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Under what temperature conditions are the following reactions spontaneous?

mikael

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14 years ago

Under what temperature conditions are the following reactions spontaneous?

Under what temperature conditions are the following reactions spontaneous?
a. NH4Cl(s) + OH–(aq) Rightwards Arrow

NH3(g) + H2O(l) + Cl–(aq) endothermic
b. 2 NO(g) + O2(g) Rightwards Arrow

2 NO2(g) exothermic
c. CaCO3(s) Rightwards Arrow

CaO(s) + O2(g) endothermic

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chemb

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14 years ago

a. Because the reaction is endothermic,

${\Delta}$ H is positive. Entropy increases as the solid becomes a gas and a liquid, so

${\Delta}$ S is positive. Since

${\Delta}$ S favors a spontaneous reaction but

${\Delta}$ H does not, to maximize

${\Delta}$ S, this reaction is spontaneous at high temperatures.

b. Because this reaction is exothermic, the sign on H is negative. Going from more moles of gas to fewer decreases entropy, so

${\Delta}$ S is negative. The enthalpy encourages spontaneity; entropy discourages it. To minimize entropy, this reaction will be spontaneous at low temperatures.

c. Because this reaction is endothermic,

${\Delta}$ H is positive. A solid turning to a gas results in an increase in entropy, so S is positive. While

${\Delta}$ S is favorable for a spontaneous reaction,

${\Delta}$ H is not. To maximize the effect of

${\Delta}$ S, high temperatures are required. Therefore this reaction will be spontaneous at high temperatures.

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