Overview of rates of reaction

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Rate of reaction

if when a reactant is increased ,say doubled, and  the rate of reaction also dsoubles this reactant is said to be a first order reactant , the amount by which the reactant changes is equal to that of the change in the rate of reaction

If the reactant is doubled and the rate of reactantion is quadrupled this means that the this is a second order reactant, as the rate of reaction is increasing by the change in the reactant squared. So if a second order reactant was tripled the rate of reaction will be increased 9 times.

a reactant with zero order has no effect on the rate of reaction when it is increased or decreased.

The overall order of a reaction is the sum of the orders of each of the reactants and if a reaction has an order which is not a whole number this means that there is more than one rate determining step and that it is a chain reaction

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Rate of reaction

The rate of reaction is the change in concentration divided by the change in time, the speed with which a reactant is used up or that a product is formed.

Rate of reaction can be measured in various ways for example:

  • Colourimetry- if there is s clour change between the reactants and the products
  • decrease in mass/ volume of gas produced- if gas is produced
  • electrical activity- if ions are formed
  • look at an infared spec- if different functional groups form
  • look at optical activity if optical isomers are formed

Remeber that a reaction can only take place if the molecules collide with than energy greater than or equal to the activation energy and in the correct orientation - if an equation such has respiration has many molecules of reactants present then you can assume that the reaction takes place in many steps as it is very hard for say 14molecules to have the right energy and orinitation

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Rate of reaction

In a reaction such as SN1 there is a rate determining step ( the slowest step) By increasing the speed with which this reaction occurs the rate of the overall reaction will increase.In the SN1 mechanism there is only one molecule in the rate determining step ( hence the name SN1) In the SN2 mechanism there are 2 molecules in the rate determining step.

The transition state formed in the SN2 mechanism with a primary halogenoalkane has the maximum energy of anything in the reaction so appears at the peak of the graph. Carbocations such as the ones formed in SN1 mechanisms with tertiary halogenoalkanes create dips in the graph as they are at a slightly lower energy as they are more stable.

Quenching a reation means stopping a reaction this can be done by cooling the reaction very quickly or removing a cataylst such as an acid by removing a sample with a pipette and titrating with NaCO3 to stop the reaction.A problem witrh the titration method is that the reaction will continue until titrated so the time at which you took the sample my not be accurate.

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Rate of reaction

Rate units and equations.

The units vfro the rate of reaction change all of the time- these are calulated by dividing mol dm3 s-1 by the units of your reactants that change the rate so if the total of the order of your reactants was 2 you would divide by mol-2 dm-6 to give the answer of mol-1 dm -3 s-1 ( hope that made sense!)

The rate constant can be worked out if you have reaction data all you need to do is work out the rate equation and sub in your concentrations of your reactants and the rate of reaction divide the rate by the total for the reactants and volia

The is the Arrhenius equation where       - activation energy/ the gas constant is equal to the gradient of the ln(k) 1/T graph. Top tip use the data which will be given to you in a table in an exam question and choose a wide range ( points which are spread out) to calulate the activation energy of the reaction by rearranging this equation where the gas constant is equal to 8.31 

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