Automobiles that use hydrogen gas for fuel are currently being developed. They e
ID: 557678 • Letter: A
Question
Automobiles that use hydrogen gas for fuel are currently being developed. They emit mainly water vapor as exhaust and are less harmful to the environment than gasoline powered cars. A chemist combusts a certain amount of hydrogen gas in a constant pressure calorimeter filled with 1.50 L of water. (The PM will explain in a couple of minutes the fundamentals of how a calorimeter works). The temperature of the water is raised by 3.70 °C. Assume the reaction goes to completion & the water in the calorimeter absorbs all of the heat. Also, 0.1 mol of water vapor is produced in the chemical reaction.
a) How much heat is given off in this reaction?
b) From your answer to ‘a’, determine the Hf for H2O(g).
Explanation / Answer
a)
volume of water = 1.50 L = 1500 mL
density = 1.00 g / mL
mass = density x volume
= 1 x 1500
= 1500 g
a) Q = m Cp dT
Q = 1500 x 4.184 x 3.7
Q = 23221 J
Q = 23.22 kJ
heat is given off = 23.22 kJ
b)
Hf = - Q / n
= -23.22 / 0.1
= -233.3 kJ / mol
Hf = -233.3 kJ / mol
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