Elecctrochemistry Flashcards
Electrochemical cells
Contained systems where oxidation-reduction reactions occur
Galvanic/Voltaic cells house what kind of reaction?
Spontaneous
Electrolytic cells house what kind of reaction?
Nonspontaneous
Electromotive force (emf)
Voltage or electrical potential difference of the cell
If emf is positive
The reaction is spontaneous
Delta G<0
If emf is negative
The reaction is nonspontaneous
Delta G>0
Half cells
Two electrodes placed in separate compartments
Salt bridge
Made of inert salt - permits exchange of cations and anions so that the excessive charge won’t accumulate on both sides
Rules of a cell diagram
- )Reactants and products are always listed left to right in anode, anode soln, cathode soln, cathode
- ) Single vertical line means phase boundary
- )Double vertical is the presence of a salt bridge
How does the current flow?
From cathode to anode
Electrolysis
Oxidation-reduction drivedn by external voltage source where chemicals are decomposed
Electrodesposition equation
molM=It/nF
n=oxidation state of the compound needed
Concentration cell
A galvanic cell that ahs two half-cells connected by a conductive material, allowing spontaneous oxidation-reduction to proceed
Chemically identical electrodes
Rechargable cell/battery
Functions as both a galvanic cell and electrolytic cell
Lead-acid battery/lead storage battery
Rechargeable battery Voltaic/Galvanic cell- two half-cells Pb anode and PbO2 cathode connected by H2SO4 When fully charged- Two PbSO4 with dilute H2SO4
What happens when a lead-acid battery is charging?
It is part of a electrolytic circuit and equations change directions
Energy density
Measure of a battery’s ability to produce power as a function of its weight
What is the energy density of lead-acid batteries compared to others ?
Low
Anode in lead-acid batteries?
Pb
Cathode in lead-acid batteries?
PbO2
Anode in nickel-cadmium batteries?
Solid cadmium
Cathode in nickel-cadmium batteries?
Nickel (III) oxide-hydroxide
What has a higher energy density, nickel-cadmium batteries or lead-acid?
Nickel cadium
Surge current
Peridos of large current early in the discharge cycle
Anodes considered what charge in galvanic cell?
Negative (low electron potential)
Cathode considered what charge in galvanic cell?
Positive (high electron potential)
What way does current flow in the anode-cathode galvanic cell?
From Cathode to anode
Anode considered what charge in electrolytic cell?
Positive
Cathode of electrolytic cell is what charge in electrolytic cell?
Negative
Which way do anions always flow?
To anode
Which way do cations always flow?
To cathode
Where do reduction and oxidation always take place?
Anode=oxidation
Cathode=reduction
Which way do electrons always flow?
From anode to cathode?
Which way does current always flow?
Cathode to anode
Reduction potential
Measured in volts compared relative to the standard hydrogen electrode (SHE) which has a potential of 0 V
Reduction potential
Defined as the tendancy of a species to gain electrons and be reduced
More positive potential means?
Greater tendency to be reduced
Standard reduction potential (E(red))
Measured under standard conditions
25 degrees C
1 atm
1 M
In galvanic cells, because electrons flow from anode (less positive) to cathode (more positive=wants to be reduced):
Reaction is spontaneous and Delta G is negative
In electrolytic cells, electrode with more positive reduction potential is forced to be oxidized, so it is the:
Anode
In electrolytic cells, electrode with less positive reduction potential is forced to be reduced and it is the?
Cathode
What is the Delta G of the electrolytic cell?
Positive, non-spontaneous
Negative Ecell value
How to get oxidation potential from reduction potential?
Reverse the sign
Standard electromotive force (emf)
E(cell)=E(red,cathode)- E(red, anode)
What is the Delta G of galvanic cell?
Negative Delta G Positive E(cell) values
What species are included in the reaction quotient
The ones in solution
If equilibrium constant for the reaction is between 0 and 1
E(cell) will be negative because ln between 0 to 1 is negative
Electrolytic cell, +G, nonspontanteous
If equilibrium constant is greater than 1
E (cell) will be positive because ln greater than 1 is positive
Galvanic cell, -G, spontaneous
What does a positive Keq mean?
Equilibrium lies far right
What charge is reduced in electrolytic cells?
More positive reduction potential means it will be oxidized at the anode
