What is the Difference Between Rate Law and Law of Mass Action?



The rate law is characterized by the following properties:

  • This is also known as Rate Equation and this is the equation for a chemical reaction that links the reaction rate with pressure concentrations for both reactants and parameters which are constant.
  • The Rate equation for a reaction which has multi step mechanism is derived in theory using the quasi steady state assumptions from some other elementary reactions and thus are compared with rate reaction which have been experimented.
  • This equation also involves fractional order and may also depend on concentration of intermediate species.
  • For a given chemical reaction the rate equation and the order for the given reaction can not be deduced from the stoichiometry and must however be calculated by experiments and since the reaction is unknown and the mechanism is either elementary or complex. While the rate equation which is experimental has been calculated it is often used for reaction mechanism deduction.
  • Single step elementary reactions and reaction steps have reaction orders which are equal to the stoichiometric coefficients for each of the reactant. The complete reaction order which is the sum of stoichiometric coefficients of the reactants is same to the molecular capacity of the elementary reaction.
  • Multistep reactions however may have or may not have reaction orders which are equal to the stoichiometric coefficients.
  • Suppose we take one constant W which is the reaction rate constant or rate coefficient of the reaction. The constant value may depend on conditions like temperature, ionic strength and surface area for the adsorbent. When the reaction is completed then the rate equation for the reaction rate is applied all through the course of the reaction.
  • For a Rate reaction he equation for the particular reaction with some multi step mechanism is often done by theory methods using a quasi state of steadiness and this is done from earlier reactions and it is compared to the rate equation for the assumed mechanism. This equation may also involve a fractional order and may depend on the species concentration.
  • If we have Zero order reaction the reaction rate is not dependent on the reactant concentration which is if there is concentration change there is no speed effect. Thus the concentration will change with time. Eg- when the reaction requires contact with an enzyme or surface which is catalytic.

Law of Mass Action:

The Law of Mass action has the following properties:

  1. It is the proportion where the rate of the reaction is directly proportional to the product of the reactant activities or concentration. They explains the solution behaviour in a dynamic equilibrium. For a Chemical reaction which is in equilibrium state, the ratio between the reactant concentration and the products is constant.
  2. This law is based on two things- First it is the equilibrium aspect which actually helps compose the reaction mixture at the point of equilibrium.Second is the kinetic energy which involves the rate equations of a reaction at point of equilibrium.
  3. Both the above conditions were derived using kinetic data and rate equation.
  4. The chemical reaction equilibrium is the dynamic process where the reaction rates for all the reactions must be equal to the chemical equilibrium. The expression for the rate equations were discovered by Jacobus Henricus.
  5. The law of mass action gives statement on equilibrium which is part of any chemical reaction. The Chemical equilibrium can also be derived from equilibrium thermodynamics and potential.
  6. Let us take example one chemical reaction where many reactive intermediates and through some reaction paths. We can define all reactions as a series of some elementary reactions and the rate reaction for each step is expressed by rate equation and can be derived from each step in the process. Then we correctly determine the equilibrium constant.
  7. The Law of Mass Action is used for Mathematical Ecology, Mathematical Epidemiology and Sociophyiscs.
  8. In the field of Biochemistry we see great development for the chemical reaction models. When the initial work on kinetics was done it was different and it was simple systems and it had reactants which were dilute and in aqua solutions.
  9. In more complex environments where we have more bound particles and it was dissociated from their surroundings and the diffusion is slow and anomalous and the model of mass action cannot be used always. Mass action model have been modified many times and not completely defined. Certain modifications have been made to it by replacing rate constants and with different concentrations. One such thought of school is that it is present in intracellular environments and under certain conditions but with certain other rates.

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