A student performs a combustion experiment by burning a mixture of fuel and oxyg
ID: 2260484 • Letter: A
Question
A student performs a combustion experiment by burning a mixture of fuel and oxygen in a constant-volume metal can surrounded by a water bath. During the experiment, the temperature of the water is observed to rise. Regard the mixture of fuel and oxygen as the system. Steps (a-c) help you think about the problem before calculating the answer in (d).
(a) Has heat been transferred to/from the system? How can you tell?
(b) Has work been done by/on the system? How can you tell?
(c) What is the sign of ?U of the system? How can you tell?
(d) 1g of fuel is placed in the can for combustion. During combustion, the 200mL water bath is observed to increase in temperature by 35.5C. What is the heat of combustion (energy released per gram) of the fuel?
Explanation / Answer
(a) from the system
This is because the outside (water) has received that heat and become hotter
(b) neither
since the volume is constant, work = P(V2-V1) = 0
(c) positive
the gas becomes hotter, its energy in., its temp. inc.
(d) heat of combustion = heat transferred to water = Mwater x cp x deltaT
= 0.2 x 1kCal x 35.5 = 7.1kCal / g
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