In the reaction of sodium thiosulphate and hydrochloric acid temperature should have been kept constant, no catalyst was added to the solution, no solid particles were present and only the concentration of the solution was changed.
In the reaction of sodium thiosulphate and hydrochloric acid, as the concentration of sodium thiosulphate is decreased, by adding more water to form 50cm3 of solution, there are fewer particles of reactant to collide and react with each other. Collisions will happen less often and they would have less energy because they have further to travel. Some particles will collide and will bounce away from each other. In a collision where the particles react, bonds are broken and bonds are formed. If there are fewer reacting particles and less energy, there will be fewer collisions resulting in a reaction and thus the sulphur particles would form less often. The solution would therefore become cloudy more slowly and the cross at the bottom of the beaker would take longer to disappear. This means that the reaction is happening at a slower rate.
If the volume of sodium thiosulphate was increased, and the volume of water decreased in order to make 50 cm3 of solution; there would be more frequent collisions of particles with more energy. In this case, the hydrochloric acid and sodium thiosulphate particles would collide and more particles would react. Because the rate of collision is higher the rate of formation of sulfhur and of the reaction is increased. As the sodium thiosulphate collide and react with the hydrochloric acid, the sulphur is precipitated out of the solution. This solid sulphur makes the water look cloudy.
Variables table
These factors will make it a fair test.
Apparatus
Hydrochloric acid (0.5M)
Sodium thiosulphate (0.5M)
10cm3 Cylinder
100cm3 Cylinder
250ml Conical flask
Stopwatch
Thermometer
Goggles
Piece of paper with fine cross
Diagram
Method
- Mark a cross, (with a fine pen), on a piece of paper. Place on the table.
- Place a 250ml conical flask on to the cross, (see the cross through the glass).
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Measure out 50cm3 of sodium thiosulphate in to a 100cm3 cylinder, and pour into the flask.
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Measure out 5cm3 of hydrochloric acid in a 10cm3 cylinder.
- Have a stopwatch at hand.
- Quickly pour the acid into the flask, and at the same time, or immediately after, start the stopwatch.
- Once you can no longer see the cross through the flask, stop the stopwatch.
- Record the time taken.
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Repeat these steps four more times, using 10cm3, 20cm3, 30cm3and 40cm3 of water, lowering the volume of the sodium thiosulphate by the same amount, to make 50cm3 of solution.
- Record all the results and do repeats to check.
Always take safety precautions; wear goggles, tie back hair, clear the work area, if anything is spilt, clean it immediately.
Results
Room temperature of the reactants = 22o C
Graphs
Analysis
Looking at the graph of Sodium Thiosulphate (cm3) versus Reaction Rate (1/sec) one can see that there is a steady rise in the reaction rate when the volume of sodium thiosulphate in the solution is increased.
The graph of Mean Reaction Time (sec) versus Volume of Sodium Thiosulphate (cm3) shows that the higher the concentration of sodium thiosulphate the shorter the time taken for the cloudy water to cover the cross.
The garphs show that my prediction was correct.
Evaluation
The main downfall of this experiment could be in the amount of accuracy used. This plays a major part when doing experiments of this kind. If a measurement or time span were one unit ‘out’, this could affect the results greatly, and therefore affect the graphs, and the final conclusion. In my results though, there were no anonymous results, which could have affected the conclusion greatly.
The way in which we did the experiment could have been improved. Firstly, to arrive at a mean time, more data should be collected for each point plotted on the graph.
The time at which the stopwatch was started and stopped could be improved. By using an instrument to detect the quantity of light coming through the solution, from a fixed constant light source shining throughout the solution to the light strength detector, we could stop the stop watch at a constant light intensity. The rate at which the acid is added to the sodium thiosulphate should be constant. This means that it should always take the same amount of time to add the acid to the sodium thiosulphate.
The temperature at which the reactants were used and added to each other should have been kept constant in order to avoid any changes in the reaction rate.
Conclusion
When hydrochloric acid was added to the sodium thiosulphate solution, a reaction took place. The rate of this reaction depended on how often, and hard the reacting particles collided. Particles have to collide hard enough for the reaction to be successful.
By diluting the sodium thiosulphate with water, and then adding acid, the reaction rate decreased. The reaction rate decreased because fewer molecules, with less energy of sodium thiosulphate were available to collide with the hydrochloric acid particles.
Thus, my prediction, that the lower the concentration of sodium thiosulphate, the slower the reaction rate, was correct. Put another way, the lower the concentration of sodium thiosulphate, the more time was required for the reaction to take place.
