When dilute Hydrochloric Acid and Sodium Thiosulphate are mixed the following reaction occurs:
2HCl + Na2S2O3 2NaCl + SO2 + S + H2O
The reason I am going to investigate this reaction is that both reactants are liquid so they are more accurate than using a solid where surface is a factor. Also the product, sulphur is a solid that causes the liquid to become opaque. Therefore it is very easy to see when the reaction has ‘finished’ by having a tile with a cross which would be obscured after a certain amount of sulphur had been produced.
I conducted some preliminary work to find out the best quantities of the liquids to use and the best overall amount. I also decided which concentrations to omit because they were either too slow or too fast. Here are the results:
Concentrations (20°C)
Quantities (20°C)
Prediction
The particles in a fluid undergo random collisions in which energy is transferred between colliding particles. As a result there will be particles with differing energies. Maxwell-Boltzmann energy distribution curves show the distribution of the energies of particles in a fluid.
The activation energy for a given reaction can be marked on the distribution curve. Only those particles with energy equal to or greater than the activation energy can react when a collision occurs.
The two predictions that I will make are:
- The increase in concentration will cause the reaction rate to increase because there will be a greater number of particles of solute so there will be a greater chance of collisions taking place. The rate of increase will be constant.
- The second prediction is that as the temperature increases, so will the reaction rates because the particles will start with a higher energy so there will be more chance of collision and the collisions will be more violent. By looking at the Maxwell Boltzmann energy curves, I predict that for every 10 degree rise in temperature, the reaction rate will double
Method - Concentration
- The apparatus should be set up as shown in the diagram.
- From my preliminary work I know that I need 40cm3 of liquid to cover the bulb of the thermometre, made up of 30cm3 of sodium thiosulphate and 10cm3 of hydrochloric acid. This is because it produces a faster reaction rate.
- Measure out the quantities of the liquids, and poor the sodium thiosulphate into the beaker.
- Add the hydrochloric acid and start the stopwatch.
- When the black cross is obscured, stop the clock and record the time and the temperature.
- Wash and dry all equipment to prevent contamination.
- Repeat with a different concentration of sodium thiosulphate. They are: 10, 20, 25, 30, 40, and 50 g/dm3
- To make it a fair test, only the concentration of sodium thiosulphate should be changed.
Method - Temperature
- The apparatus should be set up as shown in the diagram.
- From my preliminary work I know that I need 40cm3 of liquid to cover the bulb of the thermometer, made up of 30cm3 of sodium thiosulphate and 10cm3 of hydrochloric acid. This is because it produces a faster reaction rate and also because one quantity has to be greater than the other so that when it is heated, there is as small a temperature as possible when the cooler solution is added.
- Measure out the quantities of the liquids, and poor the sodium thiosulphate into the beaker.
- Heat the sodium thiosulphate over a blue bunsen burner flame to the required temperature. These are
- Add the hydrochloric acid and start the stopwatch.
- When the black cross is obscured, stop the clock and record the time and the temperature.
- Wash and dry all equipment to prevent contamination.
- Repeat with a different temperature of sodium thiosulphate. They are: 20, 30, 40, 50, 60 and 70 degrees celcius
- To make it a fair test, only the temperature of sodium thiosulphate should be changed.
Results
Concentration
Temperature
Analysis
The main thing that I can conclude from my results is that both increasing the concentration of sodium thiosulphate and raising the temperature of the reaction increase the rate of the reaction. This fits perfectly with my prediction, because in the concentration experiment, there are more particles of sodium thiosulphate, so the chance of collision increases and with the temperature experiment, the particles have more energy and are moving faster, which increases the chances of collision and the energy of the collisions.
The graph for concentration has a best fit line that is straight which fits with my prediction that the reaction rate would be a constant increase. This is because the energy is not affected by increasing concentration, it only provides more particles that can collide. As the amount also stays the same, the increase is constant. The steady increase can be seen on the graph, for each rise of 10g/dm3 in concentration, the reaction rate goes up by around 6.
The graph for temperature is a curve that’s gradient increases. This also fits my prediction that for every 10 degree rise, the reaction rate would double. Between 34 and 42 degrees, the reaction rate doubles from 32.3 to 60.6. This fits with the Maxwell-Boltzmann energy distribution curves. The reason that the reaction rate doubles each time, instead of being a constant increase is twofold. Firstly the rise in temperature means that there are more particles in the reaction with energy higher than the activation energy meaning that they move faster and increase the chances of collision, and secondly, when the collisions occur, they are more violent because the particles have more energy.
The results back up both my predictions.
Evaluation
The investigation was a success because I recorded enough reliable results on which to base a firm conclusion. I recorded a suitable range of values in both experiments in order to plot graphs and repeated each experiment and took the average to help accuracy.
However, there was room for improvement in many areas:
Firstly, the end of the reaction was determined only my judgement, which may have varied. To improve this, a light meter could be used to indicate when a certain amount of sulphur had been produced, and this would be much more reliable than human judgement.
Secondly, accuracy in the temperature experiment was quite difficult, because doing repeats was difficult due to the problems with getting the water bath to the same temperature. Also, when the acid was added it would have cooled the reaction down. These problems could be overcome by using a large, thermostatically controlled water bath where all the glasswear could be kept at a constant temperature.
Thirdly, the experiments can only be as accurate as the least accurate measuring device, which in this case, was the thermometer which cold only be read to half a degree. Therefore a more accurate thermometer would have allowed the investigation to be more precise.
Further work
Further work on this topic could include:
- Using different quantities of sodium thiosulphate and hydrochloric acid.
- Investigating the affects of a catalyst on the reaction.
- Repeating the experiment with the above improvements.
- Changing the concentration of hydrochloric acid.