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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: justplaingoodoldme on December 29, 2011, 08:08:03 AM

Title: Expression to relate enthalpy of dissociation at 2 temperatures
Post by: justplaingoodoldme on December 29, 2011, 08:08:03 AM

I am revising and am a bit stumped by this thermodynamics questions-

Question

The molar constant pressure heat capacities (Cp) of molecular and atomic iodine in the temperature range 900-1100K are approximately 9R/2 and 5R/2 respectively.

Explain why this should be so and write down an expression that relates the enthalpy of dissociation of I₂ at a temperature T (in the range 900K-1100K) to the enthalpy of dissociation at 1000K.

Thoughts

I thought that at higher temperature there were more degrees of freedom (not less) and that for a linear molecule the maximum degrees of freedom was 5R/2 due to translations+rotations. Is Cp= Cv+nR relevant?

For the second part is it just Kirchoff's law?

Any help really appreciated  :)