A diagram showing the relative standard @[email protected] of reactants, @[email protected], reaction @[email protected] and products, in the same @[email protected] as they occur in a chemical reaction. These points are often connected by a smooth curve (a '@[email protected]', commonly still referred to as a '@[email protected] profile') but experimental observation can provide information on relative standard Gibbs energies only at the maxima and minima and not at the configurations between them. The abscissa expresses the @[email protected] of reactants, products, reaction intermediates and transition states and is usually undefined or only vaguely defined by the @[email protected] (extent of bond breaking or bond making). In some adaptations the abscissas are however explicitly defined as @[email protected], Brønsted exponents, etc. Contrary to statements in many text books, the highest point on a Gibbs energy diagram does not necessarily correspond to the @[email protected] of the @[email protected] For example, in a @[email protected] consisting of two reaction steps: one of the transition states of the two reaction steps must (in general) have a higher standard Gibbs energy than the other, whatever the concentration of D in the system. However, the value of that concentration will determine which of the reaction steps is rate-limiting. If the particular concentrations of interest, which may vary, are chosen as the @[email protected], then the rate-limiting step is the one of highest Gibbs energy.