Oxidation- Reduction Reactions
The Loss of Electrons occurs during an Oxidation reaction.
The Gain of Electrons occurs during a Reduction reaction.
To remember this,
think of a lion: LEO says GER!
ex. Label the oxidation and
reduction reactions, show the changes in oxidation numbers, label the oxidizing
and reducing agents.
2K(s) + MgCl2(aq) → Mg(s)
+ 2KCl(aq)
First
begin by writing out the total ionic equation (remember that all
aqueous substances are broken apart into their ions. All solid, liquid
or gas substances are left well enough alone). Second, cross out any
spectator ions.
2K(s) + Mg2+(aq) + 2Cl-(aq)
→ Mg(s) + 2K+(aq) + 2Cl-(aq)
Third, Write out what is left as the net ionic equation:
2K(s) + Mg2+(aq) → Mg(s)
+ 2K+(aq)
Notice that:
- potassium - ox # goes from 0 to +1, ∴ 1 electron lost ↝ oxidation
- magnesium - ox # goes from +2 to 0, ∴ 2 electrons gained ↝ reduction
- chlorine begins and ends with an ox # of -1, so it is neither oxidized nor reduced; it is simply a spectator ion
Every redox reaction can be broken into 2 half-reactions:
K0(s) →
K+(aq) + e-
oxidation half reaction
Mg2+(aq)
+ 2e- →
Mg0 (s) reduction half reaction
Notice that each half-reaction is balanced in terms of elements (as usual), but also in terms of charge.
↦ The oxidation half-reaction needed a negative electron placed on the product side so that the total charge on each side is zero.
↦ The reduction half-reaction needed two negative electrons placed on the reactant side so that the total charge on each side is zero.
Please note that it's not about making the charge zero on each side of the half-reaction, but rather just making sure the charge is equal on each side.
↦ The oxidation half-reaction needed a negative electron placed on the product side so that the total charge on each side is zero.
↦ The reduction half-reaction needed two negative electrons placed on the reactant side so that the total charge on each side is zero.
Please note that it's not about making the charge zero on each side of the half-reaction, but rather just making sure the charge is equal on each side.
Oxidizing & Reducing Agents
A
substance that reduces another is called
a reducing agent; it gets oxidized
in the process. A substance that oxidizes another is called an oxidizing agent; it gets reduced in the process.
In
the above example,
- K(s) is the reducing agent (because it causes Mg2+(aq) to undergo reduction and it gets oxidized in the process)
- Mg2+(aq) is the oxidizing agent (because it causes K(s) to undergo oxidation and it gets reduced in the process).
Homework:
Answers:
