rates of reaction

following edexcel 2008 spec

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  • Created by: danni :)
  • Created on: 05-06-13 08:09

key terms

rate of reaction:  the rate of change of concentration of a reactant or product with time

average rate= change in concentration

                         change in time

rate equation:  Rate (r)         r\; =\; k[\mathrm{A}]^x[\mathrm{B}]^y (http://upload.wikimedia.org/math/2/3/a/23a06c4ee205d7ea62ff463805c6b085.png)

order of reaction: the sum of the powers to which the concentrations of reactants are raised  in the experimentally determined rate equation

rate constant: k is the rate constant and varies with the nature of the reaction, the temperature and the presence of a catalyst

 half-life: the time taken for the concentration of the reactant to half

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key terms 2

rate-determining step: the slowest step in a multistep mechanism

activation energy:  the minimum amount of energy two particles need to collide with in order  to react successfully

heterogeneous catalyst: a catalyst in the same phase as the reactants (e.g., liquid catalyst in a liquid mixture)

homogenous catalyst: a catalyst in a different phase to the reactants (e.g., liquid catalyst in a gas mixture)

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experiments to determine rate 1


  • measure out samples of the reactants with known concentrations
  • mix them together, start the clock and stir thorougly
  • at regular intervals, withdraw samples using a pipette and quench (stop) the reaction using ice cold water or with something that will react with one of the reactants to stop the reaction. note the time at which half the contents of the pipette has been added to the quenching solution
  • titrate the quenched solution against a suitable standard solution
  • the titre is proportional to the concentration of the reactant or the product being titrated
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experiments to determine rate 2


  • if a reactant or product is coloured, the concentration can be measured using a spectrophotometer (this follows the absorption of light)
  • as concentration of the coloured substance decreases, the mixture will get paler

infrared spectroscopy

  • the spectrometer is set to a particular frequency and the amount of infrared radiation absorbed at the frequency is measured at regular time intervals
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experiments to determine rate 3


  • if a reactant is optically active and the product has different optical activity or is a racemic mixture, the reaction can be follwed b measuring the extent to which plane polarised light is rotated
  • the reaction mixture is placed in a cell in the polarimeter and the angle of rotation is measured at regular time intervals
  • the angle of rotation is proportional to the concentration of the optically active substance
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iodine clock reactions

in clock reactions, the time taken to produce a fixed amount of product is measure and the experiment is repeated several times using different reactant concentrations

H2O2 + 3 I + 2 H+ → I3 + 2 H2O

  • a known amount of hydrogen peroxide is added to a solution of acid, iodide ions, starch and sodium thiosulfate.
  • iodine is prodeuced which immediately reacts with thiosulfate ions.
  •  when all thiosulfate ions are used u, the iodine reacts with the starch, turning the solution blue/ black. 
  • at this point, the clock is stopped. 
  • the experiment is repeated either at a different temperature or a different concentration. 
  • rate of reaction is proportional to 1/ time
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sulfur clock

S2O32− + 2 H+ →  S + SO2 + H2O

  • sodium thiosulphate is decomposed by acid, producing a ppt of sulphur

  • the number of moles of sulfur produced is the same in all experimetns so the average rate of reaction of each experiment is proportional to 1/t

  • temperature could be altered instead of concentration of sodium thiosulfate, this enables activation energy to be calculated
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concentration-time and rateconcentration graphs


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iodine- propanone in acid

`(http://t0.gstatic.com/images?q=tbn:ANd9GcSZsm21aQmfiPAU13P-EGnOSuMyyou4NzIMjETTyBtBiWSIsQTS)• Hydrogen ions catalyse this reaction.

• The rate equation has the form of rate 5 k[CH3COCH3]x[I2]y[H1]z

• Varying the concentration of each species in turn gives the order of each species –x, y and z.

• The reaction can be followed by titrating the remaining unreacted iodine with standard sodium thiosulfate using starch indicator. Alternatively, a colorimeter can be used to monitor the brown colour of the iodine.

  • Experimental data shows the reaction to be fi rst order with respect to both propanone and hydrogen ions, but zero order for iodine.
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arrhenius equation


lnk = -Ea/RT    where: k = rate constant,  Ea = Activation Energy, R= gas constant (8.31 J K-1mol-1)

gradient=  -Ea/RT

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rate determining steps

if the reaction is Sn2 mechanism then the rate equation will be rate = k [X] [Y] because the reaction depends on the concentration of more than one species. these reactions occur in more than one step

if the reaction is Sn1 mechanism then the rate equation will be rate = k [Z] because the reaction depends on the concentration of  one species, these reactions occur in one step.

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nucleophilic substitution mechanisms of halogenoal


primary halogenoalkanes react by Sn2 mechanisms. 

(http://t0.gstatic.com/images?q=tbn:ANd9GcSrRjDyjPclyX2FrDQ3mCO67rfCyr6-90bkZBOlWHJTpMsPCvFY:alevelchem.com/img/SN1-nucleophilic_substitution.gif)tertiary haloalkanes= Sn1

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