According to Houston 2013 (p. 106) in any chemical reaction, it is vital to note the extent to which the reaction takes place as well as its feasibility. Houston further states that whereas the feasibility of a reaction can be predicted using thermodynamics, the extent of a chemical reaction is determined by chemical equilibrium. Besides the extent of a chemical reaction and its feasibility, the rate of a reaction and the factors affecting a chemical reaction is important to note. Chemical kinetics is the branch of chemistry that deals with the above i.e. study of the rates of a chemical reaction and its mechanism.
A chemical reaction occurs when reactants are consumed and new products formed as well (Steinfeld, Francisco, and Hase 1989, p. 87). The rate at which a concentration of any reactants decrease or the concentration of the product increase is known as the rate of a reaction. For example, we consider a hypothetical reaction where reactant A reacts to form product B i.e.
A B
Where [A] and [B] are the concentrations of A and B respectively.
Rate of disappearance of A = = -
Rate of appearance of B = = +
Therefore,
Rate of reaction = - = +
Factors that affect the rate of a reaction
The rate of a chemical reaction can be influenced by the following factors:
- Temperature- as the temperature of reactants are increased then rate of a chemical reaction increase. The increase in temperature causes increase in the kinetic energy of the reacting molecules causing them to move at higher velocity causing more collision per unit time. Hence, the rate of the reaction increases.
- Concentration of the reactant- increasing the concentration of reactants increases the rate of a chemical reaction. Increasing the concentration of the reactants means that the number of reacting molecules has increased. The direct effect of increase in the number of molecules is that the number of collision between the reacting molecules increases as well, increasing the rate of the reaction.
- Physical state- the nature of the reactants plays a key role in the rate of reaction. The reactants can exist as either solid or liquid. The physical state of reactant determines the surface area of the reactants exposed for the reaction to take place. Therefore, a liquid reactant will have more area for reaction to take place than a solid reactant. Similarly, a finely divided solid reactant has more area for the reaction to occur than coarsely divided solid. Hence, increase in the rate of a chemical reaction
- Catalyst- catalysts are substances that increases the rate of chemical reaction without itself being consumed. They act by reducing the activation energy the reaction increasing the rate of a chemical reaction. Some of the most widely used catalysts are the Vanadium(IV)Oxide, Iron, and Potassium Permanganate.
Reaction between Sodium thiosulphate and Hydrochloric acid
Sodium thiosulphate reacts with hydrochloric acid to form Sodium chloride, Sulphur dioxide, Sulphur and water as shown below:
Na2S2O3 (aq) + 2HCl(aq) 2NaCl(aq) + H2O(l)
+ S(s) + SO2(g)
The Sulphur produced in the reaction above is vital to determine the rate of reaction (Amrita.olabs.edu.in 2013). Both Sulphur dioxide and sodium chloride formed both dissolves completely in the aquaesis solution. Accoring to Semenev 2013 (p. 8), however, Sulphur is insoluble, hence existing in the mixture as white or pale yellow precipitate or a colloid giving the mixture a milky appearance making the solution opaque. Therefore, by monitoring the opaqueness of the reaction, we can determine the rate of reaction. It is done by measuring the time taken (t) to form a certain amount of Sulphur.
AIM
The aim pf this experiment is to determine the effect of concertation on the rate of reaction between sodium thiosulphate and hydrochloric acid.
HYPOTHESIS
The rate of reaction between sodium thiosulfate and hydrochloric acid is directly proportional to the concentration of sodium thiosulphate solution.
VARIABLES
There are various variables in this experiment. These variables include
- Dependent variable- it is the time between the addition of hydrochloric acid and sodium thiosulfate and the complete yellow opaqueness of the solution.
- Independent variables- the concentration of sodium thiosulphate that is changed.
- Controlled variable- these are four variables that affect the experiment and the results. They include
a. Concentration and volume of hydrochloric acid- by increasing the concentration of hydrochloric acid we increase the rate of reaction resulting from more collision as a results more hydrochloric acid molecules in the mixture.
b. Stopwatch manned by the same person- this is due to different reaction time of different persons. By maintaining the same person to record the time taken for the reaction to occur, we reduce the chance of errors in reading the time.
c. The same apparatus- by setting the same conditions for the reaction to occur, we ensure that the rates of reactions are not affected by impurities as well as different calibration errors that may hinder the volume of the reactants or the rate of reaction as whole.
d. The temperature- the temperature is responsible for how fast or slow particles of the reactants move. A higher temperature means that faster moving molecules of the reactants hence more collisions causing higher reaction rate. A lower temperature, on the other hand, causes particles to move at a slower rate hence slower rate of reaction.
MATERIALS AND METHOD
Materials
- 1M Hydrochloric acid
- 0.1M Sodium thiosulfate solution
- White paper
- Marker
- Conical flasks
- Stirrer
- Stopwatch
- Deionized water
- Wash bottle
- 5ml, 10ml and 20ml measuring cylinders
- Burretes
Procedure
Using 5mls of Hydrochloric Acid - Total Volume 25 mls
1. Take five clean conical flasks and label them as A, B, C, D, and E respectively.
2. Add 25, 20, 15, 10 and 5 ml of sodium thiosulphate into flasks A, B, C, D and E respectively.
3. Add 0, 5, 10, 15, and 20 ml of distilled water into flasks A, Band B respectively.
4. Add 5 ml of hydrochloric acid to flask A.
5. Start the stopwatch as soon as half the hydrochloric solution has been added to the conical flask.
6. Stir the contents of the flask and place it on the white paper marked X in the middle.
7. Observe the contents of the flask from the top and as stop the stopwatch just when the X mark becomes invisible and record the time.
8. Repeat this experiment using 5 ml of the hydrochloric acid to flask B, C, D and E and record the time taken for the mark ‘X’ to disappear.
Using 10mls of Hydrochloric Acid - Total Volume 25 mls
1. Take two clean conical flasks and label them as A, and B respectively.
2. Add 10 and 5 ml of sodium thiosulphate into flasks A and B respectively.
3. Add 15 ad 20 ml of distilled water into flasks A and B respectively.
4. Add 10 ml of hydrochloric acid to flask A.
5. Start the stopwatch as soon as half the hydrochloric solution has been added to the conical flask.
6. Stir the contents of the flask and place it on the white paper marked X in the middle.
7. Observe the contents of the flask from the top and as stop the stopwatch just when the X mark becomes invisible and record the time.
8. Repeat this experiment using 10 ml of the hydrochloric acid to flask B and record the time taken for the mark ‘X’ to disappear.
Using 5mls of Hydrochloric Acid - Total Volume 50 mls
1. Take five clean conical flasks and label them as A, B, C, D, and E respectively.
2. Add 50,40, 30, 20 and 10 ml of sodium thiosulphate into flasks A, B, C, D and E respectively.
3. Add 0, 10, 20, 30, 40 and 50 ml of distilled water into flasks A, Band B respectively.
4. Add 5 ml of hydrochloric acid to flask A.
5. Start the stopwatch as soon as half the hydrochloric solution has been added to the conical flask.
6. Stir the contents of the flask and place it on the white paper marked X in the middle.
7. Observe the contents of the flask from the top and as stop the stopwatch just when the X mark becomes invisible and record the time.
8. Repeat this experiment using 5 ml of the hydrochloric acid to flask B, C, D and E and record the time taken for the mark ‘X’ to disappear.
Using 10mls of Hydrochloric Acid - Total Volume 50 ml
1. Take two clean conical flasks and label them as A, and B respectively.
2. Add 20 and 10 ml of sodium thiosulphate into flasks A and B respectively.
3. Add 30 ad 40 ml of distilled water into flasks A and B respectively.
4. Add 10 ml of hydrochloric acid to flask A.
5. Start the stopwatch as soon as half the hydrochloric solution has been added to the conical flask.
6. Stir the contents of the flask and place it on the white paper marked X in the middle.
7. Observe the contents of the flask from the top and as stop the stopwatch just when the X mark becomes invisible and record the time.
8. Repeat this experiment using 10 ml of the hydrochloric acid to flask B and record the time taken for the mark ‘X’ to disappear.
RISK ASSESSMENT
The apparatus must be thoroughly clean.
Measure the volumes of sodium thiosulphate solution, hydrochloric acid and distilled water very accurately.
Use the same paper with the ‘X’ mark for all observations.
Complete the experiment at one time only so that there is not much temperature variation.
When half of the hydrochloric acid has been added to sodium thiosulphate solution, start the stop-watch immediately.
View the ‘X’ mark through the reaction mixture from top to bottom and from same height for all the observations.
RESULTS
Concentration of Sodium Thiosulphate
Mol dm3
Volume of Sodium Thiosulphate
Cm3
Volume of water
Cm3
Time
(Seconds)
Rate
(Seconds -1)
0.2
25
0
20
0.050
0.16
20
5
27
0.037
0.12
15
10
59
0.017
0.08
10
15
105
0.010
0.04
5
20
246
0.004
a. Using 5mls of Hydrochloric Acid - Total Volume 25 mls
b. Using 10mls of Hydrochloric Acid - Total Volume 25 mls
Concentration of Sodium Thiosulphate
Mol dm3
Volume of Sodium Thiosulphate
Cm3
Volume of water
Cm3
Time
(Seconds)
Rate
(Seconds -1)
0.08
10
15
106
0.009
0.04
5
20
264
0.004
c. Using 5mls of Hydrochloric Acid - Total Volume 50 mls
Concentration of Sodium Thiosulphate
Mol dm3
Volume of Sodium Thiosulphate
Cm3
Volume of water
Cm3
Time
(Seconds)
Rate
(Seconds -1)
0.2
50
0
30
0.033
0.16
40
10
40
0.250
0.12
30
20
50
0.200
0.08
20
30
80
0.013
0.04
10
40
203
0.005
d. Using 10mls of Hydrochloric Acid - Total Volume 50 mls
Concentration of Sodium Thiosulphate
Mol dm3
Volume of Sodium Thiosulphate
Cm3
Volume of water
Cm3
Time
(Seconds)
Rate
(Seconds -1)
0.08
20
30
68
0.015
0.04
10
40
65
0.015
a.
b.
c.
d.
DISCUSSION
With increase in the concentration of sodium thiosulphate, the rate of reaction increases in direct proportion to it. This evidently shown by the increasing trend line along the x-axis with the exception if graph labelled d. the diagrams a, b and c all show a straight trend line while diagram b produces a horizontal line parallel to the x-axis. It can be due to either error due to timing or measuring the sodium thiosulphate solution. The values obtained for both the flasks are approximately 0.015 which creates a straight line.
The straight lines show that the rate of a chemical is directly proportional to the concentration of sodium thiosulphate. Increasing the concentration of sodium thiosulphate implies that the molecule or the particles of sodium thiosulfate increase causing more collision between the particles. The higher the frequency of the collision of the particles, the higher the rate of a chemical reaction.
EVALUATION
The investigation of the relationship between the concentration of the sodium thiosulphate solution and rate of the chemical reaction was a success despite a few obstacles that hindered the accuracy of the result. The timing of the appearance of the Sulphur colloid is a tough job because of the instant reaction that is needed to ensure an accurate result. Also, there are many reactions that needs to completed in a single sitting which strains the individual doing the practical’s. This is evident with the deteoritating accuracy of the results towards the end of the practical. It is therefore recommended that the practical be made shorter or increase the time span, which will also compromise the practical due to changing practical (atmospheric conditions).
CONCLUSION
The graph obtained is a straight line.
1/t is directly proportional to the concentration of sodium thiosulphate.
Rate of the reaction between sodium thiosulphate and hydrochloric acid is directly proportional to the concentration of sodium thiosulphate, which is one of the reactant.
REFERENCES
Amrita.olabs.edu.in,. (2013). Kinetics Study on the Reaction between Sodium Thiosulphate and Hydrochloric Acid. Retrieved 8 February 2018, from amrita.olabs.edu.in/?sub=73"brch=8"sim=142"cnt=2
Houston, P.L., 2012. Chemical kinetics and reaction dynamics. Courier Corporation.
Semenov, N.N., 2013. Some problems of chemical kinetics and reactivity (Vol. 1). Elsevier.
Steinfeld, J.I., Francisco, J.S. and Hase, W.L., 1989. Chemical kinetics and dynamics (Vol. 3). Englewood Cliffs (New Jersey): Prentice Hall.
