Investigation of Factors Effecting Reaction Rates

Sodium thiosulphate + Hydrochloric acid → Sulphur dioxide + Water + Sodium chloride

Na2S2O3 + 2HCl → 4SO2 + 2H2O + 6NaCl

The sulphur is produced slowly as a pale yellow precipitate and makes the reaction mixture cloudy. The speed of the reaction may be found by finding the time taken to to reach a particular level of cloudiness of the reaction mixture. There are factors which can affect the speed of the reaction, including

temperature
catalyst
surface area
concentration

I am going to investigate the effect of the concentration of thiosulphate on the speed of the reaction.

Preliminary Work

I have carried out a preliminary experiment to help me plan out this investigation. I used a 25cm3 measuring cylinder to measure out 25cm3 of sodium thiosulphate. This is because if I had used a 10cm3 measuring cylinder, I would have had to measure out the sodium thiosulphate three times. This would not be very accurate because every time I pour out the liquid some is still left in the cylinder. I did not use a bigger cylinder than 25cm3 because that would not be accurate either to measure a small amount. The concentration of the sodium thiosulphate was 40g/dm3. It is not worth using a pipette to measure out the sodium thiosulphate because all the effort needed to use it is not worth the accuracy it will give because such accuracy is not needed. From this, I decided to use 25cm3 measuring cylinder to measure out the sodium thiosulphate for my investigation.

I poured the sodium thiosulphate into a small conical flask. I used a small conical flask because it is easier for me to hold and swirl it. A bigger conical flask has got a bigger neck, which I found difficult to hold and swirl. I used a glass flask so that I could see what was happening inside. From this I decided to use a small conical flask for my investigation.

I drew a cross on a sheet of paper and put the flask on top of the cross so that I could observe the cross through the mixture in the conical flask.

To measure out 5cm3 of hydrochloric acid, I used a 10cm3 measuring cylinder. I have explained already why it would not be good to use a larger or smaller measuring cylinder. The concentration of the acid was 2M. I therefore decided to use a 5cm3 measuring cylinder in my investigation to measure out 5cm3 of hydrochloric.

I poured out the hydrochloric acid into the conical flask. As soon as I poured in all the hydrochloric acid into the flask, I started the stopwatch. I swirled the conical flask quickly but gently, so that hydrochloric acid and sodium thiosulphate would mix together and react. I put the flask back on top of the cross and looked at the cross vertically through the neck of the conical flask.

I couldn’t put my eye near the flask because sulphur dioxide was given off from the reaction and it is a toxic gas, so I would have had ...

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I couldn’t put my eye near the flask because sulphur dioxide was given off from the reaction and it is a toxic gas, so I would have had to inhale much of it. I couldn’t stand far away from the flask either because I needed to see the cross well, so I stood about 20cm above the flask, as a compromise. I decided to use a stopwatch because it is more accurate. Once I could not see the cross anymore, I stopped the stopwatch and recorded the time. From this I decided that for this investigation I would stand 20cm vertically above the flask and that it is best to use a stopwatch rather than a stop clock.

Everyone in the class carried out this experiment. The time it took for the cross in my experiment to disappear was 30.57s. The following are the times it took for other people, in seconds (s):

This shows that the results for the same experiment were varied, so in the investigation I have to carry out the experiment for each concentration three times so that I can get an average result, and also to check that my results are reliable.

Prediction

I predict that when the concentration of the sodium thiosulphate doubles, the time it takes for the cross to disappear will halve. The reaction rate will double. This is because when there is twice as much sodium thiosulphate, there are twice as many sodium thiosulphate molecules. They are twice as likely to collide hard enough with the hydrochloric acid for a reaction to occur, so the reaction will take place twice as fast.

Equipment

goggles – to protect eyes
overall – to protect clothes
small glass conical flask – for the reaction to occur
hydrochloric acid
sodium thiosulphate
25cm3 and 10cm3 of measuring cylinders
sheet of paper with a cross drawn on
distilled water – to add to sodium thiosulphate to make up different concentrations

Fair Test

I will use the same equipment all the time. I will use the same cross and start the stopwatch every time when all the hydrochloric acid is poured in. I will try to do the experiment on the same day so that the temperature is constant. I will carry out the experiment for each concentration three times to get an average. In this way my results will be more reliable. I will measure out the liquids as accurately as possible.

Safety

I will wear goggles and overall to protect eyes and clothing. I will keep the room well ventilated so that I will not breath in a lot of sulphur dioxide. I will throw away the sodium chloride solution as soon as I have finished the experiment so that I will not breath in a lot of the sulphur dioxide.

Concentration

I will use five different concentrations of sodium thiosulphate. The highest concentration is 40g/dm3. I will use concentrations of 8g/dm3, 16g/dm3, 24g/dm3, 32g/dm3 and 40g/dm3. After I have obtained results for these, if I do not find a pattern, I will make up other concentrations and carry out the experiment on them until I find a pattern in my results. To make up the concentrations I want, I need to do the following calculation:

total volume x concentration wanted = volume needed in

concentration given measuring cylinder

I will be given sodium thiosulphate of concentration 40g/dm3. To make up sodium thiosulphate of 8g/dm3 concentration, the calculation would be,

25cm3 x 8g/dm3 = 5cm3 of sodium thiosulphate

40g/dm3

I need to add 5cm3 of sodium thiosulphate of concentration 40g/dm3, with 20cm3 of distilled water, to get 25cm3 of sodium thiosulphate of concentration 8g/dm3.

To obtain the following concentrations, this is what I need to mix together:

Method

I will clear my desk so that I will have space to work. I will open all windows and doors to keep the room ventilated. I will keep the sodium thiosulphate and hydrochloric acid at different ends of the room so that they are far away because they can react very easily. I will draw a cross on a sheet of paper.

I will first make up sodium thiosulphate of concentration 8g/dm3, by pouring 5cm3 of sodium thiosulphate 40g/dm3 in a in a 25cm3 measuring cylinder and then adding 20cm3 of distilled water to it. I will always measure to the top of the miniscule every time I measure out a liquid so as to get the most accurate result possible. After swirling the measuring cylinder to mix the liquids together, I will pour the mixture into a conical flask, and put the flask on the cross on the sheet of paper.

I will then measure out 5cm3 of hydrochloric acid using a 10cm3 cylinder and pour it into the conical flask. Once all the acid is poured in, I will start my stopwatch. I will stand 20cm above the conical flask and observe the cross through the neck of the flask. When I can see the cross no more, I will stop my stopwatch and record the time it took for the cross to disappear. I will repeat the experiment for this concentration two more times. I will carry out this procedure for all other concentrations.

As soon as I have measured the time taken for the reaction to occur, I will throw away the liquid produced so that the sulphur dioxide does not have to be inhaled for a long time.

Results

The results in pink do not seem to fit in with the other results of the same concentration. I have not included them in my average and I am treating them as anomalous results.

Analysis and Evaluation

My experiment does not measure concentration against reaction rates; rather it measures concentration against time taken for the cross to disappear. To work out the reaction rate, I divided 1 by the time taken, as shown by the table below. I then drew a graph of the reaction rate against the concentration, since this is what I am investigating.

My prediction was right. As the concentration doubles, the reaction rate also doubles. This is because of the collision theory. When there is twice as much sodium thiosulphate, there are twice as many sodium thiosulphate molecules. They are twice as likely to collide hard enough with the hydrochloric acid for a reaction to occur, so the reaction will take place twice as fast. According to my graph, sodium thiosulphate of concentration 15g/dm3 has a reaction rate of 0.014s-1. Sodium thiosulphate of concentration 30g/dm3 has a reaction rate of 0.028s-1. Since the reaction rate doubles as the concentration doubles, the graph supports my prediction.

I have got two anomalous results. For 8 g/dm3, I measured a time of 244.83s. For 16g/dm3, I measured a time of 81.64s. I have not included them in my average. These could have been anomalous results because I did not measure the hydrochloric acid or the sodium thiosulphate as accurately as possible. I might have been distracted and not seen that the cross had already disappeared, so I might have stopped the stopwatch a long time after the cross had disappeared. There might have been a temperature change as I was taking those two results, possible because someone was opening or closing a door or a window.

I’m not that sure my results are accurate and someone else would get the same results as me since there are many errors and mistakes I could have made. I also need to conduct this experiment with at least ten different concentrations and repeat them at least five times to be certain that my results are correct and someone else would get the same results. The two anomalous results show that this experiment is not very accurate.

To get better results, the cross needs to be laminated as it got wet and smudged, which would have affected the results as the visibility of the cross would have decreased. Further work is needed to confirm my results as I have already explained.