To investigate the effect of heat on the reaction between Sodium Thiosulphate and Hydrochloric Acid
Rates of Reaction
The rate of reaction is the rate of loss of a reactant or the rate of formation of a product during a chemical reaction. It is measured by the time taken of the reaction¯1. The speed of a reaction can be increased by:
. Raising the Temperature
2. Adding a Catalyst
3. Increasing the concentration
4. Increasing the Pressure (gases)
5. Increasing Surface Area
All of these raise the speed of reaction because of the particle collision theory.
Particle Collision Theory
For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area. See bibliography for where I obtained my information.
. When the temperature of the reacting substances is increased, the particles move quicker. As the particles are moving quickly, the particles are going to have more collisions. As the particles also have more energy, they will collide with more energy. As the temperature is raised by 10ºC the rate of reaction will double, causing the time taken for the reaction to take place to halve.
2. Adding a Catalyst.
A catalyst will lower the activation energy of a reaction thus making the particles move faster. This will increase the number of useful reactions
3. Increasing the Concentration.
If the solution is highly concentrated there are more particles of the reactive substances in the solution. As there is less amount of water in a high concentration solution than in a low concentration solution, collision between the important particles becomes more frequent; increasing the number of collisions.
4. Increasing The Pressure (gases).
In a gas if you increase the pressure inside the container that holds the reactive substances, the particles are squashed up. As they have less room to move, there are going to be more collisions between the particles.
5. Increasing Surface Area
A solid has less surface area than a powder of the same substance and of the same weight. If one of the reactants is a solid then crushing it into a powder will give it more surface area. This mean the particles around it in the solution will have a bigger area to work on, increasing the number of useful particle collisions.
Aim
I must produce a piece of coursework investigating the rates of reaction, and the effect different changes have on them. I have chosen to investigate the effect temperature ...
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5. Increasing Surface Area
A solid has less surface area than a powder of the same substance and of the same weight. If one of the reactants is a solid then crushing it into a powder will give it more surface area. This mean the particles around it in the solution will have a bigger area to work on, increasing the number of useful particle collisions.
Aim
I must produce a piece of coursework investigating the rates of reaction, and the effect different changes have on them. I have chosen to investigate the effect temperature has on a reaction. This is because it is the most practical to investigate - it would take longer to prepare a solid in powdered and un-powdered form, and it is difficult to get accurate readings due to the inevitabilities of human errors, and as gas is mostly colourless it is difficult to gauge a reaction changing the pressure, and if a substance is added to give the gas colour, it may influence the outcome of the experiment. Similarly the use of a catalyst complicates things, and if used incorrectly could alter the outcome of the experiment.
I am going to investigate the effect of heat on the reaction between Sodium Thiosulphate and Hydrochloric Acid
The reaction that will be used is:
Na2S2O3 (aq) + 2HCL (aq) --> 2NaCl (aq) + H2O (l) + SO2 (g) + S (s)
Pre-test
We carried out a pre-test of this experiment to test which temperatures would be best to use in the experiment. Shown below are an equipment list, method and set of results for my pre-test.
Fair Test
I will have to keep the concentration, amounts and temperature the same. To ensure greater accuracy I will be using the same equipment throughout. To increase accuracy I will need to do four repetitions and make sure that the results for each temperature are no more than three seconds apart. If they are I will re-do all anomalous results.
Pre-test Equipment List
0.5 Molar Hydrochloric Acid
Sodium Thiosulphate
Boiling Tube
Measuring Cylinder 25ml
Measuring Cylinder 5ml
Conical Flask
Stop Clock
Thermometer
Water Bath
Paper with a red cross on.
Pre-test Method
. Use measuring cylinder to measure 25ml of Sodium Thiosulphate. Pour into a boiling tube.
2. Place boiling tube into water bath and heat Sodium Thiosulphate to selected temperature
3. Pour heated Sodium Thiosulphate into a conical flask and place conical flask onto Paper with red cross on.
4. Pour Hydrochloric Acid into conical flask and start stop clock.
5. Look through solution and when you cannot see the red cross through the solution anymore stop the stop clock.
6. Record selected temperature and time taken for reaction to occur.
7. Wash out conical flask.
8. Repeat steps 1-7 for three more different temperatures
Pre-test Results Table
Temp
Time Taken (s)
7°C
60
9°C
57
28°C
35
80°C
0
Prediction
I have chosen a range of temperatures between 80°C and 15° to test for the smallest rate of reaction.
The reaction between Sodium Thiosulphate and Hydrochloric Acid produces Sulphur precipitate. This makes the solution of Sodium Thiosulphate and Hydrochloric Acid turn cloudy.
I predict that the experiment carried out at the temperature closest to 80°C will take place in the smallest amount of time to complete because of things I have found out about the Particle Collision Theory.
I have found out that when the temperatures of one or both of the reactants are raised the particles will move faster as they have more energy to move around. As the particles are moving around faster there will be more collisions between the particles. I predict that my results will rise proportionally. When the temperature is increased by 10ºC the rate of reaction doubles, thus the time taken for the reaction to take place will halve.
I have also found that there is a minimum amount of energy that the reacting particles need to collide with each other so that they do react. This minimum energy is called the activation energy.
From this information I have reached a conclusion that heating the particles will give them more energy. Because the particles have more energy to move around they will not only move faster and have more collisions, they will also collide with more energy ensuring that most of the collisions between the reacting particles will result in the reaction happening.
All of the above information supports my prediction.
Safety
As we are using acids goggles must be worn through out the experiment. We also have to be careful of spilling the acids and wiping up any spillages.
Do not heat up Hydrochloric Acid.
Equipment List
0.5 Molar Hydrochloric Acid
Sodium Thiosulphate
Boiling Tube
Measuring Cylinder 25ml
Measuring Cylinder 5ml
Conical Flask
Stop Clock
Thermometer
Water Bath
Paper with a red cross on.
Method
. Use measuring cylinders to measure 25ml (use 5ml measuring cylinder for accuracy) of Sodium Thiosulphate. Pour into a boiling tube.
2. Place boiling tube into water bath and heat Sodium Thiosulphate to selected temperature
3. Pour heated Sodium Thiosulphate into a conical flask and place conical flask onto Paper with red cross on.
4. Pour Hydrochloric Acid into conical flask and start stop clock.
5. Look through solution and when you cannot see the red cross through the solution anymore stop the stop clock.
6. Record selected temperature and time taken for reaction to occur.
7. Wash out conical flask.
8. Repeat steps 1-7 for the same temperature three times.
9. Repeat steps 1-7 four times for each of the selected temperatures.
Results Tables
Temp
Time taken (in s)
(°C)
st
2nd
3rd
4th
Ave.
6°
71
69
69
71
70
9°
61
63
60
62
61.5
30°
33
34
34
35
34
45°
9
9
8
8
8.5
78°
2
1
2
1
1.5
Gradient Table
Temp (X)
Time (Y)
Diff X
Diff Y
DY/DX = Gradient
6.00
70.00
-3.00
8.500
-2.83
9.00
61.50
-11.00
27.500
-2.50
30.00
34.00
-15.00
5.500
-1.03
45.00
8.50
-33.00
7.000
-0.21
78.00
1.50
Graphs
Graphs are shown on next pages.
Analysis
In this experiment, I have found that as the temperature increases, the time taken for the reaction to take place decreases. This means that as the temperature increases, the rate of reaction would increase. This is because as a reaction takes less time, more reactions between particles would take place per second. From my graphs, using the lines of best fit, I can draw a conclusion from my experiment.
The graph plotting temperature against time has negative correlation. This means that as the temperature increased, the time taken decreased. From this graph I decided that the graph for rate against temperature, when plotted, would have positive correlation. This would show that when the temperature is increased, the rate of reaction would also increase.
To understand this conclusion better, we must recap on the particle collision theory:
For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same.
As shown roughly in my graph as the temp increases by 10ºC the rate of reaction doubles, which means that as the temperature goes up by 10ºC, the time taken for the reaction to take place should half. On my graph, at 20ºC, the time taken is 60 seconds, so in theory at 30ºC the time taken should be 30 seconds. However this is the not the case. The actual time taken is 34 seconds. This shows that there were some anomalies in my results. Still, my results are fairly accurate as my graph is true to the graph you would expect to get from a perfect set of results.
Evaluation
Overall, I think the experiment went quite well. Although I have a set of results that are slightly inaccurate, they are still accurate enough for me to give a firm conclusion. My results support my prediction and I think I used a significant range of temperatures and repetitions to give a firm set of results.
As I used the same equipment through out the experiment any errors due to the equipment I used can be eliminated. The only error that cannot be eliminated is when I judged whether or not I could see the cross through the solution. This cannot be eliminated because although I could not see the cross anymore, some of the reactions could have been further on than others carried out at the same temperature even if they had been ongoing for only a few seconds, or even a hundredth of a second, more or less than each other.
Also I may have been able to get more accurate results by using light detectors which record the amount of light that passes through the solution and then displays the results (in real time) on a graph on a computer. By using these light detectors, instead of when a cross can no longer be seen through the solution, my results would be more accurate.
Bibliography
Books:
GCSE Double Science, Chemistry, The Revision Guide, Higher Level, Edited by Richard Parsons, Published By CGP
Websites:
http://www.gcsechemistry.com/rc.htm
http://www.wpbschoolhouse.btinternet.co.uk/page03/3_31rates.htm
Elizabeth Holden 10NW, 10 DSC1
