Method
- Make the correct concentration of acid if needed to, by diluting the acid with water. This will decrease the concentration of the acid. Measure the water and acid separately by using measuring cylinders, and then dilute them together in one of the 100ml beakers. When diluting the acid and water you must make sure that it adds up to five centimetres cubed (refer to table).
- Measure out five centimetres cubed of Sodium Thiosulphate, in a measuring cylinder then place in the other 100ml beaker.
- Draw a lightly drawn pencil cross on a piece of paper and place it under the 250 ml beaker, which you will pour the reactants in.
- Pour the reactants into the beaker and press start on the stopwatch.
- When you can no longer see the cross at the bottom of the beaker, press stop on the stopwatch and record your findings.
- Repeat this method three times for each concentration of acid.
Safety
- When diluting the acid, make sure that you are adding the acid to the water, because when reacting these two solutions they create heat, therefore when adding them some access solution may spit out. If you were adding the water to the acid then the acid would be spitting out and could damage skin, clothing or even eyes. However because I will be adding the acid to the water, only access water will be spitting and that is harmless to our bodies.
- You must always wear apron and goggles when conducting these experiments.
- When reacting the two solutions make sure that you are conducting the experiment on a heatproof mat. This will avoid any damage to desktops.
- Standing up to conduct the experiment, therefore reducing the risk of tripping and spilling chemicals.
- Hydrochloric acid and Sodium Thiosulphate are both irritants; therefore if they come in contact with my skin, I will wash it thoroughly immediately.
Hypothesis
My hypothesis for this experiment is that as the concentration of hydrochloric acid increases, so will the rates of reaction. Therefore, the cross will disappear more quickly due to the cloudiness of the solution. The reason I predict this hypothesis is because of two main reasons:
- Collision Theory states that a chemical reaction can only occur between particles when they collide. The faster the particles are going, the more energy they have or the more particles there are, the more likely they are to collide. Fast moving particles are more likely to react when they collide. When the concentration of a liquid increases, it means that there are more particles per cubic centimetre and because there are more particles there is a higher chance of collision.
- I undertook a preliminary experiment before I began this one. It also investigated rates of reaction, however the reactants of this experiments were Magnesium and Hydrochloric acid. In this experiment I also varied the concentration of the acid and I concluded in that experiment that the higher the concentration of acid the quicker the rate of reaction. Therefore I believe that I will come out with the same conclusion as I did last time.
Diagram
Equipment (For each single experiment)
Results
Analysis (See Graph)
When reacting the two clear reactants, Sodium Thiosulphate and Hydrochloric Acid they produced a yellow precipitate, sodium chloride and sulphur. The sulphur is the product, which turned the solution yellow. As we increased the concentration of the Hydrochloric Acid the solution became cloudy faster, we meant that the rate of reaction was increasing. This is due to collision theory. When the concentration of the hydrochloric acid is higher there are more particles of acid present in one cubic centimetre of hydrochloric acid; therefore there are more successful collisions with Sodium Thiosulphate particles. When the concentration is lower there are not as many successful collisions because there are not as many particles to collide with.
Low Concentration High Concentration
From the results that were obtained from the experiment, I have produced one graph, which shows the time it takes for each reaction to take place when the concentration of the hydrochloric acid varies. When producing the graph I found no obvious anomalies, although there are a few points slightly off the curve of best fit.
This graph shows that the higher the concentration, the less time it takes for the reaction to finish. Therefore meaning the higher the concentration the faster the rate of reaction. In turn this means that the concentration is inversely proportional to time. My observational results and graph have both proved my hypothesis to be correct.
Conclusion
From the reactants I use in every experiment the following products were produced:
Hydrochloric acid + sodium thiosulphate sodium chloride + sulphur dioxide + sulphur + water
HCl (aq) + Na2 S2 O3 (aq) NaCl (aq) + SO2 (g) + S (s) + H2O (l)
From the evidence I have obtained I have come to the conclusion that as the concentration of hydrochloric acid increases so does the rate of reaction, therefore my hypothesis is correct.
Evaluation
I believe that my results are quite reliable because they are clear and structured and there are no obvious anomalies shown in my graph, however like I have said there are a few points slightly off the curve of best fit e.g. the results obtained from experiments regarding acids with the concentrations of 2 molar and 1.2 molar. The reason for this may have been because the temperature of the room could have fluctuated, or the concentration or amount of either reactant could have been slightly wrong. The method I used for this experiment gave us good results. It was quite simple to follow and was completed quickly.
To make the results more accurate, I could have taken more readings and used a more precise measuring instrument to indicate when to stop timing the reaction. When we did it with this experiment we used our own judgements to tell when the cross could no longer be seen, however if we used a more accurate instrument, e.g. a light sensor we would have obtained more accurate readings. Another reason why we should have used a light sensor is because if we repeated the experiment to get similar results it may not be possible because the cross which was drawn on by pencil could be lighter or darker than the original one because different people apply pressure differently when writing.
If I did this experiment again, I would definitely take more readings so that I got a wider range of results that would give me a clearer graph and basically a better, more productive experiment. I would also use a light sensor instead of the cross technique as it is more accurate and reliable.
If I wanted to investigate further I would re do this experiment again but at different temperatures. Therefore I will vary the concentrations at one temperature and vary the concentrations again at another temperature. From this I could be able to find an optimum temperature and concentration in which the rate of reaction is at it’s fastest.
