A Report Exploring Collision Theory through an Iodine Clock Reaction

Introduction:

An Iodine clock reaction is a chemical reaction discovered by Hans Heinrich Landlot in 1886 and made possible through collision theory. The practical involves merging two colourless solutions. One being named solution A (potassium iodate) and the other being solution B (acidified sodium bisulphate). Eventually, this reaction produced a dark blue solution because of the triiodide starch complex that was formed. However, the reaction time can be varied by changing the concentration of Solution A (IO3–) by diluting it through the addition of tap water.

Collision Theory:

Collision theory is the concept that explains how chemicals react with each other. Collision theory suggests that molecules have to collide in order to react and collisions must occur in the proper orientation. The frequency of collisions can be altered depending on temperature, the concentration of the reactants and pressure. Increasing the temperature will increase the reaction rate and the energy of the molecule, allowing it to collide more easily during a reaction. The concentration of reactants can also affect the occurrence of collisions, as more of a substance will increase the reaction rate, hence, increasing the chance of potential collisions. Likewise, modifications in pressure affects the reaction rate as an increase in pressure corresponds to a greater density in gases, and thus a higher number and frequency of collisions. So, a rule of collision theory is that for any reaction to occur, molecules need to collide or, ‘A + B -> Products’.

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By reducing the volume, the concentration of gas molecules can be increased, though pressure will not have an effect on solids or liquids in most circumstances. As collision theory explains the reaction time after the dilution of both solutions A and B, the concentration is changed because of the molecules in the iodate (IO3–) and acidified starch solution, and a triiodide starch complex is formed. Molecules must also have adequate energy to collide with each other, allowing them to break chemical bonds. However, if they don’t collide with sufficient energy, the molecules will simply bounce off each other, a theory known as activation energy (represented by the symbol Ea). Activation energy is the minimum amount of energy that is necessary for a chemical reaction to occur. It states that the volume, concentration and temperature of a reactant affect its frequency of collisions, whereas its surface area and the catalyst affect its orientation.

Maxwell-Boltzmann Distribution:

Graph 1: A Maxwell-Boltzmann distribution that shows the connection between the molecules of each reactants.

This distribution shows that the occurrence of collisions in a reaction is greater depending on the molecules per unit of volume, so colliding molecules with a greater frequency will increase the overall reaction rate. As shown in Graph 1, when the number of molecules is greater, there will be higher ‘hill’ or curve and the area under the ‘hills’ or curves will be greater, though the general shape of the energy distribution remains the same, but molecules must have energy equal to or more than the activation energy for a particle to react, forming products.

Reaction Equations:

2 H+ + 5 HSO3 + 2 IO3–   ->   I2 + 5 HSO4–+ H2O

H2O + HSO3– + I2   ->   2I– + HSO4– + 2H+

When all of the bisulphite has been used up:

Solid: (S)

Aqueous: (Aq)

2 I–   ->   I2(S)

I2(S) + I– (Aq)   ->   I3–(Aq)

I3–(Aq) + starch   ->   I3– & starch complex (blue in colour)

In the first reaction, the iodate is converted into iodine and all the bisulphite is consumed. Next, the iodine reacts to become a triiodide. Finally, once the bisulphate is completely expended, a blue-black or dark blue starch complex forms.

Hypothesis:

The concentration of solution A (potassium iodate (IO3–)), will be reduced due to the addition of tap water (H20), hence, slowing the reaction rate and increasing the time taken for the colourless solution to turn blue-black or dark blue.

Variables:

The dependant variable: The concentration of Iodate (IO3–)

The independent variable: Time

Controlled variables: Temperature, light. 

Materials:

-3 Pipettes

-90mL Acidic Starch solution

-54mL Iodate

-4 Beakers (1 50mL and 3 100mL) 

-54mL Iodate

-36mL tap water

-Heat Proof Mat

-Stopwatch

-6 labels

Procedure:

Set up the equipment (not including Iodate (IO3–) or the acidic starch solution), on a heat-proof mat.

36 mL of water was added to a 100 mL beaker.

60 mL of iodate (IO3–) was added to a 100 mL beaker labelled ‘A’ (solution A) was poured.

90 mL of acidified starch solution was poured into a 100 mL beaker labelled ‘B’ (solution B).

The required amount of iodate was added to a measuring cylinder (labelled A).

The required amount of starch solution added to measuring cylinder (labelled B).

Add the required amount of water to a 50 mL reaction beaker, using a pipette.

Add the required amount of iodate to the measuring cylinder, using a pipette labelled A.

Add the required amount of starch solution to the measuring cylinder, using a pipette labelled B.

Add solution A to the reaction beaker and mix for the equal distribution of molecules.

Add solution B to the 50 mL reaction beaker.

Measure the time taken for the colourless solution (resulting from the mixture of solution A and B and water) to turn into a blue-black colour using a stopwatch.

Repeat this method for the given number of experiments.

Results:

Table 1:  A table showing the change in reaction time with various iodate (IO3–) reactions due to the change in concentration.

Mixture #

# of mL ‘A’

# of mL’s of water (H2O)

Concentration of Iodate (IO3–)

# mL of ‘B’

Concentration of (HSO3–) mol (L-1)

Reaction Time (s)

Inversed Time (s-1)

1

10

0

0.01

10

0.03

14.19

0.07047216

2

9

1

0.009

10

0.03

15.13

0.06609385

3

8

2

0.008

10

0.03

18.7

0.05347594

4

7

3

0.007

10

0.03

24.39

0.04100041

5

6

4

0.006

10

0.03

29.77

0.03359086

6

5

5

0.005

10

0.03

40.08

0.0249501

7

4

6

0.004

10

0.03

54.94

0.01820167

8

3

7

0.003

10

0.03

116.06

0.00861623

9

2

8

0.002

10

0.03

261.9

0.00381825

Graph 2: Graph showing change in reaction time with various Iodine concentrations.

Graph 3: A graph showing the increase in reaction rate with the Iodate (IO3–) concentration.

As shown in Graph 2, the reaction time of the iodine clock decreases as the iodate (IO3–) concentration increases, also showing an exponential relationship and a fairly good trendline with an R2 value of 0.9898. This is shown again in Graph 3 where the reaction rate is positively correlated with the increase in the Iodate (IO3–) concentration, resulting in a steep, constant incline of data. This data backs up the fact that there is a faster reaction rate as the concentration gets higher due to collision theory (more molecules collide with each other, resulting in faster, stronger collisions). 

Discussion:

The hypothesis of this practical was supported because a decrease in the concentration of the reactants by diluting through the addition of tap water results in a decrease in the reaction rate (see Table 1, Graph 2 and Graph 3). This was evident by the time taken for the reaction to occur and for the solution to turn dark blue/blue-black in colour, which is shown through collision theory (as the concentration gets higher, more molecules collide with each other, resulting in faster, stronger collisions). Throughout the first few experiments, the reaction occurs very rapidly after a short period of time as shown in image 1 below. Due to the higher concentration of Iodate, there is a higher occurrence of collisions and a faster reaction rate as shown in table 1. This is also evident in low concentrations, as there is a much lower occurrence of collisions and a sluggish reaction rate. The first change that is visible is a small, random ‘cloud’ of blue mixture before it becomes darker and spreads across the beaker, starting from the centre of the beaker and spiralling towards the edges. (refer to image 2). This process can have a considerably lower occurrence of possible collisions when the diluted molecules of solution A (iodate (IO3–)) collide with solution B (acidified sodium bisulphite). This connection of molecules can be observed in Graph 1.

Image 1: One of the earlier experiments, where the reaction shown is very rapid.

Image 2: One of the later experiments, where the reaction is extremely sluggish.

Errors:

One of the errors was that the R2 value was 0.9898 (shown in Graph 2). Though it is a slight error, it is still not as close to a value of 1 as it could be. Some of the errors faced include systematic errors, dilutional errors and random errors.

Systematic errors in this practical include orientation (parallax) and stopwatch mistiming. Parallax error is when the reaction observations is viewed from a specific angle where the number appears to be different, for example, someone viewing the practical from a higher angle will see a higher number than someone viewing it from a lower angle. This is also possible from a faraway angle. One way to limit this error is to view the reaction from its side, however, it is very difficult to precisely calculate where this position is. Therefore, being a slight systematic error that may have affected the data (reaction time). This is also a random error because the viewing position can hinder a person’s ability to determine when the mixture has fully turned dark blue/blue-black, appearing incomplete or complete, possibly resulting in stopwatch mistiming because there may be a slower or faster reaction time recorded. One way to minimise this error is to have the person with the stopwatch viewing the beaker from a side angle, that way, the change of random error is at least minimised.

Dilutional errors in this practical included beaker agitation and not drying the beakers correctly. Throughout the practical, the beakers were kept on the heat-proof mat, however, for the final result, the timer recorded a time above 3 minutes before the beaker was shaken, causing the mixture to stop reacting. This is because shaking the beaker causes the water to over or possibly under dilute with the solutions (random error), preventing any further chemical reactions (shown in image 2) by slowing or increasing the frequency of collisions and the reaction rate, though there isn’t a possible way to control this throughout the course of the practical. The second dilutional error was not drying the beakers correctly. Again, this has an effect on causing over or possibly under dilution (random error) and hence, slowing or increasing the frequency of collisions and the reaction rate. The cause of this error was a lack of time. To get the best possible results and average out the data for a more accurate graph, the practical was rushed. This resulted in the beakers not drying correctly, contributing to extra tap water diluting the mixture, altering chemical reactions. Using tap water instead of distilled water way another minor controlled error throughout the practical, however, it is still important as it may have caused unintended reactions due to minerals and impurities in it, affecting the concentration of the mixture, the frequency of collisions and thus, the reaction rate. Rinsing equipment with distilled water after each individual result before drying it effectively would minimise this error and decrease the possibility of under or over dilution and a wrong reaction rate recorded.

Conclusion:

The main purpose of this practical conducted was to understand how collision theory contributes to an effect on concentration and reaction rate due to the molecules in solution A (iodate (IO3–)) and solution B (acidified starch solution) colliding and reacting, collisions having adequate energy to break chemical bonds (activation energy) and how an increase in concentration increases the frequency of possible collisions. This practical involved conducting several experiments using Iodate (IO3–) and acidified starch solution that were diluted via the addition of tap water (see table 1). The hypothesis was supported as it stated that the addition of water would slow the time taken for the colourless solution to turn blue-black/dark blue. This was expected because the concentration of the reactants was reduced, as well as the frequency of collisions, causing the rate of the reaction to be much slower. In nearly all experiments the solution almost immediately turned dark blue due to the sequence of several chemical reactions such as the conversion of iodate (IO3–) into iodine (I) and then into a triiodide before forming a starch complex responsible for the blue colour change. The errors were mostly controlled though they may have slightly affected the data through systematic errors (parallax error and stopwatch mistiming), dilutional errors (beaker agitation and not drying the beaker) and random errors (possible mixture under or over dilution). One suggestion to improve this practical would be to allow more time to record the results a few times, therefore allowing the results to be averaged as shown in Graph 2 and Graph 3, making them more accurate.

Bibliography: