chemistry rate of reaction
How does the Temperature affect the Rate of Reaction between Calcium Carbonate and Acid?
PLANNING
Introduction:
We are investigating how the temperature affects the rate of reaction between Sodium thiosulphate and acid. This project will involve 4 parts, which in them will have separate sub-headings. We will be doing the planning, obtaining evidence, analysing and concluding and finally evaluating. All the different aspects of rates of reaction will also be taken into account. E.g. Other factors and how to make the test fair.
The symbol equation for the reaction we are investigating is:
Na2S203(aq) + 2HCL(aq) 2NaCl(aq) + H2O(l) + S(s) + SO2(g)
The word equation for the reaction we are investigation is:
Sodium thiosulphate + Hydrochloric acid Sodium Chloride + Water + Sulphur + Sulphur dioxide
The rate of reaction can be found with the formula below:
Reaction Rate = Change in amount of a substance
Time Taken
Theoretical Background:
The rate of reaction can be calculated as with the equation shown above.
When a reaction takes place it has to overcome a minimum energy barrier known as the Activation Energy. If the particles collide with less energy than the activation energy then nothing worth noting happens.
"You won't get a reaction unless the particles collide with a certain minimum energy called the activation energy of the reaction." (Taken from www.chemguide.co.uk). Only those collisions, which have energies equal to or greater than the activation energy result in a reaction taking place. The reason why collisions have to overcome the activation is because every chemical reaction results in bond breaking. The activation energy is all about the breaking of the original bonds. So when the collisions between particles are relatively gentle there isn't enough energy available to start the process of breaking bonds and there the particles do not take part in a reaction. If the particles collide with less energy than the activation energy they simply bounce apart. Only those collisions equal to or greater to the activation energy react.
What is the rate of reaction?
The rate of reaction is the speed of the reaction. It is how quickly a reaction takes place.
How can we measure the rate?
) Measure how quickly one of the products is made.
2) Measure how quickly one of the reactants disappears.
Factors that affect the rate:
There are many factors that can affect the rate of reaction and below they are listed:
) Concentration - The concentration of the solution is how strong the solution is. If the concentration of a substance is higher this means it contains more particles - e.g. - a stronger acid would have more acid particles. Increasing the number of particles leads to more collisions, so the rate of reaction goes up. In a less concentrated acid the number of particles would be lower, so the rate of reaction is slower whereas in a more concentrated acid the number of acid particles is greater so the number of collisions is greater and the rate of reaction is faster. When we increase the concentration, we are increasing the frequency of collisions between the particles. However changing the concentration does not change the speed at which the particles are moving. A good way of describing this is For example if you are in a 60,000 capacity football ground and there are 59,000 people the chances of you colliding with someone are much higher than if in the same capacity stadium with just 1,000 people inside.
2) Catalyst - Catalysts are there to speed up the rate of reaction by providing an alternative reaction pathway with a lower Activation Energy (EA). Catalysts though are non-reactants.
3) Stirring - The effect of stirring increases the rate of reaction because it causes the particles to move quicker and so each collision between the particles becomes more violent. As the particles move around faster they gain more energy also which in turn helps to make the collision much more vigorous. How much the stirring affects the rate of reaction depends upon the speed of which you stir at. The faster you stir the quicker the rate of reaction because the particles will gain more energy at a faster speed.
4) Temperature - This is the factor that we will be investigating and is the main factor in regards to the rate of reaction and it has the biggest effect. When we increase the temperature at which a reaction is taking place, the particles move quicker. This has two effects: 1) More collisions will occur. 2) When a collision occurs there is more chance of the collision leading to a reaction because the energy is likely to be higher than the minimum required energy, the activation energy. If a lot of heat is applied to the molecules then they will have a greater amount of energy, which will increase the speed they travel at. The greater the speed of the molecule the more violently it will react when it comes into contact with other molecules and also the frequency of collisions will be far greater. The reacting molecules however have to overcome the activation energy to react and if the activation energy is high then only the most energetic molecules will be able to overcome it making the reaction slower. If, however, the activation energy is small then more of the collisions will be effective and the reaction will be faster. For example if the sodium thiosulphate was heated at 20oC the movement energy in the particles is less, and is more likely to be below the activation energy whereas at 60oC the movement energy in the particles is higher, making it more likely to be above the activation energy. In fact "a chemical reaction cannot happen unless particles in the reacting substances collide with each other." (Quote taken from Chemistry Counts book page 230.)
5) Surface Area - The surface area would normally affect the rate of reaction but because we are using two solutions this factor will have no affect on the rate. In a solid though the bigger the surface area the faster the rate of reaction. This is because when the surface area is larger there are more surfaces for the particles to react with. Breaking it up into smaller pieces increases the surface area of a solid.
6) Pressure - Increasing the pressure is similar to increasing the concentration. The particles will collide more frequently at a higher pressure and increase the rate of reaction. The pressure though only has a big effect on gases and not liquids, which is why it will not be a major factor to control.
Prediction:
By using the theoretical background I can now make a prediction of what the results may be like.
I predict that an increase in temperature will lead to an increase in the rate of reaction. This is mainly because ...
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6) Pressure - Increasing the pressure is similar to increasing the concentration. The particles will collide more frequently at a higher pressure and increase the rate of reaction. The pressure though only has a big effect on gases and not liquids, which is why it will not be a major factor to control.
Prediction:
By using the theoretical background I can now make a prediction of what the results may be like.
I predict that an increase in temperature will lead to an increase in the rate of reaction. This is mainly because of two theories, Kinetic Theory and Collision theory. They are both similar and both deal with the movement of particles. Kinetic theory is how fast a molecule moves and the amount of energy it has and so when a molecule is heated at a higher temperature it will move at a greater speed and more vigorously. The key though to kinetic theory is when molecules are heated at a high temperature they have more energy and what the energy actually does is, it breaks up the attractions between other molecules. The structure of the molecule is being broken down. This allows the molecules to move around faster as they have fewer attractions to other molecules.
The other theory is the Collision theory. As just explained when there is an increase in the temperature the molecules gain more energy, thus moving around faster, which in turn leads to the collisions between molecules becoming more frequent. The collisions tough have to overcome a certain energy called the Activation Energy (EA). If the EA is high then only the most energetic collisions will be able to overcome it and the reaction will be slow, but if the EA is high then more of the collisions will be effective and the rate of reaction will be faster. This is also tied in with the kinetic theory because I think the molecules gain their energy from being heated. The rise in temperature will not only increase the number of effective collisions but they will also allow the particles to produce more energy, as the collisions will be more vigorous.
Below is a diagram representing the Activation Energy:
So with both theories the rate of reaction is increased as a rise in temperature leads to the reactants overcoming the energy barrier and I also predict that the reaction will approximately double the rate for every 10oC rise in the temperature.
During the course of the experiment only one variable will be measured and that is the time. After we have collected the results two graphs will be drawn:
) Time against temperature
2) Rate of reaction (1/t) against temperature
For the first graph it is just simply plotting the time against the temperature but for the second graph a calculation has to be done to find the rate of reaction. The rate of reaction can simply be found by dividing the time by 1. The time is measured in seconds and rate of reaction will be measured in seconds - 1.
The two graphs can then be compared. For the last part of the prediction I predict that the two graphs will look like this:
Rate of reaction (1/t) against temperature
Time against temperature
Trial Experiments:
We did various trial experiments to determine the optimum concentration of sodium thiosulphate and water to use. The optimum concentration was the one that came out with the least percentage of error. The temperature ranges were from 20oC to 60oC. But for the trial experiments we just did room temperature (20oC) and 60oC as they are the two extremes and we know that for the other temperatures they are going to be in between the two extreme temperatures. The experiment was also done to find out the most suitable ranges of temperature.
To determine which concentration had the least room for error, the percentage of error was worked out. The way in which the percentage error was worked out was by dividing the number of seconds the reaction took by 100. A stopwatch was used for extra accuracy in recording the time. If we had counted by ourselves the results would not have been very accurate.
The basic method for the trial experiment was as follows:
) All the equipment was taken and set up.
2) The set amount of sodium thiosulphate and water were measured out and poured into one beaker.
3) This solution was then heated to the desired temperature, which was either 20oC or 60oC.
4) After the temperature reached our target it was then taken off the tripod and put onto our piece of paper which had a cross marked on it.
5) 5cm3 of hydrochloric acid was then added to our solution of sodium thiosulphate and water and the stopwatch was immediately started.
6) As the soon as the cross had disappeared the time was recorded.
7) This was repeated for the other concentrations.
The results for the trial experiments are represented below.
Experiment at room temperature (20oC):
Experiment
Volume of stock (cm3)
Volume of water (cm3)
Concentration of thiosulphate
(g dm-3)
Time (secs)
/time (secs)
(i)
50
0
40
29
0.034
(ii)
40
0
32
37
0.027
(iii)
30
20
24
42
0.024
(iv)
20
30
6
50
0.02
(v)
0
40
8
2m 41
0.006
Experiment at other extreme (60oC):
Volume of sodium thiosulphate (cm3)
Volume of water (cm3)
Target temperature (oC)
Initial temperature (oC)
Final Temperature (oC)
Average Temperature (oC)
Time (secs)
50
0
60
58
63
60.5
4.01
40
0
60
53
59
56
5.64
30
20
60
55
58
56.5
6.72
20
30
60
59
62
60.5
7.83
0
40
60
54
58
56
9.41
As a result of doing the trial experiments I have found out that the optimum concentration to use for our experiment is 10cm3 of sodium thiosulphate and 40cm3 of water. This is because it has the least percentage error. When we come to do the actual experiment that will be the concentration that will be used and then the only variable changed will be the temperature whereas in the trial experiment the variable changed was the concentration.
THE ACTUAL EXPERIMENT
Apparatus:
* Bunsen Burner - To heat up the solution
* Tripod - To hold the beaker
* Gauze - To put on top of the tripod
* Beaker (100cm3) - To hold the chemicals in
* Measuring Cylinder (50cm3) - To measure the volumes of thiosulphate and acid and water to use.
* Thermometer - To measure the temperature
* Heat proof Mat - The apparatus will be placed on this
* Paper with a cross marked on it
* Stopwatch - To measure the time
* Different quantities of sodium thiosulphate, water and hydrochloric acid.
Diagram:
Method:
) The apparatus will be set up as shown above.
2) As discovered the optimum concentration is 10cm3 of sodium thiosulphate with 40cm3 of water. This will be measured out using a measuring cylinder.
3) The solution will then be poured into a beaker.
4) This will then be heated to 30oC using a Bunsen burner on a roaring flame.
5) When the temperature reaches 30oC the Bunsen burner will be turned off and the beaker will be taken off the tripod. The temperature will be measured using a thermometer. (Note: The Bunsen will be turned off when the thermometer reads a few degrees below 30oC as heat is still transmitted.
6) Once the beaker has been taken off the tripod it will be placed on paper marked with a cross.
7) Then 5cm3 of hydrochloric acid will be added to the solution of sodium thiosulphate and water.
8) The stopwatch will immediately be started.
9) As soon as the cross has disappeared from sight the time will instantly be recorded.
0) Exactly the same procedure will adopted for 40oC and 50oC.
This method will be carried out in the laboratory. The chemicals and the quantities used will be as follows:
Sodium Thiosulphate - 10cm3
Water - 40cm3
Hydrochloric Acid - 5cm3
I think my plan is a good way of carrying out the experiment. This is because my plan uses precise measurements and is reliable. The equipment used is also very good and is very capable of producing accurate results and also because I have made sure that my test will be kept a fair one.
Fair Test:
Many things will be done to ensure that our experiment is a fair test because there are a lot of factors that can affect the rate of reaction as talked about above. For example stirring affects the rate. The way in which we will control these factors is to keep them constant.
There are also many variables that will be measured and kept the same or changed. By breaking them down it is easier for us to follow:
Variable changed:
* Temperature
Variable measured:
* Time taken for Rate of Reaction
Variables kept the same:
* Volume of sodium thiosulphate
* Volume of acid
* Concentration of acid
* Concentration of sodium thiosulphate
* Cross on the paper
By changing any one of the variables that we kept the same would mean that our results would become very inaccurate. For example if we changed the cross on the paper, it might affect the time we see it disappear because a bright red cross might stay visible to us for a longer period of time than a pencil cross. Also the stirring will have to be consistent as different rates of stirring will mean a change in the rate due to particles moving faster or slower depending on the speed at which the solution will be stirred. Another point to make about a fair test is that a Bunsen burner is not good for maintaining a constant temperature whilst we heat the solution up. Whilst heating the solution, the water in the solution could evaporate in the duration of time it takes for the Bunsen to heat up the solution to our desired temperature. This means that the concentration of the solution will be affected. The temperature ranges were from 20oC at 10oC intervals all the way until 60oC. There is another point to think about when deciding how to set up the apparatus, where to put the paper with the cross. If it is put underneath the beaker and on top of the gauze it will get burnt so the ideal place to put it is out of the way and the best thing to do is move the beaker on top of the paper.
The final point to make about ensuring our test is fair is the repeating of results. The reason for repeating results is because it increases the reliability and also accuracy of our results. For example we will be doing five different experiments and if we repeat these once then we can compare them and increase the reliability of the results.
Safety:
Whilst doing our experiment we have to take into account the various safety measures to abide by. For example a bench mat is needed to ensure that the desk doesn't get burnt and also safety goggles need to be worn at all times for protection over the eyes. This is because the chemicals that are going to be used are irritants and also the reaction produces other irritant chemicals and one of the gases given off is toxic, though it is given off in small quantities and is therefore relatively harmless. If any acid spills on you, you must wash your skin immediately. We also need to ensure there is safe working space for us to work in.
During the course of the experiment there will be five chemicals that will take place in the reaction:
Sulphur dioxide - Toxic gas and also very corrosive. Eye protection must be worn and this gas must not be inhaled directly.
Sulphur - This is a solid that is formed from the reaction.
Hydrochloric acid - Very corrosive and eye protection must again be worn. For extra precaution gloves are also handy.
Sodium Chloride - This is an oxidising agent that is an irritant and also toxic. It is harmful as well so eye protection must be worn.
Treatment of Results:
The results will be tabulated and then two graphs will be drawn as a result. As many results will also be collected and the average of these will be taken to increase the reliability of our results. After the results are collected two graphs can then be drawn.
Below is the likely results table that will be drawn:
Temperature (oC)
Time taken for reaction (secs)
Rate of reaction (1/t)
OBTAINING EVIDENCE
Changes to planned method:
Two changes were made to the planned:
) The experiment was done for all the temperatures not just 30oC, 40oC and 50oC. It was done for 20oC and also 60oC.
2) The procedure was repeated once for all of the temperature ranges to ensure the maximum reliability.
Table of results to show the time taken for the cross to disappear when Hydrochloric Acid was added to a solution of Sodium Thiosulphate and Water
Concentration - 10cm3 of Sodium Thiosulphate
40cm3 of Water
Target Temperature (oC)
Initial Temperature
(oC)
Final Temperature (oC)
Average Temperature (oC)
Time (seconds)
/time (seconds-1)
20
9
21
20
05
0.010
30
26
33
29.5
70
0.014
40
35
44
39.5
34
0.029
50
45
53
49
27
0.037
60
58
64
61
9
0.053
Repeats:
Target Temperature (oC)
Initial Temperature
(oC)
Final Temperature (oC)
Average Temperature (oC)
Time (seconds)
/time (seconds-1)
20
8
24
21
08
0.009
30
27
33
30
66
0.015
40
39
43
41
42
0.024
50
46
53
49.5
25
0.040
60
57
64
60.5
20
0.050
ANALYSING EVIDENCE
In the results table the calculations to find out the average temperature for each range was done as well as the calculation to find out the rate of reaction. The rate of reaction was calculated by using the formula 1/t.
Graphs:
Below are the two graphs I have drawn from my results table. For the temperature I plotted the average temperature and for the values on the y axis, the dependent variable, I plotted both results, the first time round and also the repeats that we carried out.
Patterns from graph and Explanation:
There are some patterns that can be identified when looking at the two graphs above. There was also only one anomaly in my results which was plotted on both graphs.
For the 1/time (Rate of Reaction) against temperature a positive correlation can be seen. A positive correlation means that as the temperature increases the rate of reaction increases, however not at a constant value, therefore a curved line of best fit was drawn.
But between 0oC and 30oC effectively a straight line can be drawn to join up the pints and the points therefore ascend at a constant value. The values at the beginning approximately double with every 10oC rise in temperature.
For the second graph, temperature against time graph there is a negative correlation. This means that as the temperature increases, the time taken for the cross to disappear decreases. Therefore an increase in the temperature leads to an increase in the rate of reaction. This is because when we increase the temperature the cross disappears faster meaning that the reaction occurred much quicker, hence there is an increase in the rate of reaction. The shape of this graph is a curve in a negative correlation and is not a straight line of best fit, so the points do not go down at constant values. This means that the temperature and time are not directly proportional to each other across the whole temperature range but they are directly proportional between certain points, though with different gradients. Therefore the pattern that can be identified in the graph is in two steps. The first pattern that can be found is that the first three points on the graph can be joined with a straight line, meaning that they go down at a constant value. From the graph, it can be seen, that the values for the time taken for the cross to disappear decrease by approximately 35 seconds with every 10oC rise in the temperature between 20oC and 40oC. However, the gradient becomes less steep for the last three points and a straight line can be drawn to join up these points as well. This means that the time decreases at a constant value, which in this case is by 7-8 seconds for every 10oC rise in the temperature between 40oC and 60oC. Therefore at higher temperatures the rate of reaction increases as the time decreases.
So even though the two graphs have completely the opposite correlations to each other they are both telling us exactly the same thing, as you increase the temperature you increase the rate of reaction.
Conclusion:
By doing the experiment and then collecting together our results we have found out that:
The time taken for the cross to disappear decreases with an increase in temperature
This means:
The rate of reaction increases as there is an increase in the temperature
This is because of two theories, the Kinetic Theory and also the Collision Theory. Both theories deal with the movement of molecules.
Kinetic Theory is the speed at which a molecule travels when heated to a certain temperature and the energy that the molecule has. When a molecule is heated at a higher temperature it will move at a higher speed and therefore it will have more energy. Molecules get their energy from being heated at high temperatures. What the energy does to the molecule is it breaks the various bonds the molecules have together. Molecules are attracted to each other by bonds and the energy breaks up these attractions to other molecules thus allowing the molecule to move at greater speeds and more vigorously when heated at higher temperatures. The Collision Theory is very similar to Kinetic Theory as it deals with the energy a molecule contains. When there is an increase in the temperature the molecules gain more energy, which means they move around faster. As the molecules are moving at greater speeds at higher temperatures this therefore will lead to the molecules colliding with each other on a more frequent basis. This means that there are more collisions between the molecules. However to make an impact on the speed at which the reaction is taking place the molecules have to overcome an energy barrier called the Activation Energy (EA). If the Activation Energy is high then only the most energetic collisions will be able to overcome it, making the reaction slow but if the EA is low then more of the collisions between the molecules will be effective and then the collisions will be able to overcome the EA, making the reaction faster. Also when the collisions are relatively gentle, there isn't enough energy available to start the bond-breaking process, and so the particles don't react.
Therefore the reason I stated that the rate of reaction will be faster at higher temperatures is because the higher the temperature, the more effective collisions there are as the molecules have more energy and will be moving at greater speeds, so the Activation Energy will easily be overcome.
Comparison of Conclusion to Prediction:
My prediction is not exactly the same as my prediction. In my prediction I stated that for the rate of reaction against temperature graph there will be a straight line through the origin. However, when I drew my graph the correlation was a curved line of best fit in a positive correlation therefore my predicted graph was almost correct.
My results show this because where there is an increase in temperature there was an increase in 1/time (Rate of Reaction).
For my second graph, time against temperature I predicted a straight line in a positive correlation with a plateau towards the end. In fact the final graph that I drew was the opposite and was in the form of a negative correlation, but it did however plateau off towards the end in the opposite direction. It was therefore a mirror image to what I predicted.
I also predicted that for every 10oC rise in temperature the rate of reaction will double but this was not true for my results.
The scientific knowledge that I proposed in the prediction is though identical to my conclusion.
EVALUATING EVIDENCE
Anomalous results:
There was only one anomaly in the results, which was plotted on the graph. The anomaly I had was under the temperature 40oC on the repeat. It has been circled on the two graphs.
No full explanation can be given as to why there was an anomalous result. There are a few ways an anomaly can come about but no full explanation can be given as I am not certain. There are 3 ways though how they could've come about: the procedure, measurement errors and even experimental errors like using a measuring cylinder for the water and then in the next experiment using the same cylinder for the Sodium Thiosulphate. This might have caused slight inaccuracy. However the general accuracy of my results was good.
Limitations and Possible justification for anomalies:
There were quite a few limitations with the procedure we followed:
. A limited temperature range was used from 20oC to 60oC. This caused are results to be valid for only the temperature range given. At the higher temperatures for example at 80oC something very different could have occurred. The graph could have curved off.
2. The judgement of the disappearance of the cross was a major problem. The disappearance could not be seen to one hundredth of a second and could have caused a slight inaccuracy in the results and the graphs. So our error judgement can be calculated.
3. At the higher temperatures the results became less accurate because the solution starts to evaporate and therefore the concentration in the beaker increases and this can affect the rate.
Other problems we had were keeping the stirring constant, maintaining the temperature, temperature change when hydrochloric acid was added and the fact that at 60oC the water started to evaporate. All of these factors could have affected the rate.
General Accuracy and Reliability of my results:
Accuracy - How close the results are to the expected target. The accuracy can be checked by how close the results are to the line of best fit.
Reliability - How close the repeats are together.
I think my results are very accurate because on the graph the points are almost 0n the line of best fit. Also the reliability of my results is very good because again the repeats are all next to each other. This can be most clearly shown in the results table where the repeats are at most 4 apart, apart from the anomaly. My results also prove that as there is an increase in the temperature there is an increase ion the rate of reaction. Therefore the results are sufficient enough to prove my theory and are also very accurate and are a reliable source of evidence.
Improvements:
Many things could be done to improve the experiment we carried out.
Problem
Improvement
) Keeping the stirring constant.
Using a magnetic stirrer would keep the level of stirring the same therefore all the molecules will be moving at a constant speed and this will also ensure a fair test.
2) Maintaining a constant temperature
Using a water bath would ensure a constant temperature. Therefore we will not have to take the average temperature or initial and final and this would make collecting the results easier and our target temperature can then easily be reached.
3) Evaporation of solution at 60oC. This increases the concentration and therefore affects the rate.
There is no real way to escape this problem. However a closed system could be used to stop any vapour escaping but the experiment has to have an outlet. I would personally close the beaker by putting something over it as I believe this would be a good place to start to get rid of this problem
4) Judgement of the cross disappearing. This could make our results inaccurate as the disappearance of the cross is vital and 2 seconds out could cause an anomaly. Also we can't judge the disappearance to a 100th of a second.
A light sensor attached to a LogIt can be used to judge the disappearance of the cross at the precise moment. This will increase the accuracy of our results.
5) Hydrochloric Acid - When we added the hydrochloric acid into the heated solution of water and Sodium Thiosulphate there would have been a temperature change because the hydrochloric acid wasn't heated and therefore cold acid was added to a warm solution.
Using a water bath would control this factor.
6) Whether or not the reaction was exothermic or endothermic.
Using a thermostat controlled water bath would make sure that if the temperature dropped too low or too high
Validity and Reliability:
Validity - Whether or not the results answer the main title.
If I had more time to carry out the experiment I would increase the validity by increasing the temperature range and also improving the experiment with the improvements above.
I would increase the reliability by just repeating the experiments more than once. The more times you carry something out the more reliable the evidence becomes.
Further Work:
Another experiment that could be carried out to extend and improve the work that I have done is shown below.
- 1 -
