What characterizes a non-spontaneous reaction?

A non-spontaneous reaction is characterized by its requirement for an input of energy to occur, which distinguishes it from spontaneous reactions that proceed on their own without external energy. The defining characteristic of a non-spontaneous reaction is that it absorbs energy from the surroundings, meaning that the total energy of the system increases as the reaction occurs. This process is often endothermic, where the system takes in heat, but it can also involve other forms of energy depending on the context.

Spontaneous reactions typically release energy and can occur without additional energy input; thus, the concept of energy absorption directly speaks to the nature of non-spontaneous reactions. It is important to understand that while all reactions have activation energy barriers, non-spontaneous reactions generally do not proceed unless energy is continuously provided to overcome these barriers.