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Honors Chemistry 29 – Electrochemistry

The labs we did today came from the Home Scientist CKO1A instruction manual which goes along with their chemistry kit.  In Session X-1: Observe Electrolysis there are two labs, in the first one you set up two test tubes full of water (with some epsom salts) upside down in a beaker and then place the wire ends of the battery adapter into the tubes.  Once you connect a battery, current starts to flow through the water breaking it down to create hydrogen and oxygen gases in the test tubes.  We tried to set it up like the lab describes but the battery adapters I bought had short leads and we got pretty frustrated with it.  Luckily I happened to have two electrolysis set ups (see photo below)  that were loaned to us by another homeschool mom.  IMG_9692

The black stand in the bottom of the beaker holds the test tubes in place and the screws are the electrodes, so all we had to do was connect the wires to the battery (after filling the tubes with water).  The other advantage of using this set up is that the test tubes were very small so it didn’t take as long to fill up with gas (though it still took almost an hour).    We skipped measuring the volume of the gas but did notice that one test tube filled up twice as fast as the other because you make 2 hydrogen molecules for every oxygen molecule produced.  The blue arrows in the photo above point to the water level in the test tubes.

While this was bubbling away, I set up the second part of the lab which involved doing electrolysis with salt water.  We took 50 ml of water and a tsp of salt, stirred until it was dissolved and then made electrodes out of Al foil.  We took this lab outside since it produces chlorine gas.  We also put a few drops of phenolphthalein pH indicator in the salt water.  Phenolphthalein is clear in neutral solutions but turns a bright pink in basic solutions.  I hadn’t done this before so I was as suprised as the students when I hooked up the battery and the solution turned pink starting at one electrode and making its way away across the beaker.  The bubbling gas production was also quite vigorous.  We weren’t prepared for how quickly this took place so I had the students repeat it themselves and we took lots of movies and pictures.  If you leave it hooked up for more than a few seconds the Al foil starts to break apart.   When the students did it, the reaction didn’t seem quite as vigorous and that’s because I used a regular teaspoon to put in the salt and they used the chemical spatula which gave them less salt.  Here’s a video of the electrolysis of salt water.

The solution turns pink because sodium hydroxide (a base) is formed along with hydrogen gas at the foil electrode (cathode)  where you first see the solution turn pink.  Chlorine gas is formed at the anode but most of it dissolves into the water.  Very cool little experiment.

I had the students watch Tyler Dewitt’s video on Electrochemistry before class.

He also has this great video which explains exactly what was happening in both labs we did.

 

Intro Chem 26 – Acid Concentration, Color & pH

IMG_9673This is experiment is from Chapter 6, Lesson 8: pH and Color Change, in the American Chemical Society’s Middleschool Chemistry curriculum.  I almost didn’t do this lab because when I tried it the night before class it didn’t really work that well but it worked great for the kids (I think they were being more careful than I was).  The lab calls for universal indicator so I made some red cabbage indicator (you can find the recipe all over the internet) which is blue-purple around neutral, going to red for acids and green for basic solutions.   Students filled 6 wells in a 24 well plate with the indicator and then made a weak citric acid solution – 5 ml of water with 1 toothpick (used as a scoop of sorts) of citric acid (True Lemon).  They then took a pipette and dropped 1 or 2 drops of the citric acid in well 2, recorded the color and pH.  Then they added another toothpick scoop of citric acid to their solution and so on.   IMG_9654

As you can see in the photo above, the wells of indicator get steadily pinker as they increased the concentration of citric acid, indicating a decreasing pH.  The second well of blue-green is created by doing the same experiment but increasing the concentration of sodium carbonate to make an increasingly basic (higher pH) solution.

This lab didn’t take very long so the kids played some more periodic table battleship.

 

Honors Chemistry 28 – Redox Reactions

So I had a terrible time with this chapter (Chapter 19: Oxidation-Reduction Reactions in Modern Chemistry) and spent a lot of time on youtube trying to get a grip on this stuff.  I realized part of my problem was that I didn’t understand the oxidation states well enough so I backed up a bit and these videos by Khan Academy helped me the most.

Tyler DeWitt has a pretty good video as well:

Most of the labs for this subject involve watching iron wool rust over several days (lab X-2: Observe the electrochemical Oxidation of Iron)… not exactly conducive to a two hour class.  So I had my son set up one of them yesterday so the students could just observe the results in class today and then we did lab IV-1: Observe Oxidation States of Manganese, from the home scientist’s chemistry kit manual, CK01A. The first lab was just observing a bit of iron wool in 6 different test tubes with various combinations of water, salt and oxygen available to the wool.IMG_9511

The second lab involved looking at different oxidation states of manganese by putting some in 4 different wells of a reaction plate along with some other chemicals.  This was a nice short lab (only 4 wells!) and you get some significant color changes (see photo below) and even a precipitate in one.IMG_9515IMG_9522

Besides these two labs I also happened to have a chemiluminescence kit which contains luminol solution and hyrdogen peroxide.  Luminol emits light when its oxidized.  I poured approximately 20 ml of luminol into a small beaker and then used a dropper to squirt hydrogen peroxide (Salon Care 40 volume clear developer, 40% hydrogen peroxide, not the weaker stuff that came with the kit) into the beaker – this is done in a very dark room – and the solution glows a very nice blue for a minute.  It probably would have been brighter if my solutions hadn’t been so old. I got this kit a year or two ago and the box says it has a shelf life of a year.
I also spent about 20 minutes lecturing on oxidation states and going over examples from the textbook on how to figure out oxidation states and whether or not a reaction was a redox reaction or not.

 

Intro Chem 25 – Identifying unknowns & Elements 4D

IMG_4495I had two activities planned today so half the kids played with the app Elements 4D while half did the lab from the American Chemical Society’s curriculum.  The Elements 4D app apparently has been around for awhile but I just heard of it last week on a facebook page for science teachers.  Its FREE and its awesome!  You print out 6 different blocks, each block has 6 different elements on it, cut them out and fold them to make blocks – I used tape instead of glue.  After you download the app you use the device’s camera to look at one of the blocks, the app will recognize it and then ‘animate’ the block, showing what the element looks like even as you pick up the block and move it around.  The developer, DAQRI calls it augmented reality and its very cool.  Even better, they have FREE lesson plans you can download and when you put two different elements that react in front of the camera it will tell you the compound it can form.  When you push the blocks together so that they touch, they ‘react’ and the pictures change to show the compound.  You can see in the photo below zinc chloride looks like a powder.IMG_9500

I had the students fill out a table with random elements of their chosing.  They pick a block, put it in front of the camera and record the name of the element, its group number (column of periodic table), its color and state of matter (liquid, gas or solid) at room temperature.  Then they tried to find combinations that would react and had to record element names and group numbers for both.  A couple of the kids had ‘Aha!’ moments when they saw that elements from group 1 (Na, K, Li) all reacted with Cl from group 17 because group 1 elements have an extra electron they would like to get rid of while group 17 ‘wants’ an extra electron to fill their outer most shell.  I may actually have my high school class do this activity as well tomorrow.

The lab students did was Chapter 6, lesson 6: Using Chemical Change to Identify an Unknown from the ACS curriculum.  Students are to react 4 different powders with 4 different liquids and record the results – bubbles, color changes, etc.  The lab says to do it on a piece of wax paper or laminate a grid but I used 24 well reaction plates since I had them.  I already had a template I could print out for the kids but went ahead and labeled the columns and rows for them.well plate unknown

Students put one powder in the well plate at a time, so they put roughly 1/4 spatula of baking soda in the first column, wells A1, B1, C1 and D1.  Then they took a pipette and dropped 5 drops of water in A1, got a different pipette and put 5 drops of vinegar  in B1, and so on with the iodine solution (100 ml of water with a few drops of iodine) and the pH indicator. I did not have any universal indicator but had some bromothymol blue which goes from yellow (acid) to blue (base) which worked fine.  After recording their observations on the chart they filled in the next column with the next powder and tested it with the different liquids.IMG_9489

When they had tested all four powders they were given an unknown powder and tested it the same way and were able to determine what it was by comparing to their previous results.

IMG_9493

These two activities worked well together since neither took up a whole class period and it kept the kitchen counter, I mean lab table, from getting too crowded.

Honors Chemistry 27- Equilibrium

While looking for labs to go with Chapter 18: Equilibrium in Modern Chemistry, I had a hard time finding one for which I had the required ingredients.  The Modern Chemistry curriculum had this lab, Strike a Balance, which has the students come up with their own experiment to determine if your body is getting rid of excess carbon dioxide (produced by exercising) in your breath.  One of the students knew that carbon dioxide added to water would make it slightly acidic. So we could use a pH indicator in water to see when the carbon dioxide concentration goes up or down.  I had a couple of different pH indicators out and let the students decide which one to use.  I also had out a 0.1 M solution of NaOH to make the water slighly basic before they blew into it which would then neutralize it.  This could be useful depending on the indicator they chose to use.IMG_4345

In the photo above you can see a flask of tap water (roughly 100 ml) with a few drops of Bromothymol Blue indicator.  Note the pipette in the beaker with the indicator, this helps to make sure we don’t cross contaminate with the pipettes and protects my counter.  An interesting thing we found is that using distilled or filtered water makes a big difference. One group used filtered water and found the Bromothymol Blue indicator turned the water yellow, showing it was acidic, so they couldn’t get it to change color by blowing since that just makes it more acidic. So tap water works best for this experiment.

Students did different experiments, some timed how long they had to blow into the flask to change the color of the indicator after just sitting around, and then did it again after 5 or 10 minutes of exercise.  Some tried walking, then running, some did kempo (martial arts) forms, and some tried holding their breath.  Most found that after excercising they had to blow for a shorter time before the indicator changed color, indicating a higher concentration of carbon dioxide in their breath. FullSizeRender 7 The one anomaly was a student who is an Irish dancer and  therefore used to a lot of exercise.  The concentration of carbon dioxide in her breath didn’t seem to change after she exercised but she didn’t really get her heart rate up either.  I think we would have had to make her dance for quite awhile to see a change.

I found this other lab write up on the Royal Society of Chemistry‘s website which explained the chemistry in a bit more detail and with specific procedures which helped me as the teacher figure out how to do this and make it work with the indicators that I had.

Before class students were to watch Crash Course Chemistry 28: Equilibrium.  

But while preparing for class I found these videos which are also useful for this topic.

I also found this website , Chemistry LibreTexts, which has a lot of nice examples for calculating equilibrium constants, particularly when dealing with liquids and solids.

 

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