Download Redox in Electrochemistry

Redox in Electrochemistry
Previously, we have learned that all redox reactions involve a transfer of electrons from
the species that is _________________ to the species that is ________________.
For example, consider the following reaction:
Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s)
The two half-reactions that make up this process are:
What do you think would happen if you separated the oxidation halfreaction from the reduction half-reaction? Can redox occur?
Consider the figure below in which a zinc strip is immersed in a solution of zinc sulfate
and a copper strip is immersed in a solution of copper(II)sulfate.
Two problems prohibit a redox reaction in this situation.
1) There is no way for zinc atoms to transfer electrons to copper (II) ions.
This problem can be solved by connecting a metal wire between the zinc and copper
strips. The wire serves as a pathway for electrons to flow from the zinc strip to the
copper strip.
2) A positive charge builds up in one solution and a negative charge builds
up in the other. Positive zinc ions build up on the left side prohibiting the
oxidation of zinc atoms while negative ions build up on the right side
prohibiting the reduction of copper ions.
To solve this problem we add a salt bridge: a pathway constructed to allow the passage
of ions from one side to another. This consists of a tube containing a conducting
solution of a soluble salt, such as KNO3, held in place by a plug. The ions can move
through the plug but the solutions in the two beakers cannot mix.
- When the metal wire and the salt bridge are in place, electrons flow through the wire
from the _____________________ half reaction to the ________________ half
reaction while positive and negative ions move through the salt bridge.
- A flow of charged particles, such as electrons, is called an ____________________
____________________. The energy of the flowing electrons can be put to use in
lighting a bulb or running an electric motor.
- The completed device shown above is an _______________________________:
an apparatus that uses a redox reaction to produce electrical energy or uses electrical
energy to cause a chemical reaction
- a _________________________________: a type of electrochemical cell that
converts chemical energy to electrical energy by a ____________________ redox
reaction.
Chemistry of Voltaic Cells
An electrochemical cell consist of two parts, called _________________, in which
the separate oxidation and reduction reactions take place. Each half-cell contains an
_________________, which is the object that conducts the electrons to or from
another substance, usually a solution of ions. (_____________________)
In the previous diagram, the beaker with the zinc electrode is where the
_______________ part of the redox reaction takes place (__________________)
This reaction shows that zinc is being consumed. Therefore, the mass of the zinc
electrode ____________________ as the cell operates.
The beaker with the copper electrode is where the ___________________ part of
the reaction takes place (______________________). Since copper is being
produced, the mass of the copper electrode __________________ as the cell
operates.
The reaction that takes place in each half-cell is the ______________________.
The electrode where oxidation takes place is called the ________ of the cell.
The electrode where reduction takes place is called the __________ of the cell.
Tricks To Help You Remember:
Cell Notation
Instead of drawing out an electrochemical cell diagram, it can be written in a form called
cell notation to clearly describe the redox reaction.
Note that the two participants in the oxidation reaction are written first and in the order
they appear in the oxidation half-reaction (reactant|product). They are followed by a
double vertical line (||) indicating the wire and the salt bridge connecting the half-cells.
Then the two participants in the reduction reaction are written in the same
reactant|product order,
Calculating Electrochemical Cell Potential
In electrochemistry, __________________________________________ is a
measure of the amount of current that can be generated from a voltaic cell to do work.
Electric charge can flow between two points only when a difference in electrical
potential energy exists between the two points. In an electrochemical cell, these two
points are the two ___________________. The potential difference of a voltaic cell
is an indication of the energy that is available to move electrons from the ________ to
the ___________. In redox terms, the voltage of a cell is determined by the difference
in the tendency of the electrode material to accept electrons.
We have previously learned that the gain of electrons is called _________________.
Building on this fact, we can call the tendency of a substance to gain electrons its
_________________________________________. It is measured in
_______ but it cannot be determined directly because the reduction half-reaction must
be coupled with an oxidation half-reaction. When these half-reactions are coupled, the
voltage corresponds to the ______________________ in potential between the
half-reactions.
Long ago, chemists decided to measure the reduction potential of ALL electrodes against
one standard electrode, the standard hydrogen electrode. This electrode can act as
an oxidation half-reaction OR a reduction half-reaction, depending upon the half-cell to
which it is connected. Over the years, chemists have measured and recorded the
standard reduction potentials, abbreviated ______ of many different half cells. (Back
of data booklet). The values in the table are based on using the half-cell reaction that is
being measured as the cathode and the standard hydrogen electrode as the anode.
All of the half-reactions in the table are written as reductions. However, in any voltaic
cell, which always contains two half-reaction, the half-reaction with the _________
reduction potential will proceed in the opposite direction and will be an
______________________ reaction.
In other words, the half-reaction that is more ___________________ will proceed
as a reduction and the half-reaction that is more ___________________ will
proceed as an oxidation.
Consider the following lead-chromium electrochemical
CrCl3
There are TWO ways to determine the voltage of an electrochemical cell:
1) Potential difference: we consider the reduction potential (E) of the
possible reduction half reaction in the two half cells and find their
difference using the formula:
Using the table, we see that the E0 for the Pb/Pb+2 half-reaction is: ____________
and the E0 for the Cr/Cr+3 half-reaction is ____________. In the competition for
electrons, ____ will win (because it’s reduction potential is ________________) so
its considered to be the reduction half-reaction while _____ is considered to be the
oxidation half reaction:
Equation:
We have called the predicted voltage of this E0cell, making the assumption that standard
conditions are present.
2) The second, and equally effective, method of predicting the voltage of an
electrochemical cell makes use of the half-reactions that actually occur in
the cell, and ADDS their contributions to the voltage.
We once use the Table of Standard Reduction Potentials to determine the oxidation and
reduction half-reactions. As done in step one, we realize that Pb+2 is the stronger
____________ agent and therefore, reduction will occur in the Pb/Pb+2 half cell.
Equation:
How we will find the overall cell reaction by balancing electrons, as we have previously
learned, and add the E’s of the two half reactions to get the E0cell.
Equation:
Note: The E’s are NOT multiplied by the coefficients of 3 and 2 because E is the
measure of _____________ of a half reaction to occur and is not affected by the
__________ of electrons transferred.
Significance of E: Is it Positive or Negative?
Either way that the E0cell is determined, the value obtained indicated the tendency, or
potential of the reaction, as written, to occur. This is useful because it can be used to
determine the tendency of any redox reaction. In particular:
POSITIVE indicates that the reaction _____ occur spontaneously as written
NEGATIVE indicates that the reaction ______ occur spontaneously as written
Ex) Will the following reaction occur spontaneously as written, or not?
When a battery operates for a time, the decrease in voltage is caused by the changing
concentrations of the species in the cell. Just as with all chemical reactions, the reaction
is spontaneously moving towards __________________.
With some batteries, the reaction is capable of being reversed by applying an external
voltage, as in a rechargeable battery.
However, if the batter continues to discharge:
1. E0cell becomes __________________
2. The reduction (or cell) is at ____________________
3. The batter is “_______”