Exploring Enthalpy and Entropy in Exothermic and Endothermic Reactions

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In summary, in an exothermic reaction, the change in enthalpy is negative and the change in entropy is positive in respect to the surroundings, leading to an increase in positional probability. Conversely, in an endothermic reaction, the change in enthalpy is positive and the change in entropy is negative in respect to the surroundings, resulting in a decrease in positional probability. This is in accordance with the second law of thermodynamics, which states that nature spontaneously proceeds towards states with a higher probability of existing, as seen in the increase of entropy. The statistical definition of entropy is easier to understand than the classical definition, as it takes into account the number of arrangements or microstates that are available to a system in a given state. In
  • #1
Integral0
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In an exothermic rxn change in enthalpy is negative but change in entropy is positive (in respect to the surroudings)(increase in positional probability).

In an endothermic rxn change in enthalpy is positive but change in entropy is negative (in respect to the surroundings)(decrease in positional probability).

right?

Plz say so
 
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  • #2
Wrong --- reread the assigned text, go back over the class notes, delete any mumbo-jumbo and hand-waving arguments that include the phrase "positional probability" in text and notes, also any references to "order/disorder," and try it again.
 
  • #3
I don't think so . . .

I quote "In an endothermic process, heat flows from the surroudings into the system. In an exothermic process, heat flows in the surroundings from the system." -Chemistry 6th Edition, Zumdahl

I quote "an exothermic process in the system causes heat to flow into the surroundings, increasing the random motions and thus the entropy of the surroundings. For this case, Change in Entropy of the surroundings is positive. The opposite is true for an endothermic process in a system at constant temperature (Change of Entropy of the surroundings is negative )." -Chemistry 6th Edition, Zumdahl

This definitely supports my statements made previous.
 
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  • #4
The terms endothermic and exothermic apply to the state function of enthalpy only, that is delta H.

Nevertheless energy balance does not suffice to answer all questions about a chemical reaction including whether a reaction will take place at all.

And thus we have entropy. And then delta G which is the succinct criterion btw reactions that can occur spontaneously.
 
  • #5
Out of my head:

Enthalpy is the measure of energy released by a reaction (can be positive or negative).

Entropy is the measure of free energy of a system. A spontanious reaction would start from a large G to a small G, so you've got a -dG (delta G).
 
  • #6
You might want to read throught the following link:
http://www.ucdsb.on.ca/tiss/stretton/chem2/entropy2.htm [Broken]

I wonder, are G and S the same things for entropy?
 
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  • #7
I am right . . . here is my support

I entered school today immediately to ask my chemistry teacher and he says what I wrote above was correct. I also corresponded with another chemistry teacher to double-check. I also checked in another Chemistry book (Chemistry and Chemical Reactivity by Kotz and Purcell) and I quad-checked with my own by Zumdahl . . . a prominent authority in chemistry.

Bystander . . . my "waving arguments" came from the book . . . it talked profusely about "positional probability", "entropy", and "Ethalpy" and their relations to each other. Positional probability increase with an increase in entropy and vice versa (under constant temperature and pressure).

Thank you all for sending me for the "quest for the holy Grail (i.e. Chem answer)". (although it did waste A LOT OF MY TIME, I guess everything is relative )_

:wink:
 
  • #8
Well, so you came out better than huh?


So could you explain the positional probability for me then? I don't quite understand what is meant with that The probability of finding a molecule at a certain position?? That would mean a low temperature, a reaction which lowers the temperature of the surroundings is an endothermic reaction, not exothermic..

You are true that enthalphy in an exothermic reaction has a negative sign (heat is released).

You also seem to be true that systems strive to increase entropy = disorder (and thus decrease free energy (G)?), an exothermic reaction releases energy in heat and thus increases entropy (and is generally speaking spontanious).


Bystander, if you could comment more in detail what you think is wrong, since your previous post seemed to have caused some confusion as to what you meant to say :wink:
 
  • #9
deleted post...
 
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  • #10
Originally posted by Meninger
G is not entropy. Entropy is S

Delta G is used to determine whether a system will undergo a spontaneous reaction (dG is negative) or is in equilibrium (dG = 0).

In finding out whether a system will undergo a spontaneous reaction we have to take into account both chg in enthalpy (dH) and chg in entropy (dS) of the system. We cannot use only one of the two in order to find out whether a reaction will proceed or not: dG = dH - TdS.
Yes, I figured, thanks for pointing that out! :smile:
 
  • #11
positional probabilities

I am sorry Monique that I cannot tell a full account about positional probability (my teachers enjoy giving me tons of homework - about 6-7 hours per night).

Let me try to summarize what I know . . .

Entropy is a thermodynamic function that describes the number of arrangements (positions and/or levels) that are available to a system existing in a given state. Nature spontaneously proceeds toward the states that have the highest probabilities of existing.

Consider this example:

Arrangement 1 has one microstate (a particular arrangement), Arrangement 2 has four microstates. Arrangement 3 has 6 microstates. According to positional probability Arrangement 3 is most likely or probable to occur.

Here is another example ->

A solid < liquid < gas in terms of positional probability, why?

Because there is an increase in disorder from a solid state to a liquid state and so forth.

Therefore, Positional Probability depends on the arrangement (state) and the number of ways (microstates) . . . these are correlated with an increase and decrease of Entropy (situational).

I hope this helps!
 
  • #12


Originally posted by Integral0
Nature spontaneously proceeds toward the states that have the highest probabilities of existing.
OK, now I know what you mean by the term. I wonder though, isn't that the strange way to describe nature? It proceeds to the state wherein there is the highest number of probabilities of existing.. that sounds really strange to me.. an increase in positional probability.. there must be a better term?
 
  • #13
The statistical definition of entropy is a lot easier to understand than the classical definition of entropy, dQ(reversible)/T.

If a chemical reaction is spontaneous, that implies that the total entropy of the universe increases during the course of the reaction. The entropy of the reaction system may decrease if this is offset by a larger increase in entropy of the surroundings.

Or put another way by Josiah Willard Gibbs, the greatest native born American physicist, if the change in Gibbs free energy is negative, the reaction is spontaneous.
 
  • #14
Originally posted by Heathcliff


Or put another way by Josiah Willard Gibbs, the greatest native born American physicist, if the change in Gibbs free energy is negative, the reaction is spontaneous.

Richard Feynman was also born in the United States (see http://www.nobel.se/physics/laureates/1965/feynman-bio.html). Let's just say we can't compare the two b/c Gibbs got his degree in Mathematical physics while Feynman got his in Theoretical Physics.

:wink:

I.e. both were the "greatest" native born
 
  • #15
Yes, two great scientists. I think Gibbs is the darling of scientists because he had a typical reticent scientific personality--publish in obscure journals and let your work speak for you. Feynman was more of a showman and big ego, thus far more familiar to the general public. I don't know how his colleagues took him--I imagine some disliked him for his ego.
 

1. What is enthalpy and how does it relate to chemical reactions?

Enthalpy is a measure of the total energy of a system, including both its internal energy and the amount of work it can perform. In chemical reactions, enthalpy is often used to describe the heat released or absorbed during the reaction. Exothermic reactions release heat and have a negative change in enthalpy, while endothermic reactions absorb heat and have a positive change in enthalpy.

2. How does enthalpy change during an exothermic reaction?

In an exothermic reaction, the reactants have a higher enthalpy than the products. As the reaction takes place and bonds are broken and formed, energy is released in the form of heat. This leads to a decrease in the total enthalpy of the system.

3. What is the relationship between enthalpy and entropy in chemical reactions?

Entropy is a measure of the amount of disorder or randomness in a system. In chemical reactions, the change in entropy can be influenced by the change in enthalpy. Generally, exothermic reactions have a decrease in entropy, as the products are more ordered and have lower disorder than the reactants. Endothermic reactions, on the other hand, often have an increase in entropy due to the randomness of the energy absorbed.

4. How can we determine the change in enthalpy and entropy for a reaction?

The change in enthalpy and entropy for a reaction can be calculated using the Gibbs free energy equation: ΔG = ΔH - TΔS. This equation relates the change in enthalpy, entropy, and temperature to the overall change in free energy for the reaction. By measuring the temperature and using experimental data, we can determine the enthalpy and entropy changes for a reaction.

5. How do exothermic and endothermic reactions differ in terms of energy changes and spontaneity?

Exothermic reactions release energy, usually in the form of heat, while endothermic reactions absorb energy. Exothermic reactions are typically spontaneous, meaning they occur without any external energy input, while endothermic reactions often require an external energy source to occur. This is because exothermic reactions have a negative change in free energy, indicating spontaneity, while endothermic reactions have a positive change in free energy, indicating non-spontaneity.

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