A helium-filled balloon at 1.00 atm pressure has a volume of 1.350 L?

Question

It is placed in a freezer and its volume decreases, stopping at 0.980 L.
(Assume that the static internal and external pressures are equal, that the balloon contains only helium, and helium behaves as an ideal gas.)
Calculate deltaE for the gas in the balloon.

Answer 1

From the first law of thermodynamics:

Q = ΔE + w, so ΔE = Q - w

where ΔE the change in internal energy, Q the amount of heat and w the work.

Now, the change is isobaric, so: Q = nxCpxΔΤ and Cp = 5R/2 for an ideal gas.

Q = 5nxRxΔT/2 = 5pxΔV/2 (because nxRxΔT = pxΔV)

w = pxΔV

ΔE = 5pxΔV/2 - pxΔV = 3pxΔV/2

ΔE = 3x1x(0.980 - 1.350)/2 = -0.555 L.atm

1 L.atm = 101.4 J (approx) so

ΔE = -0.555 x 101.4 J = -56.28 J (approx)

Answer 2

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