A diagram showing the relative standard @G02629@ of reactants, @T06468@, reaction @I03096@ and products, in the same @ST06775@ as they occur in a chemical reaction. These points are often connected by a smooth curve (a '@G02633@', commonly still referred to as a '@F02515@ 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 @ST06775@ of reactants, products, reaction intermediates and transition states and is usually undefined or only vaguely defined by the @R05168@ (extent of bond breaking or bond making). In some adaptations the abscissas are however explicitly defined as @B00707@, Brønsted exponents, etc. Contrary to statements in many text books, the highest point on a Gibbs energy diagram does not necessarily correspond to the @T06468@ of the @R05140@. For example, in a @S05970@ consisting of two reaction steps:
G02630-1.pngone 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 @S05925@, then the rate-limiting step is the one of highest Gibbs energy.
potential-energy (reaction) surface