Confused about answers to questions in Kaplan Gen Chemistry book 3rd edition

[tenghis]

Chapter 7: Thermochemistry: Question 8: It is saying that we cannot determine the sign of delta G because we know that delta S and delta G are positive, but we don't know the sign of the temperature. But I thought that temperature is ALWAYS positive because Kelvin is always positive??

Chapter 8: The Gas Phase: Question 5: The answer says that while ideal gases have a negligible volume, they do occupy a measurable volume, so it would be incorrect to say they have no volume. But in the reading of the chapter, the text says, word-for-word: "An ideal gas represents a hypothetical gas with molecules that have no intermolecular forces and occupy no volume." Many other sources are also agreeing with the "no volume" statement.

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[JimKimSlim]

First question: we don’t know if delta H > T*(delta s) or the other way around.

Second question: yeah, this one is definitely debatable. You won’t expect to see that kind of logic on AAMC.

 

[ManhattanEliteMCAT]

Given Positive delta Entropy and Positive delta Enthalpy, when the temperature is low, Gibbs is positive (the reaction is spontaneous). Given Positive delta Entropy and Positive delta Enthalpy, when the temperature is high, Gibbs is negative (the reaction is non-spontaneous). You're absolutely correct; Kelvin is never negative. The explanations should say Gibbs is dependent on the value of the temperature when given Positive delta Entropy and Positive delta Enthalpy.

 

[MCAT-MVP]

First Question:

The question is referring to the equation

delta G = delta H - T delta S

When delta G is positive the forward reaction will be non spontaneous and when delta G is negative the reaction will be spontaneous.

If delta H is positive (endothermic reaction) and delta S is positive (a reaction that makes more disorder) then the sign of delta G can not be determine because it will depend on the temperature. If the temperature is above a certain point the reaction will be spontaneous, but if the temperature is low the reaction will be nonspontaneous.

For example, lets say delta H is +300 kJ/mol and delta S is + 10 kJ/mol.

To make delta G negative you need to make the temperature (in Kelvin) above 60 and to make the reaction nonspontaneous you need to make the temperature below 60 Kelvin.

High Temp:
Delta G = 300 - (61)(5)
Delta G = 300 - 305
Delta G = -5

Low Temp:
Delta G = 300 - (59)(5)
Delta G = 300 - 295
Delta G = +5

You can use the following table to determine if Delta G will be spontaneous or nonspontaneous.

Delta H	Delta S	Delta G
-	+	-
+	-	+
-	-	undetermined
+	+	undetermined

When delta G is undetermined you can cover up the -delta T to determine if the reaction will be spontaneous at low temperatures and when the temperature is high you can cover up the delta H to determine if the reaction will be spontaneous.

Low Temp	High Temp
Delta G = delta H	Delta G = - T delta S
When delta H is negative the reaction is spontaneous.	When delta S is negative the reaction is nonsponaneous
When delta H is positive the reaction is nonspontaneous.	When delta S is positive the reaction is spontaneous.

Second Question:

All gasses have a volume and when you use the ideal gas equation, which is PV = nRT you are ignoring the volume the gasses take up. The V in the equation represents the volume that is available for a gas to move around.

It would be like if I were to ask you to measure the volume in your room when it is empty then I asked you to measure the volume of your room after you put all your things in your room (bed, chair, desk, etc..) the volume of you room would be less after you put all your things in. The ideal gas law does not take into account the volume the gasses take up because it makes the equation a lot easier to work with.

If we were to talk about “Real Gasses” that is a different equation than “Ideal Gasses”. Technically ideal gasses do not exist because of course molecules take up space.

The pressure deals with an entire different issue than the volume.