Investigating rates of reaction between hydrochloric acid and sodium thiosulphate and how heat/ temperature of the solution affects this rate.

Investigating rates of reaction between hydrochloric acid and sodium thiosulphate and how heat/ temperature of the solution affects this rate.

Background Knowledge:

Reaction rates are explained by collision theory, this is because it states that the rate of reaction depends on how frequently and strong the reacting particles collide, in other words the particles have to collide to react and collide with strength. There are four ways that help speed up the rate of reaction using collision theory. These are temperature, concentration, size of particles (surface area) and catalysts.

Temperature increases the rate of reaction because when the temperature increases the particles move quicker, this means the particles will have more collisions. Concentration increases the number of collisions as well. If the solution is more concentrated there are more particles of reactant between water molecules, which makes collisions between the other particles more likely. The size of solid particles creates a difference in reaction rates because if you break it into smaller pieces that it will have increased the surface area meaning that the particles around it in the solution will have more area so there will be more collisions. However this does not apply to this experiment as I am using liquids and no solids. A catalyst is the final method of increasing the rate of reaction, it works by providing a surface for the particles to stick to, where they can bump into each other.

However all these only work and reactions only happen if the particles collide with enough energy. When the temperature is high there will be more particles colliding with enough energy to make the reaction take place. This energy is called activation energy; it is needed to break the initial bonds. Energy must always be supplied to break bonds, energy is also released when bonds form, this happens during a chemical reaction, old bonds break and new ones form, bond breaking is an endothermic process and an exothermic process is bond formation. In an endothermic reaction the energy released in bond formation is greater than the energy used in breaking bonds. However in an endothermic reaction, the energy required to break old bonds is greater that the energy released when new bonds are formed.

2HCL + Na2S2O3 ⇒ 2NaCl + SO2 + S +H2O

I am not sure whether the reaction will be exothermic, endothermic or whether they balance each other out. If a chemical is at a decreased temperature the particles will not have the right amount of energy for the activation of the reaction. However if the chemicals are too high in heat then the reaction wont take place and if the reaction is exothermic. This is due to Le Chatelier’s principle which states that a change in conditions, in this case temperature, will mean the position of equilibrium will alter to counteract the difference made to it. If the temperature is raised to or above certain heat it will encourage a reaction, which produces less heat.

Prediction:

The rate of a chemical reaction is the speed at which it takes place. Temperature is a major factor in this as increasing or decreasing the temperature changes the movement of particles in a substance therefore changing the rate of reaction. When a reaction mixture is heated up, each particle in it acquires more energy and collides more with other particles. From my background knowledge and research, I predict that if you increase the temperature of a reaction, you increase the speed at which it takes place. This is due to the particles carrying more energy and colliding more often so these collisions contain enough energy to break the energy barrier. When the temperature is raised particles, when moving faster, collide with greater power. The reaction therefore becomes faster. Collisions between particles are useless unless they break the energy barrier and this is exactly what increasing the temperature will allow them to do.

Method:

I will firstly measure 20 cm3 of the hydrochloric acid and 20 cm3 of sodium thiosulphate using a 10 cm3 measuring cylinder, previously washed thoroughly to prevent any remnants of other experiments affecting my results. I am using 20 cm3 because any more acid or sulphate will take too long to react and any less means it will be too quick to measure. I am going to put the cross behind the test tube rather than under it because the diameter of the beaker and amount of liquid doesn’t change. If you looked down, the volume of liquid maybe slightly ...

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Method:

Once I have started heating the solution in a water bath, as it is safer not to heat acid directly, I will start timing with the stop clock. I will time how long it takes for the acid and sulphate to react until it goes opaque and I cannot see the cross any longer. Each time I do the experiment I will raise the temperature by 5C, from 20C to 50C. I will repeat the five experiments to gain two sets of results; I will then work out the average.

To make my experiment a fair one, I had to examine many factors. Firstly, I looked at the factors that may have affected how well the investigation would work and these were things like using different equipment or doing the actual experiment in different conditions i.e. a colder/hotter environment. To withstand this, I made sure that upon repeating the experiment, I used the exact same equipment and that I did it in the exact same environment to make absolutely sure that the experiment was fair at each temperature. I will use a –10C to 110C thermometer to gain more accurate results.

I think that these were both very important factors because they could affect the results severely and leave me with an anomaly when I should only have connecting results. To ensure fair and accurate results, all of the factors that can affect my results need to be controlled. In the group we decided to have the same person measuring the time on the stop clock, as it will be the same pair of eyes, we also decided this because it will mean the same reaction times, altogether providing us with the same margin of natural human error throughout. To try and minimilise this I will make sure that the kind of background we are working at is similar, as if it is a black background one time it will be harder to see the black cross, affecting the results. Then if the experiment is carried out in front of a white background it will be easier to see the cross but this will be an unfair test and affect the results.

Other measures taken to provide fair and accurate results are, using a measuring cylinder to get the right amount of solution, using thoroughly cleaned equipment and using a pipette for measuring as well. We will use a thermometer so we can measure the heat of the liquids precisely. We will carry out the experiment at a different temperature five times, do two sets of results and use the average to provide results, which are as exact as possible. However the room condition was probably slightly different on each occasion I did the two sets of experiments, the first one was done in the beginning of November and the second experiment was done in the middle of December so the weather conditions would have affected the results, it would have been darker and maybe harder to see as well as colder. I don’t think this would have affected the results drastically though. There are also some conditions I cannot control like the pressure and air in the room and I expect as the experiment goes on the room will heat up from the use of Bunsen Burners which will then affect our results slightly. This affects it because the heat will give the particles kinetic energy before we start the experiment.

Equipment:

1-Sodium thiosulphate 8-Tripod

2-Gauze 9-2 measuring cylinders

3-2 test tubes 10-Bunsen burner

4-Beaker 11-Stopwatch

5-Heatproof bench mat

6-Thermometer

7-Hydrochloric acid

Safety:

Throughout this experiment, I made sure that safety was one of my top priorities. I wore goggles at all times to protect my eyes; I wore a lab coat to prevent getting any chemicals on my clothing or me. I used a heatproof mat and tripod when using the Bunsen burner and took extreme caution when turning it on and off. I made sure my group and I were standing throughout the experiment so that if any substances or the Bunsen burner was knocked over we wouldn’t get anything dangerous on us.

Preliminary test:

Preliminary work is the work that is done beforehand for you to know that the values of chemicals and temperatures are all within an adequate range and are guaranteed to work. It helps by showing me whether the equipment has been set up correctly for when the actual experiment begins. For my preliminary work, I did the experiment but did not repeat it, as I only wanted to see that the values that I was to be using were accurate. This preliminary work helped me to plan my actual investigation better as I learnt from my mistakes. These were things like not starting the stop clock, quick enough, which changed the times of my results. Therefore, in the actual experiment I am going to be very careful starting the stop clock at the exact time the hydrochloric acid is added to again ensure fair and precise results. For my preliminary test I had the temperature at room temperature, which I had to measure, when I carried out the test it was 22C.

Results:

(Seconds)

SET 1

>25C: 27.5 (preliminary test)

>30C: 24.64

>35C: 20.75

>40C: 14.16

>45C: 12.28

>50C: 8.90

SET 2

>25C: (preliminary test)

>30C: 24.80

>35C: 20.41

>40C: 14.31

>45C: 12.50

>50C: 9.14

AVERAGE

>25C:

>30C: 24.72

>35C: 20.58

>40C: 14.231

>45C: 12.39

>50C: 9.02

Analysis Of Results:

From my results, I notice that there is a considerable difference in the time for the cross to disappear from the lower temperatures to the higher temperatures. All of the times are pretty much the same for the two different recording indicating that the experiment was a pretty successful one. There are no anomalies and the average temperatures seem to descend, as the temperature gets higher. There is a decrease of almost 20 seconds from the starting temperature to the finishing one.
The results clearly show that the time for the cross to disappear decreases as the temperature increases. The temperatures were exactly 25, 30, 35, 40, 45 and 50.

Conclusion:

From my results, I have come to the conclusion that if the temperature of a solution is raised, so is the time for the reaction to occur. The cross disappeared more rapidly as the temperature rose and I think this was due to the increase of energy between the particles and an increase in energy between collisions that successfully passed the energy barrier. This released the sulphur quicker and in larger doses, which turned the solution cloudy and opaque, making the cross invisible. At the lower temperatures, the time for the cross to disappear was less because the particles did not have as much energy as they did at the higher temperatures. This meant that at these temperatures the particles are colliding with much more energy and the molecules were able to break the activation energy barrier and so the reaction that releases sulphur works and now releases more sulphur at a much quicker rate, increasing the rate at which the cross disappears. At the lower temperatures the particles have to break the barrier without the extra energy given by the additional heat and have to collide more which takes more time.

My prediction was that if you increase the temperature of a reaction, you decrease the time it takes to occur. From looking at my results, I can see that this hypothesis was correct as the time for the cross to disappear decreased as the temperature rose. My conclusion and my results match my prediction, and my results clearly show this. The particles were moving around with more energy; enough to break the energy barrier and for a reaction to occur as the temperature rose.

From looking at the graph the results can be seen to follow a trend, they roughly create a straight line but the result for 40 is not totally in this pattern and can be seen as an anomaly due to this. The anomaly was the result for 40 C it was 14.2 when to fit in with the line it should have been 16.2 so it was an anomaly by 2 seconds, which is quite inaccurate. This could have been due to human error or the background changing making it harder to see the reaction. My results were not entirely accurate as they only roughly followed the pattern. This could have been due to the measurements given so to stop this I could have weighed the liquids as this is an accurate method or have been extra careful when dealing out the measurements, for instance there could have been remnants at the bottom of the measuring cylinder or even the drops around it could have affected the results by their absence.

This graph illustrates the evidence drawn from the results, it shows as stated in my prediction that as the temperature rises, the time taken for the chemicals to react is decreased.

Evaluation:

I think that this experiment has gone very well. My results were of an accurate standard as I did each temperature twice and calculated an average from these times. There were no extreme anomalies but the result produced for 40 did not follow the trend as consistently as the other results. The rest of the results were accurate and this is shown by the similarity of the result for each experiment.
I think that I could have repeated my results more however for the reason that I feel although the experiment was repeated twice, the most accurate results in experiment can only come from constant repeatability which I did not display in my experiment. However, the time allocated did not allow me to do so and as an improvement of what I could do if I did the experiment again, I would repeat the results further for even more accurate and reliable results.

I feel that the experiment chosen carried out the investigation well and was suitable for the task produced to me. I believe that I could have improved the method by making it more specific. I stated clearly all the various methods I took in setting the experiment up but I could have been more specific to how all the apparatus was used and perhaps why I used the apparatus I did.

The evidence I have produced is very reliable due to the caution and care taken with measuring the acids and with the timing of the reaction. I think that despite the one slight anomaly the evidence I have formed is sufficient to create a firm conclusion as it was a minor anomaly and did not affect the shape of the results on the graph dramatically.

I believe that I did get a suitable range of results for this experiment. I recorded results from temperatures that ranged from 25C to 50C and I think that this is a very good range to see how temperature affects the rate of reaction. However, I feel that this also reduces the chance of knowing if anything changes if the temperature reaches a certain point. Perhaps doing the temperature even higher would enhance my results but this is also quite dangerous so I cannot really say that I could have improved the range of my results.

Some other areas in the experiment that I feel I could have improved on were factors like controlling the stopwatch and measuring the amount of sodium thiosulphate and hydrochloric acid. There is a lot of chance that human error would provide inaccuracies. However the inaccuracies due to them were negligible because I paid close attention to these during the experiment.

I could have changed the apparatus I chose to use to get more accurate results for instance I could have used a digital thermometer as it is harder to get an extremely precise reading from a regular thermometer. I could have used an electronic water bath rather than a Bunsen burner and water bath as this way I could have gained really accurate results, as I would have been able to tell that the temperature of the water was constant to the degree so there would have been no affect on the results from this area.

Overall, this investigation has been a very successful one. I feel my results and analysis have been as accurate and reliable as they could have been under the time allocated. However I feel with extra time, I could have repeated the experiment and made it even more accurate and adapted it to try other variables for example changing the concentration in addition to increasing the time, this would highlight any links between these tow factors. Also adding a catalyst could have speeded up the results so by using a catalyst in my experiment I could have compared the results produced when using a catalyst and when a catalyst is not present. These are the ways that I could expand the investigation.

Lucy Gould 17th