MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

Concept Summary

Electrochemical Cells

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

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

2. The anode is always the site of oxidation. It attracts anions.

3. The cathode is always the site of reduction. It attracts cations.

4. Electrons flow from the anode to the cathode.

5. Current flows from the cathode to the anode.

· Cell diagrams are shorthand notation that represent the reactions taking place in an electrochemical cell.

1. Cell diagrams are written from anode to cathode with electrolytes (the solution) in between.

2. A vertical line represents a phase boundary, and a double vertical line represents a salt bridge or other physical boundary.

· Galvanic (voltaic) cells house spontaneous reactions (ΔG < 0) with a positive electromotive force.

· Electrolytic cells house nonspontaneous reactions (ΔG > 0) with a negative electromotive force. These nonspontaneous cells can be used to create useful products through electrolysis.

· Concentration cells are 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.

· The charge on an electrode is dependent on the type of electrochemical cell one is studying.

1. For galvanic cells, the anode is negatively charged and the cathode is positively charged.

2. For electrolytic cells, the anode is positively charged and the cathode is negatively charged.

· Rechargeable batteries are electrochemical cells that can experience charging (electrolytic) and discharging (galvanic) states. Rechargeable batteries are often ranked by energy density—the amount of energy a cell can produce relative to the mass of battery material.

1. Lead—acid batteries, when discharging, consist of a Pb anode and a PbO2 cathode in a concentrated sulfuric acid solution. When charging, the PbSO4-plated electrodes are dissociated to restore the original Pb and PbO2 electrodes and concentrate the electrolyte. These cells have a low energy density.

2. Nickel—cadmium batteries (Ni—Cd), when discharging, consist of a Cd anode and a NiO(OH) cathode in a concentrated KOH solution. When charging, the Ni(OH)2 and Cd(OH)2 plated electrodes are dissociated to restore the original Cd and NiO(OH) electrodes and concentrate the electrolyte. These cells have a higher energy density than lead—acid batteries.

3. Nickel—metal hydride (NiMH) batteries have more or less replaced Ni—Cd batteries because they have higher energy density, are more cost effective, and are significantly less toxic.

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

Cell Potentials

· A reduction 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.

1. Standard reduction potentials (E°red) are calculated by comparison to the standard hydrogen electrode (SHE) under the standard conditions of 298 K, 1 atm pressure, and 1 M concentrations.

2. The standard hydrogen electrode has a standard reduction potential of 0 V.

· Standard electromotive force (E°cell) is the difference in standard reduction potential between the two half-cells.

· For galvanic cells, the difference of the reduction potentials of the two half-reactions is positive; for electrolytic cells, the difference of the reduction potentials of the two half-reactions is negative.

Electromotive Force and Thermodynamics

· Electromotive force and change in free energy always have opposite signs.

1. When E°cell is positive, ΔG° is negative. This is the case in galvanic cells.

2. When E°cell is negative, ΔG° is positive. This is the case in electrolytic cells.

3. When E°cell is 0, ΔG° is 0. This is the case in concentration cells.

· The Nernst equation describes the relationship between the concentration of species in a solution under nonstandard conditions and the electromotive force.

· There exists a relationship between the equilibrium constant (Keq) and E°cell.

1. When Keq (the ratio of products’ concentrations at equilibrium over reactants’, raised to their stoichiometric coefficients) is greater than 1, E°cell is positive.

2. When Keq is less than 1, E°cell is negative.

3. When Keq is equal to 1, E°cell is 0.