The graph below shows zero, first and second order reactions and how changes in the concentration of a reactant affect the reaction rate.
Second order
Reaction Rate Zero order
(Mol dm-3 s-1)
First order
For this coursework a reaction will take place between sodium thiosulphate and hydrochloric acid. The equation is:
2HCl(aq) = Na2S2O3(aq) 2NaCl(aq) + S(s) + SO2 (g) = H2O(l)
APPARATUS
- Conical flask.
- Beaker.
- Pipette.
- Burette.
- Clamp stand.
- Grippers.
- Funnel.
- Stop watch.
- Labels.
- Marker pen.
- White paper.
- Goggles.
SOLUTIONS
-
0.2 mol dm-3 of sodium thiosulphate.
-
2.0 mol dm-3 of hydrochloric acid.
- Distilled water.
DILUTION TABLES
2.0 mol dm-3 of hydrochloric acid.
0.2 mol dm-3 of sodium thiosulphate.
METHOD 1
- Firstly set up the apparatus.
- Get together 5 small beakers, and label each one with the concentration from the HCl dilution table.
- Set up your burette by getting a clamp stand out and securing the burette carefully on using grippers to secure it in place.
- Place a funnel on top of the burette and pour in the HCl. (goggles must be worn throughout as acid is being used.)
- Set up another burette in the same way but pour in distilled water instead of HCl. (remember to remove funnel after pouring in as this can effect the readings.)
- Using the dilution table for HCl pour the right amounts of HCl and distilled water into the corresponding beakers.
-
Then again set up another burette in the exact same way but pour in Na2S2O3. (Again remembering to remove the funnel after use.)
- Get together 5 conical glasses and label each one with the HCl concentrations.
-
In each of these conical flasks measure out exactly 50cm3 of Na2S2O3 using the burette.
- On a piece of white plain paper mark out a cross (X)
- Then place the conical flask with the first concentration you want to test on the cross.
- At this point get your stopwatch ready.
- Then pick up the beaker with the corresponding concentration to the one on the conical flask.
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Pour this into the conical flask, and as soon as the first drop hits the Na2S2O3 start the stopwatch.
- When the cross is no longer visible through the solution stop the stopwatch and record the time in seconds.
- Repeat this for all the different concentrations.
- The experiment must be done away from sunlight as this can affect the temperature of the solution-causing there to be an increase in rate. The experiment must also be done on the same day, as there may be climate changes on different days.
- Goggles must be worn throughout the experiment as acid is involved.
METHOD 2
-
Follow the same method as method 1, but instead using the dilution table for Na2S2O3 and keeping HCl constant by measuring out 10cm3 of HCl into each conical flask using a burette.
SAFETY
- Goggles must be worn throughout the experiment.
- Long hair must be tied back.
- Gloves should be worn, if necessary.
RESULTS TABLE
CONCLUSION
From my graph and table I can see that as the concentration of HCl and Na2S2O3 increase the rate of reaction also increases. I have plotted a graph of rate against time from this I will be able to deduce the order of the reaction. From this I will be able to find out the value of x and y in the rate equation:
Rate = k[HCl]x[Na2S2O3]y
From the graph of HCl I have come to the conclusion that this is a first order reaction therefore x = 1. This is not a zero order reaction as the rate is not constant throughout. It is also not second order it is not curved, as you would expect it to be. Therefore it has to be a first order reaction.
From the graph of Na2S2O3 I have come to the conclusion that this is also a first order reaction therefore y = 1. This is not a zero order reaction as the rate is not constant throughout. It is also not second order, as it is not curved, as you would expect it to be. This is a first order reaction as the rate is directly proportional to the concentration. There is a straight line passing through (0,0).
From my results I can now conclude that the rate equation for this reaction between hydrochloric acid and sodium thiosulphate is:
Rate = k[HCl][Na2S2O3]
EVALUATION
I think overall the experiment was carried out to the best of my ability, although there are a few improvements that could have been made as I had two anomalous results, one in each experiment. In the experiment when where I was varying HCl I had one anomalous result at 0.25Mol dm-3 concentration and I had another anomalous result where I was varying Na2S2O3 at 0.1Mol dm-3.
When trying to see if the cross(X) was no longer visible I was sometimes looking from above the conical flask and sometimes from the side, by doing this I could have caused errors in my time I thought the cross took to disappear. To improve this I should have always looked from one direction, as this would have made my results more reliable and accurate. The best place to see the cross disappear would have been from above as you have a clearer view.
I repeated each experiment twice so I could work out an average. My results were very similar for my repeated experiment but to get more accurate results I could have repeated it more than twice.
When pouring the solution into the conical flask to start the reaction not all the solution would go into the conical flask some is always left behind this could have also affected the results as not the exact amount of the solution is reacting that should have. To improve this I would suggest rinsing out with distilled water but this in turn would mean that the concentration would be altered so this wouldn’t work.
Also when doing the experiment the temperature was not always constant in the surroundings and in the solution. This could have affected the rate of reaction by some reactions happening faster and some slower than the others. To improve this i could make sure all the solutions were at the same temperature before starting the reaction. This would then ensure that the test was being conducted fairly and that way i could get more accurate results.
To improve the experiment and minimise errors I can try and devise a better method that would ensure that all of the above errors were reduced.
When measuring out my solutions I was taking readings from below the meniscus, this reduced the numbers of errors in my results.
REFERENCES:
- Cambridge chemistry 2 text book
- Letts revision guide.
- Class notes.
