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reaction spontaneity ques

my book says "all reactions that are highly exothermic AND lead to greater disorder are spontaneous at all tempretures..."

isnt it impossible for it to be exothermic and give higher disoder at the same time.... exothermic rxns results in products that are more stable (have less energy eg. solids) than the reactants ----> which means less disorder.. and more disorder means formation of gas for eg. which means endothermic rxn... so i dont understand the sentence it does not make sense to me
(edited 7 years ago)
Original post by pondsteps
my book says "all reactions that are highly exothermic AND lead to greater disorder are spontaneous at all tempretures..."

isnt it impossible for it to be exothermic and give higher disoder at the same time.... exothermic rxns results in products that are more stable (have less energy eg. solids) than the reactants ----> which means less disorder.. and more disorder means formation of gas for eg. which means endothermic rxn... so i dont understand the sentence it does not make sense to me


Disorder refers to the entropy, which can be considered to be the degree of disorder. Gases have more entropy than solutions, which in turn have more entropy than liquids and solids.

This is a simplification, but useful at this level.

So a reaction can be exothermic, releasing heat energy, while at the same time reducing the entropy of the system. One example would be the dissolution of HCl gas in water. This is highly exothermic but leads to a reduction in the entropy of the system.

The reaction is overal spontaneous because the energy released increases the entropy of the surroundings MORE than the reduction of entropy in the system.
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Avatar for shengoc
shengoc
12
Original post by pondsteps
my book says "all reactions that are highly exothermic AND lead to greater disorder are spontaneous at all tempretures..."

isnt it impossible for it to be exothermic and give higher disoder at the same time.... exothermic rxns results in products that are more stable (have less energy eg. solids) than the reactants ----> which means less disorder.. and more disorder means formation of gas for eg. which means endothermic rxn... so i dont understand the sentence it does not make sense to me


delta G = delta H - T * delta S

for a specific reaction that has
delta H < 0
delta S > 0

then T* delta S < 0 at all T

that means delta G is < 0 at all T

any changes in T does not affect spontaneity at all because the reaction is already spontaneous/feasible at all T.
thank you!! xxxx

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