Wednesday, April 16, 2008

Endothermic Reactions



As a continuation of our foray into heat-producing (exothermic) chemistry, we mixed up some solutions that became colder (endothermic). Endothermic reactions involve electrons jumping to higher orbitals, which requires an input of energy. The atoms absorb energy in the form of heat from the surrounding environment, thereby lowering the temperature. Unfortunately notetaking that day was not optimal, but here is an idea of what we did:

Since we didn't have the recommended styrofoam cups for mixing our solutions -- which I assume were supposed to provide some insulation between the solution and the air temperature around it -- we used doubled-up paper coffee cups (just like my favorite coffee shop). We used a meat thermometer I found around the house (purchased for a greenhouse gas experiment I never got around to doing) and a 99 cent house thermometer I picked up at Wal-Mart. All the experiments dropped a few degrees almost immediately, going from a water temperature of about 60 degrees Fahrenheit (sorry, we're working in an American kitchen, not a lab with metric measurements) to about 55 or 50 in a minute or so. You could just barely feel the difference by putting your finger in it (we totally forgot gloves and eye protection for this one), so the thermometer is a must.


First was potassium chloride, found in salt substitute. We mixed in an unmeasured proportion with tap water.


Next, we cut open a cold pack from an old first-aid kit. The cold-pack consisted of two compartments, one containing urea (or crystalized peepee, used in cigarettes, pretzels, bath oils, cloud seeding, and tooth whitening -- although I think they make it artificially!) and the other water. You're supposed to squeeze the water portion, which I guess forces it into the other portion. We just poured the crystals into a cup and added water.


The third mixture was baking soda and citric acid. We only had a small jar (scavenged from some old science kit, I believe) so we put about half a teaspoon in the cup and mixed with a little water. Then we poured in some baking soda. It fizzed up nicely, of course, as it would with vinegar. According to about.com, the reaction was:
H3C6H5O7(aq) + 3 NaHCO3(s) --> 3 CO2(g) + 3 H2O(l) + NaC6H5O7(aq)


Finally, we mixed some calcium chloride -- the kind of road salt used to melt icy sidewalks -- with water. Surprise! This one turned out to be exothermic. The temperature went up to 78 degrees. Pretty neat.

8 comments:

Lorna said...

These are great experiments. We recently did a citric acid experiment in our mouths. We used 2 teaspoons of citric acid to one of baking soda and added 6 teaspoons of sugar (for taste). It fizzed like sherbet. I didn't appear to be endothermic (although we were distracted by the fizz and foam!).
Thank you for sharing your great experiments. It is so wonderful to see things actually tried in real life!

Anonymous said...

I'm a chemist, and I just want to point out a mistake in your introduction. Endothermic reactions typically do not involve electrons jumping to higher orbitals. The energy required to move electrons to higher orbitals is much greater than the energy available at normal temperatures. Endothermic reactions absorb energy to break chemical bonds in the reaction. Otherwise, your description is OK.

Anonymous said...

One quick suggestion: You don't need to use purified citric acid in your experiment. I use just regular concentrated lemon juice and the reaction is quite endothermic (loss of about 4 degrees for about 100 ml of juice), and the ingredients are a bit easier to get.

Anonymous said...

The change that happens when a salt like calcium chloride is mixed with water is not a reaction. The mixture made is a solution. The process is a physical change. The process is reversible.
All physical changes are reversible.

Kathy Ceceri said...

I'm getting this information from Anne Helmenstine, Ph.D.: http://chemistry.about.com/od/lecturenotesl3/a/endorxns.htm

Where are you getting yours?

Saifer Phoenix said...

yes. the salt and water combination is a physical change, but calcium chloride has an enthalpy of dissolution of about -81.3kJ/mol at room temp, which means that when you dissolve it in a solvent it releases 81.3kiloJoules of energy per mole of CaCl dissolved in the solvent at room temperature (when it's very dilute...)

Trevor Walkington said...

Anonymous' comment on reversible reactions is not correct. It repeats a common myth - often found in high school textbooks unfortunately. Some but not all physical reactions are reversible (Try reversing an atomic explosion!)but plenty of chemical reactions are reversible. The distinction between physical and chemical reactions is arbitrary and does not really give great scientific insight anyway but it is more usefully based on the re-arranging breaking and creation of interatomic bonds. Thus it is perfectly reasonable to argue that dissolving of ionic compounds is indedd chemical.The other old chestnut relates to the amount of energy involved which usually argues that chemical changes involve more heat but again arbitrary and ultimately wrong - again think of an atomic bomb or the sun!

Anonymous said...

Citric acid is also available at Wal-Mart for about 3 bucks a bottle. They keep it in housewares with the mason jars, rather than with the grocery items.