LO 79 - 84 Flashcards
Summarize a spontaneous redox reaction that proceeds forward in the standard state
ΔG° < 0
E°cell > 0
K > 1
Summarize a nonspontaneous redox reaction that proceeds in reverse in the standard state
ΔG° > 0
E°cell < 0
K < 1
Draw the triangle between the equations for ΔG°, E°cell, and K
In the equation:
Ecell = E°cell - (0.0592V/n) · log(Q)
what does n represent?
the number of mols e- transferred in the redox reaction
What is Faraday’s constant?
F = 96,485 C /mol e-
Describe the behavior of a cell in where:
Q < 1
Ecell > E°cell
Voltaic cell where the [reactants] > [products], driving the reaction to the right
Describe the behavior of a cell in where:
Q > 1
Ecell < E°cell
Electrochemical cell where the [reactants] < [products], driving the reaction to the left
Describe the behavior of a cell in where:
Q = 1
Ecell = E°cell
No tendency in either direction which is why, as reactants are depleted, batteries ‘die’
_____________ cells are voltaic cells constructed in which differences in the concentration drives the direction of current flow
concentration
E°cell = ____________ for all concentration cells
0
When calculating Ecell for a concentration cell using
Ecell = E°cell - (0.0592V/n) · log(Q), what is Q?
Q = dilute / concentrated
Explain the relationship between ion concentration and e- movement in a concentration cell at the anode and cathode
e- flow spontaneously from the half-cell with the low ion concentration to the half-cell with the high ion concentration.
What are the primary properties of dry cell batteries?
- Dry cell batteries do not contain large amounts of liquid water
- Zinc case acts as the anode:
Zn(s) → Zn2+(aq) + 2 e– - Graphite rod immersed in a moist paste of MnO2 with NH4Cl acts as the cathode:
2 MnO2(s) + 2 NH4+(aq) + 2 e– → Mn2O3(s) + 2 NH3(g) + H2O(l)
What are the primary properties of alkaline batteries?
- Alkaline batteries use a basic medium
- Zinc case acts as the anode:
Zn(s) + 2 OH–(aq) → Zn(OH)2(s) + 2 e– - Graphite rod immersed in a moist paste of MnO2 with KOH acts as the cathode:
2 MnO2(s) + 2 H2O(l) + 2 e– →
2 MnO(OH)(s) + 2 OH–(aq)
What are the primary properties of lead-acid storage batteries?
In a _________, the reactants (provided from an ___________ source) constantly flow through the battery, generating electrical current as they undergo a _____________
fuel cell
external
redox reaction
Draw a hydrogen-oxygen fuel cell converting hydrogen and oxygen gases into water
In an ___________ cell, a current drives an otherwise nonspontaneous reaction through ______________. The voltage required to cause electrolysis depends on the specific __________
electrolytic
electrolysis
half-reactions
In voltaic cells the ______ is the source of e- and has a negative charge. The ________ draws e- and has a positive charge
anode
cathode
In electrolytic cells, e- are __________ from the anode which must be connected to the ______ terminal of the external power source. e- are driven towards the cathode which is connected to the _____________ terminal
drawn away
positive
negative
What happens to the anode and cathode when a pure, molten salt undergoes electrolysis?
the anode is oxidized, and the cathode is reduced
In a mixture of different cations and anions how do you determine which will react in a cell?
Compare the E°cell of each cation reduction, the more positive E°cell is more easily reduced at the cathode. Similarly for anions, the E°cell that is the least positive is more easily oxidized.
When an aqueous solution undergoes electrolysis, it is important to remember that . . .
H2O can also undergo electrolysis and, as such, is treated as a reaction that will occur if it is the one that is reduced or oxidized more easily
Give the cations of active metals that are not easily reduced from aqueous solutions by electrolysis because H2O reduces at a lower voltage
Li+, K+, Na+, Mg 2+, Ca 2+, and Al 3+
____________ occurs when more energy is needed to get a nonspontaneous reaction to occur
overvoltage
What are two useful conversions to relate charges and time back to the mass of a substance?
1 amp = 1 coulomb/ second
F = 96,485 coulomb/ mol e-
_____________ is the gradual oxidation of a metal that occurs from exposure to oxidizing agents in the environment
corrosion
The ____________ of O2 has a strong tendency to cause the _________________ of other metals
reduction
oxidation
Imperfections in metals cause __________________ where oxidation occurs which e- produced then travel to _____________ where they react with . . .
anodic regions
cathodic regions
O2 and H+ ions dissolved in air moisture
Explain why moisture must be present for rusting to occur
the presence of water is necessary because water is a reactant in the last reaction and because charge must be free to flow between the anodic and cathodic regions
Explain why additional electrolytes promote rusting
the presence of electrolytes on the surface of iron promotes rusting because they enhance current flow
Explain why acid promote rusting
since H+ ions are involved in the reduction of oxygen, lower pH enhances the cathodic reaction and leads to faster rusting
Sometimes a _________ ___________ made of a more easily oxidized metal is placed in contact to help prevent rusting
sacrificial electrode
