Use the signs of H and S to explain why ice spontaneously melts at room temperature but

Use the signs of ΔH and ΔS to explain why ice spontaneously melts at room temperature but not outside on a freezing winter day.

Strategy This problem calls for the same type of reasoning used to construct Table 10.2. We must determine whether the process is endothermic or exothermic and whether it increases or decreases the entropy of the system. Then, by considering the signs of ΔH and ΔS in conjunction with Equation 10.4, we can attempt to explain the behavior described.

Table 10.2

Equation 10.4

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The four possible combinations for the signs of AH and AS Sign of AH Sign of AS Implications for Spontaneity Spontaneous at all temperatures Never spontaneous Spontaneous only at low temperatures Spontaneous only at high temperatures + 1 + + 1 +