ChemBench

Gibbs Free Energy Calculator

Will this reaction happen on its own?

ΔH enthalpy (J)

Temperature (K)

ΔS entropy (J/K)

Gibbs free energy

-20,200 J

What you entered

ΔH enthalpy (J): -50000Temperature (K): 298ΔS entropy (J/K): -100

1
T × ΔS
298 × -100= -29800 J
2
ΔG = ΔH − TΔS
-50000 − -29800= -20200 J

Result

Gibbs free energy: -20,200 J

ΔG is -20,200 J — negative, so this reaction is spontaneous.

Gibbs free energy and whether a reaction is spontaneous

Delta G equals delta H minus T delta S. A negative result means the reaction is spontaneous in that direction at that temperature.

Delta H	Delta S	Temperature	Delta G	Spontaneous?
-285.8 kJ	-163 J/K	298.15 K	-237.20 kJ/mol	Yes
-92 kJ	-198 J/K	298.15 K	-32.97 kJ/mol	Yes
-92 kJ	-198 J/K	700 K	+46.60 kJ/mol	No
0 kJ	+50 J/K	298.15 K	-14.91 kJ/mol	Yes
+50 kJ	+100 J/K	298.15 K	+20.19 kJ/mol	No
+50 kJ	+100 J/K	600 K	-10.00 kJ/mol	Yes

The first row is the formation of liquid water, whose standard free energy is -237.2 kJ/mol. Rows two and three are the Haber process: exothermic but entropy-losing, so it turns non-spontaneous when hot, which is why ammonia synthesis trades yield for rate.

One number decides spontaneity

A negative ΔG means a reaction is thermodynamically spontaneous (it will proceed without outside energy input); a positive ΔG means it won't happen without help — this single calculation predicts reaction feasibility.

The tug-of-war between enthalpy and entropy

ΔG = ΔH − TΔS shows spontaneity is a balance: reactions that release heat (negative ΔH) or increase disorder (positive ΔS) are favored, and temperature controls how much the entropy term matters.

Frequently asked questions

A reaction has ΔH = -100 kJ/mol and ΔS = +200 J/(mol·K) at 298 K — is it spontaneous?

ΔG = -100,000 - (298 × 200) = -100,000 - 59,600 = -159,600 J/mol = -159.6 kJ/mol. Strongly negative, so yes — spontaneous at 298 K.

How is Gibbs free energy related to the equilibrium constant?

ΔG° = -RT ln K. A large negative ΔG° means a large K (products strongly favored). Use the equilibrium constant calculator to convert between ΔG° and K directly.

Can a reaction be spontaneous at one temperature but not another?

Yes. If ΔH and ΔS have the same sign, the TΔS term can flip ΔG's sign at a crossover temperature T = ΔH/ΔS. Ice melting (ΔH > 0, ΔS > 0) becomes spontaneous above 273 K.

Does spontaneous mean fast?

No. Spontaneous means thermodynamically favorable (ΔG < 0), not kinetically fast. Diamond converting to graphite is spontaneous but takes geological time. Reaction speed depends on activation energy, not ΔG.