Spontaneous Redox Reactions
lesson 6
chapter 12
Objective
You will be able to use a redox table
to predict spontaneous reactions.
A spontaneous reaction takes place without the
addition of energy or any other stimulus.
Not all redox reactions are spontaneous
Eg. If exposed to air and moisture, steel rusts
spontaneously.
Combustion of methane does not happen spontaneously,
it requires heat to start.
In a spontaneous redox reaction electrons are
transferred from the RA to the OA
Oxidizing agents differ in their ability to accept
electrons.
2+
· Recall the oxidizing agent accepts electrons and
reducing agents donate electrons
Cu
Reducing agents differ in their ability to donate
electrons.
Reducing agents also differ.
· In order for a redox reaction to be spontaneous, the
oxidizing agent must be strong enough to take
electrons away from the reducing agent
(aq) accepts electrons more readily than Zn
Zn(s) is a better donor of electrons than Cu(s)
These observations are based upon
empirical evidence.
Zinc metal reacts spontaneously with
Cu2+.
2+
(aq).
Redox reactions require OA and RA
Zn(s) ➔ Zn2+ (aq) + 2e- (oxidized, thus RA)
Cu2+(aq) + 2 e- ➔ Cu(s) (reduced, thus OA)
A redox table is used to predict the spontaneity of a redox
reaction.
A redox table is a series of reduction reactions listed with the
strongest OA at the top left.
The strongest RA is on the bottom right
The reactions are reversible (can be read both ways)
When an oxidizing agent is reduced, it becomes
a reducing agent!
Oxidizing agent + electrons reducing agent.
Cu2+(aq) + 2 e- ➔ Cu(s)
Reducing agent oxidizing agent + electrons
Cu(s) ➔ Cu2+(aq) + 2 e-
Rule for Predicting Spontaneity
A spontaneous reaction will take place if the OA
(electron acceptor) is above the RA (electron donor)
Examples
The Chem 30 data book has a redox table on page 7
1. Will a spontaneous reaction take place between Fand Ag+?
2. Use the redox table to write a half reaction where
gold acts as a reducing agent.
Will a spontaneous reaction take place between Au3+ and Ag?
3. Use the redox table to write a half reaction where
Fe2+ acts as an oxidizing agent.
4. Use the redox table to write a half reaction where
Fe
2+
acts as an reducing agent.
Assignment
Do the spontaneous redox reaction worksheet