Redox and Assigning Oxidation Numbers Chapter 22 Notes I Redox and Assigning Oxidation Numbers

Oxidation What comes to mind when you think of the word oxidation? Oxygen! The first definition of oxidation was the combination of elements with oxygen to make oxides

Oxidation Now the definition is broader, and oxidation is the complete or partial loss of electrons in a reaction.

Reduction Similarly, reduction originally meant the loss of oxygen in a reaction. Its definition is now broader; reduction is the complete or partial gain of electrons in a reaction.

ReDox Reactions Because it is the loss/gain of electrons, reduction always accompanies oxidation (they never occur separately). A reaction that involves the gain or loss of electrons during reaction is an oxidation-reduction or redox reaction.

LEO the lion goes GER!!! Losing Electrons is Oxidation! Gaining Electrons is Reduction! Mg + S a Mg2+ + S2- Mg undergoes oxidation and S undergoes reduction

Oxidizing/Reducing Agents The substance that loses electrons (that is oxidized) is the reducing agent—because it is doing the reducing. The substance that gains electrons (that is reduced) is the oxidizing agent—because it is doing the oxidizing.

Oxidizing/Reducing Agents 2Na + S a Na2S Sodium goes from 0 to +1 (loses e-) Sulfur goes from 0 to –2 (gains e-) Sodium is oxidized, so it is the reducing agent. Sulfur is reduced, so it is the oxidizing agent.

Being Oxidized vs. A Reducing Agent If an element in a compound is losing an electron, the element itself is said to be oxidzed, and the entire compound that it is in is a reducing agent.

Determining Oxidation Numbers

Rules for determining oxidation #s The oxidation number for an uncombined element is zero.

Rules for determining oxidation #s 2) The oxidation number of a monatomic ion is equal to the charge on the ion.

Rules for determining oxidation #s 3) The sum of the oxidation numbers in a neutral compound is zero.

Rules for determining oxidation #s 4) The sum of the oxidation numbers of the atoms in a polyatomic ion is equal to the charge of the ion.

Rules for determining oxidation #s 5) The oxidation number of the more electronegative atom in a molecule or polyatomic ion is the same as the charge it would have as an ion.

Rules for determining oxidation #s 6) The oxidation number for oxygen is –2, except in peroxides (H2O2, etc.) where it is –1, and when bonded to fluorine (OF2) where it is +2).

Rules for determining oxidation #s 7) The oxidation number for hydrogen is +1, except when in metal hydrides (LiH, etc.) where it is –1.

Rules for determining oxidation #s 8) The metals in groups 1 and 2 and Al in group 13 always have a positive oxidation number equal to the number of valence electrons (+1, +2 and +3).

Rules for determining oxidation #s Many elements other than those specified (including transition metals, etc.) can be found using the oxidation numbers of the other elements in the compound, knowing the total charge will be zero.

Examples N2O3 MnO41- KBrO3 Ca(NO3)2

Find the oxidation # for all elements in:

Find the oxidation # for SO2

Find the oxidation # for CO32-

Find the oxidation # for K2SO4

Find the oxidation # for S2O3

Find the oxidation # for

Find the oxidation # for Al2(SO2)3

Find the oxidation # for Na2O2

Find the oxidation # for P2O5

Find the oxidation # for NH4+

Find the oxidation # for Na2Cr2O7

Find the oxidation # for Ca(OH)2