Entropy

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Entropy

Spontaneous Reactions: reactions that given the necessary Ea, proceed to completion without continuous assistance Enthalpy and Spontaneity
* exothermic reactions tend to be spontaneous
* Endothermic reactions can be either
* Electrolysis of water is non-spontaneous it will stop if you stop the supply of energy * DD reaction between ammonium nitrate and barium hydroxide is spontaneous, even through the Entropy

* Measure of disorder or randomness
* Can apply to system , surrounding or universe as whole

* S is greater when solute dissolves in a solvent
* S is greater when a liquid evaporate in a vapour
* S is greater when solid melts into a liquid
When a change has a S > 0 the change is more likely to be spontaneous. Entropy of a perfectly ordered crystal at absolute 0 is 0 and so absolute values of entropy can be measured relative to this S > 0

The volume of gaseous system increases
The temperature of a system increases
The physical state of a system changes from solid to liquid or gas, or liquid ΔS = sigma (S – products) – sigma (S - reactants)
Ex. Consider the completion combustion of methane
S(CH4(g))= 186 , S(O2(g)) = 205 , S(H2O(l)) = 70, S(CO2(g)) = 214 (all in J/K mol) CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
ΔS = (214 + 2(70)) – (186+2(205))
=-242 J/K mol

Enthalpy, entropy and spontaneity
Known that when H < 0 and S > 0 the change will be spontaneous Exothermic reactions (H < 0) involving increasing in entropy (s > 0) are spontaneous Endothermic reactions (H>0) involving a decrease in entropy (s < 0) are non-spontaneous

What about when H < 0 and S < 0
Temperature becomes important aas well as an understanding of the Second and Third Laws of Thermodynamics Gibbs free energy
I general a change at constant temp and pressure will occur spontaneously if its accompanied by a decrease in Gibbs Free energy G, change are spontaneous if the change in G, G < 0 ( G...
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