Amount of energy that must be placed into a Definable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with to form an activated complex in a reaction. On an Convenient way of depicting the thermodynamics of reactions. Typically, the y-axis is given in units of energy, and the x-axis is defined as the reaction coordinate (see left). The reaction coordinate is a very general entity which you can think of as the progress of the reaction. It is similar, this is the difference in energy between the reactants and the Maximas (peaks) on an energy diagram. A transition state represents the highest energy arrangement of atoms between two more stable forms. The energy required to pass through a transition state may be called the activation energy, ΔGz.. The activation energy is often perceived as an energy barrier which must be overcome for a reaction to proceed. The presence of a catalytic agent can markedly reduce activation energy.