Because of this we can say that the lower the water concentration the quicker the reaction. The less water molecules there are, then more of the hydrochloric acid and the sodium thiosulphate molecules will collide causing a faster reaction.
The above diagram clearly shows that the more acid molecules, then the more collisions which are likely to take place.
With the control experiment, where there is 50 cm cubed of water, we can say that there will be no reaction, and if there is then the water must be contaminated. We can say this as we know that hydrochloric acid does not react with water, so if there was a reaction, then there must be something in the water.
I think that the rate of concentration is directly proportional to the rate of the reaction. I think this because the higher the concentration, the quicker the reaction will be. The lower the concentration the slower the reaction will be.
Factors that effect the rate of a reaction:
• The temperature – if heat is added then this gives the molecules more
energy, which means that they will move faster, therefore causing more
collisions, and a quicker reaction.
• Concentration – The more molecules of an acid there is, then the more
collisions there are, and an overall a quicker reaction
• Catalyst – If a catalyst is added, then it provides a ‘base’ for reactions to
happen. So a catalyst increases the rate of a reaction.
• Surface area- If the surface area is increased, then there will be a quicker reaction because there would be more of that product to react with the acid.
Key:
Here is the word equation for the reaction of Sodium Thiosulphate with hydrochloric acid:
Sodium Thiosuphate + Hydrochloric acid Sodium Chloride + Sulphate + Sulphur Dioxide + water
Now the symbol equation:
Na2S3O3 + 2HCl 2NaCl + S + SO2 + H2O
Molecular Equation:
Na2S3O3 + 2HCl 2NaCl + S + SO2 + H2O
+ + +
Na H Na S SO2 H2O
+ + +
Na H Na
2- - -
S2O3 Cl Cl
- -
Cl Cl
Each part of each product is written out individually. So it is just a case of crossing off the product if it appears on the other side as well.
From the molecular equation, we are left with another equation. This is the Ionic Equation.
Ionic Equation:
2- +
S2O3 + 2H S + SO2 + H2O
This equation shows us why the reaction actually takes place. The negative S2O3 attracts to the positive 2H. It does this because two opposite charges attract.
When the reaction occurs, ions are released. This brakes the old bonds, and creates new ones.
As we can see, the (S2) is broken up, and one of the (S) joins to the (O2) which has broken away from (O3). Therefore a new bond has been made. The 2H simply attracts to the remaining oxygen molecule to make (H2O).
Fair Test: In order for this to be a fair test we had to keep some things constant.
• The temperature – if each reaction was at a different temperature then molecules would have different energy levels, therefore we kept it the same, which was room temperature, 25°C.
• The volume of the hydrochloric acid. If this changed in each experiment, then each would react at different rates, causing different times for the reactions to occur, so we kept the volume of the hydrochloric acid the same.
• Once a reaction has occurred, then the conical flask where the reaction took place should be washed out thoroughly in order to prevent the next reaction from being contaminated. This would cause different rates of reaction to occur, causing an unfair test.
Predictions:
In our hypothesis above we have seen how the rate of a reaction is effected.
From the hypothesis I predict that:
• The more sodium thiosulphate that is added, the quicker the reaction will be. I think this because as we have seen above, the more sodium thiosulphate molecules there are, and less water molecules, there will be more collisions with the hydrochloric acid molecules which means more a quicker reaction.
• Therefore we can say that if there are more water molecules than sodium thiosulphate molecules then it will be a slower reaction.
• When we mix hydrochloric acid with just water, I predict that no reaction will take place.
Diagrams:
Side View:
Top View:
Results:
We did the same experiment twice in order to compare results.
First Attempt:
Second Attempt:
∗ Rate of reaction is equal to: 1/Time
In this case the numbers have been very small so therefore multiplied by 1000 in order to get ‘friendlier’ numbers.
Graphs:
Analysis:
The first set of graphs, Fig 1, shows the Time against Volume of Sodium Thiosulphate. I have drawn graphs for both attempts at the experiment as it shows how accurate they are. As you can see the graph for the 1st attempt and the 2nd attempt ate practically the same. This suggests that my results are accurate as I managed to achieve almost the same results in both attempts at the experiment.
The shape of the graph is important. The graph curves from the top left to the bottom right. This suggests that the Time for the reaction is indirectly proportional to the volume of the sodium thiosulphate used.
The second set of graphs, Fig 2, shows the Rate against Volume of Sodium Thiosulphate. Again I have drawn graphs for both attempts as it gives me a good idea of how accurate my results are. The general shape of each graph is practically the same, suggesting accurate results.
On these sets of graphs, each graph is going from the bottom left, to the top right. This means that the Rate of the reaction is directly proportional to the volume of sodium thiosulphate used.
The rate of the reaction increases with the concentration. This is because of the collision theory described in the hypothesis. If the concentration of sodium thiosulphate is increased, then there are more molecules of sodium thiosulphate in the reaction. Therefore there is more of a chance that hydrochloric acid molecules and sodium thiosulphate molecules to collide. If they collide then a reaction will take place.
At the end of my hypothesis I made several predictions.
The first one being that ‘The more sodium thiosulphate added, the quicker the reaction will be’. As we can see from our graph this is true. If we look at Fig 1, then we can see that the more sodium thiosulphate, the lower the time.
We also stated that ‘if there are more water molecules than sodium thiosulphate molecules then it will be a slower reaction’. This again has proven to be the case. If we look at Fig 1 again, we can see that when there is more water molecules, (or as the graph shows when there are less sodium thiosulphate molecules) the reaction is slower.
I also predicted that, ‘When we mix hydrochloric acid with just water, no reaction will take place.‘ If we study Fig 2, we can see that when there is no sodium thiosulphate, and it is just water mixing with hydrochloric acid, no reaction occurs, as the rate is '0’.
Conclusion:
From these results it is clear that as the volume of sodium thiosulphate increases, the quicker the reaction will be.
So from this we can say that the lower the volume of sodium thiosulphate, the slower the reaction will be.
This experiment proves that the concentration of an acid effects the rate of the reaction.
Evaluation:
The results that I obtained seem to be very accurate. I think this as I drew up graphs for both attempts at the experiment and drew them out side by side. (see Fig 1 and 2) This makes it easier for me to compare them. And as we can see, each of the graphs are nearly identical. This suggests that my experiment was a success and that my results are accurate.
Limitations:
During this experiment we encountered several problems.
The main problem being, that it is difficult to decide exactly when the black ‘X’, has totally disappeared. Because of this my results could be slightly off by a few seconds on each.
The other problem is that we had to rinse out the conical flask we were using, and re-use it throughout the experiment. Therefore, the conical flask may be contaminated at some point and effect the results slightly.
To overcome these problems, and to improve upon the experiment, we could change some of the ways of doing it. For example, instead of using a black ‘X’ to place under the conical flask, we could use a light sensor with timer. This would only work in a blacked out room. But it would give us even more accurate results. The light sensor would be placed under the conical flask in place of the black ‘X’. Once the reaction has started, the light will gradually fade, and once no light can be sensed, the timer will stop.
Instead of re-using the conical flask, we could have a separate, clean conical flask for each reaction that you do. This would insure that none of the flasks are contaminated and a fairer test.
