EasyChemistryHigh School (11–12)Chemical Thermodynamics › Entropy, Gibbs Energy and SpontaneityConceptual

Four reactions have these signs. (a) Δ H < 0, Δ S > 0.

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Four reactions have these signs. (a) $\Delta H < 0$ , $\Delta S > 0$ . (b) $\Delta H > 0$ , $\Delta S < 0$ . (c) $\Delta H < 0$ , $\Delta S < 0$ . (d) $\Delta H > 0$ , $\Delta S > 0$ . For each, say whether it is spontaneous at all temperatures, at none, only at low temperature or only at high temperature.

Need a reminder? The key ideaShow

Gibbs energy: $G = H - TS$ , and at constant temperature $\Delta G = \Delta H - T\Delta S$ . $\Delta G < 0$ : spontaneous; $\Delta G = 0$ : equilibrium; $\Delta G > 0$ : not spontaneous. When $\Delta H$ and $\Delta S$ have the same sign, $\Delta G$ changes sign at $T = \Delta H/\Delta S$ . Put $\Delta S$ in kJ/(K mol), or $\Delta H$ in J/mol, before combining them.

The worked solution

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← Magnesium carbonate decomposes: MgCO₃(s) → MgO(s) + CO₂(g), with Δ H° = +117.6 kJ/mol and Δ S° =…At 298 K a reaction has Δ G° = -17.1 kJ/mol. →

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Four reactions have these signs. (a) Δ H < 0, Δ S > 0.

The worked solution

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