Final answer:
A nonspontaneous reaction requires external energy input to occur and tends to be endothermic or have a decrease in entropy, unlike spontaneous reactions which occur naturally and favor product formation without continual external energy input.
Step-by-step explanation:
A nonspontaneous reaction is one that does not have a natural tendency to occur under a given set of conditions and requires an external input of energy to proceed. Contrary to a spontaneous reaction, which occurs naturally and favors the formation of products, a nonspontaneous reaction supports the reactants and typically is endothermic (absorbs energy) or results in a decrease in entropy (disorder), or both. This can be contrasted with spontaneous reactions, which release free energy and move to a more stable state without the need for continual external energy input.
An example of a nonspontaneous process is the mixture of nitrogen and oxygen gases in our atmosphere not readily combining to form nitrogen monoxide without the provision of energy. Additionally, all reactions require a form of initial energy to reach the transition state known as the activation energy, even spontaneous reactions that have a negative change in free energy (ΔG). However, once the activation energy barrier is overcome, spontaneous reactions will continue on their own, while nonspontaneous reactions will not.
It's essential to distinguish between spontaneous reactions, which can occur without continuous external energy, and nonspontaneous reactions which necessitate such energy. While nonspontaneous reactions are often energetically unfavorable, they can still be significant in various scientific and industrial contexts when appropriately driven.