Explain the concept of spontaneity in chemical ... - SS2 Chemistry Chemical Thermodynamics Question

Spontaneity refers to the tendency of a chemical reaction to occur without any external influence. A spontaneous reaction proceeds on its own, releasing energy and progressing toward equilibrium. The spontaneity of a reaction depends on two main factors: enthalpy (ΔH) and entropy (ΔS).

If a reaction has a negative ΔH (exothermic, releases heat) and a positive ΔS (increase in disorder), it will be spontaneous at all temperatures. For example, the combustion of gasoline (exothermic) and the dissolution of sugar in water are spontaneous processes.

Conversely, a reaction with a positive ΔH (endothermic, absorbs heat) and a negative ΔS (decrease in disorder) is non-spontaneous at all temperatures. An example is the decomposition of water into hydrogen and oxygen gas.

However, if a reaction has a positive ΔH and a positive ΔS, it can still be spontaneous at high temperatures. The increase in temperature allows the entropy term to dominate, driving the reaction forward. An example is the Haber-Bosch process, where nitrogen and hydrogen react to form ammonia.