Entropy of transition

$\Delta S = \frac{q_{rev}}{T}$

Jessa Maheras 4F
Posts: 121
Joined: Fri Aug 02, 2019 12:16 am

Entropy of transition

Equations 5 and 6 give the entropy change at the transition temperature. To find the entropy of transition at another temperature, the calculation has to be broken down into three steps (1). For example, to find the entropy of vaporization of water at 25 8C and 1 bar:
1. Heat the liquid from 25 8C to its boiling point, 100. 8C,
2. Allow it to vaporize.
3. Cool the vapor back to 25 8C.
Because entropy is a state function, the changes in entropy for each of the individual steps can be added together to obtain the entropy of vaporization at 25 8C.

My question: if vaporization occurs at boiling point, how can you calculate the entropy of transition at a temperature other than boiling point?

Ariel Davydov 1C
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Re: Entropy of transition

Approach a problem in calculating the entropy of transition the same as you would in calculating the qtotal of a substance being raised from a non-boiling-point temperature to its boiling point and transition. Set up two separate components for the entropy for the system and add them: 1) the entropy gained in raising the temperature of the substance to its boiling point, and 2) the delta Svap for the substance at its boiling point. 1) can be calculated by using the deltaS=q/T equation; calculate the amount of heat needed to raise the substance to its boiling point (q = gCs*deltaT) and divide it by its original temperature. Then, add it to the deltaSvap for the substance (multiplied by number of moles, since enthalpy is an extensive property), and you will reach your final answer. Hope this helps!

Anika Chakrabarti 1A
Posts: 102
Joined: Sat Aug 24, 2019 12:17 am

Re: Entropy of transition

Adding on to your comment, I think you would need a third component where you calculate q when cooling the substance back to the original temperature. Then, find the change in entropy for cooling and add all three entropy changes to find the total.

Leila_4G
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Joined: Sat Sep 14, 2019 12:17 am

Re: Entropy of transition

Thank you, Ariel! That helped a lot.

505316964
Posts: 95
Joined: Thu Jul 11, 2019 12:17 am

Re: Entropy of transition

How can you identify when you need to break the reaction down into 3 steps of heating, vaporization, then cooling?

chari_maya 3B
Posts: 108
Joined: Sat Sep 07, 2019 12:18 am

Re: Entropy of transition

When deltaH is only given at temperature of vaporization.

chari_maya 3B
Posts: 108
Joined: Sat Sep 07, 2019 12:18 am

Re: Entropy of transition

would this same concept apply to standard entropy of fusion?

Jessa Maheras 4F
Posts: 121
Joined: Fri Aug 02, 2019 12:16 am

Re: Entropy of transition

Ariel Davydov 1C wrote:Approach a problem in calculating the entropy of transition the same as you would in calculating the qtotal of a substance being raised from a non-boiling-point temperature to its boiling point and transition. Set up two separate components for the entropy for the system and add them: 1) the entropy gained in raising the temperature of the substance to its boiling point, and 2) the delta Svap for the substance at its boiling point. 1) can be calculated by using the deltaS=q/T equation; calculate the amount of heat needed to raise the substance to its boiling point (q = gCs*deltaT) and divide it by its original temperature. Then, add it to the deltaSvap for the substance (multiplied by number of moles, since enthalpy is an extensive property), and you will reach your final answer. Hope this helps!

Thank you Ariel that helps a lot!!

Jessa Maheras 4F
Posts: 121
Joined: Fri Aug 02, 2019 12:16 am

Re: Entropy of transition

505316964 wrote:How can you identify when you need to break the reaction down into 3 steps of heating, vaporization, then cooling?

In the situation like described in my question! If you need to find entropy of transition at a temperature other than the boiling/melting point, then you need to break the process into 3 steps.