Chem #12 Flashcards
____ Ecell means spontaneous
positive
the standard reduction potential is determined by the ____ of the electrode, not the ______
identity
amount of it present
salt bridge consists of ____
inert electrolytes
Keq depends on ____ not ____
Also, Keq depends on the electrolytes and the temp, not the amount of the electrode.
electrons flow from ________ in all types of electrochemical cells
anode to cathode
all batteries are influenced by ____
temperature changes
half cells
two separate compartments that contain one or the other of the electrodes.
inner working of galvanic (volataic) cells
two separate compartments that contain one or the other of the electrodes.
another name for voltaic cell
galvanic
electrolyte solution
aqueous solution that surrounds the electrodes and consists of cations and anions.
Galvanic cell: daniel cell
the cations in the two half-cell solutions can be of the same element as the respective metal electrodes.
Galvanic cell: salt bridge
connects the two solutions and contains an inert salt.
Permits the exchange of cations and anions and dissipates the charge gradient.
Anions flow to anion side and cations flow to cation side.
Galvanic cell: As the spontaneous reaction proceeds toward equilibrium, the movement of electrons results in a conversion of _____
electrical potential energy into kinetic energy
if the two reactions in a galvanic cell were not separated, what would happen?
o If the two half-reactions were not separated then no work could be done.
cell diagram
: shorthand notation representing the reactions in an electrochemical cell.
o Rules for writing it out (see equations sheet)
what is similar between electrolytic and galvanic cells?
All electrochemical cells have oxidation at anode and reduction at cathode.
Electron flow from anode to cathode
Current flow from cathode to anode.
what is different between electrolytic and galvanic cells?
House nonspontaneous reactions that require the input of energy to proceed.
DeltaG>0
electrolysis
oxidation-reduction reaction driven by an external voltage source; chemical compounds are decomposed.
do the half-reactions needed to be separated in electrolytic cells?
The half-reactions do not need to be separated into different compartments because the desired reaction is nonspontaneous. `
Faraday
the amount of chemical change induced in an electrolytic cell is directly proportional to the number of moles of electrons that are exchanged during the oxidation-reduction reaction.
• Mn+ +ne- M(s)
what is faraday’s constant
10^5. amount of charge in one mole of electrons
how does a concentration cell compare to a galvanic cell?
o Similar:
Contains two half-cells connected by a conductive material, allowing a spontaneous oxidation-reduction reaction to proceed, which generates a current and delivers energy.
o Different:
Both of the electrodes are chemically identical and so current is generated as a function of a concentration gradient established between the two solutions surrounding the electrodes.
When the concentrations are equal, the voltage is 0.
The voltage can be calculated with the Nernst equation.
o Bio: represented by the cell membrane of a neuron.
Resting membrane potential (Vm) created
• If the potential is sufficiently large, it will result in the firing of an action potential.
where do electrons move in concentration cell?
move to get equilibrium of ion gradient
rechargeable battery or cell
one that can function as both a galvanic and electrolytic cell.
lead-acid battery
specific type of rechargeable battery.
When charged: Pb anode and porous PbO2 cathode connected with concentration H2SO4
When discharged: two PbSO4 electroplated lead electrodes connected with dilute H2SO4
The electric potential is positive
Charging: the lead-acid cell is part of an electrolytic circuit.
• Just opposite, reverses the electroplating to create a concentrated H2SO4 solution again.
low energy density (produce power as function of weight)
nickel-cadmium battery
Consist of two half cells made of solid cadmium (anode) and nickel (III) oxide-hydroxide (the cathode) connected by conductive KOH.
Higher energy-density than lead-acid batteries.
Provide higher surge currents: periods of large current early in the discharge cycle.
Recently been replaced with nickel-metal hydride (NiMH) batteries
energy density
a measure of a battery’s ability to produce power as a function of its weight
do nickel-cadmium batteries or lead-acid batteries have higher energy density?
nick-cadmium batteries
electrode charge designation in galvanic cell
In a galvanic cell, current is spontaneously generated as electrons are released by the oxidized species at the anode and travel through the conductive material to the cathode, where reduction takes place.
• Anode is considered negative, so electrons move from negative (low electrical potential) to positive (high electric potential)
Isoelectric focusing
technique used to separate amino acids or polypeptides based on their isoelectric points (pI).
Positive charged (cations) will migrate toward cathode
Negative charged (anions) will migrate toward anode.
how are reduction potentials determined?
o Determined experimentally using the tendency of hydrogen ion (H+) to be reduced as an arbitrary zero reference point.
o A reduction potential is measured in V volts and defined relative to the standard hydrogen electrode (SHE), which is given a potential of 0 V by convention.
when manipulating reduction potential:
flip sign but do not multiply
the more positive the reduction potential, ___
the more likely it will be reduced
reduction potential
: the tendency of a species to gain electrons and to be reduced.
standard reduction potential
measured under standard conditions: 298K, 1 atm pressure, 1 M concentrations.
reduction potential and galvanic cells
o For galvanic cells, the electrode with the more positive reduction potential is the cathode, and the electrode with the less positive reduction potential is the anode.
reduction potential and electrolytic cells
o For electrolytic cells, the electrode with the more positive reduction potential is forced by the external voltage to be oxidized and is therefore the anode.
electromotive force
o Standard electromotive force (emf or E°cell): the difference in potential (voltage) between two half-cells under standard conditions.
Calculated using standard reduction potentials.
when deviating from standard conditions what equation is used?
nernst equation
____ cells and ____ cells house spontaneous reactions
galvanic and concentration
In ALL electrochemical cells, oxidation occurs at the _____ and reduction occurs at the _____
anode
cathode
if the emf is positive then it _____
releases energy, deltaG > 0
for all electrochemical cells, the electrons move from ____ to ____ and the current runs from ____ to _____
anode to cathode
cathode to anode
the free energy change and electromotive force always have ____ signs
opposite
batteries are common ____ cells
galvanic
the salt bridge consists of an ____ salt and is responsible for _____ of the charge gradient
inert
dissipation
plating or galvanization
when the material is plated onto the cathode
from the salt bridge, the anions flow toward where and the cations flow toward where?
anions to anode and cations to cathode
electrolytic cells house _____ reactions that require ____ to proceed
nonspontaneous
energy
one faraday is equal to _____
the amount of charge contained in one mole of electrons
1 F = 96485 C (which can be rounded up to 10^5)
in a concentration cell, the current will stop when _______
the concentration of ionic species in the half-cells are equal.
how can the voltage for concentration cells be calculated?
using the Nernst equation
what type of cell has some of the lowest energy-to weight ratios?
lead-acid batteries
surge current
periods of large current (amperage) early in the discharge cycle.
charge designations for galvanic vs. electrolytic cells
galvanic:
- anode: negative
- cathode: positive
electrolytic:
- anode: + (the positive side of the battery or external source is connected to it)
- cathode: -
in all electrochemical cells, ____ are attracted to the anode and ____ are attracted to the cathode
anions
cations
in isoelectric focusing, positively charged amino acids will migrate ____ and negatively charged amino acids will migrate ____
toward the cathode
to the anode
In Molten NaCl cells, the electrodes consist of ____
anything as long as it can resist high temperatures and the corrosion
what is at the anode and cathode for charging and discharging lead-acid batteries and Ni-Cd batteries?
NiCd
- Discharging: Anode: Cd Cathode: NiO(OH)
- Charging: Anode: Cd(OH)2 Cathode: Ni(OH)2
Lead acid
- Discharging: Anode: Pb, Cathode: PbO2
- Charging: PbSO4 for both
what is the voltage of the standard hydrogen electrode?
0 V (SHE)
reduction potential
the tendency of a species to gain electrons and to be reduced
the more ____ the reduction potential, the greater the tendency to be reduced
positive
oxidation potential is the same as reduction potential but _____
flipped sign
the standard reduction potential of an electrode will not change unless the ____
chemical identity of that electrode is changed
is not multiplied by moles
is the standard reduction potential multiplies by moles?
no
galvanic cells have a ____ emf
positive
potentiometer
a kind of voltmeter that draws no current and gives a more accurate reading of the difference in potential between two electrodes
why have nickel-metal hydride (NiMH) batteries replaced Ni-Cd batteries?
they have a larger energy density, are more cost effective, and are significantly less toxic.
E cell is dependent only on the ____ of electrodes and not the ____
identity amount present (size or mass)
