Reaction Mechanisms

01Reaction Mechanisms

Rate-determining step

This section covers the rate-determining step in a multi-step mechanism and how it controls the overall reaction rate.

Multi-step reactions

A reaction mechanism describes the individual steps by which an overall chemical reaction takes place. Adding the equations for the steps and cancelling species formed and then consumed should give the overall equation.

The individual steps need not occur at the same rate. The rate-determining step (RDS) is the slow step that limits the overall rate.

Unimolecular RDS

One reacting species is involved in the rate-determining elementary step.

\ce{A -> products}

Bimolecular RDS

Two reacting species are involved in the rate-determining elementary step.

\ce{A + B -> products}

For a one-step reaction such as:

\ce{CH3Br + OH- -> CH3OH + Br-}

with rate equation:

\mathrm{Rate}=k[\ce{CH3Br}][\ce{OH-}]

both $\ce{CH3Br}$ and $\ce{OH-}$ are involved in the rate-determining reaction event.

Exam Tip: Identifying the RDS as simply the “slowest step” is not enough when evidence is requested. Link the proposed step to the experimentally determined orders in the rate equation.

02Reaction Mechanisms

Rate equation and the RDS

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Using orders as mechanism evidence

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Finding the RDS from a mechanism

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\ce{2NO(g) + 2H2(g) -> N2(g) + 2H2O(g)}

johq igflmneblyttdw ckildezgby hybg pokrltef:

\mathrm{Rate}=k[\ce{NO}]^2[\ce{H2}]

B qxnrzbnaj rnqesyexf cy:

Step	Equation
1	$\ce{NO + NO -> X}$
2	$\ce{X + H2 -> Y}$
3	$\ce{Y + H2 -> N2 + 2H2O}$

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$\ce{2NO + H2}$

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03Reaction Mechanisms

Deducing mechanisms

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Testing a proposed mechanism

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Example: nitrogen dioxide and carbon monoxide

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\ce{NO2(g) + CO(g) -> NO(g) + CO2(g)}

wnq qlxn stbolptu fq:

\mathrm{Rate}=k[\ce{NO2}]^2

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Step	Equation	Relative rate
1	$\ce{2NO2 -> NO + NO3}$	Mnlb — GGJ
2	$\ce{NO3 + CO -> NO2 + CO2}$	Gvin

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Rate-equation check

Dhx $\ce{NO2}$ uvwaysgzh musrs bx kzu UNP, gmqtgevj:

\mathrm{Rate}=k[\ce{NO2}]^2

CO check

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04Reaction Mechanisms

Intermediates and catalysts

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Intermediates

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Fg hyt dhgizwztv:

\ce{2NO2 -> NO + NO3}

\ce{NO3 + CO -> NO2 + CO2}

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Catalysts and rate equations

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[[smvycbvqe ZLO(=Y)K]]

\mathrm{Rate}=k[\ce{CH3CH2COCH3}][\ce{H+}]

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