2HCl
which element is oxidized, which is
reduced?
0 0 +1 1
H
2
+ Cl
2

2HCl H is oxidized (increase in oxidation number)
Cl is reduced (decrease in oxidation number)
See class handout for example of balancing equations using oxidation numbers (or see pg. 451-3)


17.1, 17.2

Define electrochemistry.
The study of chemical changes that are caused by or that produce
electricity.
What must a solution that conducts
electricity contain?
Ions. For example, NaCl will conduct electricity when in water or
when melted because, in these conditions, Na
+
and Cl come apart.
However, solid NaCl cannot conduct electricity.
What are the two types of
electrochemical systems/cells
called? How are they different?
Electrolytic and Galvanic. The chemicals in electrolytic cells are
forced to react by passing a current through them. The chemicals
in galvanic cells react spontaneously and can be harnessed to
produce electricity. In other words: electrolytic cells consume
electricity, galvanic cells produce electricity.
What are the electrodes/poles in an
electrochemical cell called?
Anode and Cathode.
>How are these defined?
Anode: where oxidation occurs
Cathode: where reduction occurs (remember red cat)
What is the significance of labeling
electrodes as positive or negative?
Electrons always flow from the negative electrode (excess
negative charge) to the positive electrode (less negative charge).
What is a half-reaction?
A hypothetical reaction showing the gain or loss of electrons from
either the oxidation or the reduction part of a redox equation.
Write the half-reaction for the
reduction of Na
+
.
Na
+
+ e
Na (gain of electrons is reduction).
Diagram the electrolysis of NaCl.
Indicate half-reactions.

Why is molten NaCl but not aqueous
NaCl used in electrolysis?
When a salt is mixed with water, the products of electrolysis are
difficult to predict. A reaction with the water (to produce H
2

and/or O
2
) may occur instead of a reaction with the salt ions.
What is a cell reaction?
The addition of two half-reactions (note: when adding half-
reactions to get a cell reaction, electrons must cancel).
Explain why the cathode is negative
in electrolytic cells.
The negative pole has excess electrons forced onto it by the source
of current. These electrons can only be removed by (i.e. gained
by) positive ions. Gain of electrons is reduction (GER).
17.5
Galvanic cells
Electrolytic cells


Produce electricity
Require electricity
Anode (), Cathode (+)
Anode (+), Cathode ()
How are galvanic cells different
from electrolytic cells (3 ways)?
Certain chemicals are separated All chemicals in same container