III. Finding Redox in Chemical Reactions
A. Oxidized and Reduced atoms
1. Determining Redox
If reduction occurs, atoms ____________________________________________________________
If oxidation occurs, atoms ____________________________________________________________
We know redox has occurred in a chemical reaction if _____________________________________
______________________________
If no atoms change their oxidation states _______________________________________________
#1 Example – Determine if redox has occurred in the following reactions.
Redox?
a. KBr + NaCl  KCl + NaBr
b. Sr + MgO  Mg + SrO
c. NiCl3 + CoO  NiO + CoCl3
Whenever reduction occurs __________________ must ______________________________
If you only detect one atom that changes its oxidation state. _________________________________
________________________________________
2. Determining oxidized and reduced atoms
To determine if an atom is oxidized or reduced, look at ______________________________
If the oxidation state increases ________________________________________
If the oxidation state decreases ________________________________________
When stating which atom is oxidized or reduced, we state only look at the ____________________ and
remember to give the atoms ______________________________.
#2 Examples – State the oxidized atom and the reduced atom in each of the following reactions
Oxidized
Reduced
a. KBr + Na  K + NaBr
b. Sr + MgO  Mg + SrO
c. NiCl3 + CoO  NiO + CoCl3
d. FeCl2 + CuCl  FeCl3 + Cu
B. Oxidizing and Reducing Agents
Oxidizing agents are compounds that ____________________________________________
Reducing agents are compounds that _____________________________________________
If an atom is oxidized then its compound is ____________________________________
If an atom is reduced, then its compound is ____________________________________
Remember to look only at ________________________________________________
#3 Examples
Determine the oxidizing and reducing agents
Ox agent
a. NiBr2 + NaBr  NiBr3 + Na
b. Sr + MgO  Mg + SrO
c. NiCl3 + CoO  NiO + CoCl3
red agent
Practice – For the following reactions, assign oxidation numbers, then determine which atoms are oxidized
and which are reduced. Then determine the oxidizing and reducing agents.
2 Mg + O2  2 MgO
a.
b. K + LiOH  Li + KOH
c. Mg + 2 HCl  MgCl2 + H2
d. PbO + MgO  PbO2 + Mg
Answers to questions
#1
a. No
b. Yes
c. Yes
2.
Oxidizd
a Na
b. Sr
c. Co+2
d. Fe+2
Reduced
K+
Mg+2
Ni+3
Cu+
3. Ox agent
Red Agent
a. NaBr
b. MgO
c. NiCl3
NiBr2
Sr
CoO
Did you include the proper charges?
Practice
Oxidized
a.
b.
c.
d.
Mg0
K0
Mg0
Pb+2
Reduced
O0
Li+
H+
Mg+2
oxidizing agent
O2
LiOH
HCl
MgO
Reducing agent
Mg
K
Mg
PbO
As we have studied, redox refers to the movement of electrons. If an atom loses electrons, it is oxidized, if it
gains electrons it is reduced. Now we will look at what happens in a actual chemical reaction. In some reactions,
there is no loss or gain of electrons. The atoms may just rearrange themselves, but keep their own electrons. In other
reactions, some atoms lose and gain electrons to create a redox reaction. To determine if redox exists, we need to
assign oxidation numbers to all atoms on both the products side and the reactants side. If we see a change in oxidation
states of an atom, then we can check to see if redox has occurred. For example
MgO + CaS  MgS + CaO.
Assign oxidation numbers to all elements in the reactants and the products. Which elements changed
their oxidation states?______________________________
Since the four atoms (Mg, O, Ca and S) all keep the same oxidation states, there is no redox
occurring here.
Look at this example
CaS + Mg  MgS + Ca
Assign oxidation numbers to all elements in the reactants and products. Which elements changed
their oxidation states?______________________________ (Did you remember that a single atom has an oxidation
state of zero?)
We know that redox has occurred because oxidation states changed. Some atoms lost control of more
electrons, others gained control of more electrons. One important factor to remember: Reduction can never
occur without oxidation. The two processes must go together in a complete equation. If someone loses
electrons, someone else gains electrons. If you ever notice that only one atom changes its oxidation state,
then we have a problem. (Either the equation is messed up, or perhaps the oxidation numbers are wrong.)
Now that we can recognize if redox has occurred, we need to determine which atoms are oxidized
and which are reduced. The easiest way to determine this is to watch what happens to the oxidation states.
In the reaction
CaS + Mg  MgS + Ca,
Ca changes its oxidation state from ____________________ to ____________________
Mg changes its oxidation state from ____________________ to ____________________
The only way this can happen is if calcium gains two electrons and magnesium loses two electrons.
(Remember, if you lose electrons, you become more positive, and if you gain electrons, you become more
negative. ) Some of you are now becoming more negative, Are you gaining electrons?
An easy rule to remember is this:
If the oxidation state increases, the atom is losing electrons. This is OXIDATION
If the oxidation state is decreasing, the atom is gaining electrons. This is REDUCTION.
(This is why it is called reduction, the oxidation state is going down.)
When indicating which atoms are oxidized or reduced, we name the REACTANT atom with its charge. So for
our practice equation
CaS + Mg  MgS + Ca
We would say that the Ca+2 atom is reduced and the Mg atom is oxidized. Remember to pick the reactant, and
state its oxidation state.
Another example would be:
+2
-2
+2
-2
0
+4
-2
CO + MgO  Mg + CO2
(check with me if you don’t understand where I got these numbers. )
In this case, we would say that C+2 is oxidized and Mg+2 is reduced. Again, we name the reactant
atom with its oxidation state.
Now do the notes on finding redox in equations.
Determining Oxidizing and Reducing Agents
Look at this reaction again.
+2
-2
+2
-2
0
+4
-2
CO + MgO  Mg + CO2
Now let’s ask ourselves another question. Can we go home now? Now let’s ask ourselves a chemistry
question. Why is C+2 oxidized to C+4? The reason is because the Mg+2 in MgO takes the electrons from the C+2. So we
can say that the substance or agent that causes the oxidation is the MgO. This is why the MgO is called the oxidizing
agent.
If we look at it another way, why is the Mg+2 reduced? The reason is because the C+2 in the CO forces its
electrons onto the unsuspecting Mg+2 atom. So we can say that the agent which causes reduction is the CO. Thus, the
CO is referred to as the reducting agent.
If we look at it another way, the atom that is oxidized is part of the reducing agent, and the atom that is
reduced is part of the oxidizing agent. Watch how this works.
Assign oxidation numbers to all atoms in the reaction
______ ______
______ ______
______ ______
_______
FeBr2 + NaBr  FeBr3 + Na
We know that the Fe+2 atom is oxidized because it becomes Fe+3. Also, the Na+ ion is
reduced to Na, so we say that
Fe+2 is oxidized
Na+ is reduced
FeBr2 is the reducing agent
NaBr is the oxidizing agent
In conclusion, atoms are oxidized and reduced, but whole compounds are oxidizing and reducing
agents. Also, remember to get your answers from the reactants side. These are the atoms and compounds
that will react.
Now do the notes on Oxidizing and Reducing Agents