Calculate Gibbs free energy change.
Gibbs free energy combines the enthalpy drive with the entropy drive at a given temperature: delta G equals delta H minus T delta S. The sign of delta G, not of delta H, determines whether a reaction proceeds. Because the entropy term is multiplied by temperature, reactions with opposing enthalpy and entropy signs switch direction at a crossover temperature, and the free energy also fixes the equilibrium constant through delta G equals minus RT ln K.
Gibbs free energy and equilibrium
delta G = delta H - T x delta S / 1000; crossover T = delta H x 1000 / delta S; K = exp(-delta G x 1000 / (R T))
Because enthalpy is conventionally tabulated in kJ/mol while entropy is in J/(mol K). Forgetting that conversion is the single commonest error in these calculations and inflates the entropy contribution a thousandfold.
It is where the enthalpy and entropy terms exactly cancel. Below it one direction is favoured and above it the other. Ammonia synthesis, exothermic with negative entropy change, is spontaneous only below about 464 K.
No. Free energy describes the thermodynamic destination, not the rate. A strongly favourable reaction with a high activation barrier — diamond converting to graphite, for instance — proceeds immeasurably slowly.