8303141-049-Rate-Ordersmech - Chem Factsheet April 2003...

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Rate Orders and Mechanisms of Chemical Reactions C hem F actsheet April 2003 Number 49 1 Before working through this Factsheet you should: understand the principles of kinetics covered so far at AS and A2 (Factsheets 10, 43 and 44). ensure you are fully competent in writing rate expressions and finding orders of reactions. After working through this Factsheet you will be able to: identify the mechanism for the nucleophilic substitution of halogenoalkanes by hydroxide ions from the rate expression and its order; write the two mechanisms for nucleophilic substitution and be able to explain how each type gives rise to its rate order; recognise which halogenoalkane is likely to undergo which type of mechanism; quote the Arrhenius Equation and explain its use in reaction kinetics. Reaction kinetics 1. The balanced (stoichiometric) chemical equation can tell you nothing about how fast or how slow (kinetics) the reaction will proceed – it has to be from experiments using the reactants. 2. The data from experiments is obtained by measuring the ‘initial rate’ whilst varying the concentrations (mol dm -3 ) of the reactants. 3. Only the reactants involved in the slowest step (the ‘rate determining step’) appear in the rate expression. 4. The rate expression is of the form rate = k × [A] a × [B] b k = rate constant a and b are called ‘orders’ a + b = ‘overall order’ of the reaction the units of k need to be worked out for each expression (rate units = mol dm -3 s -1 , [ ] = mol dm -3 ) 5. At A2 level ‘orders’ are 0,1 or 2 only. 6. The Maxwell-Boltzmann curve shows that increasing the temperature speeds up the rate of reaction, not just because the number of collisions increase (particles moving faster) but more of the particles have the minimum energy (activation energy = E A ) to produce a reaction when they do collide. ‘nucleophilic substitution’ reactions/mechanisms Substitution - when an atom/group in a molecule is removed and then replaced by another atom/group. Basically – ‘swapping parts’ .
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8303141-049-Rate-Ordersmech - Chem Factsheet April 2003...

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