Determining stability of a compound/reaction when there's an increase in temperature






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Joanna Pham - 2D
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Joined: Fri Apr 06, 2018 11:04 am

Determining stability of a compound/reaction when there's an increase in temperature

Postby Joanna Pham - 2D » Wed Feb 20, 2019 7:48 pm

What is the best way to determine if a compound or reaction is stable or unstable?

For example, for 9.65, the question asks us to determine which of the compounds is less stable when temperature is raised: PCl5, HCN, NO, and SO2. How w ould I determine the stability of the compounds?

Kim Tran 1J
Posts: 62
Joined: Fri Sep 28, 2018 12:24 am

Re: Determining stability of a compound/reaction when there's an increase in temperature

Postby Kim Tran 1J » Wed Feb 20, 2019 8:51 pm

You can calculate the standard entropies of formation (delta S)
If delta S is negative, then the compound will be less stable at a higher temperature.
If delta S is positive, the compound will be more stable at a higher temperature

Steven Garcia 1H
Posts: 79
Joined: Fri Sep 28, 2018 12:16 am

Re: Determining stability of a compound/reaction when there's an increase in temperature

Postby Steven Garcia 1H » Wed Feb 20, 2019 8:51 pm

For problems such as these, you will have to use the equation, standard change in Gibbs free energy= standard enthalpy of formation (of given compound) - T* standard change in entropy of rxn. First, set up a chemical equation that produces the given compound as the product. Make sure the elements (reactants), you use in the chemical equation are pure elements. Also make sure this chemical equation is balanced.

The enthalpy of formation of the compound should be provided for such problems (look in the appendix section of textbook). The standard change in entropy of the rxn can be calculated by subtracting the sum of the change in entropy of the reactants from the sum of the change in entropy of the products. The individual molar entropies of the elements and compound should also be found in the appendix of the book.

Once you calculated the standard change in enthalpy and entropy, plug these values in the Gibbs free energy equation. Then imagine the temp being raised. Is the change in Gibbs free energy a negative or a positive value when temp is raised? If it's a negative value, then the rxn occurs spontaneously and that the compound is favored, thus more stable than its elements. If it's a positive value, then the rxn is NOT spontaneous and does not occur spontaneously, so we would conclude that the elements of the compounds are more stable than the compound itself.


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