I think that the concentration of a solution effects the rate of reaction because 'the rate of reaction depends on how frequently the molecules of the reacting substances collide. A more concentrated substance has more molecules for a given volume than a more dilute substance. Because there are more molecules about, the frequency of successful collisions is greater, and the reactions happen faster.'
Safety
There are a lot of safety issues we must abide by in this experiment also. We must remember that the substances which 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 beaker during the experiment, one must still be cautious of spills.
We must make sure that coats and bags are all out of the way while doing the experiment. Ties and hair should be tucked out of the way, so they do not make contact with any of the chemicals. It would also be preferable to wear a scientific apron, however this is not essential.
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.
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 chosen to keep hydrochloric acid at a constant concentration (5cm3). We could have, however, used Sodium Thiosulphate as a constant, but we had chosen 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 beaker 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 beaker. 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. Also The person timing the experiment will look for the disappearance of the cross, otherwise there would be a time lapse between seeing the cross disappear and telling the other person to stop the clock and then eventually stopping the clock.
It is also important to keep the temperature the same to obtain fair results, however we do not have to worry about this as the temperature will be kept at room temperature through out the experiment. It is also important we keep the colour and size of the cross the same this will help keep the results fair for the experiment
Another thing we must do is to make sure that the beaker is completely clean and free of any water or any other substance before we attempt to start the next experiment.
Method
Firstly, we drew a black cross on a white piece of paper. Then, we put 5 cm3 of hydrochloric acid in a beaker, and then put the beaker on the black cross on the paper. We then added 50 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 cloudy. Then we recorded what the time was.
We then emptied out the contents of the beaker, cleaned it out and dried it out. Next, we added 45 cm3 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.
Results
Table of results
Table 1
Na2S2O3(aq) HCl(aq) H2O(l) Time
(cm3) (cm3) (cm3) (Seconds)
Table 2
Na2S2O3(aq) HCl(aq) H2O(l) Time
(cm3) (cm3) (cm3) (Seconds)
Table 3
Na2S2O3(aq) HCl(aq) H2O(l) Time
(cm3) (cm3) (cm3) (Seconds)
I have also come up with a set of average results for all 3 experiments: -
Average Results Table
Concentration Na2S2O3(aq) HCl(aq) H2O(l) Time
(M) (cm3) (cm3) (cm3) (Seconds)
After collecting the results I will be in position to analyse and use graphical format to evaluate the results. When the results are collected there are two methods that could be adopted to find the order of the reactants in order to find the rate equation.
1st Method- Concentration-Time graph
In this we will plot the graph of concentration against time. The time is the time taken for the reaction to finish at particular concentration of the solution. Then to find the order we will need to plot another graph by obtaining information from the concentration-time graph. Then draw straight line tangents to this graph at points corresponding to a variety of concentrations. Each of these tangents has a gradient which is the rate of the reaction at that concentration. You now have a set of rates and corresponding concentrations which can be used to plot the second graph to find the order of the reaction.
The concentration time graph looks something like the one on the right. The graph has negative curve. Various tangents could be found on the graph and then we can calculate and plot the rate against concentration graph to find the order. The rate graph has three curves which differentiates the three orders the following three graphs show the three curves.
This rate graph is showing zeroth order. If your curve looks like this then your solution has zeroth order.
This rate graph shows 1st order. If your graph shows a linear line then your solution has 1st order.
Finally if your graph seems to have power relationship than the solution has 2nd order.
This method can be used to find rate equation. The rate equation is equal to
Rate = k [HCl]m [Na2S2O3]n
k= constant
m & n= the order of that particular solution
Rate = k [HCl]m [Na2S2O3]n
I have worked the values for m & n. Both of them have value of 2.
So our rate equation is
Rate = k [HCl]2 [Na2S2O3]2
We can also calculate the units for constant k when we have the full rate equation. Thus I have calculated the units for the constant k.
mol m-3 s-1 = k [mol m-3]2 [mol m-3]2
mol m-3 s-1 = k mol2 m-6 mol2 m-6
mol m-3 s-1 = k mol4 m-12
mol m-3 s-1 = k
mol4 m-12
mol-3 m9 s-1 = k
Analysis (graphs are on separate sheets)
My results show that as concentration decreases, time taken for the solution to go cloudy increases. This can be explained using the collision theory. The rate of reaction is the speed at which a chemical reaction takes place. It is usually expressed in terms of the concentration (e.g.- moles per litre) of a reactant consumed, or product formed in unit time. Therefore the units would be moles per litre per second (mol 1-1 s-1). It may be affected by the concentration of reactants, the temperature of reactants, and the presence of a catalyst.
Increasing the concentration means there are more collisions and more successful collisions. Consequently the rate of collision increases. The time it takes for a change to take place can be measured; the rate of reaction can be approximated by taking the reciprocal of this time (1/time). For a reaction to take place two reactants must collide and the collision must have sufficient energy.
In the higher concentration there are more particles, so there is a higher chance of a reaction with the necessary amount of energy occurring.lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
My prediction was correct to a certain extent, as I predicted that as concentration increased the time taken for the solution to go cloudy decreased. However, I was not entirely correct as I predicted that time taken would double when concentration halved, but this was incorrect. When the concentration was 50 (M) the average time was 0.47.07 seconds, but when the concentration was 25 (M) the average time was 1.52.27 seconds. This is not half the time; it had an error margin of 1.05.20 seconds. This may be because my prediction was incorrect, but it may also be due to human error in the measuring of the liquids or the timing.lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
Evaluation
I think most of the experiment I have done has been successful with as little anomalous results as possible. However, there are still a few anomalous results which I will now point out.
Na2S2O3(aq) HCl(aq) H2O(l) Time
(cm3) (cm3) (cm3) (Seconds)
This is the only anomalous result that I have recorded. The result may have turned out anomalous because of basic human error, or maybe because we measured the substances wrong. It may have even been because we did not clean the apparatus properly.
Apart from this, the accuracy of my experiment has been more or less accurate. Although there are a number of ways in which we could have made the results more reliable. For instance, we could have used better measuring equipment, because the apparatus we used was mainly basic equipment.
Another thing we could have done to bring more evidence is to have tried to use the Hydrochloric acid as the variable substance, and used the Sodium Thiosulphate as the constant substance. This would have brought more evidence to support the idea that the higher concentration of a substance, the faster it will react.
As detected one of the main source of the error of time was to measure and record the time taken for the cross to disappear. To detect when cross disappears and stopping clock at that sudden instance was very hard to judge effectively. Also when the lower concentrations were used it was detected that the cross disappears in some areas of the flask but is still visible in a corner or in a little area. This made it very hard to examine and to keep the test to a fair level.
The experiment was done over a period of 5 days; I used the same cross I used for the first day however the light intensity was different on each day. This made it hard to keep it fair as it would be easier to see the cross on some days rather than others.
One of the best methods to work out the rate equation is to find out the concentration of the solutions after at different time intervals. We do not know the concentration of the reactants after a given time interval which was the reason this method was not introduced in the procedure. A school lab does not have enough sophisticated equipment which will allow us to measure the concentration of a reactant after a given period of time when the reaction is still continuing. Carrying out the experiment in a chemist lab which has more detailed technology available will make the experiment more reliable, accurate and will minimise the errors.
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 Sodium Thiosulphate decreases and the amount of water in the solution there are less atoms to collide and therefore less successful collisions causing 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.
Overall I think that this experiment was a success as I have proved that concentration does affect the rate of reaction, and I have also found that when the concentration is doubled the rate of reaction is not necessarily halved.
I think I could have improved my investigation by:
- Obtaining more results to get a better overall result.
- Take times for a lower concentration than 5cm3 (we could not due to time restrictions).
- I used ICT to display my coursework, but I did not use it in anyway that affected the experiment.
- I would like to do a further experiment to confirm my results. However I am restricted by time and the available facilities which means I cannot repeat it.
- Also instead of using a cross on a piece of paper I could use a single beam of light until it could no longer be seen
- Use of computer to aid analysis of results
- Carry out all of the experiments on same day to improve accuracy
- Calculate more than ten tangents to improve accuracy
List of sources to aid work
Various Textbooks and Internet sites were used in providing background information.
Cambridge Chemistry 2 Book – Brain Ratcliff, Helen Eccles
Do Brilliantly at ... A2 Chemistry-
Chemistry A-Level through Diagrams-
http://www.chemguide.co.uk/physmenu.html#top
http://www.chemguide.co.uk/physical/basicrates/arrhenius.html