elementary step

Examples of elementary step in the following topics:

• Rate Laws for Elementary Steps

  • The rate law for an elementary step is derived from the molecularity of that step.
  • The sum of each elementary step in a reaction mechanism must yield the overall reaction equation.
  • The molecularity of the elementary step, and the reactants involved, will determine what the rate law will be for that particular step in the mechanism.
  • For now, just keep in mind that the rate laws for each elementary step are determined by the molecularity of each step only.
  • The molecularity of an elementary step in a reaction mechanism determines the form of its rate law.

• Rate-Determining Steps

  • The rate of a multi-step reaction is determined by the slowest elementary step, which is known as the rate-determining step.
  • However, most chemical reactions occur over a series of elementary reactions.
  • The complete sequence of these elementary steps is called a reaction mechanism.
  • The reaction mechanism is the step-by-step process by which reactants actually become products.
  • In kinetics, the rate of a reaction with several steps is determined by the slowest step, which is known as the rate-determining, or rate-limiting, step.

• Steady-State Approximation

  • In our discussion so far, we have assumed that every reaction proceeds according to a mechanism that is made up of elementary steps, and that there is always one elementary step in the mechanism that is the slowest.
  • This slowest step determines the rate of the entire reaction, and as such, it is called the rate-determining step.
  • In such a case, we must assume that the reaction rate of each elementary step is equal, and the overall rate law for the reaction will be the final step in the mechanism, since this is the step that gives us our final products.
  • Now, both of these rates can be written as rate laws derived from our elementary steps.
  • The first term takes into account the disappearance of N2O2 in the reverse reaction of the initial step, and the second term takes into account the disappearance of N2O2 as a reactant in the second elementary step.

• Heterogeneous and Multiple Equilibria

  • Both steps must be included in the equilibrium constant equation.
  • These equilibria can be split into two steps:
  • K1 and K2 are examples the equilibrium constants for each step.
  • Thus, for a reaction involving two elementary steps:
  • Calculate the equilibrium constant of a multiple-step reaction, given the equilibrium constant for each step

• The Equilibrium Constant

  • Assuming this reaction is an elementary step, we can write the rate laws for both the forward and reverse reactions:

• The Rate Law

  • For the general reaction$aA + bB \rightarrow C$ with no intermediate steps in its reaction mechanism, meaning that it is an elementary reaction, the rate law is given by:

• Overall Reaction Rate Laws

  • As discussed in the previous concept, if the first step in a reaction mechanism is the slow, rate-determining step, then the overall rate law for the reaction is easy to write, and simply follows the stoichiometry of the initial step.
  • Since the first step is the rate-determining step, the overall reaction rate for this reaction is given by this step: $\text{rate}=k[H_2][ICl]$.
  • If the rate-determining step is not the first step in the reaction mechanism, the derivation of the rate law becomes slightly more complex.
  • Step two is the slow, rate-determining step, so it might seem reasonable to assume that the rate law for this step should be the overall rate law for the reaction.
  • Combine elementary reaction rate constants to obtain equilibrium coefficients and construct overall reaction rate laws for reactions with both slow and fast initial steps

• Molecularity

  • If a chemical reaction proceeds by more than one step or stage, its overall velocity or rate is limited by the slowest step, the rate-determining step.
  • When we describe the mechanism of a chemical reaction, it is important to identify the rate-determining step and to determine its "molecularity".
  • The molecularity of a reaction is defined as the number of molecules or ions that participate in the rate determining step.
  • A mechanism in which two reacting species combine in the transition state of the rate-determining step is called bimolecular.

• Avogadro's Number and the Mole

  • Avogadro's number is defined as the number of elementary particles (molecules, atoms, compounds, etc.) per mole of a substance.
  • So, 1 mol contains 6.022×1023 elementary entities of the substance.

• Electrolytic Cells

  • A cell used in elementary chemical experiments to produce gas as a reaction product and to measure its volume.