Need of entropy

hoay SG

the heat change measured at constant pressure is Enthalpy change. It tells the energy of the system before and after the reaction. Then why we need entropy? What does it tells?

UltimaOnline SG

the heat change measured at constant pressure is Enthalpy change. It tells the energy of the system before and after the reaction. Then why we need entropy? What does it tells?

Entropy change tells you about the measure of disorderliness before and after a chemical reaction. Why do you need entropy, in addition to enthalpy?

When considering the choice of a job, profession or career, you need to consider both job satisfaction, as well as how well it pays you. Both factors are important.

Of course, which factor is more important at this point in time to you, also depends on your family financial needs, eg. how much you need to support your family.

Your family financial needs is represented by temperature. Job satisfaction is represented by enthalpy. Job pay is represented by entropy.

If your family financial needs (temperature) is high, then job pay (entropy) takes priority over job satisfaction (enthalpy). If your family financial needs (temperature) is low, then job satisfaction (enthalpy) takes priority over job pay (entropy).

For the choice of a job / career / profession that successfully suits you and allows you to proceed forward in your life, both job satisfaction and job pay need to be considered, and is affected by a 3rd variable, family financial needs. For a chemical reaction to successfully proceed in the forward direction (ie. for Gibbs free energy to be negative and thermodynamically feasible and favorable), both enthalpy and entropy need to be considered, and is affected by a 3rd variable, temperature.

hoay SG

Q . For each of the following reactions predict the sign of G. If a prediction is not possible because the sign of G will be temperature dependent, describe how G will be affected by raising the temperature.

(a) An endothermic reaction for which the system exhibits an increase in entropy

(b) An exothermic reaction for which the system exhibits an increase in entropy

Ans (a) For the reaction to be spontaneous and feasible the enthalpy change must be exothermic and the entropy change must be positive i.e G negative. The first factor does not support here so for G to be –ve the reaction should be carried out at high temperature.

Ans (b) The sign of G would be negative since both the conditions are right for the reaction to be feasible and spontaneous

One point if we forget here the need for the temperature control is to look at the sign of enthalpy change if it is +ve then high temperature is applied or -ve low temperature should be applied. Are my answers correct??

UltimaOnline SG

(a) An endothermic reaction for which the system exhibits an increase in entropy

(b) An exothermic reaction for which the system exhibits an increase in entropy

Ans (b) The sign of G would be negative since both the conditions are right for the reaction to be feasible and spontaneous

Yes, you're correct. To be more precise :

if enthalpy change is endothermic and entropy change is positive, then high temperature needs to be applied ; or if enthalpy change is exothermic and entropy change is negative then low temperature needs to be applied.

hoay SG

are feasibility and spontaneity of a reaction same terms? spontanous means the reaction must occur without the help of external indicators. Feasibility is about what?

UltimaOnline SG

are feasibility and spontaneity of a reaction same terms? spontanous means the reaction must occur without the help of external indicators. Feasibility is about what?

The correct usage of these 2 closely related terms is as follows :

A chemical reaction is spontaneous if it is thermodynamically feasible, ie. Gibbs free energy change is negative.

Metanoia SG

Would a system be expected to experience an increase in entropy when it undergoes an endothermic reaction?

UltimaOnline SG

Would a system be expected to experience an increase in entropy when it undergoes an endothermic reaction?

Hi Metanoia, are you a H2 Chem student or a H2 Chem teacher/tutor?

For a chemical reaction that is thermodynamically feasible and hence spontaneous, if the reaction is endothermic, the reaction *must* have a positive entropy change, eg. dissolving solid ammonium salts. As far as the A level Chemistry syllabus is concerned, that's about it.

However, at higher levels and in practice, it's still possible for a thermodynamically non-feasible (ie. non-spontaneous) reaction to occur, eg. in the worst case scenario (thermodynamically speaking), an endothermic reaction with negative entropy change, resulting in a positive Gibbs free energy change regardless of temperature, can still occur *provided* that such a thermodynamically non-feasible reaction is *coupled* with a separate, external chemical reaction that *is* highly thermodynamically feasible and favorable, and hence highly spontaneous, ie. a large magnitude of negative Gibbs free energy change.

Such a coupling would ensure that the *overall* Gibbs free energy change of both coupled chemical reactions would, in totality, still be negative, thermodynamically feasible and hence spontaneous.

Examples of thermodynamically non-feasible or non-spontaneous reactions from H2 Biology that certainly continue to occur (otherwise you wouldn't be alive to read this), would be photosynthesis and protein synthesis.

hoay SG

Free gibb's energy depends upon temperature since entropy depends upon temperature??

the Enthalpy change of formation of elements in their standard states have a zero value. Why the same is not true for standard entropy change?

UltimaOnline SG

Free gibb's energy depends upon temperature since entropy depends upon temperature??

the Enthalpy change of formation of elements in their standard states have a zero value. Why the same is not true for standard entropy change?

Yes, Gibbs free energy change depends on 3 variables : enthalpy, entropy and temperature.

By definition, *enthalpy change* of formation of elements in their standard states have a zero value, while the *entropy* of a perfect crystal at absolute zero is exactly equal to zero.

Here's why standard entropy change for most reactions isn't zero : consider (as an example) the formation of ammonia : notice that in the balanced equation for N2 and H2 elements to combine chemical to form NH3, there are 4 moles of gases on the LHS, and only 2 moles of gas on the RHS, which therefore results in a negative entropy change (whether at standard or non-standard conditions).

Metanoia SG

Hi there, I do tutoring on the secondary level, occassionly rereading A-level stuff to refresh the concepts.

Thanks for the detailed explanation. You brought up an important point that systems can be viewed in either isolation or coupled.

UltimaOnline SG

Ah ok. Welcome to the Homework Forum, Metanoia :)

Hi there, I do tutoring on the secondary level, occassionly rereading A-level stuff to refresh the concepts.

Thanks for the detailed explanation. You brought up an important point that systems can be viewed in either isolation or coupled.

qdtimes2 SG

love that satisfaction-pay-needs analogy

UltimaOnline SG

love that satisfaction-pay-needs analogy

Thanks, glad you enjoyed that