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Multi-Step Reactions and Steady State Approximation (25.7)

Multi-Step Reactions and Steady State Approximation (25.7). When more than two steps are involved in a mechanism, the rate equation can become very complicated Multiple intermediates may form, and their concentrations depend on each other

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Multi-Step Reactions and Steady State Approximation (25.7)

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  1. Multi-Step Reactions and Steady State Approximation (25.7) • When more than two steps are involved in a mechanism, the rate equation can become very complicated • Multiple intermediates may form, and their concentrations depend on each other • Sometimes intermediates react with other species (e.g., reactants, intermediates) • If the intermediates are fairly reactive, then they do not attain an appreciable concentration • If an intermediate’s concentration is always small, then its change in concentration is also small throughout the experiment • The steady-state approximation (SSA) utilizes the zero change in concentration for intermediates to solve multi-step mechanisms • For every intermediate, a rate equation is formulated and set equal to zero • The overall rate is expressed in terms of rate of production of product

  2. Hydrobromic Acid from Hydrogen and Bromine • The formation of hydrobromic acid from hydrogen and bromine gas has a very complicated rate expression • The rate depends on the concentration of product (HBr) and on fractional orders of some reactants (Br2) • Many steps are proposed in the mechanism, some involving highly reactive radicals • SSA is used to eliminate radicals from the rate law • Start by setting up an equation for the rate of formation of product (HBr) • Rate of formation of each radical is set to zero according to SSA

  3. Intermediate Concentration Profile

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