AQA Chemistry A-level Year 2
the rate constant can be derived, it is enough to recall that a reaction of order n + m has the rate equation: rate = k × (concentration)n+m Table 1 Order of reaction and the units of the rate constant
Notes The initiation step involves only a single molecule reacting on its own; this is called a unimolecular step. Both the propagation steps involve two species reacting together; these are called bimolecular steps. Termolecular steps (three species reacting simultaneously) are possible, but extremely rare.
Notes Although the rate equation for a reaction cannot be deduced from the stoichiometry of the reaction equation (the rate equation is always derived experimentally), the rate of any step in a mechanism is always proportional to the concentrations of the reactant species in that step, raised to the appropriate powers. Thus, in the chlorination chain reaction, it is possible to write for the initiation step:
Order of reaction = n + m
Rate equation rate = k × [reactants]n + m
Units of the rate constant
0
rate = k × concentration0
mol dm−3 s−1
1 2 3 4
rate rate rate rate
= = = =
k k k k
× × concentration2 mol−1 dm3 s−1 × concentration3 mol−2 dm6 s−1 × concentration4 mol−3 dm9 s−1 concentration1 s−1
It is by considering kinetic data that the orders of reactions can be deduced. Information of this kind is very important in deciding how to maximise the rate of a reaction, for example in an industrial process. Increasing the concentration of Q in the example above, for instance, would have no effect on the rate and to do so would be a waste of money.
Reaction mechanisms and the rate-determining step Reaction mechanisms Chemical reactions rarely occur by the simple and straightforward route suggested by the overall stoichiometric equation. Most reactions occur in two or more steps which, when combined, produce the equation for the overall reaction. One of the major tasks of reaction kinetics lies in providing evidence to support or refute the validity of such proposed reaction steps. A proposed sequence of simple reaction steps is known as a reaction mechanism.
Definition The reaction mechanism for a reaction consists of a proposed sequence of discrete chemical reaction steps that can be deduced from the experimentally observed rate equation.
Reactions that occur in steps, and reaction mechanisms, have already been introduced in Collins Student Support Materials: AS/A-Level year 1 – Organic and Relevant Physical Chemistry, section 3.3.2.4, where a free-radical substitution mechanism is invoked to account for the observed chain-reaction kinetics of the direct chlorination of methane: Initiation: Cl2 → 2Cl• Propagation:
Cl• + CH4 → •CH3 + HCl
rate = k1[Cl2]
•CH3 + Cl2 → CH3Cl + Cl•
which is unimolecular, and therefore a first-order step, and for the first propagation step:
Overall: CH4 + Cl2 → CH3Cl + HCl
rate = k2[CH4][Cl•] which is bimolecular, and therefore a second-order step.
The rate-determining step In some reactions, the experimental rate equation seems directly related to the process as written in the overall stoichiometric equation. A good example of this is the hydrolysis of 1-bromobutane: CH3CH2CH2CH2Br + OH− → CH3CH2CH2CH2OH + Br−
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