How does varying the catalyst in the reaction between 20 mL of Sodium Thiosulphate and 20 mL of Iron (III) Nitrate solution affect the time taken for a cross to appear?

Rationale

The rate of reaction is the speed at which a chemical change occurs. Rates of reaction of a chemical experiment can be affected by many factors such as temperature, concentration, surface area and by adding a catalyst (Reference 1). In order to increase the rate of reaction between sodium thiosulfate and iron nitrate, a catalyst was added at the beginning of the reaction.

Fe³⁺(aq) + 2S₂O₃^2-(aq) → [Fe(S₂O₃)₂(H₂O)₂]^–(aq)

The chemical nature of reactants determines the speed of a chemical reaction. Throughout a reaction, bonds are broken and new bonds are formed. The collision theory states that in order for a reaction to occur, collisions must take place between reactant particles. These collisions must have correct orientation and sufficient energy to break the bonds within reactants and allow formation of new bonds.

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The activation energy is the energy the reactants are required to overcome in order to form products. Not all reactant molecules have enough kinetic energy to overcome the activation energy barrier. Therefore, a catalyst is added in order to lower the activation energy and overcome the energy barrier (Reference 2). By lowering the activation energy, a catalyst is able to speed up the rate of reaction of an experiment. Catalysts are not consumed in the catalysed reaction and can be used repeatedly.

Transition metals are examples of good catalysts with high activity due to their ability to change oxidation states with ease in a homogenous reaction. Oxidation states represent the number of electrons that an atom is capable of gaining or donating as well as a metals ability to form chemical bonds (Reference 3). Catalysts with high activity have varying oxidations states because an electron exchange involving the catalyst can initiate chemical changes in the molecules that are bound to its surface. By initiating chemical changes and determining the ability of atoms to gain or lose electrons, the oxidation states of a metal affect its activity as a catalyst which in turn affects the rate of reaction of the experiment (Reference 4). The ability of the transition metal to be in a variety of oxidation states, be a good source for electrons and transition between oxidation states determines whether or not it is a good catalyst.

Determining the activity of a catalyst by testing which catalyst performs best in this experiment can be very valuable. For example, a catalytic converter in a car converts toxic carbon monoxide into carbon dioxide with a catalyst of platinum. The best possible catalyst that should be in this situation could be determined based on the oxidation states of the element which is being tested in this experiment. In real life situations, discovering the most active catalyst can allow a reaction to occur in the shortest amount of time possible, making it time efficient as well as cost effective.

Original Experiment

The original experiment both qualitatively and quantitatively investigated the ways in which temperature and concentration effect the rate of reaction. Hydrochloric acid reacted with sodium thiosulfate and the time taken for the solution to become cloudy and obscure the cross (drawn on a piece of paper) was measured to determine the rate of reaction. The mixture becomes cloudy due to the produced sulphur precipitate.

S₂O₃^2-(aq) + 2H⁺(aq) -> S(s) + SO₂(g) + H₂O(l)

It was identified that as the concentration or temperature increased, the rate of reaction also proceeded to accelerate. The following research question was the developed from this experiment in order to further investigate the factors affecting the rate of reaction.

Research Question

How does varying the catalyst in the reaction between 20 mL of Sodium Thiosulphate and 20 mL of Iron (III) Nitrate solution affect the time taken for a cross drawn on a piece of paper to appear?

Modifications to the methodology

In order to collect appropriate data, the original experiment was:

Refined by

- Increasing the number of trials that each catalyst was tested to 15 times each – this will allow a mean to be calculate and will provide more accurate results.

Redirected

- Using three different catalysts – which further tested different factors (other than temperature and concentration which were in the original experiment) that affect the rate of reaction.

- Different reactants were used in the experiment – iron (III) nitrate reacted with sodium thiosulfate. These were changed from the original experiment where the solution became cloudy and obscured the cross because it was identified that it would be easier to observe the cross appearing as it proved to be difficult to determine when the cross was fully hidden from the cloudy solution.

Figure 1 – Diagram of Experimental Setup

Management of Risk

Inhalation of iron (III) nitrate, Sodium Thiosulfate, Iron (II) sulphate, Cobalt (III) Chloride and Copper (II) Sulphate can cause unconsciousness if inhaled, ulceration and perforation of the nasal septum. This can cause respiratory tract irritation with possible burns if swallowed or inhaled which will affect the liver. If any of these chemicals are to come into contact with eyes and skin, irritation may occur as well as possible burns. If ingested, central nervous system may be damaged in which may cause gastrointestinal irritation with nausea, vomiting and diarrhea.

If inhaled and breathing difficulties occur, medical attention will be sought. If ingested, vomiting will be induced immediately directed by medical professional. If chemicals come into contact with eyes or mouth and irritation occurs, they will be rinsed with water for fifteen minutes. For all chemicals, gloves, lab coats, enclosed shoes and safety glasses will be worn at all times. Waste materials will be handled in prep room and will be left in clearly labelled beakers to be disposed of carefully. Sweeping, cleaning and washing of all benches, surfaces and materials will be conducted.

Qualitative Observations

As soon as the sodium thiosulfate reacted with the iron nitrate, it immediately became dark. As the experiment continued, the solution began to change to a light brown colour where the cross was clearly visible and this was when the timer was stopped at the end of each trial.

Raw Data

Table 1 – Raw Data

Catalyst

Time ( $\pm$

0.04s) *

Trial

Trial 10

Trial 11

Trial 12

Trial 13

Trial 14

Trial 15

No Catalyst

47.0s

50.9s

51.1s

52.2s

55.8s

56.8s

56.2s

55.3s

60.2s

57.2s

51.1s

47.1s

57.9s

55.1s

56.1s

Copper Sulphate Catalyst

4.5s

3.7s

4.3s

3.9s

3.3s

3.9s

4.2s

4.3s

3.3s

3.7s

3.5s

4.0s

3.1s

2.6s

Iron Sulphate Catalyst

26.8s

24.8s

26.3s

26.5s

26.9s

27.8s

29.7s

29.4s

28.3s

30.9s

33.0s

32.6s

34.7s

34.0s

34.9s

Cobalt Chloride

Catalyst

27.6s

24.2s

28.5s

26.0s

24.4s

36.4s

34.3s

33.5s

36.3s

32.9s

34.3s

34.8s

36.8s

35.8s

33.9s

*Human reaction time when operating the stopwatch

Processing of Data

Table 2 – Calculations

Calculations
Average Time	$AT = \frac{sum of all 15 trials}{15}$	$No catalyst avg . time = \frac{47.0 + 50.9 + \dots 56.1}{\begin{matrix} 15 \\ = 54.0 s \end{matrix}}$
Uncertainty for the Mean	$AU = \frac{highest value - lowest value}{2}$	$No catalyst uncertanty = \frac{60.2 - 47.0}{\begin{matrix} 2 \\ = \pm 6.6 s \end{matrix}}$
Percentage Uncertainty	$PU = \frac{Absolute uncertainty}{Measurement value}$	$No catalyst % uncertanty = \frac{6.6}{\begin{matrix} 54 \\ = 12.22 % \end{matrix}} \times 100$
Outliers	$3 Standard Deviations$ = 3 $\times$ Standard Deviation $\pm$ mean	$No Catalyst Outliers = 3.9 \times 3 \pm 54$ $= 42.3 to 65.7$
Absolute uncertainty for measuring cylinder	Measuring Cylinder Absolute Uncertainty = $\pm Smallest Increment \div 2$	Absolute Uncertainty = $\pm 1.0 \div 2 = \pm 0.5 ml$

An excel program was used to calculate standard error and standard deviation.

Table 3 – Summary Table

Catalyst	Number of Oxidation States	Average Time	Standard Deviation	Standard Error	Percentage Uncertainty (%)
No Catalyst	0	54.0 $\pm$ 6.6s	3.90	1.01	12.22%
Copper Sulphate Catalyst	5	3.7 $\pm$ 1.0s	0.52	0.13	27.03%
Iron Sulphate Catalyst	4	29.8 $\pm$ 5.1s	3.37	0.87	17.11%
Cobalt Chloride Catalyst	2	32.0 $\pm$ 6.3s	7.45	1.92	19.69%

Calculating outliers using only data within 2SD of the mean

Table 4 – Showing outliers for each catalyst

Catalyst		Outliers
No Catalyst	54.0 $\pm$ 6.6s	Between 42.3 and 65.7, therefore no outliers.
Copper Sulphate Catalyst	3.7 $\pm$ 1.0s	Between 2.14 and 5.26, therefore no outliers.
Iron Sulphate Catalyst	29.8 $\pm$ 5.1s	Between 19.69 to 39.91, therefore no outliers.
Cobalt Chloride Catalyst	32.0 $\pm$ 6.3s	Between 9.65 and 54.35, therefore no outliers.

Graph 1 – Number of Oxidation States vs Time Taken for Cross to Appear

Cobalt

No Catalyst

Copper

Iron

Graph 2 – Time taken for Cross to Appear when Sodium Thiosulphate reacts with Iron Nitrate with Error Bars

Trends, Patterns and Relationships

Graph 1 shows that as the number of oxidation states of the catalyst increases, the time taken for the cross to appear decreases. There appears to be an exponential relationship between the independent and dependant variable. The coefficient of determination can predict that 68.20% of the variation in time taken for the cross to appear can be explained by the varying catalyst.

The error bars in Graph 2 measure the uncertainty of the data collected. It is evident that the width of the error bar of Cobalt is the largest of all of the catalysts and this was highlighted in Table 3. This indicates that the results collected when Cobalt was used as a catalyst was the least accurate. It is clear that the error bars of all of the catalyst were small meaning that error was minimal.

Limitations of the evidence and error calculations. Reliability and validity of the experimental process.

Table 5 – Error and Limitations

Limitations of the Evidence		Reliability and Validity of the Experimental Process
Human error when stopping the timer and determining when the cross was considered to be visible.	The times in table 1 would be slightly altered as the time for the observer to see the cross and stop the timer must be considered.	Uncertainty was taken into consideration and was $\pm$ 0.04s and in Table 3 individual uncertainties were calculated for each average time. This random error effected the reliability of the experiment.
Measuring equipment used is not exact because readings are rounded providing imprecise results. Associated error was $\pm 0.5 ml$ .	Measurements would affect the time taken for the cross to appear if there was more of one reactant than the other then the reaction may take place at a quicker or slower rate (Table 1).	This random error effected the reliability of the measurements taken for each solution.
Due to temperature being a factor that effects the rate of reaction, the fact that the room temperature was different on the two days that the experiment was completed, the results would have been affected.	This could affect Table 1 and Table 4 and the accuracy of these results.	This random error could have affected the reliability of the experiment and presented outliers in the data as the temperature was not controlled over any of the tests, it was only set to be room temperature and assumed that this was kept the same.
The catalyst was added to the solution of sodium thiosulfate and iron nitrate slightly later as it was not possible to do this as well as start the timer therefore, minor amounts of time were lost at the start of the experiment.	This error effected the times recorded in Table 1 as these would be slower than if the chemicals were combined and the timer started at the exact same time.	This random error affected all the results in a similar way however only minorly meaning that it affects the experiment in a way that would make it unreliable.

Conclusion

The results from this experiment show that varying the catalyst in the reaction between sodium thiosulfate and iron nitrate can affect the time taken for a cross to appear on the piece of paper underneath the beaker. It is evident that the number of oxidation states are a key factor in determining which catalyst works best for this reaction. Clearly shown in Graph 2 is that as the number of oxidation states of the catalyst increases, the quicker the cross on the piece of paper becomes visible.

Each of the catalysts worked in a way that increase the rate of reaction however it was evident that copper was the most active catalyst in the experiment between sodium thiosulfate and iron nitrate. Copper has 5 oxidation states of -2, +1, +2, +3 and +4. The results indicate that the more oxidation states the element has, the more active it is as a catalyst in this experiment. Transition metals are considered to be active catalysts due to their ability to lose electrons easily, also meaning that some of these metals are able obtain numerous oxidation states.

The results indicate that 68.20% of the variation in time taken for the cross to appear can be explained by the varying catalyst. This is only a moderate relationship between the increase of rate of reaction and the varying catalyst. A possible explanation for this relationship is because there is only a select amount of catalysts that will be effective in a reaction between sodium thiosulfate and iron nitrate. These results prove that the increase in oxidation states and the ability of the element to lose electrons easily has an effect on the activity of each catalyst and its effect on the rate of reaction in the experiment.

The percentage uncertainty associated with the results are quite large, ranging between 12.22% and 27.03%. This indicates that whilst the results from this experiment are accurate, they may not be the most reliable. The systematic and random errors accompanying this experiment also add to the unreliability and invalidity of the results.

Improvements and extensions

Table 6 – Improvements to be made to minimise errors highlighted in Table 5.

Random Error

Measuring equipment providing inaccurate readings with uncertainty of $\pm 0.5 ml$

Using a buret rather than a measuring cylinder to gather the most accurate readings. This would minimise error by 10 times to $\pm 0.05 ml$

Random Error

Temperature of the room and the solutions to ensure rate of reaction is not increased due to this factor.

All the experiments should be completed on the same day with the room temperature and the temperature of the solutions being controlled the entire time.

Random Error

The timing of the catalyst being added into the solution. Also, when the timer was started should be taken into consideration.

Adding the catalyst and the iron nitrate to the sodium thiosulfate simultaneously. Also starting the timer as soon as these solutions have started reacting with each other.

An extension could be made to this experiment to eliminate the human error by changing the way measuring the rate of reaction through the time it takes for a cross to appear. Instead of this, the reactants could be changed so that the product would form a gas. The volume of this gas could be measured and the rate of reaction could be calculated from this figure. This is extension would also minimise random human error and it would increase reliability of the experiment.

Bibliography

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