Thiosulphate Acid Chloride Dioxide
Na2S2O3 (aq) +2HCL (aq) = 2NaCl (aq) + S(s) +SO2 (g) + H20 (l)
Aim: We did 6 experiments to find out how the rate of reaction changes with differing concentrations of Sodium Thiosulphate, Hydrochloric Acid and water. As an inert and stable liquid, water was used to alter concentration of Sodium Thiosulphate without changing the end amount of solution. All the atoms in a water molecule have a full outer shell, so they would not react with the other chemicals.
Prediction: I predict that the rate of reaction will increase (and get more cloudy, more quickly) when the solution of Sodium Thiosulphate and Hydrochloric Acid are strongest, and there is no water. The reason for this is that it will be easier for the Sodium Thiosulphate to react with the Hydrochloric Acid, as they are the only two chemicals in the beaker, and there is not water to hinder the rate of the reaction. There will also be more Sodium Thiosulphate to react with the Hydrochloric Acid, regardless of how much water there is.
Apparatus:
· Conical flask
· Sodium Thiosulphate (different concentrations)
· Hydrochloric acid (same concentration each time)
· Water (different concentrations)
· Stopwatch
· Paper with black cross on it
· Small meniscuses
· A large meniscus
Method: We wanted to change the concentration of Sodium Thiosulphate and Hydrochloric Acid, but without changing the overall quantities. To do this, the Sodium Thiosulphate and water were mixed at different ratios, with always a constant amount of acid. The table below shows the 6 different experiments, and what each solution composed of.
In total, there were always 25cm3 of water and Sodium Thiosulphate, with 5cm3 Hydrochloric Acid, giving a total solution of 30cm3.This reaction usually produces a yellow precipitate. Firstly, we drew a black cross on a white piece of paper. Then, we put 10 cm3 of hydrochloric acid in a conical flask (measuring the HCL with a small meniscus), and then put the conical flask on the black cross on the paper. We then added 25 cm3 of Sodium Thiosulphate to the hydrochloric acid, and at this time we do not need to add any water. Immediately, we started the stopwatch. We then waited for the black cross to be completely obscured by the mixture. When this happened we stopped the stopwatch. By this time the mixture was completely yellow. Then we recorded what the time was. We then emptied out the contents of the conical flask, and cleaned it out, and dried it. Next, we added 20cm3 of Sodium Thiosulphate; we kept the HCL acid constantly at 5 cm3. Now, because we have to keep the volume the same for it to be a fair test, we added 5 cm3 of water. (We had to add the sodium Thiosulphate and the water at the same time for this to be a fair test also). We then recorded the time for the cross to become completely obscured. We repeated this many times, adding different amounts of Sodium Thiosulphate and water each time, and recording the time taken each time.
Fair test To make this experiment a fair test, we need to make sure we do a number of things. In this experiment we are trying to find the rate of reaction using concentration as a factor, so there is a number of things we need to make sure we do to keep it a fair test.
Firstly, we need to keep a chemical at a constant concentration. So, in this experiment we have chose to keep hydrochloric acid at a constant concentration. We could have, however, used Sodium Thiosulphate as a constant, but we had chose to use hydrochloric acid.
Next we must make sure that the solution is kept at a constant volume throughout the experiment. If the volume is different, then it could give different results to if it was at a constant volume.
We must also make sure that we add both the water and the Sodium Thiosulphate at exactly the same time (into the conical flask with the hydrochloric acid in it), or it could affect the results of the experiment.
We must start the stopwatch at the exact time as we put the water and the Sodium Thiosulphate into the conical flask. To do this it is a lot easier if there are two people doing the experiment, so one person can put the two substances in the HCL, and one person can start the stopwatch.
Another thing we must do is to make sure that the conical flask is completely clean and free of any water or any other substance before we attempt to start the next experiment.
Safety
There are a lot of safety issues we must abide by in this experiment also. We must remember that the substances that we use in this experiment can be very harmful if used the wrong way.
When we do this experiment, it may be necessary to wear safety goggles, as things are very unpredictable, and even though it is very unlikely that the solution would come out of the conical flask during the experiment, one must still be cautious of spills.
We should also try our best not to spill any chemicals, and we must not eat or drink in the lab while dealing with these harmful chemicals, as they can get on to our hands.