### IODINE CLOCK COURSEWORK A LEVEL

The same argument applies if you imagine the graph inverted and you were following the depletion of a reactant. The oxidation of iodide to iodine by potassium peroxodisulfate can be followed by a method known as the ‘ iodine clock ‘. The data below are for the hydrolysis of 2—chloro—2—methylpropane in an ethanol—water mixture. 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. We can examine theoretically the effect of changing concentration on the rate of reaction by using a simplified rate expression of the form for a single reactant..

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! A single set of reaction rate data at a temperature of K. The mathematics of 1st order rate equations units. 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. The oxidation of iodide to iodine by potassium peroxodisulfate can be followed by a method known as the ‘ iodine clock ‘. Of course [4] to [7] could simply represent inaccurate data! The graph below shows what happens to a reactant with a half—life of 5 minutes.

A single set of reaction rate data at a temperature of K. A small and constant amount of sodium thiosulfate and starch solution is added to the reaction mixture.

# Iodine Clock Reaction – GCSE Science – Marked by

Some rate data for the inversion of sucrose is given below. Therefore the order with respect to A is 2 or 2nd order.

The units of kthe rate constant. This zero order reaction occurs when the enzyme invertase concentration is low and the substrate sucrose concentration is high. CH 3 3 C—Cl concentration.

## The Iodine Clock

Only then is it possible to derive a rate expressionwhich summarises what controls the speed of a particular reaction in terms of the relevant concentrationswhich is not necessarily all the reactants! Orders of reaction can only be obtained by direct experiment and their ‘complication’ are due to complications of the coursedork mechanism, which can be far from simple.

Its not a bad idea to repeat ccoursework calculation with another set of data as a double check! I’ve also shown how to calculate the rate constant.

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

The numbers in bold show the factor change in concentration s its effect on the rate. Powers of 1 are not shown by mathematical convention. The graph below show typical changes in concentration or amount of moles remaining of a reactant with time, for zero, 1st and 2nd order. All copyrights reserved on revision notes, images, quizzes, worksheets etc.

Exam revision summaries and references to science course specifications are unofficial.

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. Examples of obtaining rate data. Reminder [x] means concentration of x, usually mol dm The table below gives some initial data for the reaction: The following rate data was obtained at 25 o C for the reaction: Analysing a single set of data to deduce the order of reaction.

The 2nd order graph tends to ‘decay’ more steeply than 1st order BUT that proves nothing!

The rate of radioactive decay is an example of 1st order kinetics. From the graph the gradient relative rate was measured at 6 points.

Enter chemistry words e. 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.

To calculate the rate constant, rearrange the rate expression and substitute appropriate values into it. Therefore the reaction is 2nd order overalland the rate expression is The idea is that somehow you test for the order with an appropriate linear graph Experimental results can be obtained in a variety of ways depending on the nature of the reaction e.

# The Iodine Clock – GCSE Science – Marked by

For example, many reactions occur via a single bimolecular collision of only two reactants and no catalyst e. These pevel calculations below are based on the initial rate of reaction analysis – so we are assuming the variation of concentration with time for each experimental run has been processed in some way e.

A plot of HI concentration versus time above was curved showing it could not be a zero order reaction with respect to q concentration of HI. The concentration of iodide, peroxodisulfate or an added catalyst e. This proved that the decomposition of hydrogen iodide reaction is a 2nd order reaction.