Hi everyone, welcome to this video on the Periodic Properties Lab!
In this lab, our goal is observe two periodic trends of elements and then use these trends
to determine the identity of an unknown.
Let’s discuss the relevant concept.
Recall that when we look at elements in a group or period in the Periodic Table, they
show a trend for specific properties.
For example, for the atomic radius property, the trend is that it increases as we go down
the elements in a group and it decreases along the elements in a period from left to right.
These existence of these trends can be explained by the interplay of factors that determine
the stability of the valence electrons in the atoms of these elements.
In this lab today, we will look at two additional properties that we did not discuss in lecture.
These properties are the solubility of salts formed by cations in group IIA and the oxidizing
strengths of elements in group 7A or halogens.
What you want to find out is whether these properties follow a specific trend, for example
does solubility increase or decrease as we go down a group?
Once you determine the trend of these properties, you will use this discovery to help you identify
an unknown that is composed of group 2A and 7A elements.
There are 3 parts to the lab so let’s look at them one at a time.
Part 1 is on looking at the solubility of ionic compounds containing group 2A cations.
The cations are going to be Mg, Ca, Ba, and Sr ions.
The general procedure goes as follows: you will add 1 mL of a soluble salt containing
your group 2A cation, for example barium nitrate to a test tube containing 1 mL of a soluble
salt containing an anion, for example sodium carbonate.
You will mix these solutions very well, either by swirling the tube or using your stirring
rod.
Once you let the mixture settle, observe to see if any precipitate forms.
Record your observation in the lab report shown here.
If you don’t see any precipitate, write no precipitate or use some type of symbol.
If you see a precipitate, indicate how much precipitate is formed and the texture of the
precipitate, for example is it finely ground like powder or is it large chunky pieces.
Once you collect all your data, you will need to analyze the pattern to determine the solubility
trend of the salts containing the cations.
Let’s look quickly at the following sample data.
So you’ve mixed three cations with three anions and written down your observations.
The symbols indicate the amount of precipitate formed.
The more pluses the more precipitate.
The minus represents no precipitate.
Given this data, you can see that cation C is the most likely to precipitate, so least
soluble, followed by A and B, so that’s how we will rank the solubility of the cations.
The second property you will look at is the oxidizing strength of group 7A elements.
The first thing to know is the difference in the names used:
A halogen is the actual elemental form of group 7A, which is a diatomic molecule.
So it’s either Cl2, Br2, I2.
A halide on the other hand, the anion form of group 7A elements, so it’s Cl-, Br- and
I-.
When we mix a halogen and a halide, there’s a single displacement reaction that can take
place Let’s illustrate this.
For example if I mix halogen A2 with the halide B-, there are two possible outcomes.
One is that a reaction takes place.
The products will be A- and B2.
You can see that this is a redox reaction and A2 is reduced to A- while B- is oxidized
to B2.
That means A2 acts as an oxidizing agent.
If we compare the strengths of A2 and B2 as oxidizing agents, this result tells us that
A2 is a stronger oxidizing agent.
Now, a second outcome is that when we mix A2 and B-, we get no reaction.
This means A2 cannot oxidize B-, so that means A2 is a weaker oxidizing agent than B2.
So your goal is to determine the trend in the oxidizing strengths of three halogens:
Cl2, Br2 and I2.
To help us figure out which halogen is present, we need to look at its color.
So the first thing you will do is determine the color of each halogen.
You do this by adding 1 mL of a halogen that’s been premixed with water to 1 mL of hexane.
Hexane is a non polar solvent so it doesn’t mix with water which is polar.
Since the halogen is non polar, when we mix the two solutions, the halogen will move and
mix with the hexane instead of staying with water.
To make sure this movement occur, you have to mix your solutions vigorously.
Again, either swirl your test tube or use a stirring rod to mix the content well.
Then observe the color of the hexane layer, which is located on the top because hexane
is less dense than water.
That is the color of the halogen that you put in.
For example, in the picture shown, you see the top layer is orange, so that’s the color
you will record.
Record those halogen colors at the top of the lab report page for part 2.
See here where I highlighted the lab report in yellow, that’s where those colors have
to go.
Keep in mind that the halides, which are the ions, are all colorless.
Once you know the colors of the halogens, you’re ready to carry out your reactions.
If you look at the lab report format, each square corresponds to a specific combination
of halide and halogen.
So for example, start by adding 1 mL of NaCl (which means you’re adding the Cl- halide)
to 1 mL of Br2 (which is your halogen).
Also add 1 mL of hexane.
Mix these three components well.
Sufficient mixing is important, otherwise you won’t see the correct result.
After you’re sure the contents are mixed well, first record the color of the hexane
layer in the space marked “color”.
Based the halogen colors you identify in the previous step, determine which halogen is
present in your hexane layer and write it on the space marked “halogen”.
Lastly, write the correct reaction that occurs as a result of this mixing and based on the
observations you made.
Again, there are two possibilities, either you have a reaction or you have no reaction.
Either way, you have to write a chemical equation.
Repeat the step you just did for all the different combinations of halogen and halide in your
lab report and use the result to rank the oxidizing strength of
the halogens.
Here’s an example on how to analyze your data.
Let’s say you have two halogens, E2 which is green and F2 which is blue.
The halide versions, E- and F-, are both colorless.
Let’s say you carry out a reaction between NaE, which is a sodium salt of E- and F2 and
you see that the hexane layer is blue.
What’s your conclusion about the relative oxidizing strengths of E2 and F2?
So let’s interpret the results.
If the final color is blue, that means what we see in the hexane layer is F2.
If that’s the case, the reaction that we write looks like this: E- plus F2 giving us
no reaction, because we still see F2 at the end.
This result means that F2 is a weaker oxidizing agent than E2.
You will use the same approach to analyze the rest of the data and then rank the relative
oxidizing strengths of all the halogens.
Once you complete parts 1 and 2, you now need to use the results to help you in part 3.
In part 3, you’ll be given an unknown solution, which is an ionic compound that has been dissolved
in water.
The ionic compound is made up of a group IIA cation, so one of the following: Mg, Ba, Sr,
or Ca cations and a group 7a anions or a halide, which is one of these: chloride, bromide or
iodide.
So for example the formula of the unknown could any of these combinations: MgCl2, BaI2,
CaBr2, CaCl2, or others.
Your goal here is to come up with a procedure that will allow you to determine the identity
of the cation and anion in the unknown.
After that you will go ahead and carry out your strategy to determine what you have.
Here’s an example of a similar idea.
Let’s say you know the following about these three chloride salts: aluminum chloride is
soluble in water at any temperature, lead chloride is only soluble in hot water and
silver chloride is insoluble in water at any temperature.
You’re asked to come up with a procedure to identify the cation, either aluminum, lead,
or silver in an unknown solution of a nitrate salt.
So let’s start by realizing that the ion is currently soluble because it is a nitrate
salt.
So we need to cause it to precipitate.
Since we know that chloride can precipitate the ions, we will first mix 1 mL of the unknown
solution with 1 mL of HCl.
At this point, if we don’t see a precipitate, we know we have aluminum.
But let’s say we see a precipitate, that means we can have either lead or silver ion.
So we will next put the test tube with the precipitate in a hot water bath.
At this point, if the precipitate disappears then we know we have lead.
However if the precipitate remains, then we have silver.
You can also create your procedure in the form of a flow chart as shown here.
Ok, now I want you to work on this assignment, which you must turn in at the beginning of
our lab meeting.
Here are two tables similar to the result you will obtain in your experiment.
The first table represents the solubility of group 2A cations when paired with different
anions.
The second table shows the result of reactions between the halogen E2 and F2 with the halides
E- and F-, with the colors of the halogen given here.
You’re given an unknown which contains one of the cations and one of the halides.
So the possible formulas are given here.
Your assignment is to come up with a detailed procedure that will allow you to determine
the identity of your unknown.
Note that the procedure must have detailed steps.
You shouldn’t just write “I will repeat all the steps I did in part 1 and 2 and figure
out my ions”.
If you do that, you will get zero for the assignment.
Here’s an example of what your procedure should look like, one for cation and the other
one for anion.
You can see how each step is clear and leads to specific cation or anion
You can also come up with a flowchart as I did in the previous example.
This assignment will be a significant part of your lab report grade so take it seriously.
Let’s wrap up by discussing the safety and waste disposal procedure.
You will use your standard PPE and gloves for this experiment.
The halogens are toxic, so you have to be extra careful not to let it touch your skin.
Use the droppers provided in the reagent bottle to add the halogen water into your test tube.
All the waste today must be discarded in the hazardous waste container.
All the gloves you use today must be discarded in the glove waste container.
This ends the video on the Periodic Properties lab.
I’ll see you in lab.