The concentrations for HCl can be calculated using the equation above:
Similarly using the equation I can calculate the concentration for Na2S2O3, but this time 1cm³ of 0.4M Na2S2O3 contains 1/1000 * 0.4 = 4*10-4 moles. If 4*10-4 moles in total are diluted with water with 9cm³ of water to produce a total volume of 10cm³ then the concentration (molarity ) =4*10-4 / 0.01(dm³) 0.04M
Using these calculations I can plot a graph, rate of reaction against concentration of the reactants:
Rate of
Reaction
(1/ time)
Concentration of Reactant (H+ or S2O3²-)
I already know that the rate equation will be as shown below:
Rate = [H+ (aq)]? [S2O3²-(aq)]?
As you can see in the rate equation, the order of each reactant is unknown. This will be worked out by the graph produced using the inverse of time recorded against the concentration of the reactant.
After plotting the graph for each reactant, if I get a graph as shown below this will indicate a zero order with respect to the reactant:
Rate of
Reaction
(1/ time)
Concentration of Reactant (H+ or S2O3²-)
If the rate equation for a reactant is first order then I will obtain a graph as shown below:
Rate of
Reaction
(1/ time)
Concentration of Reactant (H+ or S2O3²-)
If the rate equation for a reactant is second order then I will obtain a graph as shown below:
Rate of
Reaction
(1/ time)
Concentration of Reactant (H+ or S2O3²-)
To confirm the reactant is second order, if the [Reactant]² then you should produce a graph like this:
Rate of
Reaction
(1/ time)
Concentration of [Reactant]² (H+ or S2O3²-)
From the type of graph achieved, I will be able to complete the rate equation for this reaction.
Apparatus
Stop clock
Burettes *2
Measuring cylinder (10cm³)
Measuring cylinder (100cm³)
Beakers 100cm³
Test tubes *6
Test tube rack
0.4 moldm-³ of Na2S2O3 (90cm³)
2.0 moldm-³ of HCL (90 cm³)
Water
Ball point pen
Sticky labels
Funnel
Clamp Stand *2
Diagram
Safety
Hydrochloric acid
Contact with the eyes or skin can cause serious permanent damage therefore avoid contact by wearing gloves and avoid spillages. Concentrated solutions of this acid are extremely corrosive; very dilute solutions are mildly corrosive. Toxic by inhalation – the concentrated solution releases dangerous quantities of hydrogen chloride vapour. If it does come in contact with your skin then rinse with cold water immediately and if it comes in contact with your eyes again wash with cold water and seek medical advice.
Sodium thiosulphate
This aqueous solution may be harmful if swallowed and may also irritate the eyes or lungs. If it comes in contact with Eyes then wash the eye with plenty of water. If it comes in contact with your Skin wash off with water. Sodium Chloride
This is also known as salt that we eat, therefore it fairly safe. If it does come in contact with your eyes then wash them with water.
Sulphur dioxide
Sulphur dioxide is toxic in high concentration and is a severe respiratory irritant at lower concentration. The typical exposure limit is 2 parts per million (ppm), a level which can readily be exceeded in confined spaces such as a laboratory if ventilation is poor.
Some people, especially those prone to asthma, may be especially sensitive to sulphur dioxide. In the presence of moisture, sulphur dioxide forms an acidic, corrosive solution; contact may lead to burns to the skin or eyes. If it does come in contact with your skin then rinse with cold water immediately and if it comes in contact with your eyes again wash with cold water and seek medical advice. If inhaled and the person shows signs of distress, or has a history of respiratory problems then get medical attention immediately.
Sulphur
Contact with the eyes can cause irritation. The solid may also irritate the lungs if breathed in. Sulphur burns to give toxic gases, so should not be burnt in the open laboratory. If it does come in contact with your skin then rinse with cold water immediately and if it comes in contact with your eyes again wash with cold water
Note: Throughout the experiment wear safety glasses and a lab coat. Treat breakages or any spillages with care!
Method
- Using the 100cm³ measuring cylinder accurately measure out 90cm³ of HCL and place into a clean beaker (100cm³).
-
After rinsing the 100cm³ measuring cylinder, measure 90cm³ of Na2S2O3 and place into second beaker (100cm³).
- Using water rinse out the burettes by placing rotating the burette while adding the water.
- Measure out 10cm3 of HCL using 10cm³ measuring cylinder and rinse the first burette using the HCL to remove impurities within the burette. Label it, using a sticker.
-
Measure out 10cm3 of Na2S2O3 using 10cm³ measuring cylinder and rinse the second burette using the Na2S2O3 to remove impurities within the burette. Label it, using a sticker.
- Firmly place both burettes into place using the separate clamp stands.
- In the first burette place a funnel and empty the HCl from the 100 cm³ beaker into the burette.
-
In the second burette place a funnel and empty the Na2S2O3 from the 100 cm³ beaker into the burette
-
Now measure out required amounts of HCl and place into numbered test tubes. E.g. 1st test tube, using the burette to measure 2 cm³ of HCl, then using the 10cm³ measuring cylinder add required amount of water (8 cm³). Label the test tube.
- Now repeat this procedure and vary the amount of HCl as decided (4, 6, 8, 10 cm³) into separate test tubes and add correct amounts of water (6, 4, 2, 0 cm³) and label each test tube with a number.
- On the 1st test Tube stick a small piece of paper containing a cross (X) marked on it using a ball point pen. (on the side of the test tube as shown in the diagram, so it is covered by the solution)
-
For the first experiment you will need to measure out 10 cm³ of Na2S2O3 from the second burette into a clean test tube, as this will be a constant when determining the rate for HCl.
-
Add the Na2S2O3 to the HCl in the 1st test tube and at the same time start the stop watch.
- An end point will be determined when sulphur precipitate is formed. This will turn the solution from clear colourless to cloud and the cross (X) marked on the test tube will not be visible. When this point is reached you must immediately stop the watch and record the time in a table as shown below:
-
Repeat the experiment again but this time add the 10 cm³ Na2S2O3 into the 2nd test tube and record the time. Continue each repetition at the different concentrations and complete the table above.
-
After obtaining all the readings for HCl, rinse out the test tubes and prepare them using Na2S2O3 and label them as before. (2, 4, 6, 8, 10 cm³ of Na2S2O3 into each test tube). Then add correct amount of water to each test tube, to obtain the different concentrations (8, 6, 4, 2, 0 cm³ of water)
- This time the variable that will be kept constant is HCL, from the burette measure 10cm³ of HCL into a clean test tube.
-
Add the HCl to the 1st test tube containing the Na2S2O3 prepared with proportional water. At the same time start the stop clock and observe until sulphur precipitate is formed (cloudy solution and the cross (X) is no longer visible on the test tube).
-
repeat experiments with different concentrations of Na2S2O3 and record them in the table below:
- Using the inverse of the time (1/ Seconds) and concentration plot two graphs.
Compare the graphs to the expected and decide on the rate equation by suggesting the order with respect to each reactant.
Bibliography
http://ptcl.chem.ox.ac.uk/~hmc/hsci/chemicals/hsci_chemicals_list.html
Date accessed: 06/04/2005
http://www.york.ac.uk/org/seg/salters/chemistry/ResourceSheets/thiosulphate_acid.PDF
Date accessed: 06/04/2005
Chemistry 2 by Brian Ratcliff, First published 2001: pages 108-109
Results
A results table to show the rate of HCl at different concentrations when keeping Na2S2O3 constant, during first replication:
A results table to show the rate of HCl at different concentrations when keeping Na2S2O3 constant, during second replication:
A results table to show the average rate of HCl at different concentrations when keeping Na2S2O3 constant:
A results table to show the rate of Na2S2O3 at different concentrations when keeping HCl constant, during first replication:
A results table to show the rate of Na2S2O3 at different concentrations when keeping HCl constant, during second replication:
A results table to show the average rate of Na2S2O3 at different concentrations when keeping HCl constant:
Analysis/ conclusion
The results obtained from the experiment overall were accurate in the sense that they were similar, for example when finding the rate of HCl by keeping the concentration of Na2S2O3 constant at 0.4 moldm-³ produced exact same times when the concentrations of HCl was at 0.4, 1.2, 1.6 modm-³. This shows that the results achieved are accurate because at the other two concentrations of HCl there is a difference of 1 second which further suggests accuracy achieved in this experiment.
Looking at the graph produced by using the average results, this shows a straight line. The line that I have drawn passes exactly four points plotted on the graph; one point is slightly below the line of best fit. From this I can safely conclude due to the results I achieved rate is proportional to the concentration of HCL.
This suggests Rate = [HCl] (this is first order)
When keeping HCl constant at 2.0moldm-³ and varying the concentration of Na2S2O3, produced that showed a similar trend. As the concentration of Na2S2O3 was increased the rate of the reaction also increased, as I would expect it to. From the results achieved when the concentration of Na2S2O3 was at 0.08moldm-³, the difference in times between the first and second replication was 10 seconds. This suggests less accuracy in the results as the difference is greater in this experiment than in the other when the concentration of HCl was being varied. Overall the difference is not greater than 10 seconds throughout the other concentrations of Na2S2O3 which suggests that the average values calculated should be adequate to produce a reliable graph.
The graph produced from the average results shows clear characteristics of a second order rate of reaction as the line is curve. As the concentration of Na2S2O3 increases the rate gradually increase at the start, but at higher concentrations the rate increases at a faster rate as the steepness of the line indicates.
I can suggest that Rate = [Na2S2O3]² (second order reaction)
The overall conclusion that I can make from the results that I have achieved is the rate equation can be determined by the graphs that I have produced, that suggest that HCl is a first order reaction and that Na2S2O3 is a second order reaction. Therefore the overall rate equation for this reaction is:
Rate = K [HCl] [Na2S2O3]²
Evaluation
I think the overall results achieved and the graphs produced suggest that the experiment was successful in determining the rate equation. However I did not achieve exact results each time I repeated the experiment which suggest errors (measurements or procedural), that need to be addressed to produce more reliable results.
Looking at the results, I have noticed the difference in the results is within a range of (21 -11 seconds) 10 seconds when varying the concentration of HCl. This suggests that the measurement taken had to be very accurate, which was shown by the results in this case. However the results achieved when varying the concentration of Na2S2O3 suggests that an error in timing could have prevented the difference in time. As I had to start the clock when mixing the reactants and stop the clock when reaction was over. As I have noticed 0.5 seconds affects the rate of the reaction, I should have recorded the results to + 0.5 seconds rather than the nearest whole number which would have produced more accurate results.
The equipment I used for measurements was very accurate as the burette has a percentage error of (0.05 * 100) / 50 = 0.1% and the 10 cm³ measuring cylinder has an error of +0.2cm³. This suggests that the errors in measurements are not really significant enough to affect the results.
I feel that the most significant error was in the procedure that was the time recorded when the reaction had exactly finished each time. The end point was difficult to judge each time, as the cross (X) on a piece of paper was observed until the cloudy precipitate was formed. This method of judgment was vital and could have resulted in the differences in the results as I presume an error in my judgement of + 5 seconds. This could be over come by using an electronic colorimeter that detects the change instead of the cross (X) method.
Another factor in the procedure was to start the stop clock as soon as I had added the reactants together. This was difficult to achieve simultaneously as I was doing two things at once, but I reckon this was not a great error in the procedure as I started the clock as soon as I added the reactant. An error of + 1 second, this could simply be overcome by using another person to start the clock while you add the reactants so it is done at the same time.
As I carried out the experiment in test tubes each time, this kept it fair as I looked through the same amount of solution each time. However I noticed especially when adding the Na2S2O3 to the HCl that the solutions did not mix properly, as I observed a solid precipitate forming 3/4th down the test tube. This indicated that a sulphur precipitate formed and the whole of the solutions did not react. Therefore I repeated the experiment but this time I gently moved the test tube to help mix the reactants. This error in shaking test tubes could have led to some difference in results as I did not shake all the test tubes. As I know if I shake the test tube I am providing more kinetic energy, more molecules collide due to energy provided to molecules. This could be overcome by using glassware that has a greater surface area, such as 50cm³ measuring cylinder or a conical flask.
In the method there is no indication of replication, but I have repeated the experiment twice due to the time available and the amount of reactants. (I have only accounted for 1 replication in the method concerning the quantity of each solution, therefore multiply each solution quantity by the number of replications carried out in the future). I would like to have repeated the experiment once more to achieve more accurate and reliable results.
Finally, form the results obtained I can say that I have no anomalous results, the graphs produced are sufficient enough to produce the rate equation for this reaction. If I repeated the experiment once more and at more concentrations (e.g. HCl 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0moldm-³) this would give me a greater confidence in my results and their reliability, as the graph produced would contain more data that would provide a detailed accurate account of the rate.