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UNIT 2: REDOX REACTIONS &
GAS STOICHIOMETRY
SEPT 15, 2014
CHEMICAL
MODELS
Observable
phenomena
•
•
•
What you see with your eyes
What you measure with instruments
Lab work, demos & experiments
Our basic triangle.
Sub-microscopic
depictions
•
•
•
Representational
sketches and diagrams
(to scale or not)
Verbal descriptions
Simulations
Chemical
nomenclature
•
•
•
•
Chemical symbols
Balanced chemical
equations
States of matter (s), (l),
(g), (aq)
Quantitative calculations
WHAT DOES A REDOX RXN LOOK LIKE
FROM A MACROSCOPIC LEVEL?
• Tin(II)Chloride + Zinc
• Carbon and Copper Oxide
WHAT IS A REDOX REACTION?
REDOX IS SHORT FOR “REDUCTION AND OXIDATION REACTIONS”
Oxidation-reduction reactions are those in
which an electron is transferred from one
reactant to another.
The atom that donates the electron is OXIDIZED.Think: reduced in charge.
The atom that accepts the electron is REDUCED.
The electron donor is the REDUCING AGENT.
The electron acceptor is the OXIDIZING AGENT.
REDOX RXNS
• Mnemonics for Redox rxns:
• “OIL RIG”
• Oxidation Is Loss (electrons)
• Reduction Is Gain (electrons
• “LEO GER”
• Lose Electrons Oxidation
• Gain Electrons Reduction
WHAT DOES A REDOX LOOK LIKE FROM
A SUBMICROSCOPIC LEVEL?
• Sub -Microscopic Level: Zinc + Oxygen
• Similar redox reaction:
• CuCl2(aq) + Zn(s)

Cu(s) + ZnCl2(aq)
• Which reactant is being oxidized? Which is reduced?
HOW DO WE PREDICT IF A S.R.
REACTION WILL OCCUR?
• For instance, which of the following reactions will
happen?
•
Cl2 (g) + 2HBr (aq)  2HCl (aq) + Br2 (l)
•
Br2 (g) + 2HCl (aq)  2HBr (aq) + Cl2 (l)
• In other words, which is more “reactive,” chlorine or
bromine?
• We need an ACTIVITY SERIES (lab 3) and a way of
THINKING about activity (using Coulomb’s Law).
VOLTAIC CELLS
•Voltaic cell - an electrochemical
cell that uses a spontaneous
chemical rxn to generate an
electric current.
•Components of a voltaic cell:
•2 half-cells connected by a wire
•Salt bridge
•cathode and anode
Alessandro Volta - invented
the first voltaic cell (1800)
Electrical Potential –
Voltaic Cell –
Electrode –
Electric Current Salt Bridge -
HOW DOES A VOLTAIC CELL
WORK SPONTANEOUSLY?
• When you connect 2 metals from different positions
on the activity series - it creates a electrical
potential energy difference b/w the metals.
• The greater the activity b/w the two metals, the
greater the electrical potential
• The differing tendency of metals to lose electrons
allows an oxidation -reduction to generate
electricity!
A battery consists of a number of voltaic cells.
Here’s an example of a single “wet” cell.
Zinc is
oxidized.
Sulfate
Copper is
reduced.
Cathode
(half-cell)
Copper
will be
reduced.
Consonants.
Anode
(halfcell)
Zinc will
oxidize
Vowels.
Use oxidation potential series to determine electric potential (“voltage”)
of this cell.
Zn  Zn2+ + 2e- (3.63 V)
Cu2+  Cu + 2e- (-2.53 V)
Difference in electric potential is 1.10 V.
Zinc is
oxidized.
Sulfate
Copper is
reduced.
BATTERIES - DRY VOLTAIC CELL
•.
•Voltaic Pile
A 9 V battery consists of six 1.5 V batteries (like AAA batteries) wired in
series.
Capacity refers to how many electrons the series redox reactions will
release before no more reactants remain. We will measure this quantity
in MOLES of electrons. It is related to the lifetime of the battery.
Voltage is proportional to the amount of potential energy each electron
gains in the reaction. It is related to the power the battery can provide.
Lead and lead-oxide strips are placed in sulfuric acid.
What is oxidized? Which is reduced?
Car Battery
HOW DO CAR BATTERIES
WORK?
How Batteries Work
Common battery chemistries include:
Zinc-carbon battery: common in many inexpensive AAA, AA, C and D dry cell
batteries. The anode is zinc, the cathode is manganese dioxide, and the
electrolyte is ammonium chloride or zinc chloride.
Alkaline battery: common in AA, C and D dry cell batteries. The cathode is
composed of a manganese dioxide mixture, while the anode is a zinc powder.
Lithium-ion battery (rechargeable):, such as cell phones, digital cameras and
even electric cars. A variety of substances are used in lithium batteries, such as
lithium cobalt oxide cathode and a carbon anode.
Lead-acid battery (rechargeable): This is the chemistry used in a typical car
battery. The electrodes are usually made of lead dioxide and metallic lead, while
the electrolyte is a sulfuric acid solution.
Lithium Ion Batteries
LIFE CYCLE OF BATTERIES
• Recycling Car Batteries
• Lithium Ion Rechargable
GAS STOICHIOMETRY
CHAPTER 12
GAS STOICHIOMETRY
• Many chemical reactions involve gases as a reactant or a
product
• Gas Stoichiometry – the procedure for calculating the volume
of gases as products or reactants
• Gases also have a molar volume (L/mol) rather than
concentration.
• This is the conversion factor used to convert (litres of gas) to
(moles of gas)
• The Ideal Gas Law (PV = nRT) may also be required to:
A) find the number of moles of reactant
B) Find the V, P, or T of the product
STP = 22.4L/mol
MOLAR VOLUME
• Molar volume is the same for all gases at the same
temperature and pressure (remember, all gases have
the same physical properties)
• At STP, molar volume = 22.4 L/mol (101.325 kPa and 0oC)
• This can be used as a conversion factor just like molar mass!
At STP, one mole of gas has a volume
of 22.4 L, which is approximately the
volume of 11 “empty” 2 L pop bottles.
EXAMPLE #1
• If 300g of propane burns in a gas barbecue, what volume
of oxygen measured at STP is required for the reaction?
• Remember: 22.4 L/mol for STP
C3H8(g)
m = 300g
44.11g/mol
300 g
+
5O2(g)  3CO2(g) + 4H2O(g)
m=?
22.4L/mol
x 1 mol x 5 mol O2 x
44.11 g
1 mol C3H8
22.4 L = 761 L O2(g)
1 mol
**Remember – molar volume is the conversion factor for gases just
like molar mass is the conversion factor in stoichiometry
EXAMPLE #2
• Hydrogen gas is produced when sodium metal is added to
water. What mass of sodium is necessary to produce
20.0L of hydrogen at STP?
• Remember: 22.4L/mol for STP
2Na(s)
+
2H2O (l)  2NaOH(aq) + H2(g)
m=?
22.99g/mol
20.0L
x 1 mol x 2mol x
22.4 L
1 mol
V = 20.0L
22.4L/mol
22.99g = 41.0 g Na(s)
1 mol
**Remember – molar volume is the conversion factor for gases just
like molar mass is the conversion factor in gravimetric stoichiometry
If the conditions are not STP or SATP, the
molar volume cannot be used! You must use
the ideal gas law to find the gas values using
moles determined from stoichiometry
EXAMPLE #3
• What volume of gaseous ammonia at 450kPa and 80oC can be obtained
from the complete reaction of 7.5kg of hydrogen with nitrogen?
N2(g) +
3H2(g) 
m = 7500g
M = 2.02 g/mol
2NH3(g)
m=?
P = 450kPA
T = 353.13K
7500 g x 1 mol x 2 = 2475.2475 mol NH3(g)
2.02 g
3
PV = nRT
 V = nRT
P
=
= 16150.10L  1.6 x 104 L of NH3(g)
WHAT DO YOU REMEMBER ABOUT
GASES?
GAS VOLUME AND MOLES :
AVOGADRO’S LAW
MOLAR VOLUME & GAS DENSITY
IDEAL GAS LAW