20.1 Oxidation/Reduction (REDOX) Chapter 20 - Oxidation & Reduction Reactions 20.1 Oxidation/Reduction (REDOX)

Chemistry Today we are learning to:- 1. Understand two basic definitions of oxidation and reduction 2. Recognise from a balanced chemical equation where oxidation and reduction is taking place

Oxidation and Reduction Why name the process oxidation? Originally oxidation was seen as a chemical reaction where oxygen was added to an element or compound 2Mg + O2  2MgO Magnesium and hydrogen are oxidized 2H2 + O2  2H2O

Oxidation and Reduction Why name the process reduction? Originally, reduction was seen as a chemical reaction where oxygen was removed from a compound. The compound is made simpler/smaller/reduced Iron oxide FeO + CO  Fe +CO2 Iron and hydrogen are reduced 2H2O2  2H2O + O2 Hydrogen peroxide

Oxidation and Reduction Oxidation  gain of oxygen Reduction  loss of oxygen

Mg O  MgO Oxidation and Reduction Today we look at how electrons are transferred in oxidation/reduction reactions. Look at the previous reactions again. Mg is a group 2 metal. It has 2 valence electrons. Oxygen has 6 valence electrons 2Mg + O2  2MgO 2 electrons lost 2 electrons gained +2 Mg O -2  MgO When something is oxidized it loses electrons. Similarly, when something is reduced it gains electrons. Loss of Electrons  Oxidation Gain of Electrons  Reduction

Oxidation and Reduction LEO GER

Oxidation and Reduction Oxidation and reduction occur in any reaction where electrons are lost or gained. They always occur together So they are called REDOX reactions Mg + Cl2  MgCl2 Oxidized Reduced

Oxidation Numbers Number of electrons lost or gained by an atom called oxidation number. Oxidation number can be positive, negative or neutral. Use the periodic table in the Reference tables to find common oxidation numbers Use the list of oxidation number rules to find common oxidation numbers What are the oxidation numbers of carbon and oxygen in carbon monoxide? What are the oxidation numbers of carbon and oxygen in carbon dioxide? C =+2, O=-2 C =+4, O=-2 FeO + CO  Fe +CO2 What are the oxidation numbers of iron and oxygen in iron oxide What is the oxidation number of elemental iron Fe=+2, O=-2 Fe=0

Oxidation Numbers Ex1: Determine the oxidation number of each element in K2Cr2O4 K=+1 from periodic table or rule 2 O=-2 from periodic table or rule 5 K2Cr2O4 K 2×+1=+2 Cr 2×? = ? O 4×-2=-8 From rule 6 the sum of oxidation numbers = 0 for this particular species so: 2(+1) + 2Cr + 4(-2) = 0 2Cr = +6 Cr = + 3

Oxidation Numbers Ex1: Determine the oxidation number of each element in Ba(NO3)2 Ba=+2 from periodic table or rule 2 O=-2 from periodic table or rule 5 Ba(NO3)2 Ba 1×+2=+2 N 2×? = ? O 6×-2=-12 From rule 6 the sum of oxidation numbers = 0 for this particular species so: 1(+2) + 2N + 6(-2) = 0 2N = +10 N = + 5 Questions 1-10 in review book

REDOX Reactions A change in oxidation number for any type of atom in a reaction means it is a REDOX reaction Assign oxidation numbers to all the atoms in a reaction If an oxidation number changes the reaction is REDOX No change in oxidation number means it is not REDOX +1 -1 +2 -1 Zn + 2HCl  ZnCl2 + H2 Zinc is oxidized as it has lost 2 electrons Hydrogen is reduced as each has gained 1 electron

REDOX Reactions A change in oxidation number for any type of atom in a reaction means it is a REDOX reaction If you recognize a double replacement reaction, it is not REDOX +4 -2 +1 -1 +2 -1 +1 -2 MnO2 + 4HCl  MnCl2 + 2H2O +Cl2 Mn has gone from Mn+4 to Mn+2  gained electrons  reduced Some Cl have gone from Cl-1 to Cl0 in Cl2  lost electrons  oxidized

Oxidizing Agents and Reducing Agents A substance that causes another to be reduced is called a reducing agent. This gets oxidized A substance that causes another to be oxidized is called an oxidizing agent. This gets reduced Oxidizing agent as is oxidizes HCl to Cl2 MnO2 + 4HCl  MnCl2 + 2H2O +Cl2 Reducing agent as is reduces MnO2 to MnCl2

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