Thermodynamics in Dry ice

Chemistry Level 2

Dry ice, sometimes referred to as "cardice" or as "card ice" (chiefly British English), is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue (other than incidental frost from moisture in the atmosphere). Dry ice sublimates at 78.5 -78.5 °C ( 109.3 -109.3 °F) at Earth atmospheric pressures. Which of the following statements is correct about the sublimation of dry ice? ( Δ H \Delta H , Δ S \Delta S and Δ G \Delta G mean the change in enthalpy, entropy and Gibbs free energy, respectively.)

a) Δ H > 0 \Delta H > 0 and Δ S > 0 \Delta S > 0 , and Δ G < 0 \Delta G < 0 for all temperatures.

b) Δ H > 0 \Delta H > 0 and Δ S < 0 \Delta S < 0 , and Δ G > 0 \Delta G > 0 for all temperatures.

c) Δ H < 0 \Delta H < 0 and Δ S > 0 \Delta S > 0 , and Δ G < 0 \Delta G < 0 for all temperatures.

d) Δ H > 0 \Delta H > 0 and Δ S > 0 \Delta S > 0 , and Δ G < 0 \Delta G < 0 for high temperatures.

a) b) c) d)

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3 solutions

Anish Puthuraya
Mar 6, 2014

Since it is a cooling agent, It is absorbing heat to convert into its gaseous form. Hence Δ H > 0 \Delta H >0

Also, since it is converting from Solid to Gas, the Entropy Increases. Hence Δ S > 0 \Delta S >0

Now,
Δ G = Δ H T Δ S \Delta G = \Delta H - T\Delta S

To ensure Δ G < 0 \Delta G <0 , the term T Δ S T\Delta S must dominate over the Δ H \Delta H term (since both Δ H \Delta H and Δ S \Delta S are positive)
And for that to happen, it is favorable to have the temperature as highly positive as possible..

Hence, the answer is ( D ) (D)

What an interesting equation Δ G = Δ H T Δ S \Delta G = \Delta H - T \Delta S !

Lu Chee Ket - 5 years, 4 months ago
Shubham Sameer
Mar 8, 2014

g has to be negative for reaction to be feasible and if it was for all temperatures then it would be feasible at all temperatures but its not so ..so D :D

Vaishnavi Gupta
Mar 8, 2014

It is an endothermic reaction, in which randomness of system increases since gas phase has more randomness than solid phase. Also, its is spontaneous at high temp, since change in Gibbs free energy= change in enthalpy - (temp)( change in entropy), and this value is< 0 only if T is high enough.

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