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Free energy

# Gibbs free energy and spontaneous reactions

Gibbs free energy and spontaneous reactions.

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• At several points of the video Sal describes things as being "spontaneous." What exactly does this mean? It seems like spontaneous would be synonymous with random, aka disorder or entropy. What is a good definition of spontaneous on a molecular level?
• Spontaneous just means something happens of itself.
Wiki defines it as: "performed or occurring as a result of a sudden inner impulse or inclination and without premeditation or external stimulus."
• i did not understand what is really gibbs free energy?
• Strictly speaking, Gibbs free energy change determines if a reaction is spontaneous under the conditions of constant pressure and constant temperature, which is usually the situation in biology. Under constant volume for example, then it's something else called the Helmholtz free energy. You really have to take a thermodynamics course to make sense of it. Just try to get the basic idea that going downhill in energy and increasing entropy both favor a reaction happening, and the entropy part becomes more important the higher the temperature.
For most chemical reactions at ordinary temperatues, the enthalpy term is much more important, being on the order of 100,000 cal/mole while the TdeltaS term is on the order of a few cal/mole.
• How do you use this Gibbs equation if you don't know the exact numbers to plug in?
• If I give you an expression E = a * b, and I tell you that a and b are positive, you don't need to know the values of a and b to predict that E is going to be positive (since positive * positive = positive). Similarly, in the Gibbs equation, all that matters is the sign of ΔG, you can use the various rules of signs to infer what the sign of ΔG will be.
• What does entropy-enthalpy mean? Can this lead to something?
• What are the numbers for low T and high T? Low and high are not sufficiently defined. And buzz? Too technical for me.
• Good question! 'Low T' and 'High T' relate to the specific thermodynamic process being discussed and whether or not Gibbs free energy is negative (spontaneous) or positive (non-spontaneous) for each of the different possible combinations (i.e. G, H, T, S). For example, when Mr. Khan was talking about a spontaneous Gibbs free energy process - if delta H (enthalpy) was positive and delta S (entropy) was positive, the temperature would need to have a "High T" or high enough to make delta G (Gibbs free energy) negative overall and thus a spontaneous process. So, what Mr. Khan is describing regarding temperature is that it is either low or high enough for the specific example he is describing to either be a spontaneous or non-spontaneous Gibbs free energy process. I hope this helps to answer your question!
• How can a reaction releases energy (negative delta H) but gains free workable energy (positive delta G)? Does enthalpy not represent the total energy of a system? Or does it mean free energy is not a sub group of total energy.
• the type of reaction in which energy is released is called exothermic reaction you may take an example of acid...you might have added water to acid in laboratory the reaction of water with acid release heat(thats what energy is) and by touching the beaker you will feel something really hot...the energy utalized in the reaction is called enthalpy change..and therefore it explains that is energy released(exothermic) in the reaction or absorbed(endothermic)...
• In the first example that Sal did, there were 5 atoms, to begin with, and then once they collided I only saw four atoms. Where did that atom go?
• How does knowing the spontaneity of a reaction help a scientist? What are the applications?
• That is a quite interesting question!
The first thing which comes to my mind is that it helps you in order if you want to simulate it (do it experimentally in a lab).

The second is that you have to know if a reaction happens spontaneously so you can predict or prevent a resulting reaction. Imagine what would it be if people could spontaneously combust under certain circumstances? We would avoid them, right?

Also, you have to knwo if reaction is ponatenous in terms of efficacy of work (such as electric circuits). If reactions happen spontaneously means it emits lots of thermal energy, therefore, you require a cooling system to prevent overheating.
(1 vote)
• If the volume of a close system increases while the pressure is constant, would it breaks the first law of thermodynamics?