Electron-transfer reactions are called oxidation-reduction reactions or redox reactions. Oxidation – loss of electrons by one reactant. Reduction – gain of electrons by another reactant. Oxidation and reduction always occur together. Na → Na + + e - (oxidation) Cl 2 + 2e - → 2Cl - (reduction) The substance that is oxidized is the reducing agent. The substance that is reduced is the oxidizing agent.

Oxidation Is Losing e - Reduction Is Gaining e - =OIL RIG

Oxidation numbers provide a way to keep tabs on electron transfers. Oxidation numbers – the sign is written before the number. Electrical charges – the sign is written after the number. Na ion has a charge of 1+ and an oxidation number of +1. Since not all redox reactions produce ionic products, we can redefine redox reactions as chemical reactions in which changes in oxidation numbers occur.

Rules for Assigning Oxidation Numbers 1.The oxidation number of any free element (e.g., O 2, Ag, etc.) is zero, regardless of how complex its molecules may be. 2.The oxidation number for any simple, monatomic ion (e.g., Na + or Cl - ) is equal to the charge on the ion. 3.The sum of all the oxidation numbers of the atoms in a molecule or polyatomic ion must equal the charge on the particle. 4.In its compounds, fluorine has an oxidation number of In its compounds, hydrogen has an oxidation number of In its compounds, oxygen has an oxidation number of -2.

In binary ionic compounds with metals, the nonmetals have oxidation numbers equal to the charge on their anions. Example: Assigning Oxidation Numbers Molybdenum disulfide, MoS 2, has a structure that allows it to be used as a dry lubricant, much like graphite. What are the oxidation numbers of the atoms in MoS 2 ? Solution: Binary ionic compound: S(2 atoms)x(-2)=-4(Rule 6) Mo(1 atom)x(x)=x _________________________________ Sum=0(Rule 3) The value of x must be +4 for the sum to be zero. Therefore, Mo = +4andS = -2

Let’s redefine oxidation and reduction. Oxidation is an increase in oxidation number. Reduction is a decrease in oxidation number. To balance redox equations by the ion-electron method, we divide the oxidation and reduction processes into individual equations called half-reactions that are balanced separately.