How to tell if a reaction is spontaneous

Spontaneity is set by the sign of ΔG, not by ΔH or ΔS alone. Use ΔG = ΔH − TΔS (with T in kelvin and ΔS converted from J to kJ). If ΔG is negative the reaction is spontaneous (exergonic) and releases free energy; if positive it is non-spontaneous (endergonic) and needs an input of energy; if zero the system is at equilibrium. Because temperature multiplies the entropy term, some reactions switch spontaneity at a crossover temperature of T = ΔH / ΔS. In biochemistry, the actual ΔG also depends on concentrations through ΔG = ΔG°′ + RT ln Q, which is why a reaction with a positive ΔG°′ can still run forward when products are kept low.

Constants: R = 8.314 J/mol·K = 0.008314 kJ/mol·K. 0 °C = 273.15 K. Standard biochemical conditions use ΔG°′ (pH 7).