Question

C3H8(g)   + O2(g) --> CO2(g) +   H2O (g) 
 The balanced reaction has a ΔHorxn = -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).


Answer 1

C3H8(g)   + 5O2(g) --> 3CO2(g) +   4H2O (g)
The balanced reaction has a ΔHorxn = -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).

Answer:
Required heat by steak, qsteak = 7.2E-4 J
Heat produced by barbeque = q
15% of q = 7.2E-4J
0.15*q = 7.2E-4J
q = 7.2E-4/0.15J = 48E-4 J = 48E-7 kJ

This heat q = 48E-7 kJ will be supplied by the combustion reaction of n moles of propane C3H8
q = -n*ΔHorxn = n*2217 kJ
n = q/2217 = 48E-7/2217 = 2.165E-9 moles of C3H8

C3H8(g)   + 5O2(g) --> 3CO2(g) +   4H2O (g)
1 mole of C3H8 produces 4 moles of H2O
2.165E-9 moles of C3H8 will produce 4*2.165E-9 = 8.66E-9 moles of H2O
Molar mass of H2O, MW = 18 g/mol
Mass of H2O produced, w = n*MW = 8.66E-9*18 = 0.156E-6 g = 0.156 microgram