Thursday, January 18, 2007

Boiling water is hard

Hydrogen bonds are strong. It takes a lot of energy to break them. Especially when trying to boil 11 L of wort (Wort, BTW, is beer before it gets fermented). Thanks to the high heat capacity of water, getting to the boiling point takes a lot of hydrocarbon fuel.

Tonight I brewed a Brown Ale, and while waiting nearly 30 minutes for the brew kettle to get it's boil on, I had some time to consider the thermochemistry (I linked that page because it has some good math in it and it hasn't been updated in 8 years!!! That is my pet peeve... the lack of updates, not the math.).

When my wife and I bought the stove, we went with for the JGB900SEF GE stove complete with "Power Boil!!!" This baby kicks out 15,800 kJ per hour (15,000 BTU/hr). I was under the impression that this thing should be able to boil 11 L (3 gal) of water in about 3 minutes. Not so (and how silly of me).

Assuming the water starts at 19°C and boils at 104°C (> 100°C due to the boiling point elevation colligative property) the amount of heat needed to raise the temperature by 85° C is: 11,000g x 4.184 J/g °C x 85°C x .001 kJ/J = 3910 kJ.

If the stove kicks out 15,800 kJ/hr, assuming 100% efficiency, it should take about 15 minutes. Obviously, some heat is lost to the surroundings. What I didn't do tonight was measure the amount of time needed to boil the solution. If I do measure that, I will be able to determine how efficient my burner really is.

I don't know how I can survive without knowing this piece of information. Next time I will keep track of the time.

Once the reaction reached a boil. It was kept at a boil for 1 hour. During this one hour 15,800 kJ of heat was produced from the combustion of methane. Methane has a heat of combustion of 900 kJ/mol. During just the boil, I used 15,800 kJ / 900 kJ/mol = 17.5 moles of CH4. That means at 1 atm and 20°C (293 K), I used 420 L of methane gas. That's a lot.

4 comments:

Matt Jenks said...

My supervisor had not heard of the term "colligative" before yesterday afternoon, when I was trying to figure out if the particulate matter hanging in the air over the greater Raleigh-Durham area would cause the rain to not freeze, despite it being 30 degrees F. He was impressed with my ability to spin out an "obscure" chemistry term.

420 L of methane? I crank that out after a bowl of chili. Badda-bing.

Russ said...

I used 420 L of methane gas. That's a lot.

Well sure it seems like a lot when you put it that way.

On the other hand, you could say that it only took 280 g of methane to heat over 11000g of water (and hops and whatever) from 19 to 104 degrees...

Chemgeek said...

Russ.. Good point. It is all relative.

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