Redox Chemistry Introduction Basic Concepts AICE Chemistry Redox Chemistry Introduction Basic Concepts

What is Redox? Redox is an area of reaction chemistry that focuses on the changes in oxidation state that occur as a reaction proceeds from reactants to products.

What Types of Changes can Occur? Reduction – this is a “negative-direction” change in oxidation state. (visualize a number line) Textbooks will call this a “decrease” in oxidation state. Oxidation – this is a “positive-direction” change in oxidation state. (again, visualize a number line) Textbooks will call this an “increase” in oxidation state. ( - ) ( + )

Some Rules Regarding Oxidation State Atoms that are “by themselves” – not part of a compound – are defined to have an oxidation state = 0 . Diatomic molecules ( for example H2 , Cl2, O2 ) have oxidation state = 0 Atoms that are parts of compounds have oxidation states that you may determine using the strategies that you have always used to write chemical formulas.

Consider the Following Example: Zn + S  ZnS This is a simple composition reaction. As you will see, it also involves oxidation and reduction…

First, determine the oxidation states of all of the particles. Zn + S  ZnS +2 -2 The Zn is an un-combined atom so its oxidation state = 0 The S atom is un-combined, so its oxidation state = 0 Within the ZnS compound, each of the atoms has an oxidation state.

Next, look for the changes. +2 -2 Zn + S  ZnS Notice how the Zn has changed from 0 to +2. This is an increase in oxidation state and is described as an oxidation. Note too how the S has changed from 0 to -2. This is a decrease in oxidation state – a reduction.

2 Na + Cl2  2 NaCl Now for Example #2 Again, a composition reaction, but this time we see a diatomic element as a reactant. Let’s see if we can determine what is oxidized in the reaction and what is reduced.

First – the oxidation states: 0 0 +1 -1 2 Na + Cl2  2 NaCl Again, the Na is by itself and has an oxidation state = 0. The Cl2 is a diatomic and is defined as having an oxidation state = 0. The NaCl is a compound and the atoms in that compound have their typical oxidation states.

2 Na + Cl2  2 NaCl Now for the changes: 0 0 +1 -1 2 Na + Cl2  2 NaCl As you can see, the Na has been “oxidized” from 0 to +1. In addition, the Cl has been “reduced” from 0 to -1. The fact that Cl is diatomic is not really part of the discussion here.

One more example: 2 AlBr3 + 3 Cl2  2 AlCl3 + 3 Br2 This is a replacement reaction. Note that there is a diatomic on each side and both will have an oxidation state = 0 as a diatomic. Let’s look at the oxidation states and the changes.

Following the steps: 2 AlBr3 + 3 Cl2  2 AlCl3 + 3 Br2 +3 -1 0 +3 -1 0 +3 -1 0 +3 -1 0 2 AlBr3 + 3 Cl2  2 AlCl3 + 3 Br2 Several things to notice here: The Br has changed from a -1 in the reactant compound to a 0 as a diatomic. That is an increase in the oxidation state, so the Br is oxidized. The Cl has changed from a 0 as a diatomic to a -1 in the product compound. That is a decrease in oxidation state, so the Cl has been reduced. Note that the Al is unchanged in the reaction. Remembering that this reaction will occur in solution, the Al+3 is a spectator.

Your task: figure out what is oxidized and what is reduced in the following reactions. S + O2  SO2 Mg + 2 HCl  MgCl2 + H2 2 KClO3  2 KCl + 3 O2 Zn + 2 Ag(NO3)  Zn(NO3)2 + 2 Ag