Download Predicting Spontaneous Reactions

Predicting Spontaneous Reactions
Chapter 19 Pages 768-773
• An electrochemical (voltaic) cell consists of
an oxidation reaction and a reduction
reaction to produce a voltage for the cell.
Example: Batteries
• Need to know: which electrode is the
cathode, which is the anode, and whether
the chemical reaction is spontaneous.
•Anode
Anode : negative electrode where
oxidation occurs
•Cathode
Cathode :positive electrode where
reduction occurs
Electrons travel from the anode to the
cathode, along a conductive wire . This
is the voltage = electricity.
Anode
Cathode
When choosing your anode and
cathode follow this rule:
• Look at the reduction potential for the ions,
the cathode will be the more positive
reduction potential (p.11 of data booklet)
• Reason:
Reason higher reduction potential means
more easily reduced, therefore reduction
takes place there
Table of Standard Reduction Potentials
• Shows the relative ability for a substance to
be reduced (gain electrons).
• It expresses this potential to gain electrons
by assigning a voltage for each reduction
half reaction in the table.
Found on pages 10 & 11 in your data booklet
Eonet= Eooxidation + Eoreduction
• Eonet = total net voltage of the cell (electric
potential energy of the cell)
• Must be greater than zero for the reaction to
be spontaneous
• If less than zero, then non-spontaneous
reaction and no electricity produced.
• Eooxidation = oxidation half reaction taking
place in the cell
• Oxidation half reactions are the reverse
reaction from the table on p.11, so their
sign changes!
changes
Example:
If reduction potential of Ag+ = 0.80V, then the
oxidation potential of Ag(s) = - 0.80V
• Eoreduction = reduction half reaction taking
place in the cell
• The larger the reduction potential (more
positive), the greater the tendency for the
reaction to occur.
• The smaller the reduction potential (more
negative), the least likely for a reaction to
occur.
Predicting Spontaneity
• Looking at the table on pg 10/11, anything
on the left of the arrow will react with
anything below it to the right.
• Will the following reactions occur?
(a)
2 Ag
(b)
Mg 2+ ( aq ) + Cr( s ) → Mg ( s ) + Cr 2+
+
( aq )
+ Cu( s ) → 2 Ag ( s ) + Cu
2+
Shorthand Notation
• The shorthand notation for redox rxns start
with the oxidation first, (LR from the half
rxn) then the reduction (LR from the half
rxn) separated by a double line.
Ex:
Zn | Zn
2+
+
|| Ag | Ag
Oxidation || Reduction
Sample Problem 1:
1
• Design an electrochemical cell that uses
the reactions of the metals Zn and Ag, and
solutions of their ions, Zn2+ and Ag+.
a) Identify the anode and cathode
b) Write the oxidation and reduction half
reactions
c) Calculate the cell voltage
Solution:
Zn+2 + 2e- Zn(s)
-0.76V
Ag+1 +1e- Ag(s)
+0.80V
• Ag+ more easily reduced, therefore Ag(s) is the
cathode, so oxidation takes place at the anode
(Zn) and the oxidation potential half reaction is:
Zn(s) Zn+2 + 2e+0.76V
a) and b)
c) Eonet = Eooxidation + Eoreduction
= (+0.76V) + ( +0.80V)
= +1.56V
Sample Problem #2:
An electrochemical cell was designed using the
metals Mg and Al and solutions containing
Mg2+ and Al3+.
a) According to the table, which ion/metal will be
reduced? Oxidized?
b) Identify the anode and cathode
c) Write the oxidation and reduction half reactions
d) Calculate the cell voltage
e) Write the shorthand notation