# Generally a reaction is said to occur spontaneously

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Generally, a reaction is said to occur spontaneously if the entropy increases.Entropy (S) is a measure of the disorder in a system.Spontaneous reactions or processes happen “automatically”, and do notrequire an input of energy to occur. Spontaneous processes have a great tendency toproceed in one direction. In a closed system, entropy always increases over time. Inan open system, energy can be added to a system to cause a decrease in entropy,but this is not necessarily a spontaneous reaction.You know already that entropy may be definedas a measure of how much the energyof the particles become more spread out in a process. Now, how is entropy related tothe spontaneity of a reaction?
Determination of Spontaneity based on Entropy:There are three factors that combine to predict spontaneity.They are:1. Energy Change2. Temperature3. Entropy ChangeThe driving force for a spontaneous process is an increase in theentropy ofthe universe. The best indicator of spontaneity in a reaction is the change inEntropy. (S orS).Remember:Spontaneity of a process can be determined from the change in entropyand change in free energy of a system.Applicationof SpontaneityThe application of spontaneity with respect to equilibrium is as follows:A reaction will never spontaneously move away from equilibrium and willalways move spontaneously towards equilibrium.Although spontaneous reactions are often exothermic, this is not a criteria forspontaneity.Entropy change measures the dispersal of energy: how much energy is spreadout in a particular process, or how widely spread out it becomes (at a specifictemperature).This is reflected in the idea that a spontaneous change alwaysresults in the dispersal (spreading out) of energy (and also matter).You see now
how hot pans cooling and chemical reactions belong to the ‘how much' categorywhere energy is being transferred. Coffee in cream and gas expansion and perfumein air are ‘how widely' processes where the initial energy of the molecules stay thesame but the volume occupied by the molecules increases. You have probably heardthe phrase “the ever expanding Universe”, although this phrase is often taken tomean our ever increasing knowledge of our Universe, it also is quite true that ourUniverse is expanding and this effect is entropic in nature.ACTIVITIES

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