In redox reactions, since there's a transfer of electrons, there's also a change of oxidation state for two different elements.
If an element is oxidized, its oxidation state will increase.
If it is reduced, its oxidation state will decrease.
Oxidation and reduction must occur together, because if one species loses electrons, another species must gain electrons.
We can also refer to the oxidizing agent or reducing agent.
The oxidizing agent is the reactant containing the element that undergoes reduction.
Since oxidation and reduction must occur together, if an element undergoes reduction, it causes something else to undergo oxidation.
So when we add the word "agent" it means the species is causing that process in something else.
Likewise, the reducing agent causes reduction in something else by itself undergoing oxidation.
So the reducing agent is the reactant containing the element the undergoes oxidation.
Let's practice identifying redox reactions, oxidizing agents, and reducing agents.
To determine if this reaction is a redox reaction, we can assign oxidation states to all atoms in the reaction.
If this is a redox reaction, one element will have an increase in oxidation state and another will have a decrease in oxidation state.
Solid aluminum has an oxidation state of zero because it is a free element.
The aqueous silver ions are monatomic ions so the oxidation state is equal to the charge, which is +1.
Aqueous aluminum ions are also monatomic ions so the oxidation state is equal to +3.
And solid silver is a free element so it has an oxidation state of zero.
Based on these oxidation states, we see that aluminum changes from 0 to +3.
This means aluminum is oxidized because its oxidation state increased.
It also means that solid aluminum is the reducing agent.
By undergoing oxidation, which is the loss of electrons, it forces something else to undergo reduction, which is gaining electrons.
We also see that silver changes from +1 to 0.
This means silver is reduced, because the oxidation state decreases.
It also means that the silver ions in aqueous solution are the oxidizing agent.
By undergoing reduction, silver forced aluminum to be oxidized.
Let's try another example.
We'll start by signing oxidation states.
For sulfur trioxide, the most important rule is that the sum of the oxidation states of all atoms in a neutral molecule is zero
so sulfur plus three oxygen equals zero.
Both sulfur and oxygen are in our table for nonmetals, but oxygen is higher in the table, so we assigned -2 to oxygen and solve for sulfur.
Sulfur has an oxidation state of +6.
For H2O, our most important rule is that the sum of the oxidation states of all atoms in the neutral molecule is zero
so two hydrogen plus oxygen equals zero.
Hydrogen is higher in our table of nonmetals so we assign it an oxidation state of +1 and calculate oxygen's oxidation state.
Oxygen is -2.
And finally, for sulfuric acid, the most important rule is that the sum of the oxidation states of all atoms in the neutral molecule is zero,
so two hydrogen plus sulfur plus four oxygen equals zero.
All three elements are in the table of nonmetals, but hydrogen and oxygen are highest on the list so we assigned +1 to hydrogen,
-2 to oxygen, and we calculate sulfur.
Sulfur's oxidation state is +6.
Since there was no change in oxidation state for any of our atoms, this is not a redox reaction.