Question 6 A gas expands in volume from 15.0 L to 90.0 L against a constant pres

Question

Question 6 A gas expands in volume from 15.0 L to 90.0 L against a constant pressure of 1.75 atm. At the same time that it expands, the gas absorbs 38.0 kj from the surroundings. What is the change in internal energy, AU, of the gas in k? 175k 2 155 k 46.9 kj 4 24.7 k 5 1.93 k 6 187 K 2) 457 k) 8) 594 k Question -16 0.803 g of SiH4 is reacted with excess oxygen in a bomb calorimeter. The water's temperature changed from 20.00 to 24.12 oC. The heat capacity of the calorimeter is 9.20 kC. What is q rxn? In other words, what is q for the combustion of 0.803 g SiH4? The reaction is: SIH4(g) + 2 02 Si02(s) + 2 H20(0). ?-62.9k] 2 -2090k) 542k 4 -37.9 k)

Explanation / Answer

6.

W=?p??V

Then, if a V1 volume of gas expands to V2 under a pressure p, the work done is:

W=?p?(V2?V1).

We have:

V1=15.0.L=1.5x10?2.m3

V2=90.0.L =9.0x10?2.m3

p=1.75.atm=1.75x101,300.Pa=177275.Pa

Then:

W=-p??V=-p?(V2?V1)= - 177275 (9.0x10?2 - 1.5x10?2 ) = -13295.6 J = -13.3 kJ

Change in Internal energy ?U= q + W

q= heat absorbed from surroundings = +38.0 kJ

Change in Internal energy ?U= q + W = 38.0 – 13.3 = +24.7 kJ

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