Oxidation numbers

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Objective

Students will be able to assign oxidation numbers to elements in compounds.

Big Idea

Oxidation numbers are numbers that represent the loss or gain of electrons from atoms in a compound or chemical reaction.

Introduction

In this unit on redox reactions students are learning about tracking the flow of electrons. In this lesson students will learn background information that supports this larger goal. Specifically, they will learn how to use rules to assign oxidation numbers to elements.

This lesson also aligns to the Disciplinary Core Idea found in HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties because redox reactions focuses on a chemical reaction in which the outermost electrons are transferred from one element to another.

This lesson aligns to the NGSS Practices of the Scientist of Using mathematics and computational thinking because students will need to use math in many cases in order to assign to the correct oxidation number to an element.

It aligns to the NGSS Crosscutting Concept of Stability and Change because understanding oxidation numbers is the first step in constructing an explanation for how redox reactions occur—oxidation numbers are the method for tracking change to elements when they enter into compounds, and for how electron changes occur in redox reactions.

This lesson also aligns to the HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties because redox reactions focuses on a chemical reaction in which the outermost electrons are transferred from one element to another.

In terms of prior knowledge or skills, students should have already had an introductory lesson about redox reactions that covers some of the vocabulary used in today’s lesson.

There are no special materials needed for this lesson.

Do Now/Activator

10 minutes

Do Now: Students begin class by identifying where they are with the Electrochemistry Reading Questions from the previous class. If they completed them, then their job is to compare answers and then quiz one another on the answers. Their goal is to highlight any disagreements or confusion so that we can discuss these issues at the start of my lesson. Students who still did not complete the work are instructed to work on it at this time.

This choice reflects my desire to get students working with the redox vocabulary that we will use in class today.

Activator: After students have looked over their work I ask if they have any questions. Students did not have any questions after they talked with one another, so I asked a student to tell me what an oxidation number is. A student volunteers that it is a number that shows how many electrons an atom has lost or gained. I note that there are rules for assigning these numbers and that is the subject of today’s class. 

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: I start the lesson by having students look at a list of rules that I have written in a column on the board. The column reads “Rule 1, Rule 2, Rule 3, Rule 4, Rule 5a, Rule 5b, Rule 5c.” I tell students that rule 1 is more important than rule 2 and rule 2 is more important than rule 3. I then ask students to tell me what they can about rule 4. They note that it is more important than rule 5. Similar questioning in this vein establishes the fact that the higher the rule, the more important the rule.

I explain that this is how the rules for oxidation numbers work. The higher rule must be followed. If two rules cannot both be followed, then the higher rule must prevail

I then model how the rules work by assigning oxidation numbers to each of the atoms in the H2O, HClO2, and PO43- using the table of rules found on page 480 of the student text book. For each compound, I go through the rules. I note that there are no pure elements as described in rule 1. I note that the first two compounds are neutral, and so all of the oxidation numbers have to add up to zero (rule 2). I note that for the third compound all of the oxidation numbers have to add up to -3 (rule 3).

I then note that we have to assign numbers to each element using the other rules. For H2O, H comes first on the rule list, and so we assign it a +1. O comes second, and so we assign it next, and the rule says it gets a -2. I now point out that there are 2 H’s and 1 O. This is where students have to do a little math—2 (+1) for hydrogen equals a +2, and 1 (-2) for oxygen equals -2. I note that because +2 and -2 add up to zero (rule 2) we can keep these assigned numbers.

I teach HClO2 in a similar manner, going through the rules. However, I chose this example because in this case Cl does not follow the rule that says that Cl has a -1 oxidation number. It cannot because then the all of the oxidation numbers would not add up to zero. In this case, chlorine has to have an oxidation number of +5.

I teach PO43- similarly by going through the rules. In this case I note that the oxidation numbers have to add up to -3 (rule 3). I use this as one of my teaching examples for this reason. I also like the fact that there is not rule for P. Therefore, we have to add up the four oxygen atoms, which total -8, and then derive a +5 for the P in order to have all the oxidation values add up to -3.

During this lesson students take Notes that they can refer to later.

Guided Practice: I ask students to assign oxidation numbers for the first 3 problems on the oxidation numbers practice. I chose this particular focus so that students would get a chance to wrestle with the ideas I just presented. Then they will be a in a better position to ask questions and seek clarity on this skill. After I give students a few minutes to wrestle with this, I explain how I got the answers I got for each of the problems. I use a document camera to show my work. 

Application

25 minutes

Student Activity: During the bulk of class time students work on finding the oxidation numbers for elements in compounds by first finishing the Oxidation Numbers Practice from the mini-lesson and then working on the Redox Reaction Practice handouts. During this time some students work independently while others work with a partner so that they have a sounding board and collaborator. While students are working I walk around and answer questions, and I observe student work so that if there are general misconceptions or areas of confusion I will be able to address them.

Catch and Release Opportunities: From my learning walk there are two things that I notice are challenging students. First, some students have gotten lost in the details and forgot the fact that this is a rules based task. I review my method of assigning oxidation numbers as shown in this Teaching using the rules video. The other challenge students have is remembering which rule to use when the rules contradict. They tell me that Cl has a -1 charge, but I explain that sometimes a lower rule must be broken in order to satisfy the higher rule. How I teach this is shown in the Teaching to honor the higher rule video.

Debrief

10 minutes

To wrap this lesson up I invite a student to the front of the class to show her work. Notice in the Debrief video that the student has a pretty good idea of how to assign oxidation numbers, but it is through my questioning of her work that we are able to bring the rules into the discussion. This seems to be one of the challenges students are still having with this skill--remembering to stick to the rules and the order in which they are listed.

Another challenge that they are facing is confusing oxidation number with the total charge of all the atoms for the element in a compound. For example the oxidation number of oxygen in H2O2 is -2, but students multiply the subscript by -2 and claim that the oxidation number is -4. For further example, I notice this mistake occurring in the student work namely in problems 4 & 5 in the Redox Reaction Practice. I will need to address that in a subsequent class.