ΔH is negative and −TΔS is also negative. ΔG stays negative at every positive temperature.
Feasible at all temperaturesYear 13 · AQA & OCR A
Gibbs free energy and temperature
Change the temperature and watch ΔG move. The graph turns ΔG = ΔH − TΔS into three things you can read directly: the y-intercept is ΔH, the gradient is −ΔS, and the x-intercept is the crossover temperature.
Move along the ΔG–T line
Choose a reaction or one of the four sign combinations, then drag the temperature.
ΔG at this temperature—
Forward reaction—
y-intercept—ΔH
gradient—−ΔS
crossover——
Derive the four ΔH / ΔS sign combinations
Start from ΔG = ΔH − TΔS and ask what each term does as temperature rises.
ΔH starts negative, but −TΔS is positive and grows as temperature rises.
Feasible below the crossoverΔH starts positive, but −TΔS is negative and grows in magnitude as temperature rises.
Feasible above the crossoverΔH is positive and −TΔS is also positive. ΔG stays positive at every positive temperature.
Not feasible at any temperatureRead the graph, calculate the boundary and explain the range
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Feasible is not the same as fast
ΔG tells you whether a change is thermodynamically feasible under the stated conditions. It says nothing about how high the activation-energy barrier is.
OCR A: this kinetic limitation is explicit specification content. A reaction can have a negative ΔG and still be too slow to observe without a suitable pathway or catalyst.