MCAT Content / Electrochemistry / Batteries 2

Batteries

Topic: Electrochemistry

A battery is a device consisting of one or more electrochemical cells with external connections for powering electrical devices.

The Nernst equation can be used to determine the total voltage, or electromotive force, for a full electrochemical cell. The Nernst Equation enables the determination of cell potential under non-standard conditions. The equation relates the electromotive force (emf) of a nonstandard cell to the standard electrode potential, temperature, and activities (often approximated by concentrations) of the chemical species undergoing reduction and oxidation. The equation can be simplified as:

In this equation:

– E is the electromotive force of the non-standard cell
– Eo is the electromotive force of the standard cell
– n is the number of moles of electrons transferred in the reaction

where the uppercase letters are concentrations, and the lowercase letters are stoichiometric coefficients for the overall redox reaction: aA + bB → cC + dD

Electromotive force (emf) is the potential difference of a source, or a battery, when no current is flowing. Terminal voltage is the voltage output of a device measured across its terminals.

A lead storage battery, also known as a lead-acid provides high currents and stores charge for long periods of time, making them essential for vehicles. A lead storage battery is the oldest type of rechargeable battery and one of the most common energy storage devices.

Although the batteries are reliable, they have a limited life, are heavy to ship, and contain toxic materials that require specific removal methods at the end of their useful life. Lead-acid batteries have moderate power density and good response time. Depending on the power conversion technology incorporated, batteries can go from accepting energy to supplying energy instantaneously. Lead-acid batteries are affected by temperature and must be maintained in order to achieve maximum life expectancy.

Nickel-cadmium (NiCd) batteries, nickel-metal hydride (NiMH) batteries, lithium-ion (Li-ion) batteries, and lithium-ion polymer (LiPo) batteries are a few commonly used rechargeable batteries. As with all batteries, rechargeable batteries consist of an anode, a cathode, and an electrolyte. During charging, the anode material is oxidized and produces while the cathode is reduced and consumes electrons.


Key Points

• When a battery is supplying electric power, its positive terminal is the cathode and its negative terminal is the anode.

• The Nernst equation can be used to determine the total voltage, or electromotive force, for a full electrochemical cell (a battery).

• Lead-acid batteries are reliable sources which provide high currents. However, they have a limited life, are heavy to ship, and contain toxic materials.

• Nickel-cadmium batteries are commonly used as rechargeable batteries.  During charging, the anode material is oxidized, producing electrons, and the cathode is reduced, consuming electrons.


Key Terms

Electromotive force (emf): is the potential difference of a source when no current is flowing in a circuit.

Electrochemical or Voltaic cell: A cell, such as in a battery, in which an irreversible chemical reaction generates electricity; a cell that cannot be recharged.

Electrolyte: a substance containing free ions that carry electric current.

Anode: The electrode of an electrochemical cell at which oxidation (loss of electrons) occurs.

Cathode: The electrode of an electrochemical cell at which reduction (gain of electrons) occurs.

Nernst Equation: Enables the determination of cell potential under non-standard conditions.

Voltage: An electromotive force or potential difference expressed in volts.



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