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

I will be conducting an experiment to find out how the concentration of an acid affects the speed of a reaction. For this experiment, I will be using the reaction of Sodium Thiosulphate and Hydrochloric acid.

Sodium Thiosulphate –> sodium chloride + sulphur dioxide + Hydrochloric acid + Water + Sulphur

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Na�S�O� (aq) + 2HCl (aq) –> SO� (aq) + 2NaCl (aq) + H�O (l) + S (s)

I chose to use this reaction especially because of the product Sulphur (s). Because the reaction produces a precipitate; solid sulphur atoms, which makes the product solution go cloudy. This precipitate is very useful for measuring the speed of a reaction. By placing an ‘X’ below the beaker I am going to put the reactants into, and by measuring the amount of acid that is needed to make the ‘X’ disappear, I can get an amount which tells me about how fast the reaction took place. This can be used in comparison with acids of different strengths.

Background Science

A reaction is when one or more substances are changed into different substances. There are four major factors that affect the speed of a reaction; Temperature, Concentration, Catalyst and Pressure/Concentration. In this experiment, the effect that a Concentration has on the speed of a reaction will be tested. This means, however, that the other variables will have to remain unaltered throughout all the tests. If they were to vary, the tests could not be considered reliable. I will make sure the temperature does not vary by checking that the room is at room temperature. I will not use catalysts, and make sure that all equipment is not contaminated by any other chemicals. I will use the same equipment in each experiment to make sure that the pressure/concentration does not vary.

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 Na�S�O� – 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:


-Sodium Thiosulphate

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

-Clamp stand


-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:

Test 1

Test 2

Test 3


0.5 Molar Acid





1 Molar Acid





1.5 Molar Acid





2 Molar Acid





2.5 Molar Acid






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 10�� 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 10��, 6 x 10��, 9 x 10��, 1.2 x 10�? and 1.5 x 10�?. However, this would only apply if I was using 1 mole of acid (1 litre of acid). I measured the acid in sections of 0.5cm�, which is 3 x 10��, for 1 mole. This means that actually, the number of atoms is as follows (respectively); 1.5 x 10��, 3 x 10��, 4.5 x 10��, 6 x 10��, and 7.5 x 10��.

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 10��) x 4.5 = 6.75 x 10��

1 molar acid – (3 x 10��) x 3.25 = 9.75 x 10��

1.5 molar acid – (4.5 x 10��) x 2.5 = 1.125 x 10��

2 molar acid – (6 x 10��) x 1.5 = 9 x 10��

2.5 molar acid – (7.5 x 10��) x 1.25 = 9.375 x 10��

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.


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.


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 0.1cm�, which was definitely small enough for the experiment I was conducting. However, I did not use this measurement; I measured in 0.5cm�. I did this because I believed that 0.1cm� 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.