Chapter 20
Electrochemistry
Electrochemical Reactions
In electrochemical reactions, __________________________________ from one
species to another.
Oxidation Numbers
In order to keep track of what loses electrons and what gains them, we assign oxidation
numbers.
Oxidation and Reduction
A species is _____________________ when it ___________ electrons.
Here, zinc _________________________ to go from neutral zinc metal to the Zn2+ ion.
A species is _____________________ when it ___________ electrons.
Here, each of the H+ ________________________ and they combine to form H2.
What is reduced is the __________________________ agent.
H+ oxidizes Zn by taking electrons from it.
What is oxidized is the __________________________ agent.
Zn reduces H+ by giving it electrons.
Balancing Oxidation-Reduction Equations
Perhaps the easiest way to balance the equation of an oxidation-reduction reaction is
via the _______________________________________.
This involves treating (on paper only) the oxidation and reduction as two separate
processes, balancing these half reactions, and then combining them to attain the
balanced equation for the overall reaction.
Half-Reaction Method
Assign oxidation numbers to determine what is oxidized and what is reduced.
Write the oxidation and reduction half-reactions.
Balance each half-reaction.
Balance elements other than H and O.
Balance O by adding H2O.
Balance H by adding H+.
Balance charge by adding electrons.
Multiply the half-reactions by integers so that the electrons gained and lost are the
same.
Add the half-reactions, subtracting things that appear on both sides.
Make sure the equation is balanced according to mass.
Make sure the equation is balanced according to charge.
Consider the reaction between MnO4− and C2O42−
Balancing in Basic Solution
If a reaction occurs in basic solution, one can balance it as if it occurred in acid.
Once the equation is balanced, add OH− to each side to “neutralize” the H+ in the
equation and create water in its place.
If this produces water on both sides, you might have to subtract water from each side.
Electrochemistry
Defined as the interchange of _________________ and __________________ energy
It is primarily concern with 2 processes that involve oxidation-reduction reactions:


The generation of an electric current from a spontaneous reaction
The use of a current to produce chemical change
Voltaic or Galvanic Cells
In __________________________ oxidation-reduction (redox) reactions, electrons are
transferred and energy is released.
We can use that energy to do electrical work if we make the electrons flow through an
external device.
We call such a setup a voltaic (or galvanic) cell. A typical cell looks like this.
The oxidation occurs at the ______________________.
The reduction occurs at the ______________________.
Once even one electron flows from the anode to the cathode, the charges in each
beaker would not be balanced and the flow of electrons would stop.
Therefore, we use a _____________________, usually a U-shaped tube that contains a
____________________________, to keep the charges balanced.
Cations move toward the ________________________
Anions move toward the ________________________
Water only spontaneously
flows one way in a waterfall.
Likewise, electrons only
spontaneously flow one way
in a redox reaction—from
higher to lower potential
energy.
Electromotive Force (emf)
The potential difference
between the anode and
cathode in a cell is called
the electromotive force
(emf).
It is also called the cell
potential, and is designated
Ecell.
Cell Potential
Cell potential is measured in ____________________
Standard Reduction Potentials
Reduction potentials for many electrodes have been measured and tabulated.
Standard Cell Potentials
The cell potential at standard conditions can be found through this equation:
Cell Potentials
For the oxidation in this cell,
For the reduction,
Oxidizing and Reducing Agents
The strongest oxidizers have the most positive reduction potentials.
The strongest reducers have the most negative reduction potentials.
The greater the difference between the two, the greater the voltage of the cell.
Summary



Voltaic cells use redox reactions that proceed spontaneously
Voltaic cells produce a positive emf
Thus a POSITIVE E indicates a spontaneous process (A NEGATIVE E indicates a
nonspontaneous process)
Free Energy
The change in Gibbs free energy, G, is a measure of the spontaneity of a process that
occurs at constant temperature and pressure.
G for a redox reaction can be found by using the equation
n is the number of moles of electrons transferred
F is Faraday’s constant
E is the cell potential
Under standard conditions, (1.0M solutions)
A spontaneous process will have a ____________ thus a ____________
The Equilibrium Constant
Because standard free energy, G, is related to the equilibrium constant, K, we can
relate the standard potential to the equilibrium constant for the reaction
Concentration Cells
What would happen if we changed the concentration of one or both solutions in a
galvanic cell?
↑ [reactants]
↓ [reactants]
↑ [products]
↓ [products]
Electrolysis
Electrolysis reactions take place in ____________________________.
Uses _____________________________ to cause ___________________________
redox reaction to occur.
Mostly used for electroplating – depositing a thin layer of one metal on another metal to
improve beauty or resistance to corrosion.
Quantitative Aspects of Electrolysis
Common questions:


How many g or mol of the metal are produced?
How many amps are necessary?
Amperes = Coulombs/second

How long will it take?