Explaining why some reactions are spontaneous at certain temperatures

In summary, the conversation discusses a question about spontaneity on an exam and the equation ΔG = ΔH - TΔS is mentioned as a way to explain it mathematically. The speaker is unsure about how to answer the question and mentions the need for examples of reactions and plotting on a graph. Another person suggests plotting ΔH, -TΔS, and their sum or T vs. their sum as possible solutions.
  • #1
ampakine
60
0
On an exam I got this question:
[PLAIN]http://img849.imageshack.us/img849/5732/spontaneity.png
but at the time I had no idea how to answer it. I still don't really know if I can answer it. I can explain it mathematically with the equation: ΔG = ΔH - TΔS seeing as a spontaneous reaction requires a negative change in G and therefore if the sign of ΔH and ΔS are different then temperature will be a determining factor as to whether or not ΔG will be negative. I have no idea what I would need to plot on a graph though. As for examples of reactions of each type I suppose I needed to have a few example reactions memorised.
 
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  • #2
ampakine said:
I have no idea what I would need to plot on a graph though.

How about plotting ΔH, -TΔS, and their sum?
 
  • #3
Oh yeah didn't think of that. I could also plot T vs. their sum.
 

1. What is meant by a spontaneous reaction?

A spontaneous reaction is a reaction that occurs without any external influence or input of energy. This means that the reaction will occur on its own without needing any additional energy to get started.

2. How does temperature affect the spontaneity of a reaction?

Temperature plays a critical role in determining whether a reaction will be spontaneous or not. In general, as temperature increases, the likelihood of a reaction being spontaneous also increases. This is because higher temperatures provide more energy for the reactant molecules to overcome the activation energy barrier and initiate the reaction.

3. What is the relationship between entropy and spontaneity?

Entropy is a measure of the disorder or randomness in a system. In spontaneous reactions, the total entropy of the system always increases. This means that the products of a spontaneous reaction will have a higher entropy than the reactants. Therefore, increasing the temperature can increase the randomness of the system and make the reaction more spontaneous.

4. Can a reaction be spontaneous at one temperature but not at another?

Yes, a reaction can be spontaneous at certain temperatures but not at others. This is because the spontaneity of a reaction depends on a balance between the change in enthalpy and the change in entropy. At higher temperatures, the change in entropy has a greater effect on the spontaneity of the reaction, while at lower temperatures, the change in enthalpy is more significant.

5. Are all spontaneous reactions exothermic?

No, not all spontaneous reactions are exothermic. While exothermic reactions release energy in the form of heat, some spontaneous reactions can actually be endothermic, meaning they absorb heat from the surroundings. The spontaneity of a reaction is determined by the overall change in energy, not just the release or absorption of heat.

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