Question

1. 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 DH^o_rxn = -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)

2. 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?

Answer 1

The solution is as shown below.

1. convert enthalpy of reaction in kJ to kJ/mol

molar mass of propane = 44.1 g/mol

enthalpy delta Horxn = -2217/44.1 = -50.27 kJ/mol

Now, 1 mole of propane gives 1 mole of H2O

actual heat absorbed = 15/100 x -50.27 = -7.5 kJ/mol

thus, mass of water = 7.5 x 18.01 = 135.08 g

2. energy per gram of marshmallow = m x Cp x delta T

                                                         = 50 x 4.18 x 10 / 0.4

                                                         = 5230 J/g