Download Redox Reactions: Transferring Electrons

Redox Reactions: Transferring Electrons
When we think about chemical reactions, in reality they are all about moving
around electrons.
Let’s think about something that most people don’t think about as
“chemistry”
A battery is basically a chemical reaction that’s producing electron flow for
whatever gizmo you have it hooked up to
When a reaction transfers an electron from one substance to another we call
it an “Oxidation-Reduction” reaction or Redox for short.
If a substance is losing the electron it is being oxidized
If a substance is gaining, it is being reduced.
LEO says GER
How are Redox reactions different from other reactions we have studied so
far?
The acceptance of an electron (reduction) by a material is matched with the
donation of an electron (oxidation).
Each part is called a half-reaction, but they occur at the same time.
Because electrons are being transferred, the number of electrons gained must
equal the number of electrons lost.
Important Terminology
If a substance is losing the electron it is being oxidized
If a substance is gaining, it is being reduced.
LEO says GER
Ag +(aq) + e-  Ag(s) (reduction)
Cu (s)  Cu 2+ + 2e- (oxidation)
Because electrons are being transferred, the number of electrons gained must
equal the number of electrons lost.
Reducing Agent: the one that does the reducing.
Oxidizing Agent: the one that does the oxidizing.
Be Careful Here!
Oxidation States: Electron Bookkeeping
How do we know when a redox reaction is occurring?
Oxidation Numbers: positive or negative numbers assigned to atoms
depending on their electronegativity.
Note: oxidation numbers are different from charges! We put the sign before
the number for oxidation states.
Rules for Assigning Oxidation Numbers
1.
2.
3.
4.
5.
6.
7.
8.
Free elements (ie. Diatomic, solids) are zero.
The sum of oxidation numbers in an uncharged molecule must be
zero or equal to the molecule’s charge.
Hydrogen always +1 (except -1 in hydrides)
Oxygen always -2 (except -1 in peroxides)
Group 1 = +1
Halogens = -1
Group 6 = -2
group 2 = +2
Remember the total oxidation numbers must equal the total charge.
Using Oxidation Numbers
By assigning oxidation numbers we can identify oxidation and reduction
reactions.
Basically we assign oxidation numbers to all atoms or groups in an equation.
If we notice one atom or group has increased in oxidation state (more
positive) it has been oxidized. If it has a lower oxidation state (more
negative) it has been reduced.
This makes sense logically because gaining an electron makes something
more negative and losing one makes something more positive.
Example:
2 KCl + MnO2 + 2H2O4  K2SO4 +MnSO4 + Cl2 + 2H2O
Let’s assign oxidation numbers:
K +1 Cl-1 + Mn +4 O2 –2 H2 +1 S +6 O4 –2  K2 +1 S +6 O4 –2 Mn +2 S +6 O4 –2 Cl2
0
H2 +1 O –2
Notice that Cl goes from –1 to 0. It has been oxidized.
Mn goes from +4 to +2. It has been reduced.