Transcript DEPENDENCE OF CELL POTENTIAL ON CONCENTRATIONS LAB 10 PURPOSE • In this experiment students will construct half-cells of Cu2+ / Cu and Zn2+ /

DEPENDENCE OF CELL POTENTIAL
ON CONCENTRATIONS
LAB 10
PURPOSE
• In this experiment students will construct half-cells
of Cu2+ / Cu and Zn2+ / Zn in contact with KNO3
solution (salt bridge).
• They will be able to show that there is a linear
dependence of cell potential on concentration
as per the Nernst equation.
• The Nernst equation is used for calculations
when non-standard conditions and/or
concentrations are involved in a Voltaic setup.
ELECTROCHEMISTRY
• A study of the interchange of electrical and
chemical energy.
• An electrical current can be established FROM a
spontaneous chemical reaction.
• Chemical change can be produced FROM an electrical
current.
GALVANIC (VOLTAIC) CELLS
• Galvanic cells use a redox reaction (chemical
reaction) to generate an electrical current.
• When both reagents are in the same solution, electrons are
transferred directly when reagents collide, so no useful work
is obtained (heat may be released).
• When the reagents are separated, but connected through
a salt bridge and metal electrodes, the electron transfer
occurs through a wire and can, for example, run an electric
motor (useful work obtained).
GALVANIC (VOLTAIC) CELLS
This is a traditional Galvanic cell setup. Ours will look slightly different.
GALVANIC (VOLTAIC) CELLS
• Without a salt bridge:
• Current flows from the anode to the cathode but builds up a
negative charge (on the cathode).
• Without a large external influx of energy, the current ceases its
flow.
• With a salt bridge:
• Electrons are transferred from the reducing agent (anode) to
the oxidizing agent (cathode).
• The salt bridge ions neutralize the charge build-up (cations to
the cathode, anions to the anode).
• The circuit is complete, the net charge in each compartment
becomes zero.
• Current flows until the cell is discharged and equilibrium is
reached. At that point, the components in the two cell
compartments have the same free energy. (G = 0, Q = K, E = 0)
CELL POTENTIAL
• Ecell (unit V) is the cell potential or electromotive
force responsible for driving electrons from the
reducing agent (anode) to the oxidizing agent
(cathode)
• We measure Ecell with a voltmeter which draws
current through a known resistance.
• When current flows through a wire, frictional heating
results in lost energy.
• A voltmeter therefore always reads a potential less
than the maximum cell potential (E0cell). This occurs
less so with digital voltmeters compared to analog
voltmeters.
STANDARD REDUCTION POTENTIALS
• Half-reactions are written as REDUCTION reactions in
reduction potential tables.
• Each half-reaction has its own reduction potential,
which can be positive, or negative, depending on
how it compares to the standard hydrogen
electrode:
2H+ + 2e-  H2 which has an E0 = 0.00 V
• Our half-reactions are:
Zn2++ 2 e-  Zn
Cu2+ + 2 e-  Cu
E0 = - 0.76 V
E0 = 0.34 V
STANDARD REDUCTION POTENTIALS
• When the reduction potentials are added together, you
get the standard reduction potential for the cell (E0).
• A cell runs spontaneously in the direction that produces a
positive cell potential. (E0 has to be positive for the
reaction to work.)
Zn  Zn2++ 2 e0.76 V
Cu2+ + 2 e-  Cu
+ 0.34 V
E0 = 1.10 V
• Both cell compartments must be in their standard states to
obtain this “theoretical” value. (1 M, 1 atm, 25 C)
• Experimentally we can find our E0cell value by plotting E, V
vs. log Q and then solving for E when log Q = 0.
STANDARD REDUCTION POTENTIALS
• Because of nonstandard concentrations (and other
conditions), experimentally:
Ecell < E0cell < E0
Ecell = the cell potential we will measure
E0cell = the experimental standard state potential
difference from E,V vs. log Q. This is the largest
potential we can possibly observe before the
current flows.
E0 = the theoretical standard state potential
difference (1.10 V)
THE NERNST EQUATION
 The Nernst equation demonstrates a linear
relationship between galvanic cell potential and cell
concentration.
Ecell =
E0
cell
-
RT
nF
ln Q
where R = gas constant,
T = temperature in Kelvin
F = Faraday’s constant
n = number of mole electrons
THE NERNST EQUATION
 Adjusted for lab conditions (substituting in the
values for R and F and 25C), with a few other
conversions, we get:
Ecell = E0cell -
0.0591
n
log Q
E=-
0.0591
n
log Q + E0
y =
m
x
+
b
NERNSTIAN RESPONSE
• A reversible electrode responds in a Nernstian
fashion when E, V vs. log Q gives a straight line
with a slope of  0.0591
n
• To calculate number of electrons transferred,
we simply use:
n
0.0591
slope
PROCEDURE
Prepare your Cu2+ solutions.
Collect your Zn2+ and KNO3 solutions.
“Calibrate” your voltage probe.
Set up your experimental apparatus and
perform your experiment as detailed in your lab
manual.
• Make up the required spreadsheet and graph
based on your results.
•
•
•
•
SAFETY CONCERNS
• Reagents:
•
•
•
•
Cupric sulfate
Zinc sulfate
Potassium nitrate (1 M)
Copper / Zinc solids
• Eye Contact:
• Irritation, pain, redness, conjunctivitis, ulceration, mechanical harm,
clouding of cornea
• Skin Contact:
• Irritation, redness, pain, itching
• Inhalation:
• Coughing, sore throat, shortness of breath, ulceration,
methemoglobinemia, cyanosis, convulsions, tachycardia, dyspnea,
dizziness, drowsiness, headache, perforation of the respiratory tract and
death. Fumes from heating may cause symptoms similar to a cold.
• Ingestion:
• Burning of the mouth, esophagus, and stomach, hemorrhagic gastritis,
nausea, vomiting, abdominal pain, metallic taste, tachycardia,
hypotension, pulmonary edema, kidney damage, liver damage,
hemorrhagic pancreatitis and diarrhea. Systemic copper poisoning
with capillary damage, headache, cold sweat, weak pulse, CNS
excitation, depression, jaundice, convulsions, blood effects, paralysis,
coma and death.
WASTE
• Zinc solutions are toxic and MUST be disposed in
the appropriate waste container in the fume
hood.
• Copper solutions are toxic and MUST be
disposed in the appropriate waste container in
the fume hood.
• KNO3 solutions may go down the drain.
LAB 11 REMINDER
• Lab 11 in 2 weeks.