Affinity and Reaction Rates: Reconsideration of Theoretical Background and Modelling Results

Abstract

The phenomenological affinity approach to chemical kinetics based on mass-action rate expression is revised. It is shown that the reaction rate is not an unambiguous function of affinity and that in ideal mixtures with only elementary reactions thermodynamic coupling, i. e. a positive reaction rate and negative affinity of some reaction step at the same time, is not possible. Neither does thermodynamic coupling occur in a non-ideal system of elementary reactions where the mass-action rate equation is written with activities in place of concentrations. The non-ideality and/or non-equality of reaction orders to stoichiometric coefficients lead to more complex affinity-rate relationships than commonly given which puts no explicit restrictions on affinity and rate signs. Theoretical considerations are completed with results of numerical modelling made on several simple mechanisms. Various combinations of affinity and rate signs and complex affinity-rate profiles were obtained. Modelling results support the idea that affinity is much more a result of the time evolution of a reacting system and corresponding time profiles of concentrations than the actual cause of reaction rates.

Path from Reaction Control to Equilibrium Constraint for Dissolution Reactions

“…The symbols r⃗ and r⃖ refer to the rates of the forward and reverse reactions, respectively, and R and T are the universal gas constant and the absolute temperature, respectively. Inspection of eq 1 confirms that the chemical affinity is defined by eq 2, 10,11 which is sometimes referred to as the De Donder equation. 12…”

“…Inspection of eq confirms that the chemical affinity is defined by eq , , which is sometimes referred to as the De Donder equation The chemical affinity can be interpreted as a driving force for chemical reactions in irreversible thermodynamics and has units of kJ/mol.…”

“…For an elementary reversible reaction, the ratio of the rates of the forward to the reverse reaction is equal to the activity quotient ( Q ) divided by the equilibrium constant ( K ) so that the chemical affinity can also be written as eq . − Elementary reactions by definition must obey the Guldberg and Waage mass action kinetics, , in which the orders of reaction are equal to the absolute value of the stoichiometric coefficients written as positive integers. Thus, the combination of eqs and describes the path between reaction control and equilibrium constraint for an elementary reversible reaction. At equilibrium, Q / K is equal to 1, A is equal 0, and the net rate of dissolution is 0.…”

A Non-Electroneutral Model for Complex Reaction-Diffusion Systems Incorporating Species Interactions

“…The local affinity quantifies how far a particular reaction is from equilibrium at a point in space as well as the direction the system must move to reach equilibrium, under which condition its value is zero. The affinity is related to the ratio of the forward and reverse elementary rates, R f j and R r j respectively, by the relationship [23][24][25]…”

The mechanism of dissolution of minerals in acidic and alkaline solutions: Part IV equilibrium and near-equilibrium behaviour

“…It should be noted that the chemical affinity has the same mathematical form as the Gibbs free energy, but differs from it because it contains Q. Eq. (6) is exact for elementary reversible reactions, and succinctly expresses the relationship between thermodynamics and kinetics, defining the thermodynamic driving force for chemical reactions in terms of irreversible thermodynamics (Prigogine et al, 1948;Boudart, 1976Boudart, , 1983Helgeson et al, 1984;Prigogine and Kondepudi, 1999;Pekar, 2009). It has been used almost exclusively as the basis of the analysis of dissolution under conditions close to equilibrium (Oelkers, 2001).…”