
Atoms Pairs Angles Shape
Example
4
4
0
109
tetrahedral CH4
4
3
1
~109
Pyramidal NH3
4
2
2
~109
Bent
H 2O
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2
Last Time: Polar Molecules
• contain polar bonds.
• have a separation of positive and negative
charge called a dipole indicated with + and -.
• have dipoles that do not cancel!
+ -
••
H–Cl
H—N—H
dipole
H
dipoles do not
cancel
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Chapter 5!!!
Chemical Reactions
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24 Good Practice Problems (Ch 5)
5.01, 5.05, 5.07, 5.11, 5.13, 5.17,
5.25, 5.27, 5.33, 5.37, 5.41, 5.47,
5.57, 5.69, 5.73, 5.75, 5.79, 5.81,
5.83, 5.85, 5.91, 5.93, 5.95, 5.97
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Last Time: Conservation of Mass
Revisited – Balanced Reactions (Ch 5)
In a balanced
chemical
reaction
• atoms are not
gained or lost.
• the # of reactant
atoms is equal to
the number of
product atoms.
6
Steps in Balancing an Equation
To balance the following equation,
Fe3O4(s) + H2(g)  Fe(s) + H2O(l)
• work on one element at a time.
• use only coefficients in front of formulas.
• do not change any subscripts.
Fe:
Fe3O4(s) + H2(g)  3Fe(s) + H2O(l)
O:
Fe3O4(s) + H2(g)
 3Fe(s)
+ 4H2O(l)
H:
Fe3O4(s) + 4H2(g)  3Fe(s)
+ 4H2O(l)
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Equations with Polyatomic Ions
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Balancing with Polyatomic Ions
MgCl2(aq) + Na3PO4(aq)  NaCl(aq) + Mg3(PO4)2(s)
Balance PO43- as a unit
MgCl2(aq) + 2Na3PO4(aq)  NaCl(aq) + Mg3(PO4)2(s)
2 PO43=
2 PO43Balance Mg and Cl
3MgCl2(aq) + 2Na3PO4(aq)  6NaCl(aq) + Mg3(PO4)2(s)
3 Mg2+
=
3 Mg2+
6 Na+
=
6 Na+
6 Cl=
6 Cl9
The “mole”…. Thinking about
reactions on a real-world scale!
 1 mole is:


a collection of stuff that equals the # of atoms
present in exactly 12.000 g of 12C (6.022  1023
atoms)
always 6.022  1023 somethings (Avogodro’s #, NA)
 New term: Molar Mass (MM)

the mass of 1 mol of an element/compound in gms.

e.g. the MM of 12C is 12.000 g/mol

b/c 1 mol is defined as 6.022  1023 atoms, the MM
for any element can be determined
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Collection Terms
A collection term states
a specific number of items.
• 1 dozen donuts
= 12 donuts
• 1 ream of paper
= 500 sheets
• 1 case
= 24 cans
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11
Some One-mole Quantities
One-Mole Quantities
32.1 g
55.9 g
58.5 g
294.2 g
342.2 g
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Using Avogadro’s Number
Avogadro’s number is used to convert
particles of a substance to moles.
How many moles of CO2 are in
2.50 x 1024 molecules CO2?
2.50 x 1024 molecules CO2 x
1 mole CO2
6.02 x 1023 molecules CO2
= 4.15 mole CO2
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Subscripts and Moles
The subscripts in a formula show
• the relationship of atoms in the formula.
• the moles of each element in 1 mole of compound.
Glucose
C6H12O6
In 1 molecule: 6 atoms C 12 atoms H
6 atoms O
In 1 mole:
6 mole C 12 mole H 6 mole O
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Molar Mass
• is the mass of one
mole of an element
or compound.
• is the atomic mass
expressed in grams.
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Molar Mass from Periodic Table
Molar mass is the
atomic mass
expressed in grams.
Remember atomic
masses???
1 mole Ag
= 107.9 g
1 mole C
= 12.01 g
1 mole S
= 32.07 g
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Determining the Molar Mass…
1S
SO2
2O
SO2
32.07 g/mol
+ 2 x 16.00 g/mol
64.07 g/mol
For any molecule
molecular mass = S all atomic masses
1 mole SO2 = 64.07 g SO2
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Putting the mole to work in
chemical reactions!
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Calculations Using Molar Mass
Molar mass factors are used to convert between the
grams of a substance and the number of moles.
Grams
Molar mass factor
Moles
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Reading Equations In Moles
Consider the following equation:
4 Fe(s)
+ 3 O2(g)
2 Fe2O3(s)
This equation can be read in “moles” by placing
the
word “moles” between each coefficient and
formula.
4 moles Fe + 3 moles O2
2 moles Fe2O3
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Steps in Finding the Moles and
Masses in a Chemical Reaction
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Calculating the Mass of a Reactant
The reaction between H2 and O2 produces 13.1 g water.
How many grams of O2 reacted?
2 H2(g)
+ O2(g)
2 H2O (g)
?g
13.1 g
The plan and factors would be
g H2O
mole H2O
mole O2
g O2
molar
mole-mole
molar
mass H2O
factor
mass O2
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Calculating the Mass of a Reactant
The setup would be:
13.1 g H2O x 1 mole H2O x 1 mole O2 x 32.0 g O2
18.0 g H2O
2 moles H2O 1 mole O2
molar
mole-mole
molar
mass H2O
factor
mass O2
= 11.6 g O2
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Several Types of Reactions (5.3)
Chemical reactions can be classified as:
• combination reactions.
• decomposition reactions.
• single replacement reactions.
• double replacement reactions.
Don’t stress about these too much; be able to
recognize the different flavors (see suggested
problems)
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Oxidation-Reduction Reactions (5.4)
An oxidation-reduction reaction
• provides us with energy from food.
• provides electrical energy in
batteries.
• occurs when iron rusts.
4Fe(s) + 3O2(g)
2Fe2O3(s)
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Electron Loss and Gain
An oxidation-reduction reaction
• transfers electrons from one reactant to another.
• loses electrons in oxidation.
Zn(s)
(LEO)
Zn2+(aq) + 2e- (loss of e-)
• gains electrons in reduction.
(GER)
Cu2+(aq) + 2eCu(s) (gain of e-)
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Oxidation and Reduction
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Writing Oxidation & Reduction Reactions
Write the separate oxidation and reduction reactions
for the following equation.
2Cs(s) + F2(g)
2CsF(s)
A cesium atom loses an electron to form cesium ion.
Cs(s)
Cs+(s) + 1e−
oxidation
Fluorine atoms gain electrons to form fluoride ions.
F2(s) + 2e2F−(s)
reduction
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Cu and Ag1+
COOL DEMO!
Cu(s)
Orange metal
Ag1+(aq) + 2eColorless
Cu2+(aq) + 2eBlue
Ag(s)
Silver
oxidation
reduction
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Collision Theory of Reactions
A chemical reaction occurs when
• collisions between molecules have sufficient
energy to break the bonds in the reactants.
• bonds between atoms of the reactants (N2 and
O2) are broken and new bonds (NO) can form.
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30
Activation Energy
• The activation
energy is the
minimum energy
needed for a
reaction to take
place.
• When a collision
provides energy
equal to or
greater than the
activation energy,
product can form.
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Exothermic Reactions
• heat is released.
• the energy of the
products is less than
the energy of the
reactants.
• heat is a product.
C(s) + 2 H2(g)
CH4(g) + 18 kcal
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32
Endothermic Reactions
• Heat is absorbed.
• The energy of the
products is greater
than the energy of the
reactants.
• Heat is a reactant
(added).
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N2(g) + O2 (g) + 43.3 kcal
2NO(g)
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Summary
Reaction
Energy
Heat
Type
Change
in Reaction
Endothermic Heat absorbed
Reactant
Exothermic
Heat released
Product
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Rate of Reaction
• is the speed at which reactant is used up.
• is the speed at which product forms.
• increases when temperature rises because
reacting molecules move faster providing
more colliding molecules with energy of
activation.
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Reaction Rate and Catalysts
A catalyst
• increases the
rate of a
reaction.
• lowers the
energy of
activation.
• is not used up
during the
reaction.
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Learning Check
State the effect of each on the rate of reaction as:
1) increases
2) decreases
3) no change
A. increasing the temperature.
B. removing some of the reactants.
C. adding a catalyst.
D. placing the reaction flask in ice.
E. increasing the concentration of one of the
reactants.
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