http://www.ck12.org Chapter 23. Electrochemistry
The electrons are passed through the circuit, providing energy to the system, while the hydrogen ions move across
the proton exchange membrane (the red area), which is permeable only to cations. At the other electrode, oxygen
from the air reacts with electrons that have gone through the external circuit and the protons that pass through the
red membrane area according to the following equation:
O 2 + 4 H++ 4 e−→2 H 2 OOne major advantage of a hydrogen fuel cell is that the only waste product is water (and leftover nitrogen from the
air, which is not involved in any chemical reactions). However, a major drawback is the need for pure hydrogen,
which is flammable and difficult to store. A variety of techniques are being explored in an attempt to solve this
storage problem.
Lesson Summary
- The activity series (or electromotive series) ranks metals in terms of their ability to be oxidized.
- Batteries are based on galvanic (voltaic) cells, which use oxidation-reduction reactions to generate a flow of
electrons that can be employed for useful work. - The lead-acid battery in a car is a “wet cell” battery in which there is a net transfer of electrons from Pb to
PbO 2. - “Dry cell” batteries use a paste instead of a liquid solution. A common type of dry cell battery uses zinc as a
reducing agent and a manganese compound as an oxidizing agent. - One type of rechargeable battery uses water as the electron acceptor, resulting in the formation of metal
hydrides. These can easily be converted back to the original metal by running the reaction in the reverse
direction using an external energy source. - A common type of fuel cell uses the platinum catalyzed reaction between hydrogen and oxygen gases to
generate power.
Lesson Review Questions
- What does the metal reactivity series indicate about the ability of cations of a given metal to be reduced back
to their elemental form? - Sketch a basic galvanic cell, indicating the location of the anode, cathode, salt bridge, and the direction of
electron flow. - What causes the electrons to flow through a wire in a lead-acid battery?
- Write half-reactions for a dry cell battery composed of nickel(II) oxide and cadmium.
- In a dry cell battery that uses oxidation-reduction reactions involving silver and zinc, which metal will be
spontaneously oxidized by cations of the other metal? - Explain how a car battery works.
- Write the oxidation and reduction half-reactions for the lithium battery shown in theFigure23.7, and explain
how it generates electron flow. - What characteristic of rechargeable batteries (such as nickel-metal hydride batteries) allows the batteries to
recharge? - Write the half-reactions that would occur when each neutral element below is placed in an aqueous solution
of the following compound. If no reaction would occur, indicate why.
(a) Ca + H 2 O