What a learner can do afterwards
- Calculates a free energy change from enthalpy and entropy values with the units made consistent first
- Reads the sign to say whether a reaction is feasible at that temperature
- Finds the temperature at which a reaction becomes feasible by setting the free energy change to zero
- Explains why a feasible reaction can still be far too slow to see
1 · Read
Gibbs free energy combines enthalpy and entropy into one test. Delta G equals delta H minus T times delta S, with T in kelvin. Negative means the reaction is feasible at that temperature, positive means it is not. For pure ice and water at normal pressure, that is why melting runs on its own just above 0 degrees Celsius while freezing runs on its own just below.
Watch the units before you touch the maths. Enthalpy usually arrives in kilojoules per mole while entropy comes in joules per mole per kelvin. Convert one of them first or the answer lands wrong by a factor of a thousand.
Set delta G to zero and solve: T equals delta H divided by delta S. Endothermic changes that gain entropy turn feasible when hot. Exothermic changes that lose entropy stay feasible only when cold.
Feasible never means fast. Diamond turning to graphite is feasible at room temperature yet far too slow to see, because few particles clear the activation barrier. Thermodynamics says if, kinetics says when.
Balance enthalpy against temperature weighted entropy, fix the units first, and never read speed from the sign.
2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.