What is Spontaneity: Definition and 16 Discussions

In thermodynamics, a spontaneous process is a process which occurs without any external input to the system. A more technical definition is the time-evolution of a system in which it releases free energy and it moves to a lower, more thermodynamically stable energy state (closer to thermodynamic equilibrium). The sign convention for free energy change follows the general convention for thermodynamic measurements, in which a release of free energy from the system corresponds to a negative change in the free energy of the system and a positive change in the free energy of the surroundings.
Depending on the nature of the process, the free energy is determined differently. For example, the Gibbs free energy change is used when considering processes that occur under constant pressure and temperature conditions, whereas the Helmholtz free energy change is used when considering processes that occur under constant volume and temperature conditions. The value and even the sign of both free energy changes can depend upon the temperature and pressure or volume.
Because spontaneous processes are characterized by a decrease in the system's free energy, they do not need to be driven by an outside source of energy.
For cases involving an isolated system where no energy is exchanged with the surroundings, spontaneous processes are characterized by an increase in entropy.
A spontaneous reaction is a chemical reaction which is a spontaneous process under the conditions of interest.

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  1. il postino

    Chemistry Chemistry equilibrium - Remove H2

    It is desired to remove the molecular hydrogen present in a flask. What will be better to introduce, ##Cl_2## or ##Br_2##? Why? ##2 HCl <-> H_2 + Cl_2## (1) ##k = 3,2.10^{-14}## ##H_2 + Br_2 <->2 HBr## (2) ##k = 2.10^9## I thought about turning equation (1) to make the...
  2. C

    Spontaneity and Reaction Coupling

    Greetings everyone! I have been a long-time lurker on here and have just recently signed up. I'm unsure if this was the correct forum to post this in, so I apologize in advance. My question isn't exactly homework per se - I am currently studying for my MCAT and have been trying to wrap my head...
  3. N

    Methane production and storage

    Hello members, I wish to produce and "store" "Methane" gas. I wish to accomplish this task by conducting out a reaction between carbon-dioxide an water. I have my own source of energy and I completely understand that the reaction is endothermic. The reaction is: CO2 + 2H2O + energy = CH4 + 2O2...
  4. D

    Entropy Thermodynamics: Calculate ∆S for 1 Mol of Diatomic Gas

    Homework Statement A sample consisting of 1 mol of a diatomic perfect gas with Cv,m = 3/2 R is heated from 100 ºC to 300 ºC at constant pressure. Calculate ∆S for the system. Homework Equations Cv,m = 3/2 R The Attempt at a Solution Cpm=Cvm +r because we want cp right isobaric ∆S= Cp ln...
  5. TheDarkness

    Can Virtual Particles Have Infinite and Negative Acceleration?

    Hypothetically, a particle p of mass M is sitting at velocity v. Assuming M = 0, and the vi of p is 0 m/s, 0 seconds passes and vf 100 m/s is reached. Since time t is 0, acceleration cannot exist, as no t has passed between the vi and vf states. And, knowing that a = (vf - vi)/ Δt, a = (100 -...
  6. R

    Predicting Spontaneity of 2CO(g)+O2(g)=2CO2(g)

    Homework Statement Will the following processes be spontaneous only at high temperatures, only at only temperatures, at all temperatures, or at no temperatures? 2CO(g)+O2(g)=2CO2(g) Delta(H)=-566kJ Homework EquationsThe Attempt at a Solution So i just need to figure out how to predict the...
  7. MexChemE

    Gibbs free energy and entropy inconsistency

    Homework Statement For a certain reaction, ΔG = 13580 + 16.1 T log10(T) - 72.59 T. Find ΔS and ΔH for the reaction at 298.15 K. Homework Equations ΔG = ΔH - TΔS \left[\frac{\partial (\Delta G)}{\partial T} \right]_P = - \Delta S The Attempt at a Solution For the sake of this thread's length I...
  8. Y

    Predicting the Spontaneity of 2BH3(g)->B2H6(g): Help Needed!

    2BH3(g)->B2H6(g) Which is true? a) Reaction is always spontaneous. b) Reaction is always non-spontaneous. c) Reaction is sometimes spontaneous sometimes not. d) Reaction is endothermic. e) Two of the above are true. I know DSsystem is negative. But I would need more information on...
  9. M

    Spontaneity of a reaction with ∆G=0

    Homework Statement Note: the following question is from a recent biochemistry exam, for which the grades have been posted and the exam questions and answers have been made available to students. I asked my professor about the answer to this question, and when I explained the issue, we resolved...
  10. M

    Spontaneity of physical processes

    hello! there is no law that says that processes occur physically towards the energy minimization of the system however, there is a law (second thermodynamic) that says that processes occur physically towards the entropy maximization 1) so, why do we need ΔG (Gibbs free energy) to predetermine...
  11. B

    Can I prove spontaneity using the entropy change?

    According to the second law of thermodynamics, for any spontaneous process, \Delta S_{uni}> 0 . And for any reversible process, \Delta S_{uni}= 0 . This means that no process can be reversible and spontaneous at the same time. However, what I don't understand is the connection to \Delta G...
  12. L

    Why is Reaction Spontaneousity Not Occuring at 298K?

    Homework Statement Sorry - this was on a test I took today, so I don't remember it exactly. A reaction (I believe a combustion reaction) is known to be spontaneous at 298 K. When the reactants are placed are placed into a container, no reaction occurs. Why? Info: The reactant side had 15 moles...
  13. J

    Spontaneity of reactions and free energy

    Does anyone know of a clean proof that a reaction will occur at constant temperature if the change in Helmholtz free energy is negative, or at constant temperature and pressure if the change in Gibbs free energy is negative? The only `proofs' I've found rely on the fact that the entropy...
  14. J

    Standard Entropy Change and Spontaneity

    Homework Statement Quartz, SiO2(s), does not spontaneously decompose to silicon and oxygen at 25C in the reaction: SiO2(s) -> Si(s) + O2(g) even though the standard entropy change of the reaction is large and positive (delta S = +182.02 J K-1). Explain. -------------------------...
  15. B

    Why is energy the ultimate determinant of spontaneity

    I understand that a chemical reaction is spontaneous if it has a (-) gibbs free energy = if it is endergonic. My question is: Why is energy the ultimate determinant of spontaneity
  16. O

    Can Spontaneous Events Exist Without a Cause?

    Must everything have a cause? Or are spontaneous events possible, things that arise without any cause? First, it seems clear that whatever exist must have an effect of some sort on other things, otherwise they cannot be called real. Claiming otherwise is like saying that an infinite number of...