This easy to use exercise displays the key functions of
the Oxidation Numbers Tutor. Once this exercise is completed,
students will be familiar and comfortable with the Tutor,
giving them maximum benefit as they progress with their chemistry
studies.
The best opportunity to learn occurs when mistakes are made
— and that is when the full power of the Quantum Tutor
is realized. To demonstrate the Tutor's capabilities and features,
some of the more common errors made by students are a key
focus of this exercise.
Similar to a human tutor, the Quantum Tutor deliberately
offers a variety of wordings and numbers in its examples and
explanations every time you use it. The ideas and concepts
are the same. So, be prepared — you may not see the
exact same words or numbers that appear in these sample screens.
Know that you are learning the concepts and the why
behind the answers, not just memorizing facts and procedures.
Teachers may choose to print out this document and use it
as an assignment in the classroom or for homework. Space has
been provided for students to write in their responses.
Learning Objectives:
- Students will be able to assign oxidation numbers using
the Tutor, either by choosing a formula from the menu or
by entering their own original work.
- Students will learn to use the Tutor’s hints and
comments in order to learn more about assigning oxidation
numbers.
Overview of Exercise:
Steps 1 through 5: Students will enter a
formula of their own choosing, ask the Tutor to solve the
problem step by step, and receive individualized tutoring
along the way.
Step 6: Students will select a formula from
the Tutor’s menu and, with the help of the Tutor, assign
oxidation numbers to each element.
Step 1: Open the Tutor
Open your favorite web browser, such as Internet Explorer
or Netscape. Enter the web address for the Oxidation Numbers Tutor
(provided by your teacher) in the location window. At
any time you may click Instructions
at the upper left of the screen for information about the
operation of the Tutor. The main window will look like this:
Figure 1: Oxidation Numbers Tutor
main window.
There are two ways for the Tutor to help you assign oxidation
numbers. You can either enter your own formula into the top
box labeled Enter a Formula, or you can pick
a formula from the Choose a Formula menu
below it. Either way, the Tutor will provide you with the
same detailed hints and comments. Start by entering a sample
formula (see Step 2).
Step 2: Enter a formula
The Tutor allows us to enter the formula unformatted, or
as plain text. In the box labeled Enter a Formula,
enter the following formula as shown:
MgS04
Figure 2: Entering
a formula.
The formula you have just entered is the formula for a common
compound available at many supermarkets and most drugstores.
The common name for this chemical is Epsom salts. Notice that
you don’t need to type in subscripted numbers because
the Tutor will properly display them for you.
Step 3: Take the first step
When you have typed the formula, click on the green OK
button to begin. You will see a window that looks like this:
Figure 3: The Tutor
is ready to begin work on your formula.
In this first problem, we will ask the Tutor to assign the
oxidation numbers step by step and explain how it’s
done.
Each time you click the green Next Step button,
the Tutor will take another step towards a complete solution.
Now, take the first step by clicking on the Next Step
button. The result will look like this:
Figure 4: The Tutor
takes the first step in assigning oxidation numbers.
All the work done by the Tutor will follow a set of oxidation
number rules as provided in Table 1 below. These rules can
be accessed at any time by clicking the Instructions
button located at the upper left of the page.
Table 1: Oxidation
Number Rules
First Priority |
|
Free Element Rule |
The oxidation number of an atom of a free element
equals zero. |
Simple Ion Rule |
The oxidation number of a monatomic ion equals the
charge on the ion. |
Fluorine Rule |
The oxidation number of fluorine in compounds equals
-1. |
Hydrogen Rule |
The oxidation number of hydrogen in combination with
nonmetals equals +1. |
Group 1 Metal Rule |
The oxidation number of Group 1 metals in compounds
equals +1. |
Group 2 Metal Rule |
The oxidation number of Group 2 metals in compounds
equals +2. |
Sum Rule |
The algebraic sum of the oxidation numbers of all
the atoms in a chemical formula equals the net charge
on the species. |
Separate Ions Rule |
In ionic compounds, the oxidation numbers in the
cation and the anion are independent and can be assigned
separately. |
Second Priority |
|
Oxygen Rule |
The oxidation number of oxygen in compounds equals
-2. |
Nonmetal Rule |
In binary combinations of nonmetals, the more electronegative
element is given a negative oxidation number, equal
to the charge on its common monatomic ion. |
Notice these rules are divided into two groups, first priority
and second priority. It is very important to remember that
a second priority rule is only used when there are no applicable
first priority rules.
Whenever the Tutor takes a step it always provides an explanation.
Carefully following these explanations is the best possible
way to learn the rules and become familiar with the logic
and concepts involved in assigning oxidation numbers.
Is the order of assigning elements optional? Was there another
element that could have been assigned first?
Step 4: Ask for a hint
The Tutor will provide you with useful hints in addition
to explanations of its work. These hints relate to the application
of the rules and the nature of ionic compounds. Click the
green Hint button and you will see a window
that looks like this:
Figure 5: The Tutor
provides a hint.
The particular hint you get will vary, but all hints remind
you of something you can use to determine the next best step.
Not only will this help you determine the next step, but it
will also help you to realize that assigning oxidation numbers
is never random or haphazard. Instead, the procedure is a
logical and reasonable process based upon a particular set
of rules. It is very important for you to realize that the
concepts and procedures you master here will be of enormous
benefit to you as you continue your study of chemistry.
Click the green Hint button again and you
will see a more detailed hint. However, if you select Hint
a third time the computer will take the next step for you
and encourage you to try to proceed with the rest of the problem
on your own. To see how this works, click Hint a third time
and allow the Tutor to take the next step.
Figure 6: The Tutor
takes the next step.
Step 5: Splitting an ionic compound
The step the Tutor has taken is to split the cation and anion
so they can be solved independently. This is a necessary step
but it is easy to overlook since it is frequently done as
a mental step. Since the Tutor will not allow you to skip
steps, it will insist that you complete this step, especially
when a polyatomic ion is involved. It is not difficult to
do because it involves exactly the same information used to
name and write the formula for the compound. To name this
compound using the rules for nomenclature of ionic compounds
you must recognize that it is composed of the magnesium ion
and the sulfate ion. To write the formula, you must recognize
the magnesium ion carries a charge of +2 and the sulfate ion
has a charge of -2. You simply instruct the Tutor to
separate these two ions by entering:
Split Mg(+2) SO4(-2)
These two ions are now of equal importance and, since magnesium
was previously assigned, the Tutor proceeds to assign the
elements in the sulfate ion.
When you are working with an ion rather than a neutral compound,
what special factors must you consider?
In the sulfate ion, which element do you think the Tutor will
assign first?
Continue to click Next Step until the problem
is finished.
Figure 7: The Tutor
confirms the correct answer.
Now that you are finished, scroll down to the bottom of the
screen. There, you will see a record of every step taken and
every hint given by the Tutor. This record is a “transcript”
of the work you have done so far.
Figure 8: The Tutor
always displays the transcript.
It is always there if you need to review when you use the
Tutor. Because of the valuable information contained in this
transcript, you will find that reviewing it will be very worthwhile.
In addition to the correct answers and how to arrive at them,
the Tutor also offers excellent suggestions on useful ways
to think about the concepts.
Step 6: Start a New Problem
From the lower left of this screen, click on Please
enter another formula. This will bring you back to
the Oxidation Numbers Tutor main window. You could also click
on the New Problem
button, located in the upper left corner of the page.
This time, select a formula from the Tutor’s Choose
a Formula menu. To do this, click (highlight) the
selected formula and then click the green OK
button to the immediate right. Choose any formula you wish,
but one of the more complex examples near the bottom of the
list will make a more interesting test of the Tutor’s
capabilities. The Tutor makes no distinction between the formula
you entered before and this formula selected from the menu.
The Tutor will consistently analyze and help you to solve
both using the same techniques.
Figure 9: The Choose
a Formula option.
Rather than asking the Tutor to take a step for you, enter
your own best answers by typing your entries into the Enter
a step box and clicking the OK button.
Feel free to experiment with the features of the Tutor so
you will feel completely comfortable with it. Try any series
of entries including some answers you know are incorrect,
some you are uncertain about, and some you feel are correct
and examine the Tutor’s response in each case. Be sure
to try some answers you know are unreasonable, such as assigning
a value of +5 to a metal or -6 to a nonmetal.
Entering intentional errors like this is one of the very
best ways to gain knowledge of the Tutor and the concepts
governing the assignment of oxidation numbers. The Tutor will
always respond to your work, regardless of the accuracy, and
guide you toward productive ways of thinking about the problem.
Also, feel free to ask for a Hint at any
point in the problem.
What element did the Tutor assign first?
Was there a specific first priority rule applicable to this
element?
How does the Tutor respond when you intentionally assign the
wrong element first?
What element was assigned last?
Was there any rule, either first or second priority, applicable
to this element?
Was it necessary to “split” the ions in order
to assign oxidation numbers to this formula?
Explain why the “split” step was or was not necessary
in this problem.
How did the Tutor respond when you intentionally entered an
unreasonable answer?
Was this response something you should have been aware of
already if you had considered it carefully?
The two problems you have just completed illustrate many of
the major features of the Quantum Tutors. The program has
tutored you through the entire process by encouraging you
to bring together related background information and concepts
to combine with the rules. In addition to learning the rules
for assigning oxidation numbers, you have reviewed ionic charges,
chemical symbols, polyatomic ions, formula writing, and how
both ionic charges and oxidation numbers are related to the
periodic table.
Your challenge is to continue to bring this knowledge together
for each new example. With these skills and concepts now clearly
associated in your thinking you are much better equipped to
handle more and more complex formulas. As you move on to other
problems, begin to predict what element should be assigned
next and what rule or concept the Tutor will cite as an explanation.
When you reach the point where your predictions and explanations
begin to match those of the Tutor, then you have mastered
this concept and you are ready to apply oxidation numbers
to a variety of interesting topics you will encounter as you
continue your study of chemistry.
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