Thermodynamics Enthalpy and Entropy

[justhanging]

Can you deduce the sign of enthalpy and entropy just by changing the temperature of the reaction and observing whether the equilibrium shifts forwards or backwards?

For example,
If I increase the temperature of a reaction and it produces more products then I know that it is endothermic. Can I take another step and say because Keq increases, dG becomes more negative therefore (dG = dH - TdS) dS has to be positive? I feel like am making an incorrect assumption somewhere.

Other questions that tie in:
dG is just a measure of spontaneity of reaction not a measure of stability of products and reactants?
dH is a measure of stability of products and reactants, and the stability does not change with temperature? ( I know that dH changes very slightly with temperature but that is out of scope for the mcat)

This EK chem question is the source of my confusion:

Reaction I
dH= -160 dS= -140

Chemist A:
Chemist A argues that reaction I will be spontaneous in the forward direction because the products are more stable then the reactants. Furthermore, if temperature is raised, the reaction will run in reverse because the reaction is exothermic.

QQ:
Which of the following is a logical conclusion of chemists A's argument?
A: The enthalpy change of reaction is an indicator of the relative stabilitiy of reactants and products.
C: At higher temperatures, the reactants of reaction I will be more stable then the products.

Can someone clarify my confusion, all the questions are kinda related cause I wanna make sure my concept is consistent.

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[whiteshadodw]

Can you deduce the sign of enthalpy and entropy just by changing the temperature of the reaction and observing whether the equilibrium shifts forwards or backwards?

For example,
If I increase the temperature of a reaction and it produces more products then I know that it is endothermic. Can I take another step and say because Keq increases, dG becomes more negative therefore (dG = dH - TdS) dS has to be positive? I feel like am making an incorrect assumption somewhere.

Other questions that tie in:
dG is just a measure of spontaneity of reaction not a measure of stability of products and reactants?
dH is a measure of stability of products and reactants, and the stability does not change with temperature? ( I know that dH changes very slightly with temperature but that is out of scope for the mcat)

This EK chem question is the source of my confusion:

Reaction I
dH= -160 dS= -140

Chemist A:
Chemist A argues that reaction I will be spontaneous in the forward direction because the products are more stable then the reactants. Furthermore, if temperature is raised, the reaction will run in reverse because the reaction is exothermic.

QQ:
Which of the following is a logical conclusion of chemists A's argument?
A: The enthalpy change of reaction is an indicator of the relative stabilitiy of reactants and products.
C: At higher temperatures, the reactants of reaction I will be more stable then the products.

Can someone clarify my confusion, all the questions are kinda related cause I wanna make sure my concept is consistent.

yes and no.

if you increase temperature, and Keq increases, then you know deltaS is positive. an increase in temp, and a decrease in Keq implies a negative deltaS.

however, if at low temperatures (0 K, absolute zero), a reaction has Keq > 1, then you know deltaH has to be negative. Keq<1 then deltaH is positive, when @ low temperatures.

the idea is to use 'limiting cases'. at a sufficiently high temperature, if deltaS is positive it should be able to make the reaction spontaneous (Keq>1) because TS >>> H. then at extremely low temp, the deltaS value doesn't really matter, ie T=0, then TS = 0.

 

[justhanging]

yes and no.

if you increase temperature, and Keq increases, then you know deltaS is positive. an increase in temp, and a decrease in Keq implies a negative deltaS.

however, if at low temperatures (0 K, absolute zero), a reaction has Keq > 1, then you know deltaH has to be negative. Keq<1 then deltaH is positive, when @ low temperatures.

the idea is to use 'limiting cases'. at a sufficiently high temperature, if deltaS is positive it should be able to make the reaction spontaneous (Keq>1) because TS >>> H. then at extremely low temp, the deltaS value doesn't really matter, ie T=0, then TS = 0.

I understand what your saying but your not really answering my question(s). I know how dG = dH - TdS works. Yes and no, to what exactly?

 

[whiteshadodw]

I understand what your saying but your not really answering my question(s). I know how dG = dH - TdS works. Yes and no, to what exactly?

"For example,
If I increase the temperature of a reaction and it produces more products then I know that it is endothermic. Can I take another step and say because Keq increases, dG becomes more negative therefore (dG = dH - TdS) dS has to be positive? I feel like am making an incorrect assumption somewhere."

like i said, you'd have to be given to Keq values, one for one temperature, another for another temperature. if Keq increases with Temp then yes, you are safe to assume dS is positive.

"QQ:
Which of the following is a logical conclusion of chemists A's argument?
A: The enthalpy change of reaction is an indicator of the relative stabilitiy of reactants and products.
C: At higher temperatures, the reactants of reaction I will be more stable then the products."

reactant and product stability is more of an intrinsic property of a substance, so i can't really agree with C. the temperature doesn't affect the "stability" but it does affect whether a compound or substance can react with something else or decompose. however, enthalpy change is a direct correlation to stability of reactant and products.

enthalpy of a reaction is looked at as enthalpy = sum of bonds broken - sum of bonds formed. of course, temperature plays some role in breaking bonds, but for the most part an H-H bond at one temperature is still an H-H bond at another temperature, and breaking it will require heat, and forming a new bond with say, C, to make C-H releases heat.

was the answer A?

dG is simply the spontaneity of a reaction as you said, but keep in mind that it depends on temperature. you already knew this, but dH and dS are values that cannot change for a given reaction. those are all pretty much intrinsic values of certain substances.

 

[justhanging]

"For example,
If I increase the temperature of a reaction and it produces more products then I know that it is endothermic. Can I take another step and say because Keq increases, dG becomes more negative therefore (dG = dH - TdS) dS has to be positive? I feel like am making an incorrect assumption somewhere."

like i said, you'd have to be given to Keq values, one for one temperature, another for another temperature. if Keq increases with Temp then yes, you are safe to assume dS is positive.

Do you need to have K values? If you have a reaction at equilibrium and you know that increasing temperature increases the products then dG has to be negative with the increase in temperature, otherwise how would it go forward. With that you can then say that dS has to be positive, since at equilibrium 0 = dH - TdS.

"QQ:
Which of the following is a logical conclusion of chemists A's argument?
A: The enthalpy change of reaction is an indicator of the relative stabilitiy of reactants and products.
C: At higher temperatures, the reactants of reaction I will be more stable then the products."

reactant and product stability is more of an intrinsic property of a substance, so i can't really agree with C. the temperature doesn't affect the "stability" but it does affect whether a compound or substance can react with something else or decompose. however, enthalpy change is a direct correlation to stability of reactant and products.

enthalpy of a reaction is looked at as enthalpy = sum of bonds broken - sum of bonds formed. of course, temperature plays some role in breaking bonds, but for the most part an H-H bond at one temperature is still an H-H bond at another temperature, and breaking it will require heat, and forming a new bond with say, C, to make C-H releases heat.

was the answer A?

dG is simply the spontaneity of a reaction as you said, but keep in mind that it depends on temperature. you already knew this, but dH and dS are values that cannot change for a given reaction. those are all pretty much intrinsic values of certain substances.

The answer is actually C, which doesnt really make sense. Their explanation: Chemist A chooses the direction of the reaction based upon chemical stability, and then says that the direction will change at higher temperatures. This is tantamount to saying that the stability will switch at higher temperatures, which, by the way, is also correct. Chemists A statement is correct and does not contradict the second law of thermodynamics. A is contradicted because chemist A says the direction is temperature dependent.