Frequently Asked Questions
What does a negative Gibbs free energy mean?
A negative change in Gibbs free energy means the forward reaction is thermodynamically spontaneous under the given conditions. A value of zero means the system is at equilibrium, and a positive value favors the reverse direction.
Why is entropy divided by 1000 in the formula?
Enthalpy is usually given in kilojoules per mole while entropy is given in joules per mole per kelvin. Dividing entropy by 1000 converts it to kilojoules so the two terms share the same units.
Does spontaneous mean the reaction is fast?
No. Spontaneity only tells you the reaction is energetically favorable. The speed depends on kinetics and activation energy, so a spontaneous reaction can still be extremely slow.
At what temperature does a reaction become spontaneous?
Set ΔG = 0 and solve for temperature: T = ΔH ÷ ΔS, using ΔH and ΔS in matching energy units. This crossover temperature is where the sign of ΔG flips. When ΔH and ΔS share the same sign, the reaction is spontaneous above this temperature if both are positive, or below it if both are negative. For example, ΔH = 100 kJ/mol (100000 J/mol) and ΔS = 200 J/(mol·K) give T = 100000 ÷ 200 = 500 K as the switch point.
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