Redox Reactions

Redox is the abbreviated way to write reduction- oxidation reaction. Redox is the abbreviated way to write reduction- oxidation reaction. Redox reactions are a very broad type of reaction and many of the types we have already discussed can also be classified as a redox reaction. Redox reactions are a very broad type of reaction and many of the types we have already discussed can also be classified as a redox reaction. In a redox reaction, reduction and oxidation must occur together. In a redox reaction, reduction and oxidation must occur together. But what is reduction and oxidation? But what is reduction and oxidation? Reduction – chemical change in which electrons are gained. Reduction – chemical change in which electrons are gained. Oxidation – a chemical change in electrons are removed. Oxidation – a chemical change in electrons are removed.

So, we need to Review Oxidation Numbers What is the oxidation number? What is the oxidation number? Book definition – the number of electrons that must be added to or removed from an atom in a combined state to convert the atom into the elemental form. Book definition – the number of electrons that must be added to or removed from an atom in a combined state to convert the atom into the elemental form. In other words, the number of electrons gained or lost when a chemical bond is made. In other words, the number of electrons gained or lost when a chemical bond is made. Where can we find possible oxidation numbers? Where can we find possible oxidation numbers? The Periodic Table

Assigning Oxidation Numbers – The steps from page Identify the chemical formula If no formula is provided, write the formula of the formula or ion from the name. 2. Assign known oxidation numbers - The oxidation number of an atom of any free element in atomic or molecular form is zero. Examples: Cu, H 2, Ag, Na, S 8 - The oxidation number of monatomic ion is equal to the charge of the ion

Assigning Oxidation Numbers – Continued - Fluorine always has an oxidation number of -1 since it is the most electronegative element. - In a compound: Group 1 atoms = Group 2 atoms = Aluminum = - Hydrogen = +1, unless it is with a metal, then it is

-Oxygen is usually -2, exceptions: -If combined with Fluorine, Oxygen’s oxidation number becomes +2 -In peroxides, H 2 O 2, Oxygen’s oxidation number becomes -1 -When comparing nonmetals, the more electronegative atom (page 194) gets the negative charge based on the column it is located in on the periodic table. -Group 17 =, Group 16 =, Group 15 = __ Assigning Oxidation Numbers – Continued -2 -3

Calculate any remaining numbers, and verify results. Calculate any remaining numbers, and verify results. –Use the total oxidation number of each element’s atoms = oxidation number times subscript –The sum of the oxidation numbers for all the atoms in a molecule is zero. –The sum of the oxidation numbers for all the atoms in a polyatomic ion is equal to the charge on that ion. Assigning Oxidation Numbers – Continued

Examples NH 4 + PbSO 4 Ca(OH) 2 AlH 2 Fe 2 (CO 3 ) 3 H 2 OKClO 3 H 2 PO 4 -1 Pb +2 BF 3 S 2 O = = = = = = = = =

Now that we reviewed oxidation numbers, what do we do with them? Remember, in a redox reaction, something has to be oxidized (loses electrons, oxidation number increases) and something else has to be reduced (gains electrons, oxidation number decreases). Remember, in a redox reaction, something has to be oxidized (loses electrons, oxidation number increases) and something else has to be reduced (gains electrons, oxidation number decreases). If we assign oxidation numbers to all atoms in a chemical equation, we can determine whether it is a redox reaction. If we assign oxidation numbers to all atoms in a chemical equation, we can determine whether it is a redox reaction.

Are the following redox reactions? 2 H 3 O + + Zn  H 2 + 2H 2 O + Zn +2 C 3 H 8 + 5O 2  3CO 2 + 4H 2 O 2KI + Pb(NO 3 ) 2  PbI 2 + 2KNO 3 2K + Cl 2  2KCl

Some other terms Oxidizing agent – the substance that gains electrons in an redox reaction and is reduced. Oxidizing agent – the substance that gains electrons in an redox reaction and is reduced. Reducing agent - the substance that loses electrons in an redox reaction and is oxidized. Reducing agent - the substance that loses electrons in an redox reaction and is oxidized. Notice that the oxidation agent oxidizes another substance and becomes reduces in the process and visa versa Notice that the oxidation agent oxidizes another substance and becomes reduces in the process and visa versa Also note that these refer to the whole compound or ion. Also note that these refer to the whole compound or ion.

Let’s look at the following redox reaction First, write the oxidation number for each atom: 8 H 3 O + + MnO Fe +2  Mn Fe H 2 O What was reduced? What was oxidized? What is the oxidizing agent? What is the reducing agent? Mn Fe MnO 4 - Fe +2

Homework Page 611: 6, 7, 8, 9 Page 611: 6, 7, 8, 9