Chem 106 Tuesday 19 April 2011

A few OWL problems

Chapter 20: Electrolysis

Interchapter: Chemistry of Nitrogen

Makeup of the atmosphere

Aurora borealis: Role of N2 and O2

Nitrogen oxides – Major air pollution players

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4/19/2011 2

20.2b Homework: Electrochemical Cells

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20.5c Homework: Effect of Concentration on Cell Potential

ElectrolysisUsing electrical energy to produce chemical change.

Sn2+(aq) + 2 Cl-(aq) ---> Sn(s) + Cl2(g)

⊕

4

BATTERY

+

Na+Cl-

Anode Cathode

electrons

BATTERY

+

Na+Cl-

Anode Cathode

electrons

• NOTE: Polarity (+/-) of

electrodes is reversed from

electrochemical cells.Sn2+

• Electrolysis of aqueous SnCl2.

• Here, a battery pushes electrons

“uphill” from Cl- to Sn2+ in the non-

spontaneous direction.

5

e-e-

BATTERY

+

Na+Cl-

Anode Cathode

electrons

BATTERY

+

Na+Cl-

Anode Cathode

electrons

Sn2+

6

Why is the cathode labeled NEGATIVE in an electrolysis cell, but

POSITIVE in electrochemical cell?

The NEGATIVE terminal is where the ELECTRONS “PILE UP”.

This occurs “naturally” at the anode of an electrochemical cell.

But, in an electrolysis cell, electrons are driven there by the

battery voltage.

e-e-

e-e-e-e-e-e-e-e-e-e-

e-e-e-e-e-e-e-e-e-

The difference is the driving

force for the production of

electrons:

Electrochemical c.

Spontaneous linked half-

reactions.

Electrolysis c. Voltage from the

attached battery.

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Cu2+(aq) ionsAg+(aq) ions

Cu Ag

Electrons-->

Cu ����

Cu2+ + 2e-

Ag+ + e-

���� Ag

Oxidation

Anode

Negative

Reduction

Cathode

Positive

Salt bridge

e-e-e-e-e-e-e-e-e-e-

e-

e-e-e-e-e-

e-e-e-e-e-

e-

Electrochemical cell

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(Spontaneous direction) (Electrolysis direction)

Anode (+)

2 Cl- ---> Cl2(g) + 2e-

Cathode (-)

Sn2+ + 2e- ---> Sn(s)

BATTERY

+

Na+Cl-

Anode Cathode

electrons

BATTERY

+

Na+Cl-

Anode Cathode

electrons

Sn2+

Eo for cell = E˚cathode - E˚anode (in the electrolysis direction)

= - 0.14 V – (+1.36 V)

= –1.50 V

This much external voltage is required to reverse the electrochemical cell.

9

e-e-

Electrolysis of aqueous sodium iodide NaI (aq) produces I2 and OH- ion.

Anode (+): 2 I- ---> I2(g) + 2e- (-0.54 V)

Cathode (-): 2 H2O + 2e- ---> H2 + 2 OH- (-0.83 V)

Eo for cell = -1.45 V

10

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Na metal is not produced because the reduction potential Eo of Na+ (-2.71 V) is more

negative than that of WATER (-0.83 V). So the water molecules get the electrons,

not the Na+ ions.

Say you electrolyzed

aqueous solutions of FeI2,

ZnI2, and AlI3, which one

would not form a METAL

at the cathode?

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FeI2

ZnI2

AlI3

9

25

4

1. FeI2

2. ZnI2

3. AlI3

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Say you electrolyzed

aqueous solutions of FeI2,

ZnI2, and AlI3, which one

would not form a METAL

at the cathode?

1. FeI2

2. ZnI2

3. AlI3

Stoichiometry of Electrolysis

Consider electrolysis of aqueous silver ion.

Ag+ (aq) + e- ---> Ag(s)1 mol e- ---> 1 mol Ag

If we could measure the moles of e-, we could know the quantity of Ag formed.

But how to measure moles of e-?

14

time(sec)

lomb)charge(couofAmount)( =ampCurrent

Current (Coul/s) x time (s) = Charge (Coulomb)

mole e- x (mol species/1 mol e-) = mol species

mol species x atom or molecular weight (g/mol) = mass (g)

15

Stoichiometry of Electrolysis

−

−

−

=

=

emoleeoleCoulombs/m96,500

Coulombs

eoleCoulombs/m96,500Faraday1

1.50 amps flow thru a Ag+(aq) solution for 15.0 min. What mass

of Ag metal is deposited?

Solution

(a)Calculate charge

Charge (C) = current (A) x time (t)

= (1.5 amp)(1Cou/s-amp)(15.0 min)(60 s/min) = 1350 C

seconds

coulombs = (amps) I

seconds

coulombs = (amps) I

16

Stoichiometry of Electrolysis

Solution

(a) Charge = 1.50 Coul/sec x 15.0 min x 60 sec/min = 1350 Coulombs

(b) Calculate moles of e- used

seconds

coulombs = (amps) I

seconds

coulombs = (amps) I

1350 C • 1 mol e -

96,500 C==== 0.0140 mol e -1350 C •

1 mol e -

96,500 C==== 0.0140 mol e -

0.0140 mol e - • 1 mol Ag

1 mol e -==== 0.0140 mol Ag or 1.51 g Ag0.0140 mol e - •

1 mol Ag

1 mol e -==== 0.0140 mol Ag or 1.51 g Ag

1.50 amps flow thru a Ag+(aq) solution for 15.0 min. What mass

of Ag metal is deposited?

(c) Calculate quantity of Ag

17

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20.8b Homework: Counting Electrons

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Atmospheric Chemistry

Troposphere ~ 10 km

Stratosphere ~ 30 km

Together these contain ~90% of atmosphere

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Atmospheric

gases

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390. ----

* = an excited state (electron in a

high energy atomic or molecular

orbital)

N2 + e- � N2+* + 2e-

N2+* � N2

+ + UV photon

UV photon + O2 � O2*

O2* � 2 O* (O excited states)

O* � O + red, green photon

O atom fluorescence

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Fluorescence can only occur at high altitudes (> 300 km) where there are about

10 atoms/cm3.

At low altitudes:

-No electrons

-O* excited states lose energy as heat in collisions with other molecules

(“quenching” of fluorescence).

(There are two different excited states for O atoms, one yielding the red photons, the

other green. The red ones occur only at higher altitudes. This state of O requires a longer

time interval before it can “spit out” its red photon, and the thinner atmosphere up there

allows fewer intermolecular collisions that would otherwise quench the red fluorescence.)

(What is the number density at 1 atmosphere?)

1 mole of gas molecules/22.4 L

= 6.022 x 1023 molecules/2.24 x 104 cm3

= 2.29 x 1019 molecules/cm3

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Aurora studies verify the chemistry of the upper

atmosphere. This is a type of remote spectroscopy

(with the eyes!) that can also be used to identify

molecules on distant planets and stars.

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N2 originated from

outgassing of molten

rock. N2 is a common

molecule throughout

the universe.

N2 is one of more

than 100 molecules

observed so far in

studies of interstellar

dust clouds.

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(nitrous oxide, laughing gas)

(nitric oxide)

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Dinitrogen tetroxide

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Nitrogen fixation by the root nodule bacterium azotobacter vinelandii

Nitrogenase enzyme from protein data bank (www.rcsb.org/pdb/, 1M34.pdb)

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Iron-sulfur clusters

Molybdenum atom

Adenosine

diphosphate (ADP)

Two identical halves

N2 + 6 H+ + 6 H:- + nATP ���� 2 NH3 + nADP + nPO4

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N oxides (NOx) in the atmosphere

Nitrogen oxides N2O, NO, and NO2 are produced

by reaction of N atoms or N2 with O atoms or O2

N2 + O � N2O

O2 � 2 O

N2 + 2 O2 � 2 NO2

N2 + O2 � 2 NOProduced in any high

temperature combustion using

air as O2 source.

These are not removed by

automobile catalytic converters.

In upper atmosphere

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N2O is a stable molecule that contains several π-bonds.

The necessity of drawing resonance forms in the

Lewis formula points to multi-atom π-bonds.

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A multi-atom π-bond in N2O

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Agree or disagree: N2O4 is resonance-

stabilized.

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Yes

No

Abst

ain

18

3

141. Yes

2. No

3. Abstain

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yes

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Santiago, Chile winter 2003

colorless brown

N2O4 � 2 NO2

∆Ho = +57.1 kJ/mol

∆So= +176 J/mol-K

∆Go = ∆Ho - T ∆So

Cold Warm 4/19/2011 36

T (°C) T (Kelvins) ∆∆∆∆G° (kJ/mol)

K=

[NO2]2/[N2O4]

-30 243.15 14.306 0.0008

-20 253.15 12.546 0.0026

-10 263.15 10.786 0.0072

0 273.15 9.026 0.0188

10 283.15 7.266 0.0457

20 293.15 5.506 0.1045

30 303.15 3.746 0.2262

40 313.15 1.986 0.4664

50 323.15 0.226 0.9195

60 333.15 -1.534 1.7402

Peroxyacetyl nitrate (PAN) – eye irritant in smog

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Nitrogen oxides are unpleasant, and in large enough concentrations, toxic.

The larger problem is reaction with acetaldehyde to make PAN. The latter is a

significant irritant at low concentrations.