MCAT Gen Chem Chapter 12: Electrochemistry ***

What does a vertical line and a double vertical line represent in a cell diagram?

1) a phase boundary 2) a salt bridge or other physical boundary.

What are the Nickel-cadmium batteries (Ni-Cd) when discharging and then charging?

1) consist of a Cd anode and a NiO (OH) cathode in a concentrated KOH solution. 2) the Ni(OH) 2- and the Cd(OH)2- plated electrodes are dissociated to restore the original Cd and NiO(OH) electrodes and concentrate the electrolyte.

What is the change in free energy, when the electromotive force is positive? What cell type is this?

1) negative. 2) galvanic cells.

What is the change in free energy, when the electromotive force is negative? What cell type is this?

1) positive 2) electrolytic cells

What are lead-acid batteries? When discharging and charging:

1) when discharging, they consist of a Pb anode and a PbO2 cathode in a concentrated sulfuric acid solution. 2) When charging, the PbSO4- plated electrodes are dissociated to restore the original Pb and PbO2 electrodes and concentrate the electrolyte.

What is the change in free energy, when the electromotive force is zero? What cell type is this?

1) zero. 2) concentration cells.

What flows from cathode to anode?

current.

Electrochemical cell:

describes any cell in which oxidation-reduction reactions take place. Certain characteristics are shared between all types of electrochemical cells.

What flows from anode to cathode?

electrons.

What are in cell diagrams? how are they written?

from anode to cathode with electrolytes ( the solution) between them.

What is the energy density of Nickel-Cadmium bateries?

higher than lead-acid batteries.

Anode:

is always the site of oxidation.

Cathode:

is always the site of reduction.

What is the standard electromotive force (E knot cell)?

is the difference in standard reduction potential between the two half-cells.

What is the Nernst equation?

it describes the relationship between the concentration of species in a solution under nonstandard conditions and the electromotive force.

What is a galvanic (voltaic) cell?

it houses spontaneous reactions ( delta G < 0) with a positive electromotive force.

What is a surge current?

it is an above-average current transiently released at the beginning of the discharge phase; it wanes rapidly until a stable current is achieved.

What is the energy density of lead-acid batteries?

low.

What is the difference of the reductive potentials of the two half reactions for electrolytic cells?

negative.

When the equilibrium constant Keq is less than 1, what is the electromotive force of the cell?

negative.

What is the sign of electromotive force and change of free energy always?

opposite signs.

What is the difference of the reductive potentials of the two half reactions for galvanic cells?

positive.

When the equilibrium constant Keq is greater than one, what is the electromotive force of the cell?

positive.

What is a reduced potential?

quantifies the tendency for a species to gain electrons and be reduced. The higher the reduction potential, the more a given species wants to be reduced.

Electrodes:

strips of metal or other conductive materials placed in an electrolyte solution.

What is energy density?

the amount of energy a cell can produce relative to the mass of battery material.

What are the charges for galvanic cells?

the anode is negatively charged and the cathode is positively charged.

What are the charges for electrolytic cells?

the anode is positively charged and the cathode is negatively charged.

What is the equilibrium constant Keq?

the ratio of products' concentrations at equilibrium over reactants', raised to their stoichiometric coefficients.

What are standard reduction potentials (E knot red)?

they are calculated by comparison to standard hydrogen electrode (SHE) under the standard conditions 298 K, 1 atm pressure and 1 M concentration.

What are rechargeable batteries?

they are electrochemical cells that can experience charging (electrolytic) and discharging (galvanic) states.

What are cell diagrams?

they are shorthand notation that represent the reactions taking place in an electrochemical cell.

What are electrolytic cells?

they house nonspontaneous reactions ( delta G > 0) with a negative electromotive force. Can be used to create useful products through electrolysis.

When the equilibrium constant Keq is equal to 1, what is the electromotive force of the cell?

zero.

What ions does the anode attract?

It attracts anions.

What ions does the cathode attract?

It attracts cations.

What is the standard reduction potential of the standard hydrogen electrode?

0 V.

For a cell with the following half-reactions: Anode: SO2 + 2 H2O -> SO4 2- + 4 H+ + 2e- Cathode: Pd 2+ + 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) The emf would decrease. 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, cat - E red, an.

The anode of a certain galvanic cell is composed of copper. Which of the following metals can be used at the cathode, assuming equal concentrations of the two electrolyte solutions? Hg2+ + 2e- -> Hg E red= +0.85V. Cu+ + e- -> Cu Ered= +0.52V. Zn2+ + 2e- -> Zn E red= -0.76 V. Al 3+ + 3e- -> Al E red= -1.66V A) Hg B) Cu C) Zn D) Al

A. Hg. Oxidation occurs at the anode and reduction occurs at the cathode. Because Cu is the anode, it must be oxidized. The reduction potential of the cathode cannot be less than that of the anode for a galvanic cell. Therefore, mercury must be the cathode. In a concentration cell, the same material is used as both the cathode and anode; however, this question assumes equal concentrations. If both electrolyte solutions have the same concentration, there will be no oxidation-reduction reaction and, therefore, no anode or cathode.

Given the standard reduction potentials: Zn 2+ + 2e- -> Zn E red= -0.763 V. and Ag+ + e- -> Ag E red= + 0.337V. What is the standard electromotive force of the following reaction? Zn2+ + 2 Ag -> 2Ag+ Zn A) -2.2 V B) -1.1 V C) + 1.1 V D) +2.2 V

B) -1.1 V To determine the standard electromotive force of a cell, simply subtract the standard reduction potentials of the two electrodes. In this case, the cathode is zinc because it is being reduced; the anode is silver because it is being oxidized. Thus, E cell = E red, cat - E red, an = -0.763 V - 0.337 V = -1.10 V. While we must multiply the silver half-reaction by two to balance the electrons, the actual value for the reduction potential does not change. Remember that the standard reduction potential is determined by the identity of the electrode, not the amount of it present.

If the surface area of electrode material in an electrochemical cell is tripled, what else is necessarily tripled? I. E cell II. Current. III. K eq A) I only. B) II only. C) I and II only. D) II and III only.

B) II only. Potential, as measured by E cell, is dependent only on the identity of the electrodes and not the amount present. Similarly, the equilibrium constant depends only on the identity of the electrolyte solutions and the temperature. However, as the electrode material is increased the surface area participating in oxidation-reduction reactions is increased and more electrons are released making statement II correct.

Which of the following statements could be true about a Na-Cd cell, based on the information below? Na+ + e- -> Na Ered= -2.71 V. Cd 2+ + 2e- -> Cd E red = -0.40 V. A) It is a galvanic cell, and sodium is the cathode. B) It is an electrolytic cell, and cadmium is the anode. C) It is a galvanic cell, with E cell = 3.11 V. D) It is an electrolytic cell, with E cell = -3.11V.

B) It is an electrolytic cell, and cadmium is the anode. If this were a galvanic cell, the species with the more positive reduction potential (cadmium) would be reduced. The cathode is always reduced in an electrochemical cell, so sodium could not be the cathode in such a galvanic cell, eliminating A. Sodium would be the cathode in an electrolytic cell, however, which would make cadmium the anode. Thus the answer is B. Note that we do not have to determine E cell because we already know the answer. However, the E cell would be -2.71 - (-0.40)= -2.31V for an electrolytic cell, and + 2.31 V for a galvanic cell. Eliminating choices C and D.

Which of the following is LEAST likely to be found in the salt bridge of a galvanic cell? A) NaCl B) SO3 C) MgSO3 D) NH4NO3

B) SO3. Salt bridges contain inert electrolytes. Ionic compounds in A, C and D are known to be strong electrolytes because they completely dissociate in solution. B cannot be considered an electrolyte because its atoms are covalently bonded and will not dissociate in aqueous solution. B and C may be similar, but there is an important distinction to be made. C implies that Mg 2+ and SO3 2- are the final, dissociated ionic constituents, while B implies that neutral SO3 would have to be dissolved in solution.

An electrolytic cell is filled with water. Which of the following will move toward the cathode of such a cell? I. H+ ions. II. O2- ions. III. Electrons. A) I only. B) II only. C) I and III only. D) II and III only.

C) I and III only. In an electrolytic cell, ionic compounds are broken up into their constituents; the cations (positively charged ions) migrate toward the cathode and the anions (negatively charged ions) move toward the anode. Cations are H+ ions (protons), so I is correct. Electrons flow from anode to cathode in all types of cells, so III is correct. Option II is incorrect because it is unlikely that the anions in any cell would be O2- rather than OH-. Second, these anions would flow to the anode, not the cathode.

Rusting occurs due to the oxidation-reduction reaction of iron with environmental oxygen. 4 Fe (s) + 3 O2 (g) -> 2 Fe2O3 (s). Some metals cannot react with oxygen in this fashion. Which of the following best explains why iron can? A) Iron has a more positive reduction potential than those metals, making it more likely to donate electrons to oxygen. B) Iron has a more positive reduction potential than those metals, making it more likely to accept electrons from oxygen. C) Iron has a less positive reduction potential than those metals, making it more likely to donate electrons to oxygen. D) Iron has a less positive reduction potential than those metals, making it more likely to accept electrons from oxygen.

C) Iron has a less positive reduction potential than those metals, making it more likely to donate electrons to oxygen. In the oxidation-reduction reaction of a metal with oxygen, the metal will be oxidized (donate electrons) and oxygen will be reduced (accept electrons). This fact eliminates B and D. A species with a higher reduction potential is more likely to be reduced, and a species with a lower reduction potential is more likely to be oxidized. Based on the information in the question, iron is oxidized more readily than those metals, this means that iron has a lower reduction potential.

An electrolytic cell necessarily has: A) Delta S > 0 B) Delta G < 0 C) Keq < 0 D) E cel > 0

C) Keq < 0. An electrolytic cell is nonspontaneous. Therefore, the delta G must be positive and E cell must be negative, eliminating B and D. The change in entropy ay be positive or negative depending upon the species, eliminating A. According to the equation Delta G = - R T x ln K eq, K eq< 1 would result in ln Keq < 0, which means that Delta G >0.

Which of the following best describes why overcharging a Ni-Cd battery is not detrimental? A) The energy density of a Ni-Cd battery is high, so it can store more charge than other batteries per its mass. B) The electrodes of a Ni-Cd battery can discharge through the circuit when they are fully charged. C) The Ni-Cd battery will stop accepting electrons from an outside source when its electrodes have recharged. D) Ni-Cd batteries have a high surge current and can dissipate the overcharge before damage can occur to electrodes.

C) The Ni-Cd battery will stop accepting electrons from an outside source when its electrodes have recharged. During the recharge cycle, Ni-Cd cells will accept current from an outside source until the Cd and NiO(OH) electrodes are pure; at this point, the reaction will stop because Cd(OH)2 runs out and no more electrons can be accepted. (A) and (A) are both true statements, but they fail to explain why overcharging the battery (continuing to try to run current into the battery even when the electrodes are reverted to their original state) is not a problem with Ni-Cd batteries. Finally, surge current refers to the initial burst of current seen in some batteries; once charged, the surge current will not increase even if the power source continues to be run because no additional charge will be stored on the electrodes, eliminating (D).

Which of the following expressions correctly describes the relationship between standard electromotive force and standard change in free energy? A) Delta G= -nF (E red, an - E red, cat) B) E cell= nF/ RT x ln Keq C) E cell= - RT/ nF x ln Keq D) Delta G = nF (E red, an - E red, cat)

D) Delta G = nF (E red, an - E red, cat). There are only two equations involving standard change in free energy in electrochemical cells: delta G= -nF (E red, an - E red, cat) and Delta G = nF (E red, an - E red, cat). Substituting E cell= E red, cat - E red, an into the first equation and distributing the negative sign gives us choice D. A: would be the opposite of Delta G. Setting the two equations equal to each other, we get RTx ln Keq = n x F x E cell. Solving for E cell, we get E cell= RT/ nF x ln Keq, which is the opposite of B. C: incorrectly solves the algebra.

If the value of E cell is already known, what additional data is sufficient to calculate delta G knot? A) The gas constant (R) B) Reaction quotient C) Temperature of the system D) Half-reactions of the cells

D) Half-reactions of the cells. This answer comes directly from the equation relating Gibbs free energy and E cell. Delta G know = -n x F x Ecell, where n is the number of moles of electrons transferred and F is the Faraday constant, 96,485 C/ mol e-. to determine n, one must look at the balanced half-reactions occurring in the oxidation-reduction reaction.

Which of the following 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) K eq> 1.

D) K eq> 1. A spontaneous electrochemical reaction has a negative delta G. Using the equation Delta G = -RT x ln Keq, Keq> 1 would result in ln Keq >0, whic means Delta G <0. A negataive electromotive force (A) or equilibrium state (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 (C). When an answer choice may be true, but does not have to be, it is the wrong answer on test day.

Which of the following is the best explanation for the fact that a larger mass of electrodes are required for lead-acid batteries, as compared to other batteries, to produce a certain current? A) The lead-acid electrolyte, sulfuric acid, is diprotic and incompletely dissociates in solution. B) The energy density of lead-acid electrodes is higher than that of other batteries. C) The electrolytes in other batteries less readily dissociates than those of lead-acid batteries. D) The energy density of lead-acid electrodes is lower than that of other batteries.

D) The energy density of lead-acid electrodes is lower than that of other batteries. Compared to other cell types, the lead-acid batteries have a characteristically low energy density (D). While (A) is a true statement, the incomplete dissociation of sulfuric acid does not fully explain the low energy density of lead-acid batteries. (C) is likely to be an opposite; the more easily the electrodes dissociate, the easier it is to carry out oxidation-reduction reactions with them.

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.

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 x ln Keq = n x F x E cell. The temperature, T, appears in this equation; thus, a change in temperature will impact the E cell.

How are rechargeable batteries ranked

by energy density.

What are nickel-metal hydride (NiMH) batteries?

They have more or less replaced Ni-Cd batteries because they have higher energy density, are more cost effective and are significantly less toxic.

What are concentration cells?

a specialized form of a galvanic cell in which both electrodes are made of the same material. Rather than a potential difference causing the movement of charge, it is the concentration gradient between the two solutions.