An investigation into a factor affecting the rate of reaction of sodium thiosulphate (Na2S2O3) and hydrochloric acid (HCl).

CHEMISTRY

Aim:

I aim to investigate how altering the concentration of sodium thiosulphate will affect its rate of reaction with hydrochloric acid. The sodium thiosulphate (before dilution) is at a concentration of 40g/litre of water, and the hydrochloric acid is at 1M (1Molar).

1M designates a solution containing 1 mole of solute per litre of solvent. A mole is the molecular weight of a substance expressed in grams.

I will probably use concentrations ranging from 100% to 10% in my experiment, and will repeat each concentration at least twice in order to obtain an accurate and representative average.

Scientific Theory:

When Na2S2O3 and 2HCl are mixed together, a yellow precipitate of sulphur is produced. This is suspended in a solution of all the other products of the reaction, making the resulting solution opaque when the reaction is complete.

The equation of the reaction is:

2Na2S2O3 + 2HCl → S2 + 2SO2 + H2O + NaCl

sodium + hydrochloric → sulphur + sulphur + water + sodium

thiosulphate acid dioxide chloride

All rates of reaction are governed by the basic principles of the collision theory. This states that the rate of a reaction relies on how hard and how often the active particles collide with each other.

In my investigation, I will alter the concentration of the sodium thiosulphate. This means that in terms of collision theory the variable will be how often the particles collide. The less active particles there are in the solution, the less often they will collide with anything. Therefore, the lower the concentration of sodium thiosulphate, the longer the reaction will take, and the slower the rate of reaction will be.

Range of results:

I will refer to the concentrations using their value in percent. This makes it easier to refer to as this is how I recorded my results.

Prediction:

I believe that as the concentration of sodium thiosulphate increases, so the rate of reaction will increase, thereby showing a positive correlation. This prediction is partly based on my preliminary findings and partly on common sense.

Preliminary Results:

My preliminary results show exactly what I expected them to show – that ...

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Range of results:

I will refer to the concentrations using their value in percent. This makes it easier to refer to as this is how I recorded my results.

Prediction:

Preliminary Results:

My preliminary results show exactly what I expected them to show – that is, the time decreasing as the concentration increased. This helped me in my prediction as it gave me a better idea of how strong the correlation would be and set in my mind how to continue with my investigation.

I constructed a graph around these results as well, and it showed me that there was a negative correlation between time and concentration. My points were also on a curved line. I then decided that I was going to use rate and concentration, in order that my results would fit a straight line of best fit. This would make them easier to read from the graph.

I chose rate as opposed to time because I knew that rate was the inverse of time (the reciprocal, or 1/x). This made the curved line straight.

Method / Apparatus:

conical flask

beakers

measuring cylinders

an “X” card

stopwatch

sodium thiosulphate

hydrochloric acid

Firstly I placed the conical flask on top of the “X” card. I then poured out 10ml of hydrochloric acid into a measuring cylinder and 50ml of sodium thiosulphate into another, bigger measuring cylinder.

I poured the sodium thiosulphate into the conical flask and added the hydrochloric acid, starting the stopwatch as I did so. I looked down through the mixture at the “X” on the card and stopped the stopwatch when I could no longer see it.

When this reaction was complete, I rinsed out the flask and dried it off.

I next poured out 45ml of sodium thiosulphate and added 5ml of water. Another 10ml of hydrochloric acid was then poured into its measuring cylinder. I decanted the sodium thiosulphate/water mixture into the flask and swirled it around to ensure it was mixed. I then repeated as for the previous concentration, stopping the stopwatch when I could no longer see the “X”.

I repeated this, using each different concentration. (Obviously I repeated each concentration twice, in order to obtain enough results to produce an accurate average.)

Diagram:

Safety precautions:

I wore safety goggles so that if any chemicals were spilt they would not harm my eyes.

Fair Test:

In order that my results were as accurate as possible, it was necessary that I ensured I only changed one variable – concentration. I made sure that I used the same size and shape conical flask each time, so that the depth of the liquid was not affecting our decision about when the “X” was obscured from view. I also attempted to keep each experiment at the same temperature because I only wanted the concentration to change. The “X” card I used was the same each time, as different pieces could have a darker mark on than others. I performed the experiment on each concentration three times, in order that I could average my results and thereby increase accuracy and reduce anomalous readings.

Table of Results:

Analysis:

From my results I can gather that my prediction was correct – the concentration of a solution in a reaction does affect the rate of reaction. I observed this from my graphs.

I constructed two graphs, one of time/concentration and the other of rate/concentration. My time/concentration graph showed a definite trend, as I could draw a line of best fit. This line was a curve, and showed a negative correlation between time and concentration – as the concentration increased, the time decreased.

However to provide an answer for my investigation, I had to find the rate of the reaction. This is the reciprocal of time, or 1/t. I constructed this graph and found an even better trend – the points almost made a perfect straight line. This showed a definite positive correlation between rate and concentration – as the concentration increased, so did the rate of reaction. This proved my theory undeniably correct: when the concentration was higher, there were more particles of reactant to collide with the hydrochloric acid, therefore the collision theory shows that the rate of reaction was higher due to how often the particles collided.

Evaluation:

I believe my experiment was a resounding success, as both graphs showed a clear line of best fit and my prediction was proved correct. There was no anomalous data to be found in my results, which is both bad and good, as it means I now haven’t got much to write about.

I possibly could have done the 10% concentration, but that would have taken too long, and it would take so long, that the point would distort the graph and make it difficult to compare the other concentrations with it. Also the issue of constant temperature would be a more pressing issue if I had had time enough to consider it!

To extend my investigation I could have kept the sodium thiosulphate as it is and changed the concentration of the hydrochloric acid. However this would have been limiting as an extension as the results would have followed almost exactly the same theory as the one above.

To improve on my experiment I could have done the following:

used a different conical flask for each repeat of each concentration (reason not done this time: not enough equipment)
used a new set of measuring cylinders for each repeat of each concentration (reason not done this time: not enough equipment)
extended the range of my concentrations, for example used 5%, 15% and 25% etc. (reason not done this time: not enough time)
used a light sensor (and other automated equipment) instead of just human eyesight to judge when the cross was no longer visible – light sensor would not entail the possibility of human error (reason not done this time: necessary equipment not available)