Unit 3: Electrochemistry
Outcome: Investigate the chemistry of oxidation and reduction (redox) reactions
https://www.youtube.com/watch?v=9OVtk6G2TnQ
Indicator: Define oxidation and reduction and oxidizing and reducing agents in terms of
electron transfer.
An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer
of electrons between two species.
Oxidation:

The losing of electrons

OIL = oxidation is loss
Reduction:

The gaining of electrons

RIG = reduction is gain
Chemicals that are oxidized:
-
lose electrons
promote reduction of something else (gaining of something else)
are also referred to as a reducing agent
Chemicals that are reduced:
-
gain electrons
promote oxidation of something else (losing of something else)
are also referred to as a oxidizing agent
Simple example of a redox between metals:
Zn:
Cu+2:
- loses electrons to copper ion
- gains electrons from zinc
- is oxidized by copper ion
- is reduced by zinc
- reduces copper ion
- oxidizes zinc
- is called a reducing agent
- is called an oxidizing agent
Redox half reactions:
-
every redox reaction can be separated into two half reactions. One is the oxidation half
reaction and the other is the reduction half reaction
Oxidation half reaction:
-
this shows the loss of electrons by the zinc metal to become the zinc ion reduction half
reaction:
Reduction half reaction:
-
this shows the gain of electrons by the copper ion to become copper metal
When zinc and copper react how do you know which is oxidized and which is reduced?
** standard electrode potentials for half reactions **
- shows half reactions with voltage potentials
- all are written as reductions
- when placing two together, the lower half reaction is flipped to be shown as an oxidation
(lower voltage reaction will lose electrons)
Because the Zn half reaction is lower, it will lose electrons to Cu...Zn is oxidized and Cu is
reduced:
and because Zn loses 2 electrons and the Cu ion gains 2 electrons the reaction is balanced
already and doesn't require you to balance it. When the two are placed together, the electrons
are treated as spectators and you are left with:
But often they will need to be balanced:
Indicator: Write and balance redox equations using the half reaction and oxidation number
method.
Balancing redox reactions:
There are three types of redox reactions to balance:
1) neutral state redox reactions
2) acidic state redox reactions
3) basic state redox reactions
1) neutral state redox reactions
Example: balance the redox reaction between copper and iron in neutral conditions
Steps:
1) find the half reactions on the table and flip the lower one
2) balance the half reactions for number of electrons
3) place the reactions together and cross out the spectator electrons
2) acidic state redox reactions
some of the half reactions on our table are more complex looking and involve hydrogen ions
and water.
MnO2(s) + 4H+ + 2e-
Mn+2 + 2H2O
These are acidic state half reactions. Although we can find a lot on the chart we will usually
have to create our own
Example: balance the following redox reaction in acidic conditions
Steps:
1) separate the half reactions and assume the reduction potential flipping is already done
2) balance elements except O and H in each half reaction separately
3) balance O atoms in each half reaction separately by adding water molecules (each water
contains one O)
4) balance H atoms in each half reaction separately by adding H+ ions
5) balance the half reactions so they each have the same charge. This is done with electrons (e-)
6) balance the number of electrons in the half reactions so they are equal (you must multiply
every piece of the half reaction)
7) add the half reactions together and cross out spectators (e-, water, and H+)
3) basic state redox reactions
This is similar to redox reactions in acidic state except there is the addition of OH- ions (making
it basic conditions)
Example: balance the following redox reaction in basic conditions
Steps:
1) separate the half reactions and assume the reduction potential flipping is already done
2) balance elements except O and H in each half reaction separately
3) balance O atoms in each half reaction separately by adding water molecules (each water
contains one O)
4) balance H atoms in each half reaction separately by adding H+ ions
5) add equal number of OH- ions to BOTH sides to equal the number of H+ ions
6) on the side containing both OH- and H+, create water (1H+ and 1OH- = 1 water)
7) balance the half reactions so they each have the same charge. This is done with electrons (e-)
8) balance the number of electrons in the half reactions so they are equal (you must multiply
every piece of the half reaction)
9) add the half reactions together and cross out spectators (e-, water)
Practice:
1) balance the redox reaction between lead and aluminum in neutral conditions
2) balance the following redox reaction in acidic conditions
MnO4- + SO2
Mn+2 + SO4-2
3) balance the following redox reaction in basic conditions
Cr(OH)3 + ClO3-
CrO4-2 + Cl-