Unit 11: Classification of Chemical Reactions Cypress Creek HS: Chemistry 1K Chapter 10

Part 3: Redox

Redox Reactions Electron transfer reactions are called oxidation-reduction or redox reactions. Occurs when oxidation numbers change. Results in the generation of an electric current (electricity) Therefore, this field of chemistry is often called electrochemistry.

Terminology for Redox OXIDATION REDUCTION OXIDIZING AGENT loss of electron(s); increase in oxidation number REDUCTION gain of electron(s); decrease in oxidation number OXIDIZING AGENT electron acceptor; species oxidizes another and is itself reduced. REDUCING AGENT electron donor; species reduces another and is itself oxidized. 4

Ways to Remember Redox Reduction can’t happen without an oxidation to provide the electrons – they are companions! OIL RIG Oxidation is loss Reduction is gain LEO the lion says GER Lose electrons – oxidation Gain electrons – reduction

Oxidation Numbers The oxidation number of an atom is the number of electrons lost or gained when it forms ions. Oxidation numbers are written with the sign before the number, whereas ionic charge is written after the number. Oxidation number: +3 Ionic charge: 3+

Common Oxidation Numbers 7

Rules for Oxidation Numbers 1. The oxidation number of an uncombined atom is zero. Ex: Mg, Ca, O2, Cl2, S 2. The oxidation number of an uncombined ion is equal to the charge on the ion. Ex: the oxidation number of a Ca2+ is +2, and Br– is –1

Rules for Oxidation Numbers 3. The oxidation number of oxygen in compounds is –2. 4. The oxidation number of hydrogen in compounds is +1. 5. The oxidation number of group 1A elements in compounds is +1. 6. The oxidation number of group 2A elements in compound is +2.

Rules for Oxidation Numbers 7. The oxidation number of the more electronegative atom in a compound is the charge of its ion. Ex: In SiCl4, chlorine is more electronegative, so chlorine has an oxidation number of –1.

Rules for Oxidation Numbers 8. The sum of the oxidation numbers in a neutral compound is zero. 9. The sum of the oxidation numbers of the atoms in a polyatomic ion is equal to the charge on the ion.

Determining Oxidation Numbers What is the oxidation number of chlorine in KClO3 (potassium chlorate)? Oxidation numbers add up to zero. The oxidation number of oxygen in compounds is –2. Group 1A elements have a +1 oxidation number. +1 ? -2 (+1) + (?) + (-2●3) = 0, so Cl = +5 KClO3 What is the oxidation number of sulfur in SO32– (sulfite ion)? Oxidation numbers add up to –2. ? -2 (?) + (-2●3) = -2, so S = +4 SO3

Oxidation and Reduction Zinc is oxidized from zinc metal to the Zn2+ ion. H+ is the oxidizing agent. Each H+ is reduced and combine to form H2. Zn is the reducing agent. 13

Oxidation and Reduction To see how oxidation numbers change, start by assigning numbers to all elements in the balanced equation. There is no change in the oxidation number of potassium – it is called the spectator ion.

Oxidation and Reduction Practice Identify what is oxidized and what is reduced: Aluminum is oxidized, iron is reduced Identify the oxidizing agent and the reducing agent: Aluminum is the reducing agent, iron is the oxidizing agent. Identify the spectator ion: Oxygen

Half-Reactions The oxidation process and the reduction process of a redox reaction can each be expressed as a half-reaction. For example, consider the unbalanced equation for the formation of aluminum bromide.

Half-Reactions The oxidation half-reaction shows the loss of electrons by aluminum. The reduction half-reaction shows the gain of electrons by bromine.