Flashcards in Chapter 12: Electrochemistry Deck (42):
Three types of electrochemical cells?
1. Galvanic cells (voltaic)
2. Electrolytic cells
3. Concentration cells
Which cells house spontaneous reactions? Which house nonspntaneous reactions?
- galvanic and concentration cells
- electrolytic cells
What do all of the cells contain? Two types? What occurs at each and which way do electrons flow and which way does current flow?
Electrodes: anode, where oxidation takes place and cathode, where reduction takes place. electrons move from anode to cathode, current runs from cathode to anode
What does electromotive force respond to? If it's positive? Negative?
If emf is positive, the cell is able to release energy (spontaneous). If negative, the cell must absorb energy, so nonspontaneous
What type of cells are typical non-rechargeable batteries? Delta G of reaction? EMF of the cell?
Galvanic cells. negative delta G (spontaneous) and positive EMF (as it always has the opposite sign of delta G
Makeup of a galvanic cell?
Two electrodes placed in separate compartments (half-cells) connected by a conductive material (which may contain resistors/capacitors). Electrodes surrounded by aq electrolyte solution (in which cation can be same element as electrode) which are connected by an inert salt bridge
What happens once the electrodes are connected by a conductive material? How can this be used?
Charge begins to flow spontaneously as a result of redox reaction between two half-cells. Proceeds towards equilibrium, converting electrical potential into kinetic energy, which can be harnessed to do work by connecting devices into circuit between electrodes
Components of a Daniell cell?
Galvanic cell where zinc is the anode, copper is the cathode and each electrode is in an electrolyte solution containing each respective cation and sulfate
What is plating/galvanization?
When the electrolyte precipitates out of solution onto the electrode eg. copper plating the cathode in a Daniell cell
What does a salt bridge do?
Anions in the salt bridge flow toward the anode and cations flow toward the cathode to balance the charge in the electrolyte solutions, which dissipates the charge gradient that would otherwise stop the reaction
Cell diagram notation rues?
anode | anode solution (concentration) || cathode solution (concentration) | cathode
single vertical line is phase boundary, double is a salt bridge or some other barrier
Free energy change for an electrolytic cell? What drives the redox reaction?
Positive - a nonspontaneous reaction. Driven by an external voltage source (electrolysis) in which chemical compounds are decomposed eg water into oxygen and hydrogen or NaCl (l) into Na(l) and Cl2(g)
Do the half reactions in an electrolytic cell need to separated? Why?
No, they are nonspontaneous
General half-reaction equation for a reduction?
M^n+ + ne- --> M(s), whereby one mole of metal will be produced if n moles of electrons are supplied to M^n+
What is the Faraday constant?
the amount of charge in one mole of electrons.
1F = 96485 C. Use 10^5 on MCAT
mol M = It/nF, where mol M is the amount of metal ion being deposited at a specific electrode, I is current, t is time in seconds, n is the number of electron equivalents for a specific metal ion and F is the Faraday constant
What is a concentration cell? How is current generated?
A special type of galvanic cell in which the electrodes are chemically identical and current is generated as a function of a concentration gradient between the two electrolytic solutions. Similar to resting membrane potential of a cell
How can the voltage of a concentration cell be calculated?
The voltage, as a function of concentrations, can be calculated by the Nernst equation
A rechargeable battery can function as what types of cells? Two examples?
Both a galvanic cell and an electrolytic cell. Lead-acid batteries and nickel-cadmium batteries
What does a lead-acid battery consist of when fully charged? When discharged?
Two half cells - a Pb anode and a porous PbO2 cathode connected by a conductive material (4M H2SO4). Two PbSO4 electroplated lead electrodes with a dilute H2SO4 solution
What do the two electrodes attract and form in a lead-acid battery? Mechanism in case of cathode?
Negatively charged lead anode attracts anionic bisulfate. Porous cathode allows sulfuric acid in, solvates into lead and oxygen, which react to form water and remaining sulfate reacts with lead. Both electrodes form lead sulfate
Net equation for discharging a lead-acid battery?
Pb(s) + PbO2(s) + 2H2SO4 -> 2PbSO4(s) + 2H2O
How do lead acid batteries charge?
An external source reverses the electroplating process and concentrates the acid solution
How is battery energy density measured? How is lead-acid battery energy density?
Energy to battery weight ratio. Very low
Electrodes and conductive material in a nickel-cadmium battery? (AA, AAA batteries)
Composed of two half-cells made of solid cadmium (anode) and nickel(III) oxide-hydroxide (cathode) connective by a conductive material (usually potassium hydroxide).
Nickel cadmium oxidation at anode and reduction at cathode?
Cadmium reacts with hydroxide to form cadmium hydroxide. Nickel oxide-hydroxide reacts with water to form form nickel(II) hydroxide and free OH-
Nickel-cadmium battery net equation?
2NiO(OH) (s) + Cd(s) +2H2O -> 2Ni(OH)2(s) + Cd(OH)2 (s)
How does a nickel cadmium battery charge?
External source reverses the electrolytic cell potenticals (as with lead-acid batteries)
Difference in performance with a Ni-Cd battery? Ideal for?
Higher energy density and higher surge current (period of large current early in discharge) than Pb-acid. Ideal for appliances that require a rapid response, like remote controls
What have Ni-Cd batteries been replaced with recently?
More efficient nickel-metal hydride batteries, which are more energy dense, cost effective and less toxic
Electrode charges of a galvanic cell? Of an electrolytic cell?
- anode is negative (source of electrons) and cathode is positive
- anode is considered positive as it is connected to the positive pole of he external voltage source and attracts ions from solution. Cathode is negative (for same reason)
--however red at cathode and ox at anode for both
What happens in isoelectric focusing?
positively charged amino acids migrate towards the cathode, negatively charged amino acids travel towards anode, separating based on pH on the gel
What is reduction potential?
Measured in volts, and defined relative to the standard hydrogen electrode, which is given a potential of 0 V. When compared, allows you to determine which species will be oxidized and which will be reduced. The more positive the potential, more likely to be reduced
Conditions for standard reduction potential?
25 degrees celcius, 1 atm of pressure and 1M concentrations
For galvanic cells, the electrode with the more positive reduction potential is? less positive is? For an electrolytic cell?
Electrode with more positive reduction potential is forced by external source to be oxidized and is anode. Electrode with less positive reduction potential is forced to be reduced - nonspontaneous
How do you get oxidation potential?
Flip sign of reduction potential
How is electromotive force calculated?
Is is the difference in potential between two half cells under standard conditions. Equals reduction potential of the cathode minus the reduction potential of the anode
How is delta G related to EMF? Units of F and delta G?
delta G = -nF(E^o_cell), where n is the moles of electrons exchanged, F is the faraday constant, and E^o_cell is the standard emf of the cell. F is in J/V, so delta G must be in J
How can you calculate EMF when a cell isn't at standard conditions?
E_cell = E^o_cell - (RT)/(nF) * lnQ
Where Ecell is the emf of the cell under nonstandard conditions, E^o_cell is the emf of the cell under standard conditions, R is the ideal gas constant, T is the temp in kelvin, n is the number of moles of electrons, F is the faraday constant, Q is the rxn quotient at a given time. Only aq species included in Q calculation
Simplified version of the nonstandard emf calculation?
Ecell = E^o_cell - (0.06/n) * logQ
Other equation for delta G? By eliminating delta G, how else can you solve for standard EMF?
delta G = -RTlnKeq.
nFE^o_cell = RTlnKeq