Aim: To investigate how the rate of reaction between hydrochloric acid (HCI) and sodium thiosulphate (Na2S2O3) is affected by the concentration of sodium thiosulphate.
Prediction: I predict that as the concentration of sodium thiosulphate doubles, the rate of reaction will double. This is because if you double the number of particles you will also double the amount of successful collisions.
Sodium thiosulphate + hydrochloric acid ï‚® sulphur + sodium chloride + sulphur dioxide + water
Na2S2O3 (aq) 2HCl (aq) S(s) 2NaCl (aq) SO2 (g) H2O (l)
Theory: By increasing the concentration of Na2S2O3, you will also increase the rate of reaction between HCI and Na2S2O3. This is because the more particles there are the faster the reaction will take place since there is a greater chance of a sodium thiosulphate particle hitting a hydrochloric acid particle.
File:Molecular-collisions.jpg
Collision Theory: The collision theory describes how chemical reactions take place and why rates of reaction change. The theory states that for a reaction to take place the reactant particles must collide. It also states only a certain portion of the total collisions cause chemical change; these are called successful collisions. The successful collisions have enough energy (activation energy) at the moment of impact to break the existing bonds and form new bonds, resulting in the products of the reaction. Increasing the concentration of the reactants and increasing the temperature causes more collisions and therefore more successful collisions which increases the rate of reaction.
http://upload.wikimedia.org/wikipedia/commons/4/41/Molecular-collisions.jpg
Variables:
Volume of sodium thiosulphate - use a measuring cylinder or burette
Volume of distilled water - use a measuring cylinder or burette
Volume of HCL- use a measuring cylinder or burette
Concentration of sodium thiosulphate -.by diluting the amount of sodium thiosulphate with the same amount of water each time.
Temperature of room - this is because if the room is hot the faster the reaction will be and if the room is cold the reaction between HCl and Na2S2O3 will be slow.
Person watching - this is because both partners eye sight are not exactly the same so while one will see the cross, the other might not.
Height of eye from conical - this is because the closer your eye is to the flask, the more chance you have of seeing the cross. Whereas if your eye is further away from the flask, the more chance of you not seeing the cross.
Type of measuring equipment used - whether you choose to use a measuring cylinder for larger or smaller amounts or maybe a burette for the amounts in the experiment.
Concentration of HCl - by diluting the amount of HCl with the same amount of water each time.
Safety:
Wear goggles at all times to protect your eyes from any chemicals.
Wear gloves to protect your hands from any acid spillage.
Wear an apron to protect your skin and clothes.
Keep bags, coats and any other objects unrelated to the experiment out of site.
Do not eat or drink in the lab.
Before beginning work in lab, clean your work area.
Clean all equipment before use.
Never look directly into a test tube when you have no goggles on.
Always use the equipments the way your teacher taught you.
Never taste any chemicals.
Always with the labels on chemical bottles before use.
Return all lab materials and equipment to their proper places after use.
After the experiment always wash and dry you equipment as well as your work area.
Sodium thiosulphate can be very harmful when swallowed and can also irritate the eyes or lungs.
If there is a contact between HCl and the eyes or skin it can cause a serious permanent damage.
Concentrated solutions of hydrochloric acid are extremely corrosive; very dilute solutions are mildly corrosive.
Toxic by inhalation - the concentrated HCl solution releases dangerous quantities of hydrogen chloride vapour.
Equipment/ Apparatus:
6 measuring cylinders
(3) 25cm3 measuring cylinders (+ or - 0.5 cm3)
(3) 10cm3 measuring cylinders (+ or - 0.2 cm3)
3 50cm3 burette (+ or - 0.1 cm3)
3 100cm3 beakers
2 100cm3 conical flasks
2 laminated small squared papers with a thick cross
Hydrochloric acid (2 molar)- same concentration in each experiment
Sodium Thiosulphate (40g/L )
Distilled water
Stopwatch
Goggles
Gloves
Apron
3 funnels
3 different coloured (board) pens
Apparatus (burette stand)
Method:
First, clean work surface and equipments e.g. conical flask, measuring cylinder e.t.c
Collect all equipments and place them on your work area.
Place the measuring cylinders, burette, conical flasks and beakers in three groups -distilled water, hydrochloric acid (HCI) and sodium thiosulphate (Na2S2O3).
Label the equipments
Sodium thiosulphate (Na2S2O3 (aq)) (40g/L)
Distilled water (H2O)
Hydrochloric acid (HCl (aq)) 2 moldm-3
Wear your safety equipment at all times.
Experiment 1:
Measure 20 cm3 of hydrochloric acid and 20 cm3 of sodium thiosulphate in 25 cm3 measuring cylinders.
Pour the chemicals in the conical flask at the same time and let your partner start the stopwatch when the last drop of chemicals drops in the flask.
Stop the stopwatch when you and your partner are unable to see the cross underneath the conical flask.
After you have stopped your stopwatch and recorded the result, pour the mixture away and wash the conical flask. This is because the chemicals will solidify if it's not washed/ thrown away. Wash the conical flask so that the previous chemical reaction doesn't affect your next experiment.
Repeat the experiment again twice. You can repeat your experiment again if you think it's necessary.
Experiment 2:
Repeat experiment 1 changing the following:
15 cm3 of sodium thiosulphate
5 cm3 of distilled water
10 cm3 measuring cylinder for distilled water
Experiment 3:
Repeat experiment 1 changing the following:
10 cm3 of sodium thiosulphate
10 cm3 of distilled water
10 cm3 measuring cylinder for Na2S2O3 and H2O
Experiment 4:
Repeat experiment 1 changing the following:
5 cm3 of sodium thiosulphate
15 cm3 of distilled water
10 cm3 measuring cylinder for Na2S2O3
Experiment 5:
Repeat experiment 1 changing the following:
3 cm3 of sodium thiosulphate
17 cm3 of distilled water
Burette instead of measuring cylinder for all solutions
Experiment 6:
Repeat experiment 1 changing the following:
2 cm3 of sodium thiosulphate
18 cm3 of distilled water
Burette instead of measuring cylinder for all solutions
Experiment 7:
Repeat experiment 1 changing the following:
1 cm3 of sodium thiosulphate
19 cm3 of distilled water
Burette instead of measuring cylinder for all solutions
Volume of sodium thiosulphate (Na2S2O3) (cm3)
Volume of distilled water (H2O) (cm3)
Concentration of sodium thiosulphate
(Na2S203) (g/L)
20
0
40
15
5
30
10
10
20
5
15
10
3
17
6
2
18
4
1
19
2
[ ] i
V Na2S2O3 = [ ] new
VT
Initial Concentration
Volume of Na2S2O3 = New Concentration
Total Volume
1
Rate (s-1) =
Average Time (s)
Table of Results
Try
Concentration of Na2S2O2
(g/L)
Volume of sodium thiosulphate (Na2S2O3) (cm3)
Volume of Distilled water (H2O) (cm3)
Total Time
(seconds)
Average time
(seconds)
Rates (s-1)
(3 significant figures)
Rates in
standard form (s-1)(10-3)
1
1
40
20
None
46:25
34.05
0.0332
33.2 x 10-3
2
20
None
30:06
3
20
None
30:59
4
20
None
33:19
5
20
None
29:81
6
20
None
34:38
2
1
30
15
5
47:94
48:76
0.0205
20.5 x 10-3
2
15
5
48:84
3
15
5
49:50
3
1
20
10
10
76:07
74:69
0.0134
13.4 x 10-3
2
10
10
73:25
3
10
10
74:75
4
1
10
5
15
173:62
172:77
0.00579
5.79 x 10-3
2
5
15
170:56
3
5
15
174:13
5
1
6
3
17
453:53
451:26
0.0022
2.22 x 10-3
2
3
17
449:03
3
3
17
451:23
6
1
4
2
18
600:00+
2
2
18
600:00+
3
2
18
600:00+
7
1
2
1
19
1800:00+
2
1
19
1800:00+
3
1
19
1800:00+
Table of Results (for graph)
Experiment
Try
Volume of hydrochloric acid (HCl) (2 moldm-3)
Volume of sodium thiosulphate (Na2S2O3) (40 g/L)
Volume of distilled water (H2O)
(cm3)
Total Time (seconds)
Rates (s-1)
(3 significant figures)
Rates in
standard form (s-1)(10-3)
1
1
20 cm3
20 cm3
None
30:06
0.0332
33.2 x 10-3
2
20 cm3
20 cm3
None
30:59
3
20 cm3
20 cm3
None
29:81
Average time
30:15
2
1
20 cm3
15 cm3
5
47:94
0.0205
20.5 x 10-3
2
20 cm3
15 cm3
5
48:84
3
20 cm3
15 cm3
5
49:50
Average time
48:76
3
1
20 cm3
10 cm3
10
76:07
0.0134
13.4 x 10-3
2
20 cm3
10 cm3
10
73:25
3
20 cm3
10 cm3
10
74:75
Average time
74:69
4
1
20 cm3
5 cm3
15
173:62
0.00579
5.79 x 10-3
2
20 cm3
5 cm3
15
170:56
3
20 cm3
5 cm3
15
174:13
Average time
172:77
5
1
20 cm3
3 cm3
17
453:53
0.0022
2.22 x 10-3
2
20 cm3
3 cm3
17
449:03
3
20 cm3
3 cm3
17
451:23
Average time
451:26
6
1
20 cm3
2 cm3
18
600:00+
2
20 cm3
2 cm3
18
600:00+
3
20 cm3
2 cm3
18
600:00+
Average time
600:00+
7
1
20 cm3
1 cm3
19
1800:00+
2
20 cm3
1 cm3
19
1800:00+
3
20 cm3
1 cm3
19
1800:00+
Average time
1800:00+
Experiment 5,6 and 7 are close together because I started with 1cm3 sodium thiosulphate and 19cm3 distilled but the reaction was very long so I did 2cm3 sodium thiosulphate and 18cm3 distilled water which also took very long to react. I then tried 3cm3 sodium thiosulphate with 17cm3 distilled water, which went perfectly well and I decided to use low amounts of sodium thiosulphate because I wanted to know how long it took the lowest [Na2S2O3] to react with HCl also I wanted to know the highest and lowest [Na2S2O3] that I could use for my table of results.
Average Time Graph
As the concentration of Na2S2O3 increases, the time decreases.
Rates Graph
As the concentration of Na2S2O3 increases the rates also increases.
Conclusion
The pattern in my results shows that as the concentration of sodium thiosulphate increases the reaction time is decreasing which means that the rate of reaction increases due to the fact that, it takes less time for a/the reaction to take place. Using the graphs, I can make a conclusion from my experiment. I can see that with the time graph as the concentration increased the time taken for the reaction to take place decreased. There is one variable that has affected the results and that is the temperature which may have changed slightly, giving an incorrect result. This is because when the temperature is increased the particles will have more energy and so make the particles move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to react successfully and collisions between reacting particles are consequently more likely to take place, as stated in the collision theory. As the concentration of sodium thiosulphate increases, the amount of time decreases because the reaction is getting faster. This is because the higher the concentration, the more particles there are, so there is a bigger chance of successful collisions happening between sodium thiosulphate and hydrochloric acid. When the concentration of the hydrochloric acid was lower the reaction took longer, this is because there were fewer particles, so there was a smaller chance of successful collisions happening.
Evaluation
I believe that all my results are reliable because I repeated the experiment with results I think didn't fit and my table of results have only 3 anomalies which were quite close to the final results. In my experiment 1, results 1, 4 and 6 are outliers because their readings didn't lie within the range of the other three results, there may have been a fault in the stopwatch or the room temperature changed during the experiment. In experiments 6 and 7, the chemical reaction took quite a long time even though the solution went cloudy it wasn't opaque so, I decided to increase the volume of sodium thiosulphate and decrease the volume of distilled water. From my results I noticed that as the concentration of sodium thiosulphate increased, the rate of reaction also increased. This is because the more particles there are in the volume of sodium thiosulphate the faster the reaction will take place, since there is a greater chance of a sodium thiosulphate particle hitting a hydrochloric acid particle. This confirms my theory which states that increasing the concentration of the reactants and increasing the temperature causes more collisions and therefore more successful collisions which increases the rate of reaction. To improve the accuracy of my results, if I was to do the experiment again, I would use a burette to measure the correct volumes of sodium thiosulphate solution and hydrochloric acid. I realised that I may not have been precise enough when using the measuring cylinder. If I did the experiment again, instead of visually checking when the cross has disappeared, I would use a light meter to measure the amount of light left in the reaction. When the reaction turns cloudy, the light meter would detect it. This would give more fair and precise results. I would also use a water bath to keep the mixture the same temperature. The room temperature may have been different on the days I did the experiment so this could cause anomalies.
The equipment I used for my experiments were precise because for greater amounts of HCL, Na2S2O3 and distilled water I used a measuring cylinder but for smaller amounts of HCl, Na2S2O3 and distilled water I used a burette to be more precise. I am confident wit my results because
We measured everything carefully too, to ensure better results and we also found that the higher the concentration of sodium thiosulphate the faster the reaction. Overall I think my experiment was a success.
