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mikael mikael
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14 years ago
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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14 years ago
a.   Because the reaction is endothermic, Δ H is positive. Entropy increases as the solid becomes a gas and a liquid, soΔ  S is positive. Since Δ S favors a spontaneous reaction but Δ H does not, to maximize ΔS, 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  ΔS 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, Δ H is positive. A solid turning to a gas results in an increase in entropy, so  S is positive. While Δ S is favorable for a spontaneous reaction, Δ H is not. To maximize the effect of Δ S, high temperatures are required. Therefore this reaction will be spontaneous at high temperatures.
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