Sign combinations of delta H and delta S decide whether a reaction is spontaneous at high or low temperature.
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When delta H and delta S have the same sign, the spontaneity of a reaction changes with temperature, and the crossover occurs at T equal to delta H divided by delta S, where delta G is zero. Above this temperature the term minus T times delta S dominates if delta S is positive, so entropy-driven processes become spontaneous at high temperature. Below it, the enthalpy term dominates, so enthalpy-driven processes are favored at low temperature. The sign pattern of delta H and delta S therefore places a reaction into one of four categories, two of which are always spontaneous or always nonspontaneous. This analysis is useful for predicting industrial conditions such as calcination.
Key ideas: Find the switch temperature by setting delta G to zero: T equals delta H divided by delta S. If both are positive the reaction is spontaneous above that temperature, and if both are negative it is spontaneous below it. This estimate assumes delta H and delta S are temperature independent.
Common mistakes and connections: Beware of that approximation over large ranges, and of converting entropy units incorrectly. In industry the idea is used to choose temperatures, for example to decompose limestone in a kiln or to avoid decomposition of ammonia at high temperature.
Decomposition of CaCO3 has delta H = +178 kJ and delta S = +161 J/K, so it becomes spontaneous only above about 1100 K, where TS exceeds H. The interactive lesson shows this as a live animation or chart.
Calcium carbonate decomposes to calcium oxide and carbon dioxide only at high temperature, becoming spontaneous above about 830 degrees Celsius.
Sign combinations of delta H and delta S decide whether a reaction is spontaneous at high or low temperature.
Temperature Dependence of Spontaneity is a advanced topic in Thermodynamics & Energetics. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Thermodynamics & Energetics first: Entropy of Phase Change, Gibbs Free Energy, Gibbs Energy and Spontaneity.
The next topics in Thermodynamics & Energetics are Standard Free Energy of Formation and Free Energy and Equilibrium.
Yes. The interactive lesson includes a self-check question on Temperature Dependence of Spontaneity, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Temperature Dependence of Spontaneity is topic 39 of 91 in Thermodynamics & Energetics.