Transcript Document
Chapter 14 – Fundamentals of Electrochemistry Homework Due Friday, April 1 Problems: 14-4, 14-5, 14-8, 14-12, 14-15, 14-17, 14-18, 14-25, 14-26, 14-41,
CHM 320 - Lecture 23 Chapt 14
Electrochemistry Review of the Basics
•
Oxidation
– –
Loss of electrons Always occurs at the anode
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Happens because of the action of a reducing agent
Fe +3 + e • =
Reduction
–
Gain of electrons (charge is reduced)
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Always occurs at the cathode
–
Happens because of the action of the oxidizing agent
Fe +2 CHM 320 - Lecture 23 Chapt 14
Redox Reaction
ox 1 + red 2 Example: = red 1 + ox 2(e- + Fe +3 = Fe +2 ) 2 + Sn +2 = Sn +4 + 2 e ---------------------------------- ------- 2 Fe +3 + Sn +2 = 2 Fe +2 + Sn +4 So… ox 1 = Fe +3 , red 2 = Sn +2 , red 1 =Fe +2 , ox 2 =Sn +4
Electric Charge (in Coulombs) and Work
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The charge in coulombs (q) is equal to the number of moles of electrons (n)
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times the Faraday Constant (F)
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Voltage represents electrical potential (potential to do work) If some total charge in coulombs (q) is moved through some electrical potential (E, in volts V) then work is done!
q (coulombs)
n (moles) x F (Faraday Constant) F
4 9.649E10
Coulombs mole of e Work (joules)
E (volts) x q (coulombs)
CHM 320 - Lecture 23 Chapt 14
• • • •
Ohm’s Law and Power
Ohm’s law relates electrical resistance, current and potential!
Power is the work done in some unit time (e.g. joules of work per second) The units of Power are Watts (W) Ohm’s law and power are related!
E(potentia l)
I (current) x R (resistanc e, in Ohms,
) Power (in Watts)
Work (joules) second
E x q s E x q
E x q s s
E x I
CHM 320 - Lecture 23 Chapt 14
Let’s Work Some Problems
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A 6.00V battery is connected across a 2.00 K
resistor, how many electrons flow through the circuit per second?
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How many joules of heat (heat is work) are produced per electron?
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What voltage would the battery need to be to deliver a power at 100.0 Watts?
CHM 320 - Lecture 23 Chapt 14
Electrochemical Cells
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A complete cell contains:
– – –
anode cathode completed circuit (for electrons to flow)
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a salt bridge (usually!)
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an electrolyte solution chemical species that undergo reaction.
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There are two basic electrochemical cells:
– –
A
GALVANIC
cell uses spontaneous chemical reactions to generate electricity A
ELECTROYLTIC
cell requires an electrical potential to be applied to the cell to drive some reaction.
CHM 320 - Lecture 23 Chapt 14
Galvanic Cell
Cells and Cell Reactions
Overall Cell Reaction Zn (s) + Cu +2 (aq) ---> Zn +2 (aq) + Cu (s) oxidation half reaction anode Zn (s) ---> Zn +2 (aq) reduction half reaction cathode Cu +2 (aq) + 2 e + 2 e ---> Cu (s) CHM 320 - Lecture 23 Chapt 14
CHM 320 - Lecture 23 Chapt 14
CHM 320 - Lecture 23 Chapt 14
What is happening at the electrode(s) and how do we describe the cell?
Anode half rxn : Zn 0 (s)
Zn
2 (aq)
2 e Cathode half rxn : Cu
2 (aq)
2 e -
Cu 0 (s) Complete Cell reaction : Zn (s)
Cu
2 (aq)
Zn
2 (aq)
Cu 0 (s) in shorthand, we use symbols!
a single vertical line marks the phase difference a double vertical line marks the salt bridge Anode on the left, cathode on the right Including the counter ions tells us something about the solutions Zn (s) | ZnSO 4(aq) || CuSO 4(aq) | Cu (s)
CHM 320 - Lecture 23 Chapt 14
• • •
The Standard Hydrogen Electrode (SHE) The basis by which all other measurements are made.
Assigned a potential of zero by definition!
Not practical for regular use Hydrogen Half-Cell
H 2(g) = 2 H + (aq) + 2 e reversible reaction SHE consists of a platinum electrode covered with a fine powder of platinum around which H 2(g) is bubbled. Its potential is defined as zero volts.
CHM 320 - Lecture 23 Chapt 14
CHM 320 - Lecture 23 Chapt 14
CHM 320 - Lecture 23 Chapt 14
Standard Potentials
• • • • •
Standardized potentials (E o ), listed as reductions, for all half-reactions Measured versus the S.H.E (0) Used in predicting the action in either a galvanic cell or how much energy would be needed to force a specific reaction in a non-spontaneous cell Assumes an activity of one for the species of interest (usually a fair approximation) at a known temperature in a cell with the S.H.E.
Assumes that the cell of interest is connected to the (+) terminal of the potentiometer (voltmeter) and the S.H.E. is connected to the (-) terminal
CHM 320 - Lecture 23 Chapt 14
Better Oxidizing Agents in upper left hand corner.
CHM 320 - Lecture 23 Chapt 14
Better Reducing Agents in lower Right hand corner