OXIDATION & REDUCTION
TOPIC 9
OVERVIEW
• Oxidation & reduction are important chemical rxns
• They work with and against us (batteries, corrosion)
• Originally, oxidation meant the combination of a substance w/ oxygen, and
reduction meant the opposite (loss of O)
• They are interdependent (what do you think this means?)
OXIDATION & REDUCTIONS
• What is occurring when magnesium burns in oxygen? (use your periodic table)
• What about magnesium reacting with chlorine?
• Oxidation is the loss of electrons by an atom or ion
• Reduction is the gain of electrons by an atom or ion
REDOX
• Whenever one atom loses an electron, there must be another atom available
to gain that electron
• Interdependency
• Because they both must occur together, the two terms are often combined, and
rxns where both reduction and oxidation occur are called redox rxns
OXIDATION NUMBERS
• Where can you find them?
• What are they/what do they mean?
OXIDATION NUMBERS
• Positive, negative, or neutral values known as oxidation numbers (states) are
assigned to atoms
• Changes in oxidation states identify how many electrons have been
lost/gained
• Oxidation is the ____ of electrons and a ____ in oxidation number
• Reduction is the ____ of electrons and a ____ in oxidation number
OXIDATION NUMBERS
• Oxidation numbers are used to identify the path of electrons in redox rxns
• LEO says GER
• Loss of Electrons is Oxidation
• Gain of Electrons is Reduction
• Oxidation numbers are written differently than ionic charges
• Charge of a magnesium ion is 2+, but the oxidation number is +2
• Periodic table has oxidation numbers, so just remember to switch the order for ionic
charge
OXIDATION NUMBERS - RULES
• 1. every uncombined element has an oxidation number of 0 – why?
• 2. monatomic ions have an oxidation number equal to the ionic charge
• 3. in compounds, group 1 metals always have an oxidation number of +1,
and group 2 metals always have an oxidation number of +2
• 4. fluorine is always -1 in compounds, and other halogens are also -1 when
they have the greatest electronegativity
OXIDATION NUMBERS - RULES
• 5. hydrogen is +1 in compounds unless it is combined with a metal, when it will
become -1
• 6. oxygen is usually -2 in compounds, but when combined with fluorine it is +2
• 7. the sum of all oxidation numbers in all compounds must be ____
• 8. the sum of the oxidation numbers in polyatomic ions must be equal to the
charge on the ion
EXAMINING REDOX RXNS
• Not all rxns are redox rxns
• If there is a change in oxidation number for any atom from reactant to product, it is
redox
• If there is an uncombined element on one side, but in a compound on the other
side, it is redox
• If it is a double replacement rxn, it is NOT redox
IDENTIFYING OXIDATION & REDUCTION
• Use the before and after oxidation numbers of a rxn to identify which atoms
underwent oxidation, and which underwent reduction
• Whichever changes from a lower oxidation number to a higher one has been
oxidized
• Whichever changes from a higher oxidation number to a lower one has been
reduced
OXIDIZING & REDUCING AGENTS
• The substance that is oxidized is the reducing agent
• The substance that is reduced is the oxidizing agent
HALF-RXNS
• A half-reaction shows either the oxidation or reduction part of a redox rxn,
including the electrons gained or lost
• Reduction half-rxn: Fe3+(aq) + 3e- → Fe(s)
• Oxidation half-rxn: Fe(s) → Fe3+(aq) + 3e-
HALF-RXNS
• Half rxns must have balanced atoms on both sides, but don’t necessarily have
to balance charge on both sides, why?
• Sn4+(aq) + 2e- → Sn2+(aq)
HOMEWORK
• Review questions sets 9.1, 9.2, 9.3
ELECTROCHEMICAL CELLS
• One practical use of a redox rxn is an electrochemical cell, which involves a
chemical rxn and flow of electrons
• Have two surfaces that conduct electricity at the site of oxidation or reduction,
called electrodes
• Anode – electrode where oxidation occurs
• Cathode – electrode where reduction occurs
ELECTROCHEMICAL CELLS
• Two common types of these cells:
• A voltaic cell is an electrochemical cell in which a spontaneous chemical rxn produces a
flow of electrons
• An electrolytic cell requires an electrical current to force a nonspontaneous chemical rxn
to occur
SPONTANEOUS RXNS – VOLTAIC CELLS
• A spontaneous rxn can take place in one container; if a strip of zinc is placed
into a solution of lead nitrate, the zinc will be oxidized and the copper will be
reduced
• Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq)
• The exchange of electrons takes place on the surface of the zinc
SPONTANEOUS RXNS – VOLTAIC CELLS
• The materials can also be separated into two containers, with the electrons
being transferred through a wire connecting them
• In a voltaic cell, a salt bridge connects the two containers and provides a path
for the flow of ions between the two beakers
• Forms a complete circuit and allows the rxn to proceed
SPONTANEOUS RXNS – VOLTAIC CELLS
• Reference table quiz!
• Table J – Activity series
• Used to identify the anode and cathode in an voltaic cell
• Identify the two metals used in the cell, and find them on the table
• The metal that is higher on the chart will be oxidized (the anode)
• The metal that is lower will be the site of reduction (cathode), but the cathode itself will
not be reduced
SPONTANEOUS RXNS – VOLTAIC CELLS
• To help remember the anode and cathode, think of:
• RED CAT & AN OX
• REDuction occurs at the CAThode
• ANode is the site of OXidation
SPONTANEOUS RXNS – VOLTAIC CELLS
• When a voltaic cell begins to react, electrons flow from anode to cathode
• A voltmeter placed in the circuit measures the electric potential between the
metals in the electrodes in units of volts
• E0cell = E0reduction – E0oxidation
NONSPONTANEOUS RXNS – ELECTROLYTIC CELLS
• In a voltaic cell, the electrons flow spontaneously from the _____ to the _____
• Can the reverse take place?
• If so, how?
NONSPONTANEOUS RXNS – ELECTROLYTIC CELLS
• The rxn can occur in the reverse direction, but not spontaneously
• There must be some electrical generator placed into the circuit to force the
electrons to flow from the anode to the cathode
• When electricity is used to force a chemical rxn, it is called electrolysis
NONSPONTANEOUS RXNS – ELECTROLYTIC CELLS
• Think of a car & its battery – when you first start the car, a spontaneous
chemical rxn occurs in the battery, providing the electricity to start the car
• Once it’s started, the alternator (nonspontaneous rxn) recharges the battery
• Electroysis can be used to separate active elements like sodium and chlorine
(NaCl)
• Electolysis can also be used to electroplate metals onto a surface (anyone
know what this means?)
NONSPONTANEOUS RXNS – ELECTROLYTIC CELLS
• Although voltaic & electrolytic cells are different, there are still some
similarities
• Both use redox rxns
• The anode is the site of oxidation
• The cathode is the site of reduction
• The electrons from through the wire from the anode to the cathode
NONSPONTANEOUS RXNS – ELECTROLYTIC CELLS
• But don’t forget the differences between them!
• Redox rxn in a voltaic cell is spontaneous, but is nonspontaneous in an
electrolytic cell
• Voltaic cell: anode is negative, cathode is positive; electrolytic cell: anode is
positive and cathode is negative
HOMEWORK
• Review questions set 9.4