Electrochemistry Test Study Guide
IN GENERAL

metal ↔ metal ion+n + ne-
(Example: Pb(s) ↔ Pb2+(aq) + 2e-)

non-metal + ne- ↔ non-metal ion-n
(Example: Cl2(g) + 2e- ↔ 2 Cl-(aq))
OXIDATION NUMBERS
 Rules for assigning oxidation numbers
REDOX REACTIONS
 oxidizing agents vs. reducing agents
 reduction reactions vs. oxidation reactions
 be able to identify redox reactions (ie. use oxidation numbers)
 use change in oxidation numbers to identify the molecule/atom that is being
oxidized, reduced, or is just a spectator ion
DISPROPORTIONATION
 the same atom in the same molecule undergoes oxidation, while the same atom
in another identical molecule undergoes reduction
BALANCING REDOX REACTIONS
 always need the same amount of electrons being lost as there are electrons
being gained!!
1. Half-Reactions Method
o Use oxidation numbers to help identify which chemicals are being
oxidized and which are being reduced
o Use the 5 steps to balance each half reaction in acidic conditions
o Balance the number of electrons in each half reaction (this will give
the number of electrons transferred in the redox reaction)
o Add the two half reactions together to get the balanced redox
reaction
2. Oxidation Numbers Method
o Use this method if asked to determine the number of electrons
transferred per atom, per molecule, or per reaction
SPONTANEITY RULE
 Spontaneous reactions occur if the oxidizing agent is higher than reducing agent
on table on pg. 7 of data booklet
 Be able to build a reduction half-reaction table from a list of spontaneous and
non-spontaneous reactions
o Need to identify the OA and RA for each reaction and then rank the OA
and RA based on the spontaneity rule
REDOX STOICHIOMETRY
 Always need to begin with a balanced redox reaction.
 Several ways to develop a balanced redox reaction
o Oxidation numbers method
o Half-reactions method
 Half-reactions can be created by using the 5 steps if the reaction is
in acidic conditions
 Half-reactions can be simple and can be created based on your
knowledge of metals and non-metals
 Copy half-reactions from pg. 7 of data book for the strongest OA
and for the strongest RA
 Stoichiometry calculations always involve 3 steps:
1. Calculate the moles of the chemical you have the most information for
2. Use your “want over have” ratio to calculate the moles of the chemical you
are trying to find
3. Solve for the unknown
Chapter Review
Pg. 474-475
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Reduction vs. Oxidation, RA vs. OA, and Redox Reactions in general: # 1, 2, 4, 5, 10
Oxidation Numbers: #3, 6
Disproportionation: #9 (omit 9a), 16a (just complete and balance the net ionic equation),
Practice Problem
Half-Reactions Method for Balancing: #14a-c, 17a-b
Oxidation Numbers Method for Balancing: #15
Redox Stoichiometry: Practice Problem
Practice Problem
Highly toxic phosphine gas, PH3(g), is used in industry to produce flame retardants. One
way to make phosphine on a large scale is by heating elemental phosphorus with a
strong base as shown by the following reaction.
P4(s) + 3H2O(l) + 3OH-(aq)
3H2PO2-(aq) + PH3(g)
a. Show that the reaction is a disproportionation reaction.
b. Calculate the mass of phosphine that can theoretically be made from 10.0kg of
phosphorus by this method.
[Answers: see #19b-c in text book]