Chapter 22 Notes – Oxidation-Reduction Reactions

Oxidation – complete or partial loss of electrons or gain of oxygen, increase in oxidation number.

Reduction – complete or partial gain of electrons or loss of oxygen, decrease in oxidation number.
Oxidation-Reduction Reaction (Redox) – the chemical changes that occur when electrons are transferred between reactants.
"LEO the lion goes GER"
Losing Electrons – Oxidation
Gaining Electrons – Reduction
Oxidizing agent – the substance in a redox reaction that accepts electrons
Reducing agent – the substance in a redox reaction that donates electrons
Oxidized substance – the substance that undergoes oxidation (loses electrons)
Reduced substance – the substance that undergoes reduction (gains electrons)
0
Mg +		 0
S à 		+2-2
MgS
Reducing Agent Oxidizing Agent Mg – oxidized S – reduced

If given: 2AgNO3 + Cu à Cu(NO3)2 + 2Ag

Find the oxidized substance, reduced substance, oxidizing agent, and reducing agent

Step 1: Find all oxidation numbers

+1    +5 –2
2AgNO3 +		 0
Cu à 		+ 2   +5 –2
Cu(NO3)2 +		 0
2Ag

Step 2: Determine the elements that change oxidation numbers from reactant side of the equation to product side.

Cu0 to Cu+2 and Ag+1 to Ag0

Step 3: Determine which element is reduced, which is oxidized. "LEO the lion goes GER"

Cu0 to Cu+2 lost 2 electrons so oxidized

Ag+1 to Ag0 gains 1 electron so reduced

Step 4: The reduced substance is always found in the oxidizing agent. The reduced substance is an element on the reactant side of the equation. The oxidizing agent is the compound in which the reduced element is found. Therefore AgNO3 is the oxidizing agent.

The oxidized substance is always found in the reducing agent. The oxidized substance is an element on the reactant side of the equation. The reducing agent is the compound in which the oxidized substance is found. Therefore, Cu is the reducing agent.

In redox reactions that produce covalent substances, electronegativity of each element would need to be considered.

EX: NH3 ;N is reduced and H is oxidized

Rules for Assigning Oxidation Numbers can be found on p.655

  1. The oxidation number of a monatomic ion is equal in magnitude and sign to its ionic charge. Ex. Br1- is –1; Fe3+ ion is +3
  2. The oxidation number of hydrogen in a compound is +1, except in metal hydrides, such as NaH, where it is –1.
  3. The oxidation number of oxygen in a compound is –2, except in peroxides, such as H2O2, where it is –1.
  4. The oxidation number of an atom in uncombined (elemental) form is 0. Ex. N2 or K are each 0.
  5. For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal 0.
  6. For a polyatomic ion, the sum of the oxidation numbers must equal the ionic charge of the ion.

What is the oxidation number of chromium in the polyatomic ion CrO42-?

? + (4 x (–2)) = -2 so ? must be +6 ; Cr = +6

What is the oxidation number of each kind of atom in K2SO4?

(2 x (+1)) + ? + (4 x (-2)) = 0 so ? must be +6; S = +6

Not all reactions are redox reactions. Double replacement reactions are not redox reactions.