endothermic

Honors Chemistry 24 – Heat of Solution

This lab is pretty much a repeat of the lab last week but students had two different chemicals, potassium chloride (KCl) and calcium chloride (CaCl2), to dissolve in water while they observed the temperature change. Using the same equation as last time, Δ H = mc Δ T, where m is the mass of the water in the calorimeter (styrofoam cup), c is the specific heat capacity of water (4.184J/g°C)and ΔT is the temperature change, they calculated the heat of solution. The lab, IX-1: Determine Heat of Solution, in the homescientist chemistry manual used plain NaCl but a lab I did with the middle school class showed pretty big temperature changes with KCl and CaCl2 so I had the students use those instead and had them figure out how much solute and solvent (water) they should use for their experiment. There’s no real right answer for this, as long as the quantities they decide on are reasonable. The final number they’re trying to calculate is the molar heat of solution so if they use a tenth of a mole of the solute, they just have to move the decimal over one place at the end, which is what most of them decided to do.

For KCl, students used 7.5 grams of NoSalt, which is mostly KCl but not 100%, so the values we determined for the heat of solution are going to a bit different than accepted values for pure KCl. Same goes for the calcium chloride, it was not a pure chemical, but DampRid purchased at Lowes. The amount of water used varied from 50 ml to 200 ml, but all groups saw very noticeable temperature changes and got decent values for the molar heat of solution. I had each group do both chemicals because one is exothermic and one is endothermic.

February 21, 2017 1

Honors Chemistry 20 – Solubility

Unlike the last couple of labs, the Solubility Lab found in Ian Guch’s 24 Lessons that Rocked the World, was a breeze and we actually finished early enough to do a bit of the middle school lab and go over homework!

I love this little pocket scale. Its great for massing small quantities of chemicals.

The solubility lab tests whether water or isopropyl alcohol will dissolve more potassium chloride (KCl). I bought NoSalt at the local grocery store for my source of potassium chloride. It has a few other ingredients but it didn’t affect the experiment. Students write down which they think will dissolve more KCl before we start, and most agreed that water would be the better solvent. They measure 2 grams of the KCl into a small 50ml beaker and then added 10 ml of distilled water. Stir for a minute then write down their observations. They then pour off the liquid, keeping any undissolved KCl in the bottom of the beaker. Beakers were then heated on a hot plate or butane burner (with very low flame) to drive off the remaining water. Once the beaker were cooled, the beaker and remaining residue were massed. Beakers were then cleaned, dried, heated briefly and massed again. The difference in the masses is the mass of the undissolved KCl. Then they repeated the experiment using 10 ml of isopropyl alcohol (91%) and found they had a lot more KCl left over in their beaker.
When I did the lab I found only about 3% of the KCl had dissolved in the alcohol, while over 90% dissolved in the water.

We finished this lab in about an hour so I had the students do the same lab the middle school did yesterday, dissolving chemicals in water and watching the temperature as it dissolves to see if its exothermic (gives off heat) or endothermic (absorbs heat). I had each group just do one chemical, either potassium chloride or calcium chloride (DampRid, purchased at Lowes). The temperature change is pretty drastic for both of those and after the frustrations of the last couple of labs it was nice to do some short and simple labs.

January 24, 2017 1

Today’s lab went great, it was fairly simple but had pretty impressive results and the students did a great job with it. The lab, Temperature Changes in Dissolving, is from Chapter 5, Lesson 9 of the American Chemical Society’s free middle school chemistry curriculum. Before we started the lab I wrote ‘exothermic’ and ‘endothermic’ on the white board and asked if anybody knew what the words meant. Nobody answered, so I asked if they could look for clues in the words and immediately there were shouts of ‘temperature’ for thermic and ‘outside’ for exo. One students said bugs have exoskeletons which means its on the outside and we have endoskeletons – skeleton is on the inside. So I explained how exothermic means something is giving off or releasing heat and would be warm to the touch while endothermic means heat is being absorbed. I then did a few demos, including ‘hot ice’, which is super saturated sodium acetate. There are many recipes for this on the internet/youtube, basically you have a small amount of water and dissolve a huge amount of sodium acetate into the water at high temperature. Then you let it cool. I poured the super saturated solution on to a petri dish containing a few crystals of solid sodium acetate so that the solution crystalized immediately upon contact. You can see the broken stalagmite in the photo below and the glass jar behind it where the solution crystalized before I could finish pouring it. The reason I did this demo today is that when the sodium acetate turns back into a solid it becomes very hot, hence the term ‘hot ice’ for this demo. Its actually the same stuff you find in hand warmers and instant hot packs. The heart shaped HotSnapZ in the photo below is an example of an instant hot pack. I also had an instant cold pack on hand and you’re supposed to activate it by squeezing and popping the internal bag of water inside the cold pack but we just cut it open so we could see what was going on and dumped it in a large beaker. It became very cold when the pellets (probably ammonium nitrate) dissolved in the water.

The lab itself involved dissolving 2 grams of different chemicals (potassium chloride,

calcium chloride, sodium bicarbonate and sodium carbonate) in 10 ml of water and recording the initial temperature and the maximum or minimum temperature reached as each of the chemical dissolved. Two of the solutes were exothermic and two were endothermic and the changes in temperature were very noticeable, one increased by almost 20 degrees Celsius! So overall a very nice lab.