Monday, December 5, 2011

Heat of Dilution of Sulfuric Acid

Measuring heat of solution.

My thoughtful students are contemplating their data.
Calorimetry is not exactly a flashy subject.  Heat exchange into and out of water does not make for a showy demo.  However, today I did a demo for my honors class of the heat of dilution of concentrated sulfuric acid, Demo 1.6 in volume 1.  To my surprise, this demo offered an element of excitement that I was not expecting (why am I surprised that the demos from Shakhashiri are exciting?).  I decided to run three trials at the same time, dissolving 10, 20, and 30 mL of concentrated sulfuric acid in 100mL of water.  I used one of my Lab Quests from Vernier with 3 temperature probes so we could watch the temperature readings graphically projected on the big screen.  I asked for three volunteers; the hands went up immediately.  Three boys came up to pour in the acid and help monitor the reactions.  I counted to three and then they all poured at the same time.  The exciting part was the 30 mL sample, which produced enough heat to boil the water and overflow out of the cup all over the bench.  It was a surprise to us all to see this happen.  The spike in the temperature readings told the whole story of the heat produced from diluting sulfuric acid in water.  (Do as you "oughter", add acid to water.)  We followed up with the calculation of the heat produced in each cup, which led to the conclusion that this is a linear relationship.  The demo ended with me pouring baking soda onto the spilled acid, producing the classic “volcano” reaction on the bench.  Welcome to calorimetry, kids.

"What, no flames today?"

Taking his temperature with the Vernier probe.

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