Scientific explanation
In order for the sodium thiosulphate and Hydrochloric acid to react their particles must:
- Collide with each other
- The collision must have enough energy
This is shown by the explanation below:
The particles in the liquid If the collision has enough If the collision
move around continually. energy, a reaction takes place does not have
here an acid particle is enough energy
about to collide with a no reaction
sodium atom occurs. The
acid particle
bounces away again.
If there are a lot of collisions then the rate of the reaction is fast. If there are not many, the rate is low.
The rate of a reaction depends on how many successful collisions there are in a given unit of time.
There are three main ways of speeding up a reaction:
- Changing the concentration
- The temperature
- The surface area
- A catalyst
Why rate increases with concentration
If there are few particles if the substance is more
this will mean there is concentrated – there
not much chance of a are more particles
particle hitting another in it. There is now
atom. more of chance of a
a successful collision.
Atoms are used up so a reaction stops because the particles have been used up during successful collisions.
Why rate increases with temperature
At low temperatures, particles of reacting substances do not have much energy. However, when the substances are heated, the particles take in energy. This causes them to move faster and collide more often. The collisions have more energy, so more of them are successful. Therefore the rate of the reaction increases.
Why rate increases with surface area
The particles can collide For example when metal is
only with those sodium powdered, many more atoms
atoms in the outer layer. are exposed. So there is a
greater chance of
successful collisions.
Why a catalyst increases the rate
Some reactions can be speeding up by adding a catalyst. In the presence of a catalyst, a collision needs less energy in order to be successful. The result is that more collisions become successful, so the reaction goes faster. Catalysts are very important in industry, because they speed up reactions even at low temperatures. This means that less fuel is needed so more money is saved.
Prediction
I predict that if one were to have a high level of sodium thiosulphate and a low level of water, than it is obvious to see that the reaction will be very quick. If one was to have low sodium thiosulphate and a high level of water than the reaction will be slow. The reaction is explained by the graph below:
1: At this point in the graph the reaction is very quick due to a high gradient. There is a high concentration of Sodium Thiosulphate, so there are more successful collisions of particles hitting the atoms.
2: The concentration of sodium thiosulphate is low so there are fewer particles meaning less successful collisions leading to a slow reaction.
3: The graph has now started to level off because there is nearly no sodium thiosulphate in the solution, therefore there are less particles meaning much less successful collisions, and the particles which are colliding and have non-
successful collisions loose a substantial amount of energy. If there are much less unsuccessful collisions then the reaction will be a very slow reaction.
Preliminary results
Hydrochloric acid. Sodium Thiosulphate. Distilled water. Time
5 40 10 22.3secs
5 30 20 40.5secs
5 20 30 1.46secs
5 10 40 2.20secs
From my preliminary results I can see that my prediction was correct.
But there are a number of ways to improve my experiment:
- Take more than one reading
- Keep the temperature in the room the same; I will keep all the windows in the room closed.
From my preliminary experiment I can see the measuring utensils I used were good enough to lead to a successful experiment, but I have decided to start with 50cm3 of sodium thiosulphate and 5cm3 of Hydrochloric acid but no water. I have decided to take three results of time for each solution, for example:
Time
1st attempt 2nd attempt 3rd attempt
From the preliminary experiment I have a good range of results. I believe I chose the right volumes of the substances and the range of concentration of the substances was good.
I predict with a low level of sodium thiosulphate the number of particles will be low; therefore there will be a low number of successful collisions. This means the reaction will be a slow reaction.
Experiment results
Analysing evidence
From the results I can see that my prediction was correct, that as the level of sodium thiosulphate decreases and the level of water increases the time for the completion of the reaction is slow.
The experiment was successful; there were no anomalies in the experiment. As the concentration of sodium thiosulphate decreases the number of particles are low the number of successful collisions are low.
My experiment was successful therefore my experiment was well planned. My conclusion is the same as my prediction; with a low level of sodium thiosulphate a slow reaction takes place. Therefore my prediction is correct.
Evaluating evidence
My results are reliable; they are similar to my preliminary experiment which went well. There were no anomalies. On my graph there were no points on the graph which were out of place, showing my experiment was successful. I repeated my results three times, the results were similar meaning the experiment and results were successful and correct.
There were no outstanding anomalies in the experiment, but there were possibilities of a change in some things. There water used was not distilled; there would have been impurities in the solution. The apparatus used could have been improved:
- I could have used instead of a measuring cylinder to measure the thiosulphate, I could have used a burette, this would have given me more accurate results.
- The conical flask could have been kept in a water bath to keep the temperature the same.
Conclusion
I conclude that the more concentrated a reactant is, the quicker the rate of reaction time will be.
I have come to this conclusion because of several reasons. Firstly, my results give conclusive evidence that as the amount of Thiosulphate decreases, and the amount of water increases in the solution, there are less reactive atoms to collide and therefore less successful collisions causing little chemical change so the reaction rate is slower. In a more concentrated solution, there are more atoms to collide so the reaction time is quicker.
My results support the prediction I made because I said 'the greater the concentration of thiosulphate solution, the faster the chemical reaction will take place.' I believe I was correct and a secondary sources from the Internet state that the reaction time will be faster with a more concentrated solution because, 'the more molecules there are, the frequency of successful collisions is greater and therefore the reaction rate is speeded up.'
I could have also done an experiment like this one:
The experiment would consist of Magnesium ribbon and Hydrochloric acid. I would have 0.06g of magnesium ribbon in one conical flask which is connected to a glass syringe and then a conical flask containing 0.06g of magnesium, but in the first conical flask it would contain 50cm3 of 1M hydrochloric acid and the second flask would contain 50cm3 of 05 M hydrochloric acid. Using the glass syringe I can measure the rate of reaction between these two different concentrations. The apparatus would be set up as shown below.
