“Redox” is the combination of the two words Lesson 1 Reduction/Oxidation “Redox” is the combination of the two words Lesson 1

What is a redox reaction? In many reactions, from reactants to products, the way atoms are surrounded by electrons changes. One type of atom gains some electrons and another type of atom loses some electrons.

What is oxidation? Oxidation refers to losing electrons. The process is called “oxidation” because oxygen was the first element to be identified as causing another species to lose electrons. Oxygen is very electronegative. (Second only to fluorine.) It is also very abundant. We now know that other species can also cause oxidation.

What is reduction? Reduction is when a species gains electrons. All redox reactions include one species that is oxidized and one that is reduced. They cause each other to occur. This movement of electrons is electricity and is the science behind batteries and electroplating.

Redox reactions are not a new type of reaction! Synthesis, decomposition, single replacement, and combustion of a hydrocarbon are also redox reactions. Double replacements (precipitation and acid/base) are NOT redox.

Assigning oxidation numbers The process of “assigning oxidation numbers” to all atoms in a reaction is a human devised accounting method that serves two purposes: Determining for certain if it is a redox reaction (involves a transfer of electrons.) Figuring out which species was oxidized and which was reduced.

Rules for Assigning Oxidation Numbers (ONs) Use them in this order: Elements not in compounds always have an ON of zero Alkali metals are +1 Alkaline earth metals are +2. Fluorine is always -1 In binary ionic compounds, halogens are -1, group 16 elements are -2, and group 15 elements are -3 Oxygen is -2 (except in peroxides where it is -1) Hydrogen is +1 (except in metal hydrides where it is -1) The sum of the ONs in a neutral species is zero In a polyatomic ion, the sum of the ONs is the ion charge

ONs for binary ionic compounds (In a binary ionic compound, the normal ionic charges are also the ONs.) MgCl2 Mg = +2 Cl = -1 KBr K = +1 Br = -1 Fe2O3 Fe = +3 O = -2

Assigning ONs in a polyatomic ion MnO4‾ Mn = +7 O = -2 CO3‾­² C = +4 O = -2 NO3‾ N = +5 O = -2

Assigning ONs in ionic compounds that have polyatomic(s)

Sample Problem Assign oxidation numbers to each element and identify the element that is oxidized and the element that is reduced in the following reaction: +1 -2 +1 +5 -2 0 +2 -2 +1 -2 3 H2S + 2 HNO3  3 S + 2 NO + 4 H2O Oxidized: S Reduced: N

Redox Agents The element oxidized is the reducing agent. Common reducing agents are Na, K, Mg, Al, Zn, and Fe The element reduced is the oxidizing agent. Strong oxidizing agents usually contain F, O, or Cl

Practice—Assign Oxidation Numbers As + NaOH ---> Na3AsO3 + H2 0 +1 -2+1 +1 +3 -2 0 C8H16 + Cl2 ---> C + HCl -2 +1 0 0 +1 -1 Xe + F2 ---> XeF6 0 0 +6 -1

Gummy Bear Explosion— Assign Oxidation Numbers What is Oxidized Gummy Bear Explosion— Assign Oxidation Numbers What is Oxidized? What is Reduced? KClO3 + C12H22O11 → H2O + CO2 + KCl 2KClO3 → 2KCl + 3O2 C12H22O11 + O2 → 9C + 3CO2 + H2O +1+5-2 0 +1 -2 +1 -2 +4 -2 +1 -1 +1+5-2 +1-1 0 0 +1 -2 0 0 +4-2 +1 -2 Oxidized: C, O Reduced: Cl, O