How does the concentration of an acid affect the fate of reaction?

Preliminary Test

I used the preliminary test to get an idea of the amounts reactants I would need for the main test. I tested different amounts of the reactants on each other to see what sorts of speeds they reacted at, and also the speed at which the reactions took place. By using this, I could get a rough idea of how much reactants were needed, and also how long I should wait for a reaction to end. These were the results for the preliminary tests:

20ml Na²S²O³ -        16ml HCl 0.5m

                                8ml HCl 1m

                                4ml HCl 2.5m

This helped me form the main test, because it allowed me too see how accurate the tests would be. I decided that I would increase the amount of Sodium Thiosulphate I used to 30ml, so that any trends in the results would be clearer.

Main Test

For the main test, I needed some specific equipment:

                                                        -Burette

                                                        -Sodium Thiosulphate

                                                -Hydrochloric acid, in                                                         strengths of 0.5, 1, 1.5, 2,                                                        and 2.5 moles.

                                                        -Clamp stand

                                                        -Funnel

                                                        -Conical flask

                                                        -Measuring cylinder

Risk Assesment:

because I am working with acids with strengths up to 2.5 moles, it is vitaly important to be very careful whilst handeling the Hydrochloric acid, as well as the Sodium Thiosulphate. Googles must be worn at all times to protect the eyes. Used chemicals must not be poured down the drain, and instead, put into some thing separate so that it can be desposed of safely at a later time.

I decided to use a Titration system for my tests. First, I filled a burette with Hydrochloric acid. I then put 30cm of Sodium Thiosulphate into a conical flask, and put it below the burette.

For the tests, I put 0.5cm of Hydrochloric acid into the Sodium Thiosulphate, stirred it, and then waited 15 seconds before putting 0.5cm more Hydrochloric acid into the Sodium Thiosulphate. This was to allow time for the reaction to end. I did this until the ‘X’ on the bottom of the beaker of Sodium Thiosulphate was no longer visible. I did this 3 times, with 5 strengths of acid: 0.5 moles, 1 mole, 1.5 moles, 2 moles, and 2.5 moles of Hydrochloric acid.

These were the results:

Analysis

In the tests, I used hydrochloric acid in strengths of acid of 0.5, 1, 1.5, 2 and 2.5 moles. The theory of Avogadro's constant claims that there are 6 x 10²³ atoms in one mole of any substance. If I apply this to the acids I am using, I can work out that the 5 strengths of acid have the following number of atoms in them (respectively); 3 x 10²³, 6 x 10²³, 9 x 10²³, 1.2 x 10² and 1.5 x 10². However, this would only apply if I was using 1 mole of acid (1 litre of acid). I measured the acid in sections of 0.5cm³, which is 3 x 10²º, for 1 mole. This means that actually, the number of atoms is as follows (respectively); 1.5 x 10²º, 3 x 10²º, 4.5 x 10²º, 6 x 10²º, and 7.5 x 10²º.

Using those quantities, I can work out exactly how many atoms are present in each reaction. I will do so for the averages.

0.5 molar acid - (1.5 x 10²º) x 4.5 = 6.75 x 10²º

1 molar acid - (3 x 10²º) x 3.25 = 9.75 x 10²º

1.5 molar acid - (4.5 x 10²º) x 2.5 = 1.125 x 10²¹

2 molar acid - (6 x 10²º) x 1.5 = 9 x 10²º

2.5 molar acid - (7.5 x 10²º) x 1.25 = 9.375 x 10²º

Although they vary, the numbers of atoms in each reaction are moderately close, save the 0.5 molar acid tests. This makes sense, seeing as it theoretically, a certain number of atoms of Hydrochloric acid are needed to react with the atoms of the Sodium Thiosulphate to produce the precipitate (sulphur) which makes the product go cloudy.

Conclusion

From all the information collected, a conclusion is possible. I believe that it is safe to say there is a negative correlation. That is to say, the lower the strength of acid, the more acid is needed. This is because the amount of atoms in an acid of a higher mole will be higher than the amount of atoms in an acid of a lower mole.

Evaluation

In total, I collected 15 results; 3 tests for 5 different strengths of acid. I think that that number of results was definitely enough to make a conclusion because they cover a very large range. I think that my results were reliably accurate.” my results all clearly fit a pattern, and all lie along the line of best fit. There were anomalous results. Because of this, I think that none of the results need to bee repeated. I believe that the differences between the repeated tests were not substantial enough to claim that the results were not sufficiently reliable, seeing as they were all relatively close to each other. I believe that I covered a sufficiently large range of results that are accurate enough to justify the conclusion.

The only problem I had with the method I used was to do with the quality of equipment. The burette that I used was not of the highest quality, and so it was hard to release exactly the right amount of hydrochloric acid into the Sodium Thiosulphate. However, it was possible, if slightly tricky. My results were judged by eye, so they may not be as precise as they would be if they had been electronically measured. The divisions on the burette were 0.1cm³, which was definitely small enough for the experiment I was conducting. However, I did not use this measurement; I measured in 0.5cm³. I did this because I believed that 0.1cm³ was too small to have any real effect in a reaction. If I was to try and improve the reliability and accuracy of my results, I would simply use higher quality equipment.

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