• July 17, 2019

The table below gives some initial data for the reaction: Copying of website material is NOT permitted. Orders of reaction can ONLY be determined by rate experiments CH 3 3 C—Cl concentration. This proved to be a curve – compare the blue rate data curve with the black ‘best straight line’ courtesy of Excel! From experimental results you need to know how the speed of a reaction varies with respect to individual reactant concentrations. For example, many reactions occur via a single bimolecular collision of only two reactants and no catalyst e.

A graph is drawn of CH 3 3 CCl concentration versus time. Only then is it possible to derive a rate expression , which summarises what controls the speed of a particular reaction in terms of the relevant concentrations , which is not necessarily all the reactants! From runs ii and iii. Examples of obtaining rate data. The orders of reaction are a consequence of the mechanism of the reaction and can only be found from rate experiments and they cannot be predicted from the balanced equation. Then you would get two negative gradients one steeper than the other for the greater concentration.

So simplified rate data questions and their solutions avoiding graphical analysis are given below. The same argument applies if you imagine the graph inverted and you were following the depletion of a reactant. From runs ii and iii.

The graph on the left illustrates the initial rate method for the formation of product. The oxidation of iodide to iodine by potassium peroxodisulfate can be followed by a method known as the ‘ iodine clock ‘. From the point of view of coursework projects the detailed analysis described above is required, but quite often in examination questions a very limited amount of data is given and some clear logical thinking is required.


From experimental results you need to know how the speed of a reaction varies with respect to individual reactant concentrations. We can now examine theoretically, the effect of changing individual concentrations on the rate of reaction of a more complicated rate expression of the form.

A-Level Investigation – Rates of Reaction – The Iodine Clock

You just substitute the values into the full rate expression: The gradients A and B would be for two different concentrations of a reactant, the concentration for A would be greater than the concentration of B. The mathematics of 1st order rate equations units.

iodine clock coursework a level

Reminder [x] means concentration of x, usually mol dm Its not a bad idea to repeat the calculation with another set of data as a double check! A single set of reaction rate data at a temperature of K.

iodine clock coursework a level

In reality the results would be not this perfect and you would calculate k for each set of results and quote the average! The graph below shows what happens to a reactant with a half—life of 5 minutes.

A small and constant amount of sodium thiosulfate and starch solution is added to the reaction mixture.

Some possible graphical results are shown above. The hydrolysis of a tertiary chloroalkane produces hydrochloric acid which can be titrated with standardised alkali NaOH aqor the chloride ion produced can be titrated with silver nitrate solution, AgNO 3 aq.

The rate of radioactive decay is an example of 1st order kinetics. Example of a rate expression. To put this graph in perspective, a 2nd order plot is done below of rate versus [RX] 2.

Iodine clock coursework level

The rate of reaction was is then plotted against HI concentration to test for 1st order kinetics. To calculate the rate constant, rearrange the rate expression and substitute appropriate values into it. Of course [4] to [7] could simply represent inaccurate data! In the zero order graph the gradient is constant as the rate is independent of concentration, so the graph is of a linear descent in concentration of reactant.


Wherever you draw a straight line, the data does not express itself as a linear plot and cannot be a 2nd order reaction.

The Iodine Clock – GCSE Science – Marked by

Examples of obtaining rate data. You may need to use aqueous ethanol courseework a solvent since the halogenoalkane is insoluble in water and a large volume of reactants, so that sample aliquot’s can be pipetted at regular time intervals.

iodine clock coursework a level

An individual order of reaction is the power to which the concentration term is raised in the rate expression. The graph below show typical changes in concentration or amount of moles remaining of a reactant with time, for zero, 1st and 2nd order. This zero order reaction occurs when the enzyme invertase concentration is low and the substrate sucrose concentration is high.

Orders of reaction can only be obtained by direct experiment and their ‘complication’ are due to complications of the actual mechanism, which can be far from simple. Powers of 1 are not shown by mathematical convention. The graph is ‘reasonably linear’ ipdine it is a 1st order reaction.

The orders of a reaction may or may not be the same as the balancing numbers of the balanced equations.