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 the temperature and read what the line tells you
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. Track the sign of ΔH first, then see how the size and sign of −TΔS change as temperature rises.
ΔH starts negative. −TΔS is positive and grows as temperature rises.
Feasible below the crossoverΔH starts positive. −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.
ΔG positive at all temperaturesRead the graph, calculate the boundary and explain the range
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Check the activation-energy barrier separately
ΔG tells you whether a change is thermodynamically feasible under the stated conditions. Reaction rate depends on the activation-energy barrier and the available pathway.
OCR A: this kinetic limitation is required specification content. A reaction can have a negative ΔG and still be too slow to observe without a suitable pathway or catalyst.