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Question:

Carbon dioxide (CO₂) is an interesting substance. It has no liquid phase at normal pressure; it sublimates rather than melts, going directly from the solid form (dry ice) to a vapor. This sublimation occurs at -78.5 degrees C with a heat of sublimation (similar to a heat of vaporization) of 573,100 J/kg.

A. Draw the phase diagram (T vs. Q) for a 1kg sample of carbon dioxide that covers temperature range from -125 degrees C to -25 degrees C.

B. Calculate the total amount of heat required to raise the temperature of the sample from -125 degrees C to -25 degrees C. The specific heat of solid dry ice is 815 J/kg Celsius degrees and for the vapor is 2 J/kg Celsius degrees.

Answers:

Cramster Expert

dhara

(Cramster SME)

Cramster In-House Subject Matter Expert

Date Posted: 7/24/2008 8:00:54 AM Status: Live

Asker's Rating: Lifesaver

Response:

A.The heat released when 1 kg sample of carbon dioxide is at -125^oC is

Q = m * s * t = 1 * 815 * (-125 + 273) = 815 * 148 = 120620JK

The heat released when 1 kg sample of carbon dioxide is at -25^oC is

Q = 1 * 815 * (-25 + 273) = 815 * 248 = 202120JK

The phase diagram (T vs. Q) for a 1kg sample of carbon dioxide that covers temperature range from -125^oC to -25^oC is shown as below :

B.The total amount of heat required to raise the temperature of the sample from -125^oC to -25^oC

Q = m * s * Δt = 1 * 815 * (100) = 81500JK.

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