Experiments Within The Collision Model Outlined In Collision Theory

The investigation of rate of reaction evolves from Chemical Kinetics, the central premise of which is the Collision Model outlined in Collision Theory (Harper College, 2019). This theory comprises of three basic tenets, each of which all must be met for a bimolecular reaction to occur (Harper College, 2019).

The first tenant states that two particles must collide to react, the second that they must have enough energy and the third that they must collide with the correct orientation (lumenlearning.com, 2019). The number of successful collisions will impact the reaction rate, defined as the amount of product produced per unit time (UNC, 2019). Reactions are classified as either first-order, second-order, mixed-order or higher-order (Unknown, 2019).

First-order reactions proceed at a rate dependant on one reactant concentration, second-order are dependent on two, and so forth (UNC, 2019). Another classification is zero-order, where the rate is independent of concentration; any changes result from experimental conditions including only a portion of reactant reacting and consistently being replaced, or the concentration of one reactant being significantly higher (Curtis, R., Martin, J. and Cao, D, 2019).

The chosen research question for this investigation was how does altering the concentration of hydrochloric acid within a sodium thiosulfate reaction influence the rate of reaction?

The original experiment proposed to measure the rate of reaction within Na2S2O3 + 2HCl(aq) → S(s) + SO2 (g) + 2NaCl(aq) + H20(l). The reactants were added to a beaker with 35mL of water and the time it took to opacify was recorded. The rate of reaction is defined as the rate at which insoluble Sulfur particles opacify the remaining H20. As such, the experiment was modified to more conclusively investigate the rate of concentration. Modifications included changing the concentration of HCl in different trials to investigate its effect on, rather than just measure, the rate of reaction and undertaking three trials of each.

Collision Theory implies that as the concentration of HCl (the independent variable) is increased, the reaction time (dependent variable) will decrease, with more successful collisions taking place (BBC Bitesize, 2019). Reaction time is inversely proportional to rate of reaction, so a short reaction time means a high reaction rate (BBC Bitesize, 2019). The hypothesis for this investigation reflects these theories stating that, “as the concentration increases the length of reaction will decrease, resulting in a higher rate of reaction”.

To what extent does altering the concentration of hydrochloric acid within a sodium thiosulfate reaction influence the rate of reaction?

The original methodology was devised to measure the reaction between hydrochloric acid and sodium thiosulfate. A cross was marked on a sheet of paper, over which was placed a 100ml beaker. Ten (10)ml of 0.25M Na2S203[Aq] was mixed with 35ml of deionised water. Five (5)ml of 1M hydrochloric acid was added, and timing was commenced, measuring the time taken for the cross to disappear when viewed above. The solution was disposed of immediately. (Dunn, 2019).

Various modifications were made to increase reliability and accuracy and refine the experiment.

The experiment will adhere to all considerations put forward under the original experiment, including the use of safety glasses and laboratory coats, along with the correct disposal of environmentally damaging chemicals, in addition to further considerations (Graham, 2019).

Statistical analysis techniques were used to find the mean, uncertainty, relative uncertainty and the average rate of reaction for subsequent analysis and interpretation:

Within the raw data collected under experimental conditions (Table 2), much of the data (trials of 1.5M and higher) showed little to no variance in results. The greatest difference recorded was a change of 0.66 seconds between the first and third trials of 2.5M HCl. This implies that this data is accurate, and resulted in small experimental errors, as evidenced in Table 3, providing more credence to the conclusions of this investigation. While this close corroboration is true for four out of the five independent variables, the trials of 1.0M HCl provide an interesting comparison. While trials 2 and 3 have a difference of only 0.16s, there is a variance of 11.01s between trials 1 and 2. To this extent, trial 1 appears an obvious anomaly, clearly diverging from the trend. As such, trail 1 was unincluded from the average calculations, to obtain a more accurate result.

As all experiments were conducted in a similar manner, the only notable difference was that all trails for 1.0M were conducted on a previous day to the others. Therefore, it is likely that there was some external variable that influenced results that present when the experiment was re-conducted for the remaining trials. The trial was also the first of the experiment, and it is likely the routine at this point was not established, compromising the results.

Table 3 also includes the uncertainty and relative uncertainty calculation for all independent variables. 1.0M has an uncertainty of ±6s, meaning this point could realistically be anywhere in this range, numerically 103s to 115s. This is also relative uncertainty of ±5%. This is an error margin large enough to have considerable influence on the trend lines and any analysis thereof. This can be accredited to the varying results analysed above and lends further credibility to this data point being excluded in the graphs and analysis.

All other HCl molarities had an uncertainty calculation of ±1s and therefore, a relative uncertainty calculation of ±1%. From this it can be implied that there will be minimal to no experimental error, rendering these points highly reliable. The minimal error might result from the uncertainties having been rounded but will therefore be no more than ±0.99s, small enough to have minute influence.

The most obvious trend within Figure 1 consists of the increase in the concentration of HCl being congruent with a decrease in the lengths of time. At 1.0M the average length of time is 112s decreasing to 88s at 3.0M. However, as displayed in the graph (Figure 1) this relationship is not perfectly linear, with the decrease varying between points. Any linear negative trend in time is only realistic over a limited time range and is more commonly associated with zero-order reactions (Curtis, R., Nguyen, C. and Lower, S, 2019). First order reactions are exponential, so this implies that the rate is independent of the concentration (Curtis, R., Nguyen, C. and Lower, S, 2019).

There is only one significant error bar, as all other points had an experimental error of ±1s. As the first point has a large error margin of ±6s, this error bar overlaps the position of the next point of 1.5M at 110s by 4s. This means the difference between these two points is not statistically significant, and this data point will not have a significant influence on the analysis. All other points had an experimental error of ±1s and no overlap. Therefore, all are statistically significant with their difference provided statistical evidence as to the relationship.

Figure 2 appears to have an opposite trend to Figure 1, a positive linear relationship. The trend line is positive, rising from (0.5, 0.008) to (3.5, 0.012). The data goes from 0.00892 at 1.0M to 0.01128 at 3.0M. Again, this substantiates that, due to the positive linear relationship, as the HCl concentration increases so does the rate of reaction. This draws a correlation between rate of reaction and concentration; however, within the parameters of the graphs, any small change or pattern appears to have a greater affect as the y-axis goes from 0 to 0.014 s-1. When the y-axis is increased to even 1.0 s-1 the strength of this correlation is greatly diminished.

Within Figure 3 the trend line no longer appears to have positive gradient, but rather one close to zero. The concentration was tripled within the experiment, and logic suggests that this would result in a substantial change, constant with the Collision Theory. Greater molecules should result in more successful collisions. The change, however, is a mere increase of 0.00239 s-1. in relation to investigating the influence of HCl concentration, this change is not significant enough and difficult to argue the change in HCl has any statistically significant effect.

While there is variation, it is not significant enough to prove that the HCl influences the rate more so than any other factors, having more alignment with a zero-order reaction than first-order.

Limitations that could have impacted the precision of this experiment were minimal, as the raw data was very closely corroborated. However, there were several limitations that could have impacted the data. Due to having to re-use equipment such as flasks, there is a possibility that random error occurred when extra water remained. While that may have been minimal, it would have diluted the solution, potentially weakening the reaction and the small experimental scale would have resulted in this having a greater effect. A further limitation influencing reliability was that all trials for 1.0M were conducted separately. This resulted in a myriad of minor changes that could influence precision. Different equipment would have been used, perhaps flasks with a different residue or hue that changed experimental conditions or a different stopwatch with a different period of reaction. All attempts to contain the effects of this were mad; however, due to this many small variables were unable to be sufficiently controlled. A person’s reaction time can also vary. Due to this it was impossible to perfectly recreate the experimental conditions, and this would have had an effect. As the data for 1.0M was the only data point with an obvious experimental error, at ±6s, and the data collected on a separate day, it is highly likely that these limitations would have likely influenced within this data.

There were several limitations that could have resulted in systematic errors and influenced the validity of the experiment. One of these was measurement. While all attempts were made to have accurate measurements, not all the measurement were conducted by the same person, with people reading them differently. As liquids, the reactants tended to form a bubble on top. There was variation in whether the measurement was taken from the top or the bottom of the bubble. Consequently, some experiments may have been conducted with differing reactant to what was recorded, influencing accuracy.

Several HCl concentrations were also dilutions leaving a margin for error, potentially not having the ideal molarity. This effected accuracy in the same way, with the experimental actuality not matching the exact concentrations intended.

For both, as the analysis is done from the intended values, there can be some discrepancy. The values were also rounded effecting the final calculations. While all these influences would be minimal, it is important to note the likelihood of some validity issues.

The easiest improvement would be for all tests to be conducted on the same day under the same experimental conditions. This would reduce error by nullifying a vast portion of the reliability concerns above as they result from this issue.

In addition, new glassware could be used each experiment eradicating the need for washing and therefore, the danger of accidentally diluting the solution with water being left in the flask. The measurements could also be taken more accurately with a method such as a light meter being used, with a unanimous decision to record the time it takes to reach a certain point, e.g. 45.

One extension of this experiment could be the inclusion of a greater variance of HCl concentrations. The minimal change in rate was unexpected and further experimentation would allow conclusions to be drawn about the longevity of this trend.

Another possible extension could involve changing the concentration of the sodium thiosulfate while keeping the HCl consistent or changing concentrations within a separate reaction. Research suggests that the relationship between Na2S2O3 and rate is first order reaction and would allow investigation into the relationship between concentration and rate of reaction on a broader scale, forming more informed analysis.

The reaction time identified decreased by 20s from 109 to 89s, while the reaction rate increased by 0.00239 s-1 when the concentration was doubled. The data collected within this experiment clearly evidences that within the indicated reaction, the concentration of HCl has considerable impact on the reaction time; however, the change in reaction rate was so insignificant that a clear correlation could not be drawn.

These findings corroborate the background theory, with zero-order reactions providing a logical explanation for the negligible change. The results contradict the hypothesis, but the large differential between the strength of HCl and Na2S2O3 used, is an experimental condition that generally indicates zero-order reactions (Curtis, R., Martin, J. and Cao, D, 2019). Experimentation is the only way to identify zero-order reactions, so the hypothesis was a logical prediction. There were also several critical limitations, including both random and systematic errors as discussed above. Therefore, to further corroborate and investigate this, further testing and extrapolation of this experiment is required.