MCAT general chemistry Chapter 6 Electrochemical cells
Faraday's constant
(F) = charge in coulombs of 1 mol e- = 96,485 C/mol e-
12. For a cell with the following half-reactions: Anode: SO2 + 2 H2O → SO42- + 4 H+ + 2 e- Cathode: Pd2+ + 2 e- → Pd How would decreasing the pH of the solution inside the cell affect the electromotive force (emf)? A. The emf would decrease. B. The emf would remain the same. C. The emf would increase. D. The emf would become zero
A change in pH has a direct correlation to the hydrogen ion (H+) concentration. Decreasing the pH increases the H+ concentration, which means the concentration of products has increased in the oxidation of sulfur dioxide. This means it would be harder to liberate electrons, thus decreasing the emf. One could also view this decrease in oxidation potential as an increase in reduction potential. If E°red,anode increases, then E°cell must decrease according to E°cell = E°red,cathode - E°red,anode
Which of the following choices is indicative of a spontaneous reaction, assuming standard condition? A. E°cell is negative B. Q = Keq C. The cell is a concentration cell D. Keq > 1
A spontaneous electrochemical reaction has a negative ΔG. Using the equation ΔG° = -RT ln Keq, Keq > 1 would result in ln Keq > 0, which means ΔG° < 0. A negative electromotive force, choice (A), or equilibrium state, choice (B), would not correspond to a spontaneous reaction. Concentration cells can be spontaneous; however, if the concentration cell had reached equilibrium, it would cease to be a spontaneous reaction, eliminating choice (C)
13. An electrolytic cell necessarily has: A. ΔS° > 0 B. ΔG° < 0 C. Keq < 1 D. E°cell > 0
An electrolytic cell is nonspontaneous. Therefore, the ΔG° must be positive and E°cell must be negative, eliminating choices (B) and (D). The change in entropy may be positive or negative, depending on the species involved, eliminating choice (A). According to the equation ΔG° = -RT ln Keq, Keq < 1 would result in ln Keq < 0, which means ΔG° > 0.
Cell diagrams
Anode ll Cathode ex: Pt (s)|Zn (s) l Zn (aq)^2+ ll 1/2 F (aq) l F(aq) l Pt(s) *Can remember that the cathode is on the right because right and reduction both start with an "r"
How is changeinG^o given at equilibrium?
By the equation changeinG^o=-RTln(K)
15. Which of the following best describes why over-charging a Ni-Cd battery is not detrimental
C. The Ni-Cd battery will stop accepting electrons from an outside source when its electrodes are recharged
Components of a Galvanic cell
Can be remembered by : T-E-I-E-T where T=terminals (conductors like metal wires), E=electrodes (also conductors) I=ionic conductor(often a salt bridge)
Adding the two half reactions of a concentration cell gives E^o =
E^o =0
Nernst equation formula
Ecell = E˚cell - (RT/nF) * ln Q
Which of the following can alter the emf of an electrochemical cell? A. The mass of the electrodes B. The length of the wire connecting the half-cells C. The overall size of the battery D. The temperature of the solutions in the half-cells
E°cell is dependent upon the change in free energy of the system through the equation RT ln Keq = nFE°cell. The temperature, T, appears in this equation; thus, a change in temperature will impact the E°cell.
What are the two types of electrochemical cells?
Galvanic (aka voltaic) cells and electrolytic cells
If changeinG^o=0 what can be said about the equilibrium constant K?
K=1
what can be used to find the cell potential when the concentrations are not at 1 M
Nernst equation
Connecting the terminals does what?
Reduces the potential difference due to internal resistance in the galvanic cell (emf drops)
The cell potential (E)
The potential difference between the terminals when they are not connected
The reduction potential of the ____ cannot be less than that of the ___ for a galvanic cell.
The reduction potential of the cathode cannot be less than that of the anode for a galvanic cell.
Even in a galvanic cell with a salt bridge, there is some leakage of ions across the liquid junction. This causes what?
This leakage causes the battery to lose its chemical potential over time?
Salt bridge
U-tube device used to connect the oxidation and reduction half-cells; provides path for the flow of ions. Minimizes the potential difference in cases where you have two different solutions -typically made from aqueous KCl -K+ moves towards cathode at about same rate as Cl- ions move towards anode
If zinc electrode would like to get rid of its electrons what would you use for the potential here using standard reduction potentials?
Use the inverse sign of the E for the reaction: Zn2+(aq)+2e- -> Zn so here that would be -(E=-0.76V)
When do you need a salt bridge?
When a cell contains two different solutions. Don't need it for SHE because oxidation potential of H is 0, so the cell potential just depends on the other electrode
electrochemical cell
a device that transforms chemical energy into electrical energy -two distinct phases in contact but a charged species cannot cross over creates charge difference
an electrolytic cell on the MCAT will have what kind of emf?
a negative one
What happens in a galvanic cell using a standard hydrogen electrode
a platinum conductor over which hydrogen gas is bubbled catalyzes H+ions and carries an electron through the wire to the silver strip (AgCl). Ag+ accepts electrons becomes solid silver and allows Cl to enter aqueous solution
A positive cell potential indicates what?
a spontaneous reaction shown by: changeinG=-nFEmax *n=number of moles of electrons transferred *F=charge on one mole of electrons
what kind of cell requires a power source?
an electrolytic cell
electrolytic cell
an external source of electricity is used to cause a non spontaneous redox reaction to occur *forces reaction to occur in reverse
Electrons in which electrode have higher potential energy?
anode
Where does the oxidation half reaction take place?
anode
Galvanic cell
battery that turns chemical energy into electrical energy -different phases: all phases conduct electricity but at least one is impermeable to electrons (called ionic conductor, often a salt bridge)
Why does a salt bridge minimize the potential difference?
because the K+ moves towards the cathode at about the same rate as the Cl- ions move toward the anode in salt bridge carrying away any charge buildup(p216)
Where does the reduction half reaction take place?
cathode
In an electrolytic cell the anode is marked what ?the cathode is marked what ?
cathode negative and anode positive *this is the opposite of galvanic cell/like in bio
Application of nernst equation
concentration cell or to express the resting membrane potential across the membrane of a neuron
What are the units for faradays constant
coulombs/mole of electrons
electrophoresis uses what kind of cell??
electrolytic cell so it makes sense with the designations of negative for cathode and positive for anode here
Electrons won't flow if there is what ? this is why what is important?
electrons won't flow if there is a charge buildup. This is why salt bridge important to carry away charge buildup
electromotive force
emf the cell potential E
What kind of process happens in an electrolytic cell? exothermic or endothermic?
endothermic because requires energy input
In a galvanic cell electrons flow from what to what?
flow from anode to cathode current is opposite
the concentration cell is a type of what kind of cell?
galvanic cell
the cell potential depends on
half reactions, temperature and the concentration of reactants and products
What are the units for current?
i=Coulombs/second
emf drops more when the current does what
increases
concentration cell
is a limited form of a galvanic cell in which a reduction half reaction is taking place in one half cell while the exact reverse reaction is taking place in the other half of the cell. Cells differ in their ion concentrations.
If the MCAT asks in which direction will current flow in a concentration cell? respond:
nature tends to increase entropy. Electrons will flow in the direction that allows the concentrations in the half cells to become equal, they will flow towards the side that has a greater concentration of positive ions
If E^o is positive the changeinG^o is K is
negative and K>1
The anode is marked with what sign?
negative sign (in chemistry) ***From bio perspective like in electrophoresis would be seen as +
How can the potential be found when the concentrations in the cell are not all 1 molar?
nernst equation
Is the concentration cell ever at standard conditions?
no
Is there a salt bridge in SHE galvanic cells?
no
If changeinG^o is 0, is the reaction at equilibrium?
no can't assume that if changeinG^o=0 then changeinG=0 *changeinG=0 at equilibrium only
The cell potential for a galvanic cell is always what ?
positive *always has chemical energy that can be converted to work
If E^o is negative the changeinG^o is K is
positive and K<1
The cathode is marked with what sign?
positive sign (in chemistry) ***From bio perspective like in electrophoresis would be seen as -
In an electrolytic cell where do reduction and oxidation take place?
same as with galvanic cell reduction still at the cathode and oxidation still at the anode
The two terminals of a galvanic cell must be made of what?
same material
Concentration cells tend to have what kind of potentials?
small ones
When you encounter a positive Emax value or a negative changeinG think
spontaneous
The standard state emf can be found from the cell diagram by
subtracting the potential of the reduction half reaction on the left (at the anode) from the potential of the reduction half reaction on the right side (at the cathode)
the lead battery discharges in the presence of
sulfuric acid
A simple galvanic cell has two electrodes which are:
the anode and the cathode
In the lead storage battery the anode is what and the cathode is what?
the anode is lead while the cathode is lead dioxide
in which type of battery is the electrolyte not consumed?
the electrolyte is not consumed during discharge
The cell potential (E) is also called
the electromotive force (emf)
If using the Nernst equation to find the potential of a concentration cell at 25C keep in mind that the ion in solution is what?
the ion in solution is both a product and a reactant Ex: substituting ion concentrations of anode (0.01M) and cathode (0.1M) into logQ of nernst equation: E=E^o- 0.06/2*log (0.01/0.1) *n was 2 here because 2 electrons were used each time the reaction occurs *E^o=0
The change in Gibbs free energy in the following equation: "changeinG=-nFEmax" represents what?
the maximum non PV-work available to do work at constant pressure and Temp *A negative changing shows that work is being done by the system
Nernst equation expresses what?
the relationship between chemical concentrations and potential difference
If conditions are standard what happens to changeinG=-nFEmax?
the same except that now changinG and Emax are considered at standard state with symbol "o'
the lead storage battery and the nickel-cadmium batteries represent what kind of cell?
they are rechargeable so they alternate between galvanic and electrolytic function
Positive ions move across the salt bridge in what direction?
towards the cathode *since the negative electrons move this way, need positive charge to balance it out
In a concentration cell , as cell potential moves towards 0, the concentrations on both sides will tend to even out. True or false?
true
In a concentration cell electrons will move from less concentrated side to more concentrated side. True or false?
true
When the concentrations are 1 M in a galvanic cell..
we are at standard conditions and can use the reduction half reaction table
Which metal is the anode and which metal is the cathode in electrolytic cells?
will oppose prediction based on reduction potentials. with a voltage source an electrolytic cell allows the metal that would normally be oxidized to be reduced etc.
