Oxidation-Reduction (REDOX) Reactions

• Oxidation-Reduction reactions are also known as REDOX reactions
• These involve the transfer of electrons (e-)
• Reduction is the gain of electrons
  • The charge gets reduced (more negative) as electrons are added
• Oxidation is the loss of electrons
• Reduction and oxidation can each be represented by half-reactions
• Conservation of Charge applies
  • The number of electrons lost must equal the number of electrons gained

Oxidation Numbers

• Basic oxidation numbers come from the group numbers.
• It's based on gaining or losing valence electrons to earn a full outer shell.
  • Ex: Magnesium (group 2) has 2 valence electrons. It gives them away in an ionic bond to get a 2+ charge. This happens with all group 2 elements.
• Transition metals (groups 3-12) have varied oxidation states.
• All oxidation states are shown in the top right corners of your periodic table.
• Elements on their own in a reaction have an oxidation state of zero (0), even if they are diatomic.

General Oxidation Numbers

There is a general pattern of oxidation numbers for about half of the periodic table, as follows:

Group	1	2	3-12	13	14	15	16	17	18
Oxidation #	1+	2+	Varies	3+	4+	3-	2-	21	0

Redox Reaction

Example reaction: H₂ + F₂ → 2HF

• Start by assigning charges (oxid #'s) to each element on both sides.
• Reactants H2 and F2 are each on their own, so they have oxidation numbers of 0.
• In HF, hydrogen has a 1+ charge and fluorine has a 1- charge (based on oxid #s).
• We use their oxidation numbers because they are bonded together.
• We can write a half-reaction for hydrogen that shows how its charge changes from 0 (in H2) to 1+ (in HF).
• To do this, we need to add electrons to one side to balance the charges.
• A second half-reaction can be created for fluorine.

Let's breakdown the reaction: hydrogen

First, show all the charges above each element in the reaction.

0       0         1+  1-
H₂ + F₂ → 2HF

The charges on each atom are shown above. Let's use these to make the half-reactions.

H₂ → 2H⁺

The problem is, this isn't balanced by charge. The charge on the left is zero and the total charge on the right is 2+ (because there are two H⁺).

To balance this, we need to add 2 electrons to the right side.

H2 → 2H⁺ + 2e^-

This shows electrons being lost, therefore this is oxidation.

Let's breakdown the reaction: fluorine

First, show all the charges above each element in the reaction.

0       0         1+  1-
H₂ + F₂ → 2HF

The charges on each atom are shown above. Let's use these to make the half-reactions.

F2 → 2F^-

The problem is, this isn't balanced by charge. The charge on the left is zero and the total charge on the right is 2- (because there are two F^-).

To balance this, we need to add 2 electrons to the left side.

F2 + 2e^- → 2F^-

This shows electrons being added, therefore this is reduction.

Final comments

H₂ + F₂ → 2HF

So, the two half-reactions for this reaction are:

oxidation:                H₂ → 2H⁺ + 2e^-

reduction:      F₂ + 2e^- → 2F^-

If you were to add these two reactions together, the electrons would cancel out and you would end up with the original reaction.

Electrochemical cells

• Redox reactions can take place in an electrochemical cell
  • This is just a place where electrons move between chemicals
  • Conversation between chemical and electrical energy
• Anode versus Cathode
  • Oxidation occurs at the Anode (vowels)
  • Reduction occurs at the Cathode (consonants)
• There are 2 types of electrochemical cells
  • Voltaic
    • spontaneous, happens on its own
  • Electrolytic
    • requires the addition of electricity to produce chemical change
    • this process is called electrolysis

Voltaic Cell This happens spontaneously (automatically) A zinc (Zn) anode is placed in a solution of ZnSO4 the anode is negative it pushes e- out through the wire Zn loses electrons through the wire (oxidation) Zn + 2e- → Z2+ A copper (Cu) cathode is placed in a solution of CuSO4 the cathode is positive it pulls e- in from the wire Cu gains electrons through the wire (reduction) Cu2+ + 2e- → Cu Anions (negative ions) can flow through the salt bridge or porous disk

Electrolytic Cell Basically the opposite of a voltaic cell This requires a power source Here the anode is positive so it draws electrons up from the solution oxidation occurs here This means the cathode is negative and pushes electrons back into solution reduction occurs here This process is called electrolysis