Usually, an increase in temperature is accompanied by an increase in the reaction rate. If the molecules are moving faster then they will collide more which will increase the chances of them making a successful collision and reaction.
In a mixture of molecules, there may be some which do not have the required energy to react and some which do. The proportion of these molecules can be shown by the Maxwell-Boltzmann distribution curve. As shown on the graph, all the molecules in the shaded area have the required energy (activation energy) for the reaction and will react if it makes a successful collision.
The experiment is conducted at room temperature therefore can not be controlled.
Presence of Catalysts and Competitors
A catalyst is a substance that alters the rate of a chemical reaction without being used up or permanently altered. A catalyst works by changing the energy pathway for a chemical reaction. It provides an alternative route (mechanism) that lowers the Activation Energy meaning more particles now have the required energy needed to undergo a successful collision.
Catalysts not used in the experiment therefore can not be controlled
The above graph demonstrates what a catalyst does to the reaction profile.
In this experiment, I am going to find out the rate of reaction between sodium thiosulphate Na2S2O3 and hydrochloric acid HCl, by varying concentrations of both solutions.
Ionic equation: S2O32-(aq) + 2H+ (aq) → SO2 (g) + S(s) + H2O (l)
Overall equation:
Na2S2O3 (aq) + 2HCl (aq) → 2NaCl (aq) + SO2 (g) + S(s) + H2O (l)
End Point Determination
From the equation, we can see that sulphur dioxide is produced and sulphur is precipitated as products. Therefore bubbles should be produced and a cloudy solution should be observed. The product becomes cloudy once the reaction starts, so it is very difficult for human eye to judge when the end point of the reaction is. Therefore actual time taken for the reaction to complete is not easy to obtain accurately. To measure the rate of reaction I will be using a white tile which will contain a think small back cross which is visible through the test tube, to draw the cross on the white tile, black marker will be used. The cross will act as the indicator for the experiment, the solution will be placed on top of the cross to measure the rate of reaction, and we will time using a stop watch how long it takes for the solution to become cloudy and the cross to disappear through the solution from the birds eye view.
Justification for the Concentrations Used
After watching the demonstration shown by the teacher, I noticed a cloudy solution forming within 14 seconds after mixing the 50cm³ of (0.4 moldm-³) Na2S2O3 was mixed with 5.0 cm³ HCL (2.0 moldm-³) diluted with 20cm³ H2O. From this I can simply conclude that the reaction takes place quickly and further increasing concentration/ volumes would produce a faster reaction. This as a result would make it difficult to measure the exact point when a cloudy solution is formed (increasing the procedural error). If the time taken to produce the cloudy solution is faster than 14seconds this would increases the chance of error and inaccuracy as the time taken for detection from the eye and time to respond by stopping the stop clock can lead to an error of judgement.
SO2 produces a strong smell as it diffused across the room. This is important to take into consideration, because if a large amount of Na2S2O3 is used then this will cause the production of more SO2 which can be dangerous and difficult to get rid of.
Consequently after considering and thought of all different factors, I have decided to keep a total volume of 55cm³ this will enable me to vary the concentration of HCL and Na2S2O3 and obtain suitable results via dilution using distilled water. I will use 5 different concentration of one reactant and keep the other reactant constant. Using 5 ranges of concentration will provide me enough data to plot a graph in order to determine any patters or trends obtained. If I use less than 5 concentrations it will be difficult to detect an anomalous, as it’s difficult to detect a trend using less data. To increase the reliability of the collected data I will repeat the experiment for each different concentration 3 times, as it will reduce errors caused during the experiment.
Table of result (Experiment I)
Table of result (Experiment II)
I have decided to use smaller concentration of sodium thiosulphate and hydrochloric acid because the time taken for the reaction to complete will be longer compare to the teacher’s demonstration and therefore reduce the timing error. In the first experiment (result table I) I will start with 4 cm3 of hydrochloric acid and increase it by 4cm3 each time when using a new concentration, the volume of sodium thiosulphate will stay constant for each different concentration in experiment 1; 5 cm3 and the volume of water will be added accordingly to make the total volume of each concentration the same. The same procedure and reading will be used when hydrochloric acid is in excess in experiment 2
Dilution
To dilute the chemical used during the experiment, distilled water will be used. The reason why chemicals are diluted in the experiment is to change the molarity of the solutions; changing the molarity of only one chemical at a time and keeping the other constant will allow us to determine the rate of reaction.
To work out the molarity of different concentration used in the experiment I will be using the dilution equation. When we dilute a solution the number of moles remain constant, just the volume changes to reduce the Molarity. Since the number of moles equals Molarity x Volume
We can use the equation bellow to work out the molarity of any solution quantity.
Molarity dilute X Volume dilute = Molarity Conc. X Volume Conc.
For example:
To work out the molarity of HCl in the first concentration from the first result table, which is; 4cm3 of HCl, 10cm3 of Na2S2O3 and 41 cm3 of H2O, which gives the total volume of 55cm3 and the molarity of Na2S2O3 is 2.0 mol dm−3 I used the above formula.
By rearranging the formula like this:
Molarity dilute = Molarity Conc. X Volume Conc.
Volume dilute
Will allow me to work out the molarity of the HCl for the fist concentration on the result table I:
Molarity dilute = Molarity Conc. X Volume Conc.
Volume dilute
= 2.0mol dm−3 X 4cm3
41cm3
= 0.19mol dm−3
Using the same method I have worked out all the molarities of both result tables.
Apparatus:
-Chemical –
Hydrochloric Acid – HCl
Sodium thiosulphate – Na2S2O3
Distilled water – H2O; Added to dilute the solutions. Help maintain the volume of the solution the same and also dilutes the enzyme.
- Equipment -
10 Test tubes – 5 used to mix the HCl and water and 5 used for Na2S2O3.
2 test tube rack – to hold the test tubes.
3 Graduated pipette (10cm3) – used to measure the amount of HCl, Na2S2O3 and distilled water; as it is more accurate than using measuring cylinder.
2 beakers – used to collect HCl and Na2S2O3 from the main source.
1 White tile – used to place underneath the HCl and Distilled water solution.
1 Black marker – used to place the cross on the white tile and also used to label the test tubes to prevent contamination.
1 Stop watch – used to measure the time it takes for the solution to turn milky/ measure the rate of reaction.
1 Thermometer – used to check the room temperature.
Goggles – used for eye protection.
Tissue – used to wipe wet equipments and other possible uses.
Gloves – used for hand protection.
Labelle’s – to mark the test tubes to prevent contamination.
Method –
- Collect all the equipments required to carry out the experiment.
- Wash all the equipment with water and dry it to avoid contamination.
-
Lay out all the equipment as shown bellow.
- Label each test tube with the correct concentration level and organise them in ascending order, to prevent contamination.
-
Label the Graduated pipettes and the beakers for different chemicals to prevent contamination as HCl, Na2S2O3 and H2O are all clear liquid solution.
-
Pour HCl and Na2S2O3 into the correct labelled beaker from the main source.
-
Using the graduated pipette measure 4cm3of HCl and pour into the intended test tube in the test tube rack as shown below:
-
Then measure 41 cm3 of water into the HCl in the test tube and shake the solutions to dilute the HCl.
-
Measure 10cm3 of Na2S2O3 using the graduated pipette and pour it into the other labelled test tube.
- Using a black marker pen draw a thick cross on the white tile as shown below;
- Then place the white tile with the cross under the test tube of the diluted HCl as shown bellow.
-
Then add the 10cm3 of Na2S2O3 from the test tube and start the stop watch immediate.
- Allow the same person to observe through out the whole experiment to make the practical more reliable.
- Stop the stop watch when the cross is not visible through the solution from the birds eye view in other words when the solution becomes milky/cloudy.
- Record the result in the result table shown below:
-
Repeat the experiment with the same amount of HCl, Na2S2O3 and H2O another 2 times and record the result to make the experiment reliable and the data collected valid.
- Now repeat the whole experiment using different ranges shown bellow:
During these stages distilled water will be used to dilute the HCl and the volume of Na2S2O3 will be the same for the fist experiment.
-
8cm3 of HCl with 37cm3 of distilled water
-
12cm3 of HCl with 33cm3 of distilled water
-
16cm3 of HCl with 29cm3 of distilled water
-
20cm3 of HCl with 25cm3 of distilled water.
Apparatus:
-Chemical –
Hydrochloric Acid – HCl
Sodium thiosulphate – Na2S2O3
Distilled water – H2O; Added to dilute the solutions. Help maintain the volume of the solution the same and also dilutes the enzyme.
- Equipment -
10 Test tubes – 5 used to mix the HCl and water and 5 used for Na2S2O3.
2 test tube rack – to hold the test tubes.
3 Graduated pipette (10cm3) – used to measure the amount of HCl, Na2S2O3 and distilled water; as it is more accurate than using measuring cylinder.
2 beakers – used to collect HCl and Na2S2O3 from the main source.
1 White tile – used to place underneath the HCl and Distilled water solution.
1 Black marker – used to place the cross on the white tile and also used to label the test tubes to prevent contamination.
1 Stop watch – used to measure the time it takes for the solution to turn milky/ measure the rate of reaction.
1 Thermometer – used to check the room temperature.
Goggles – used for eye protection.
Tissue – used to wipe wet equipments and other possible uses.
Gloves – used for hand protection.
Labelle’s – to mark the test tubes to prevent contamination.
Method –
- Collect all the equipments required to carry out the experiment.
- Wash all the equipment with water and dry it to avoid contamination.
-
Lay out all the equipment as shown bellow.
- Label each test tube with the correct concentration level and organise them in ascending order, to prevent contamination.
-
Label the Graduated pipettes and the beakers for different chemicals to prevent contamination as HCl, Na2S2O3 and H2O are all clear liquid solution.
-
Pour HCl and Na2S2O3 into the correct labelled beaker from the main source.
-
Using the graduated pipette measure 4cm3of Na2S2O3 and pour into the intended test tube in the test tube rack as shown below:
-
Then measure 41 cm3 of water into the HCl in the test tube and shake the solutions to dilute the Na2S2O3.
-
Measure 10cm3 of HCl using the graduated pipette and pour it into the other labelled test tube.
- Using a black marker pen draw a thick cross on the white tile as shown below;
-
Then place the white tile with the cross under the test tube of the diluted Na2S2O3 as shown bellow.
-
Then add the 10cm3 of HCl from the test tube and start the stop watch immediate.
- Allow the same person to observe through out the whole experiment to make the practical more reliable.
- Stop the stop watch when the cross is not visible through the solution from the birds eye view in other words when the solution becomes milky/cloudy.
- Record the result in the result table shown below:
-
Repeat the experiment with the same amount of HCl, Na2S2O3 and H2O another 2 times and record the result to make the experiment reliable and the data collected valid.
- Now repeat the whole experiment using different ranges shown bellow:
During these stages distilled water will be used to dilute the Na2S2O3 and the volume of HCl will be the same for the fist experiment.
-
8cm3 of Na2S2O3 with 37cm3 of distilled water
-
12cm3 of Na2S2O3with 33cm3 of distilled water
-
16cm3 of Na2S2O3with 29cm3 of distilled water
-
20cm3 of Na2S2O3 with 25cm3 of distilled water
Safety & Risk assessment:
- Wear goggles through out the experiment.
- Wear gloves through out the whole experiment for protection.
- Long hair or any other dangling things must be tied back, so it does not fall into chemicals.
- Do not touch electrical equipment with wet hands.
- Do not look into the mixture of solution without goggle as it might affect the eye.
- Do not eat, drink, or smoke while in the laboratory.
- Do not taste any chemical.
- Long-sleeved shirts and leather-topped shoes should be worn at all time.
- If spill any chemical on you, wash the exposed area with large amounts of cold water. If skin becomes irritated, see a technician immediately.
- Clean your area thoroughly.
- Never use mouth suction when filling pipettes with chemical reagents.
- Check the label on all chemical bottles twice before removing any of the contents. Take only as much chemical as you need.
- Contact lenses should not be worn in the laboratory.
- Dispose of all chemical waste properly. Never mix chemicals in sink drains.
- Always work in a well-ventilated area.
Safety in relation to chemicals
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!
Results Table Experiment I – change in concentration of HCl
Results Table Experiment II – change in concentration of Na2S2O3
Analysis
From the results obtained I am able to conclude that the overall experiment was fairly accurate as both experiment produced similar results. For example; when the HCl was in excess and Na2S2O3 was constant at 0.4 mol dm-3 the results obtain shows that; as the molarity (Concentration) of HCl increases the time taken for the cloudy solution to form decreases hence increasing the rate of reaction. Similarly when Na2S2O3 was in excess and HCl volume was constant at2.0 moldm-3 the results obtained showed the same trend/ pattern.
We can see the same pattern and trend when analysing the graphs. The graphs demonstrate the reaction rate against the concentration; from this I will able to deduce the order of reaction and find out the value of X and Y in the rate equation shown below:
Rate = k [HCl] x [Na2S2O3] y
The line of best fit produced in the both graph is a vertical straight line showing that, as the concentration of the HCl or Na2S2O3 increase the reaction rate also increase therefore produces a linear line. A vertical straight line indicates that the order of the graph is first order reaction as stated in the theory above. First order = the rate of reaction depends directly upon the concentration of the reactant. If the concentration of the reactant is doubled, the rate doubles therefore from the graph shown above I can conclude that the order for sodium thiosulphate is first order and thus the rate equation of sodium thiosulphate should look like this:
The rate of reaction = K [Na2S2O3]
Using flow chart to determine the order
Using the same flowchart shown above I am also able to determine the order of reaction with respect to Na2S2O3.
Yes
No
However even though the graph for when HCl is in excess produces a straight line it doesn’t not start form the origin (0, 0 co-ordinates) therefore it is not possible to state whether the graph for HCl concentration is first order or not. Hence the rate can not be determined from just observing the graph as it doesn’t match the theory stated above.
Therefore the final rate equation looks like this;
The rate of reaction = K [HCl]? [Na2S2O3]
Evaluation
I believe the results that I have collected for the determination of the rate equation through the process of timing the reaction rate are not very accurate, the reason behind this is that when analysing the graphs and trying to determine the rate equation by analysing the graph I could only work out the order of reaction for one graph; the graph which demonstrated the reaction rate for Na2S2O3 and not for the HCl reaction rate as the line of best fit did not start from the origin. This could have been for a number of reasons, including the errors in procedure and the errors in measurement. Some of the errors in procedure and measurement are listed below;
Errors in procedure
- Difficult to start timing
- Temperature not controlled
- Experiments conducted in two parts.
- Relative amount of each reagent
Errors in measurement
- Measuring the time.
- Measuring the volume of reagent.
Identify those errors that are significant and justify your choice – justify improvements to the procedure
The experiment conducted to gather the data was carried out in two parts, first when sodium thiosulphate was kept constant and second when HCl was kept constant, therefore the results obtained for both results table were collected on two separate days, the reason behind this was due to lack of time to conduct both experiment in the same time interval. This as a result could have affected the results gained hence difference in the graphs and the reaction rate. Carrying out the experiment in two slots would have certainly affected the reaction rate as stated in the planning, if the temperature of the first day was higher than the second day (or vice versa), this difference in temperature would have had an effect on the overall rate of reaction with respect to the collision theory; which states that the higher the temperature the more energy the molecules have and hence the greater successful collision; increasing the reaction rate. Therefore to make sure that temperature does not affect the rate of reaction I will next time conduct the experiment in the water bath as the water bath will allow me to control and monitor the temperature of the water and also allow me to react to changes quickly. Using a water bath will keep my experiment fair and will increase the level of accuracy and reliability of the gathered data. To further make sure that the temperature does not affect the my experiment in will check the temperature of the each reagent concentration before starting each reaction, as this way I will further remove any errors and maintain the temperature well.
Another factor and the most significant procedural error in the procedure was to start the stop watch as soon as I had added the reactants together. This was difficult to achieve simultaneously as I was doing two things at once, although I tried to the best of my ability to maintain doing both thing right at a time I believe it is very difficult to observe when the exact point is to start the stop watch and therefore this method has huge room for errors to take place however it is not the most significant error because the experiment was repeated 3 times and an average was worked out. Working out an average reduces the error to certain extent however to further reduced the error I would next time use a data logger, this is an electronic device that records data over time in the computer. Data logger will automatically start collecting the data when the reaction starts; this as a result would reduce the error.
Identify those errors that are significant and justify your choice – justify improvements to the measurement
A noticeable effect and the most significant measurement error on the rate of reaction which could have lead errors in measurement could have being; using naked eye to approach conclusion on the rate of reaction, the main problem with using a naked eye is that different people have different eyesight, even though the same person observed the rate of reaction through out the whole experiment, I believe this was not very accurate as even one second delay on the stopping the stop watch could cause an anomaly and also the fact that the person observing the reaction will not get the same clearness in each experiment will make difference in the result therefore instead of using naked eye I will next time use a colorimeter to measure the rate of reaction. Colorimeter is equipment used to measure the colour change in a reaction. By using this devise it will improve accuracy of the experiment as it will exactly tell me how long the rate of reaction has taken place for.
Another factor which leaves a margin for error to certain extent was the equipment used to measure the reagents. The accuracies of each burette for example may differ slightly, which may affect the outcome of the experiment, as different burettes were used to measure different solutions. I noticed that it was very difficult to maintain the meniscus of the reagent on the mark as the burette was filled because some times air bubble were formed in the burette, This as a result leaves margin for error. When reading the burettes I was at the level of the graduation to avoid parallax error. The burette used to measure different reagents has an error of + 0.005 and hence may have affected the accuracy of the results obtained. To modify this error I would next time use a digital burette as it would measure the exact amount of reagent required in electric digits and therefore would improve the reliability and accuracy of the solution measured further.
Improved apparatus to measure the rate of reaction:
Bibliography:
Websites:
- [ttp://en.wikipedia.org/wiki/rateequation]
- [ttp://www.docbrown.info/page06/OrgMechs2.htm#rateequation]
-
[]
-
[http://www.chemguide.co.uk/
- [http://www.webchem.net/notes/how_far/kinetics/intro_to_kinetics.htm]
Books :
- [Brian Ratcliff, Helen Eccles, David Johnson, John Nicholson, John Raffan, Advance chemistry 1 by OCR, Brian Ratchliff, 2000, the university of Cambridge, Halogenoalkane, Page 138-139]
- [Ted Lister and Janet Renshaw, AS Essential Chemistry, Janet Renshaw, 2003. Halogenoalkane, Page 115]
- [M. J. Clugston & Rosalind Flemming Advance chemistry, 2000, Halogenoalkane Page 439 -441]
