Investigation of the rate of reaction of sodium thiosulphate with dilute hydrochloric acid.

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Yordan Agov 5th

Chemistry Coursework

Investigation of the rate of reaction of sodium thiosulphate with dilute hydrochloric acid

I am going to investigate the relationship between the rate of the reaction of sodium thiosulphate with hydrochloric acid. This is the ionic equation of the reaction:

2H+(aq) + S2O32-(aq) SO2(aq) + H2O(l) +S(s)

Dilute hydrochloric acid + Sodium thiosulphate + Sulphur dioxide +

Water + Sulphur

Scientific background

All mater made out of tiny particles. Particles, which are of the same matter, attract each other, but if these particles gain enough energy they can overcome this attraction and move freely.

This is the state at which particles of a certain matter have enough kinetic energy to stretch their intermolecular attractions to be able to move more freely. Elements or compounds in this state can fit and form different shapes. In this state, all elements and compounds are described as liquids. A meter squared of this matter in liquid form would also be lighter that the same volume of a solid of the same matter as it is less dense and has a lower concentration.

This is the state at which particles of a certain matter receive enough energy to completely overcome their intermolecular attraction. Therefore the particles of that matter are virtually not at all attracted to each other, they move independently in random directions. In such a state, elements or compounds can easily fit into different shapes. In this state, matters are also much easier to compress. Matters in such a state are described as gasses. A certain volume of gas would be lighter than that same volume of liquid or solid states of that matter as it is least dense and has the lowest concentration

This is the state at which particles of a certain matter hardly receive enough energy to separate from each other. They have low kinetic energy and so their intermolecular attraction keeps them close together. So, as they are close together, when they vibrate, they seem to vibrate at a set point. Since the particles are closely packed they stay at a certain shape and are extremely hard to compress. Matters at that state are described as solids. Their shape can be altered if the particles strongly clash into some other material, simply if you throw it into another hard solid.

At one temperature, some matters may be at solid, liquid or gas state, but that would depend on the molecular structure of these matters.

To investigate the rate of reaction I would need to understand the collision theory. For two particles, molecules, atoms or ions to react, they would firstly need to collide. However, these particles would need to collide with a certain energy level for the reaction to occur. This is because these particles would need to break the bonds or/and attractions, before they could make new different particles. To break those bonds and attractions they would need a certain minimum level of energy, called the activation energy. At room temperature (around 20oC), a minor collision can easily start up a chemical reaction of this sort. If particles collide and reach the activation energy that they would produce a fruitful collision, which means that they have collided with enough energy to break old bonds and form new ones, this is when a chemical reaction occurs. But if particles don’t reach the activation energy when they collide, than they wouldn’t be able to break old ones to form new ones and no chemical reaction would occur. This type of collision is described as an unfruitful collision. The diagram below explains this:

The rate of the reaction depends on many variable conditions such as pressure, concentration of solutions, temperature and size of particle (if the matter is a solid).

A greater concentration of the reactant means the more particles of the reactant per unit of volume. We talk about concentration when we use liquid solutions. If you originally had a flask filled with the reactant up to a certain volume, you can lower the concentration of the reactant in a solution by firstly putting a lower volume of the reactant into the flask and than filling it up to the fixed volume with another liquid like water. In liquids, particles have an extended intercellular attraction and so aren’t independent, however that doesn’t prevent them to move randomly in all directions (within their boundaries as there still is an intercellular attraction). Collisions between particles in liquids or gasses theoretically occur at random, therefore if you increase the number of particles in a defined volume there is a greater chance of particles between the two reactants colliding, so more collisions would occur, and vice versa, provided that one of the reactants is kept at a constant volume and concentration.

Pressure also affects the rate of the reaction. You could say that it doesn’t directly affect the rate of the reaction. Pressure actually affects concentration. If there is a higher pressure upon a liquid, than the liquid would be suppressed and become denser. This is because the particles are given less volume but keep their kinetic energy, which brings them closer to each other. So if they are closer, then they would be a higher chance of collisions occurring and so the rate of the reaction would be increased. It is also noticeable that one of the ways that catalysts ...

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Temperature could significantly affect the rate of the reaction. The temperature is a measurement of thermal energy. If there is more thermal energy in the room where the reaction is taking place, than the particles would be converting this energy into kinetic energy, and so they would be vibrating and moving more which would increase their chance of collisions in a certain amount of time.

If I was using a solid for an experiment as one of the reactants, that the rate of the reaction would vary with the surface area of that solid. The larger the surface area, the more particles of that solid are exposed, therefore the more collisions can occur in a fixed amount of time.

Preliminary work

I carried out a preliminary experiment to determine what equipment I should use, what sets of results I should take and also to consider safety. Also to consider variables and ways in which I would make sure that some of them are kept constant.

Method for preliminary experiment

First of all, I would have to choose what apparatus would be most accurate for the experiment. I should also consider safety. That I would have to choose what volume of dilute hydrochloric acid I would chose. As I am choosing what volume of hydrochloric acid I should also have in mind what maximum volume of sodium thiosulphate I would use. After I have decided the volumes of the solutions I would have to decide what range of results I would be taking, that would mean what the difference of volume of sodium thiosulphate would be between every reading that I take. I would also have to decide exactly at what point I should start and stop the chronometer. Knowing that I wouldn’t have as much time as I want to take my results I would also want to try to find and chose a range of results that wouldn’t take too long to record.

Basic apparatus used for preliminary experiment:

White tile with a black cross
Conical flask
A measuring cylinder
Chronometer

In the preliminary work, I would draw a black cross on the white tile. I would put the conical flask on the cross. All the chemical reactions are going to take place in the conical flask. I am going to put the volume of hydrochloric acid first and then the volume of sodium thiosulphate. I will only start off the stopwatch after I have poured the sodium thiosulphate completely. I am going to stop it when there is absolutely no sign of the cross on the tile.

Preliminary results:

*Took too long and I could still see the cross on the tile

Conclusion

I chose to start taking a range of results where I will be varying the concentration of sodium thiosulphate; I will change the volume of sodium thiosulphate in every reading by one millilitre in a range of 1ml. to 10ml. I chose to do so because I think that this range is quite sensible to the time that I got given to take results. Another reason for which I chose a range with such a small difference in every reading (of 1ml.) is because that way there would be a lower chance of anomalies.

Method for the experiment

I have chosen to use three simple flasks in which I will keep the hydrochloric acid, the sodium thiosulphate and the water. I would write on both of them with a permanent black marker not to mix them up as they are all colourless. I chose to use to measuring cylinders, one would be to measure the hydrochloric acid and the other one would be to measure the sodium thiosulphate and water, they would too be marked. I would also have tissues so that after every reading that I take, I wash the conical flask and then dry it up. I would also need to wear safety goggles and a lab coat because hydrochloric acid is obviously corrosive, and although it is diluted it would be nasty if some of it gets on cut or your eyes. Another reason for wearing those is that one of the products, sulphur dioxide is toxic and we shouldn’t have too much contact with it, we should make sure that it is poured down the sink in the fume cupboard after every reading.

Apparatus for experiment:

Conical flask
Black permanent marker pen
Three simple flasks
Two measuring cylinders
White tile (with a black cross on it)
Chronometer
Lab coat
Goggles
Tissues

To make it a fair test I would need to control certain variables. I will be changing the concentration of sodium thiosulphate by lowering its volume and adding water to it. In this process I would have to be very careful not to exceed the fixed volume of 10ml. vol. of hydrochloric acid and the fixed volume of 10ml. vol. of sodium thiosulphate and water when I am pouring them into the conical flask. If I exceed these limits than there would be no point to the experiment, especially if I change the concentration of sodium thiosulphate, because than I would not be using the scale and I will be getting random anomalies. The whole point of the experiment is to investigate the rate of reaction at different concentrations, therefore if the concentrations are not in the scale, than the results could not be used to form a conclusion.

Every time, on every trial of results the hydrochloric acid and the sodium thiosulphate are being changed for the whole class. It would be good if I could take a whole trial of results at once, surely getting my solutions from the same places. That way I would be sure that my whole set of first results could not have been affected by a different concentration of hydrochloric acid and sodium thiosulphate.

Pressure is a variable that could affect the concentration of the solutions but pressure virtually doesn’t change at all while I am taking my results. It is a variable that stays constant on its own and that I won’t need to control.

Temperature is also a variable that could affect the rate of the reaction and therefore it should be kept constant. However we cannot really control room temperature and so perhaps the best thing to do would be to try and take a whole trial of readings in at once when the room temperature would remain constant, because in another lesson the temperature may be different, which means that the results would be different, and so the trial wouldn’t be appropriate. A change in temperature in one trial will cause anomalies.

Prediction

Using my background information and the collision theory, I can predict that the rate of the reaction would be proportional to the concentration of sodium thiosulphate, provided that variables such as temperature and pressure. This is because, even if the concentration of particles of sodium thiosulphate is lower, a particle of a lower concentration of NaS2O32- would be colliding with the same amount of hydrogen ions as a particle of a higher concentration of sodium thiosulphate. This is because the difference in concentration doesn’t prevent or affect the certain freedom of the particles in the liquid solution, unless they are compressed. I would expect my graph of rate of reaction against concentration of sodium thiosulphate to be proportional, it would resemble the graph below:

Obtaining Evidence and Results

The anomalies in the table above are marked in red and are not included in the average time.

Analysis

I am encouraged by the graphs that my results are correct and accurate. I predicted that the concentration of sodium thiosulphate would be proportional to the rate of the reaction and as seen in the graph on the previous page, a line of best fit through the origin could be drawn. All of my results were in the range of two seconds, apart from three anomalies, which aren’t that far away. So I may conclude that the experiment over all went well. I had three trials of results for every experiment, which easily allowed me to identify the anomalies.

In my table of results, I have divided 1 by the average time in seconds it took for the reaction to occur. I did this because I already knew how long it took for the reaction to take place, but I wanted to know what was going on during that time, I wanted to know how fast the reaction was going at that time. Rate equals speed per unit time and rate is proportional to 1/time, which is what I was after when I was dividing one by the average time.

Using the graph I can conclude that the concentration of the reactant, in this case sodium thiosulphate, is proportional to the rate of the reaction. The rate of the reaction is the speed at which the reaction is going per unit of time. This statement is simply logical, because if you increase the concentration of the reactant in a fixed volume that would mean that there would be more reactant particles in the volume. As there are more particles, there are more collisions; more collisions per second (a second is a unit of time) would mean more particles reacting at the same time, which would explain a faster reaction. And so the rate is higher at a higher concentration. This explanation is also strongly supported by the collision theory.

In my prediction I stated that I would expect the concentration of sodium thiosulphate to be proportional to the rate of the reaction and I wrote a detailed definition of the collision theory, which states that for different materials to react, their particles would need to collide. If there are more particles of these materials in a fixed volume, a higher concentration, than they wouldn’t need search for each other to collide, which would again increase the rate as more particles would be colliding in the same amount time.

I have three anomalies overall in my experiment, they are marked in red in my results table. The possible factors for the occurrence of these anomalies could be temperature, concentration of the solutions, or my inaccuracy or error.

Evaluation

Although many things more could have gone wrong and given me anomalies, I can still say that my method was good as it gave me reliable results. I can conclude that it did so because by graph and my results stick to my scientific background and to my prediction. This method also gave me enough results to draw a conclusion; this is because I had decided to use a range of concentration from 0.1 to 1.0 mol/dm-3, which gave me enough time to take three ranges of results.

I had three anomalies in this experiment, two in the first trial and one in the second. They were all at low concentrations of sodium thiosulphate and weren’t really that far away from my other results.

There could not have been such a great change in temperature between the periods in which I took my results, nor are my anomalies too far away from my other results. Therefore my anomalies could have been caused by the change of temperature between the periods in which I took the results. However, the reason for which I do not support this possibility is that my anomalies are not in the same trial. I took a trial of results in each period, in which the temperature could not have changed. Since the temperature stayed constant during every period in which I took my results, and I took one trial of results every period, and my anomalies were in different trials, then I am less enthusiastic in believing that a change temperature is the cause for the anomalies.

A change in the concentration of liquids is a less considerable possibility as the hydrochloric acid and the sodium thiosulphate were taken from the same source on each try. But if someone has put water in the solutions than that could explain one of my anomalies: 107.26(s), on 3rd Trial for a concentration of 0.1 mol/dm-3 of sodium thiosulphate in 9ml. of water. It could explain this anomaly because I started off by taking the results of higher concentrations and finished by taking the one with the lowest concentration. So if one of the solutions got diluted with water than it would give me an anomaly. That could not have caused my other two anomalies because the result after them wasn’t an anomaly. If any of the solutions were diluted and I kept on using these solutions, then I would have been getting anomalies ever since I started using them. So obviously that wasn’t the case for my other anomalies. Even though the explanation above could be considered for one of my anomalies, if anyone poured any water in the solutions, then that person could not have put too much water in as my anomalies weren’t that far away from my results.

The most possible explanation for my anomalies is that they were caused by my inaccuracy. I had to measure the amounts of the solutions, and then I had to time the reaction. Since I kept doing that over and over again, it is highly possible that I went wrong somewhere. I do not think that I went wrong while measuring the volume of the liquids as I took a lot of care while doing so and when I messed it up I started over again. I personally think that I had problems with the chronometer and the timing. It is noticeable that my anomalies have occurred at lower concentrations of sodium thiosulphate. At lower concentrations, the reaction is slower. Therefore, when you look at the flask for the cross at low concentrations, the cross is slowly disappearing, and so it is very hard to differentiate when exactly the cross has fully disappeared and when to press the stop button of the chronometer. This helps me conclude that my anomalies most probably occurred due to my inaccuracy in timing.

Although I could draw the conclusion from this experiment that the concentration of sodium thiosulphate is proportional to the rate of the reaction of sodium thiosulphate and hydrochloric acid, there are ways in which this experiment could be improved to cancel out the few anomalies.

If more time was available I could simply continue with the same method and repeat the experiments in which I got anomalous results. But if that doesn’t work out and I keep on getting anomalous results than it would be smarter to use other ways and ideas to improve the method. That means that I would have to take in account the variation in concentration, change in temperature and also find ways to eliminate the chance of more errors caused by my inaccuracy.

To make sure that there is no change in the concentrations of the different solutions, a good idea could be to cover the flasks. If the flasks are covered, than there would be no risk of contamination or dilution with water. When I say flasks I mean the small flasks that I use as well as the bigger ones from which I get the solutions.

It would be good if there were a better way of measuring the volumes of the concentrations. If I were to continue using measuring cylinders, then I would have to use something more precise to pour the liquids into them. Pipettes would be very useful here. As in the experiment I used only two measuring cylinders, one for hydrochloric acid and one for sodium thiosulphate and water together, I had problems with the water and sodium thiosulphate as I quite so often passed the 10ml. fixed volume on the flask. Using pipettes would allow me to pour the liquids drop by drop when I need to and so makes it much harder to surpass the 10ml. fixed volume. Another way of fixing this problem would be to simply use syringes, they are very accurate in measuring the volume and they are easier to use. The best way to measure the volume would be to first take the solution with a syringe, then put it into the measuring cylinder and then add some more with a pipette.

As I do not want the reaction to affect the experiment, I would want it to be constant. I cannot control the room temperature, unless there is air conditioning in the laboratory, but I could control the temperature of the solutions. The best way to do that would be to use a water bath. You would put the solutions in the water bath and you would keep the water at a constant temperature. Doing so is also easier, because if you used something like a Bunsen burner, it would be very dangerous especially when heating hydrochloric acid.

Since I have problems with the timing of the reaction at low concentrations of sodium thiosulphate. Some sort of electronic device would help me to detect when the cross disappears completely. Perhaps something like a laser or a light bulb on top of the beaker and a receiver or a light sensor underneath the beaker would help. When the receiver shows that it doesn’t receive any more or the least possible light, than the light could not go through the residue. Therefore if light or laser could go through, than I would not have seen it with my own eye.