The initial rates method

In a nutshell

The rate of reaction when time is zero is the initial rate of reaction. To work out the orders of reaction, the initial rates method needs to be done to find the initial rate of reaction. An example is the iodine clock reaction.

Initial rate of reaction

The rate of the reaction when time is zero is the initial rate of reaction. To work out the orders of reaction, the initial rates method needs to be done to find the initial rate of reaction. This method derives rate laws.

The initial rate method can be done by changing one reactant's initial concentration. Then analyse how the initial rates changes with the changing reactant's initial concentration. The order for each reactant can be figured out after.

The initial rate of reaction is the gradient of a concentration against time graph when time is zero. Using this method requires lots of graph analysis. Another way to carry out an initial rates method is the iodine clock reaction.

Clock reaction

The clock reaction is a method when the time is observed for the formation of a certain amount of product. This is called the end point. This can be a visible change, such as a colour change to indicate the product has formed and the reactants has been used up. The concentration of the one of the reactant is changed each time.

During the clock reaction, the amount of product will suddenly increase and this will be an easy thing to spot during the reaction. The quicker the experiment, the quicker the initial rate of reaction.

There are a few assumptions that must be made. The temperature must remain constant throughout the reaction. The reaction doesn't go too far when the end point is reached. During the experiment, the reactant's concentration doesn't significantly change.

Iodine clock reaction

The iodine clock reaction is also known as the Harcourt-Esson reaction. This experiment uses starch as an indicator to detect any iodine. It turns into a blue/black colour when it detects iodine.

First excess hydrogen peroxide and iodide ions are mixed together in an acid solution.

$H_2O_2 (aq)+2I^-(aq)+2H^+ (aq)→2H_2O(l)+I_2(g)$

Then a small amount of sodium thiosulfate solution is added as well as starch. Sodium thiosulfate reacts with the iodine.

$2S_2O_3^{2-}(aq)+I_2(g)→S_4O_6^{2-}(aq)+2I^-(aq)$

This means any more iodine that is produced after all the sodium thiosulfate is used, will stay in solution. Starch will detect this iodine and turn into a blue/black colour. This is the end point of the reaction. The time taken for the colour to change to blue/black is recorded. The initial rate of reaction can be calculated for iodide or hydrogen peroxide.

The concentrations of either iodide or hydrogen peroxide can be changed, resulting in different times for the reaction to reach the end point.

Potassium iodide order experiment

Performing the iodine clock reaction can be used to find the reaction order for potassium iodide in this reaction.

Variable definitions

The control variable is the one that stays the same. The dependent variable is what you are measuring. The Independent variable is the one you change.

Control	The concentrations and volumes of hydrogen peroxide, starch, sodium thiosulfate and sulfuric acid solution.
Dependent	The time taken for the solution to change to a blue/black colour.
Independent	The concentration of potassium iodide.

Method

This is the instructions on how to complete the experiment.

1.	Using a pipette, transfer a known volume of sulfuric acid solution into a beaker. Write down the volume.
2.	Using a measuring cylinder, transfer distilled water into the same beaker. Write down the volume.
3.	Add a few drops of starch using a pipette.
4.	Using a pipette, transfer a known volume of potassium iodide solution into the beaker. Write down the volume.
5.	Using a pipette or measuring cylinder, transfer a known volume of sodium thiosulfate solution into the beaker and swirl the beaker to ensure the solution is mixed together. Write down the volume.
6.	Using a pipette, transfer hydrogen peroxide solution into the beaker, start your stop watch immediately and swirl the beaker at the same time. Write down the volume.
7.	When the solution turns into a blue/black colour, stop the stopwatch. Record the time taken.
8.	The experiment should be repeated with different volumes of potassium iodide solution and distilled water. This will change the concentration of potassium iodide. Everything else must stay the same, including the overall volume of the reaction mixture.

The initial rate of reaction for each concentration used for potassium iodide can be calculated. The reaction order for potassium iodide can be found by comparing these values.