8303123-043-Rate-Equations-and-Constants

8303123-043-Rate-Equations-and-Constants - Chem Factsheet...

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C hem F actsheet January 2003 Number 43 1 To succeed with this topic you need to: Know and understand the AS material on kinetics - collision theory activation energy Maxwell-Boltzman graph Factors affecting rates of reaction Catalysts and energy profiles covered in Factsheet No. 10 (Kinetics I) After working through the Factsheet you will be able to: understand the parts making up the rate equation (or expression); be able to find the orders for a particular chemical reaction from the data given for it; be able to perform calculations based on the rate equation; be able to work out the units for the rate constant, k . Rate expressions (equations) The balanced chemical equation (the stoichiometric equation) can tell you nothing about the rate expression. The rate expression can only be worked out from the data provided by reaction rate experiments. Let us consider a reaction of substances A, B and C where the equation is A + B + 2C 2D + E The rate expression (or equation) works out to be [ ] = concentration i.e. mol dm -3 'orders' rate = k × [A] 2 × [B] 1 rate constant units of mol dm -3 s 1 In words: The reaction is 2 nd order with respect to A and 1 st order with respect to B. Overall, the reaction is 3 rd order (2 + 1) Note the following points about this example: (1) We would mormally write [B], not [B] 1 . The 1 is put in to make it clear (2) C does not appear in the rate expression – it is ‘0 order’ and if shown would look like this [C] 0 (3) the expression would normally be shown as rate = k [A] 2 [B] with the ‘ × ’ symbol removed. Finding the orders from experimental data
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8303123-043-Rate-Equations-and-Constants - Chem Factsheet...

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