2. There is nothing like the smell of BBQ in the summertime. Many people use pro

Question

2. There is nothing like the smell of BBQ in the summertime. Many people use propane gas Barbeques to cook the perfect steak. Here is the reaction that is occurring while the steak sizzles on the grill:

C3­H8(g)   + O2(g) --> CO2(g) +   H2O (g)

The balanced reaction has a DHorxn = -2217 kJ. If a steak must absorb 7.2x10-4 kJ to reach a perfect medium rare, and if only 15% of the heat produced by the barbeque is actually absorbed by the steak, what mass of H2O is emitted into the atmosphere when this steak is cooked? (Hint: don’t forget to balance the reaction)

3. A student performed this lab with a marshmallow. He used 50.0 g of water and recorded a temperature of 21°C before heating and 30°C after heating. The marshmallow and fork weighed 21.5 g before heating and 21.1 g after heating. What was the total energy (in J) of the marshmallow? What was the energy per gram of the marshmallow?

Explanation / Answer

1. delta Hof = -2217 kJ

15% of delta Hof absorbed = -2217 x 15/100 = -332.55 kJ

heat released per mole of propane = -332.55/44.1 = -7.54 kJ/mol

mole ratio of propane : water = 1 : 1

mass of H2O formed = 7.54 x 18.01 = 135.80 g

2. q = mCpdT

       = 50 x 4.184 x 10

       = 2092 J

Thus, energy per gram of marshmallow = 2092/(21.5-21.1) = 5230 J/g

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