The effect of concentration on the rate of reaction between sodium thiosulphate and dilute hydrochloric acid

The effect of concentration on the rate of reaction between

sodium thiosulphate and dilute hydrochloric acid

This investigation is about rates of reaction and what affects them. In this case I am going to look at hydrochloric acid and sodium thiosulphate which is a precipitation reaction causing the solution to go ‘cloudy’. They react as in the equations below:

sodium thiosulphate + hydrochloric acid -> sodium chloride + sulphur + sulphur dioxide + water

Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + S(s) + SO2(g) + H2O(l)

A reaction will only occur where the particles of the reactants meet and combine. This is called the 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 rate of reaction can be achieved by increasing the frequency of collisions. Therefore to increase the rate of reaction it is necessary to cause more particles to collide harder and collide more often. There are several ways to do this and these make up the factors for this experiment. They are listed below along with predictions as to their affect on the reaction.

Possible Factors

To make sure I carry out a fair test I will only change the concentration of sodium thiosulphate. I will keep the following factors the same.

Increasing the pressure. By reducing the volume in which the same amount of particles exist the pressure is increased. Once the same number of particles are in a smaller area there is less space in which to move and so the particles are more likely to hit each other. It is therefore possible to predict that increasing the pressure will result in an increase in the rate of reaction. I will not test this variable because we don't have the facilities to test it. However pressure is a continuous variable.

Catalyst. A catalyst is a separate substance that speeds up a reaction. After the reaction has happened it gets left behind. This makes this variable unsuitable for the type of experiment I am going to do.

Temperature. By giving the particles extra energy they will move faster. This means that they cover more ground and are therefore more likely to hit each other, which in turn makes the reaction faster. The best way to give energy to a particle is as heat and so I can predict that raising temperature will make a greater proportion collide with minimum energy, which is needed to break bonds in the reactants. This is a continuous, independent variable. There is also exothermic temperature to consider. This is something I have noticed in my preliminary work . Heat is given out by the reaction, which will give more energy to the particles and cause them to reach their activation energy. Below is some evidence I obtained in a previous experiment, when investigating the rate of reaction between thio, HCl and water when changing the temperature only, which shows the affect that temperature can have on the rate of the reaction.

From these results I can say that when the temperature is increased the rate of reaction increases. It is a very important factor in this experiment. Varying the temperature in my experiment would greatly affect the rate of reaction this is why I will keep it constant.

Concentration. Just as increasing the pressure will increase the number of particles colliding, so will the concentration. By putting more particles into the reaction, the chance of them colliding increases (because of the crowding of particles) and so the rate increases. This variable is continuous and independent. I shall test this variable. I ...

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Surface area. Particles can only collide when the two sorts can meet. Therefore a reaction can only occur on the surface of the material. Therefore by increasing the area of the material, which is available to collide, the speed of the reaction will increase. I predict that doubling the surface area will double the speed of the reaction. This variable is continuous but I shall not test it because it is hard to control the exact surface area of the two reactants as they both come in an aqueous solution.

Chosen Factor

I am going to test the variable concentration. This is an independent, continuous variable. The reaction is exothermic as I found out from previous experiments (also see exothermic reaction table). Therefore even while I am testing concentration, heat will be given out by the reaction, which will give more energy to the particles and so cause them to reach their activation energy sooner. I will change the concentration of sodium thiosulphate only. The concentration of hydrochloric acid will remain constant at 2.00 mol/dm³.

Dilute solution Concentrated solution

Less frequent collisions More frequent collisions

Fair Test

The variable I am testing is the concentration of the sodium thiosulphate, therefore the only thing that I should be changing in each experiment is the concentration of the thio and nothing else. This will make all of the tests identical (apart from the thio’s concentration), which means the experiment should be accurate and fair. Because of this any differences in the results of my experiment will enable me to see that it is due to the concentration of the thio. If I was to use two variables, and there was a difference in the results, e.g. I changed the temperature as well as the concentration and the rate of reaction sped up, I would not know which of the factors had caused this and therefore I could not reach a valid conclusion. Also to make this test fair I must keep the concentration of the hydrochloric acid constant because if it were changed it would alter the rate of reaction greatly. I will use hydrochloric acid that has a concentration of 2.00 mol/dm³ each time. Another thing that may alter the rate of the reaction is the room temperature. The greater the temperature the quicker the rate of reaction. I will monitor the room temperature on each test and if there are any anomalous results I can check to see if it has anything to do with the rate of reaction. I must also make sure that on every test I stop the stopwatch at the same degree of cloudiness to show when the reaction has finished. This is the only major unreliability in the experiment because I cannot be certain if the solution is exactly the same degree of cloudiness. I must try and be as accurate as possible.

Prediction

When sodium thiosulphate and hydrochloric are mixed, a yellow precipitate of sulphur is produced. The solution becomes increasingly cloudy and difficult to see through as more and more sulphur is formed. I predict that as I increase the concentration of sodium thiosulphate the time taken for the solution to go cloudy will decrease this is because if the concentration of a solution is increased there are more reactant particles per unit volume. This increases the probability of reactant particles colliding with each other meaning the rate of reaction will be quicker.

Plan

Apparatus

Pipette, burette, measuring cylinder, sodium thiosulphate (0.01 mol/dm³), dilute hydrochloric acid (2.00 mol/dm³), distilled water, stopwatch, conical flask, paper with cross marked on, goggles.

Method

I will measure out the required volumes of sodium thiosulphate and water (see measurement table) using a burette. I will transfer these to the flask . I will then stand the flask on a piece of paper with a cross marked on it with a pen or pencil. I will measure out the acid using a pipette, as this is highly accurate for measuring, and add it to the flask at the same time I start the stopwatch. I will then record the time for the cross to be no longer visible. I will repeat the whole procedure for more reliable and average results. If after plotting a graph I may come across some anomalous results. I will repeat the certain amounts and see what results I get the second time. If the results fit in better with the line of best fit this will tell me that a mistake was made in the first set which affected the rate of reaction.

Measurement table

Diagram

Conical flask

Thio, HCl, water solution

Cross marked on paper

Predicted graphs

Graph 1 Graph 2

time 1/t

conc. conc.

The time taken for the cross to As the concentration increases disappear decreases as the concentration time decreases increases Inverse proportionality conc = 1 time

Graph 1 – This graph shows the time taken for the cross to disappear. As we increase the concentration of thio the time taken for the cross to disappear will decrease. The graph is a curved line. It is at a slope and the gradient decreases. The graph is a curved line because the gradient which shows that the rate of reaction is changing. This graph is inverse proportionally as when the concentration is increased the time decreases.

Graph 2 – 1/t is the inverse of time and it shows the rate of the reaction. From this graph we can see that as the concentration of the thio increases the time decreases. From this graph I can predict that the concentration of thio 1/time

Preliminary results

From my preliminary results I found out that the method I used was reliable enough because looking at my preliminary graph I can see that the line of best fit is like my predicted graph. I will not alter my method in any way for my proper experiment because it will help me to get successful results such as those in the preliminary work.

Experiment Results

TEST 1

TEST 2

Exothermic Reaction Results

The results above prove that there is an exothermic reaction occurring because the temperature of the solution before the reaction is less than after the reaction by 2 degrees each time. This means that the rate of reaction will be increasing when the reaction is happening because the temperature will be increasing. However, this does not make the experiment unfair in any way because it happens for each individual test.

Conclusion

After a while I noticed the solution in the flask started to turn cloudy. This indicated that the reaction between the sodium thiosulphate and the hydrochloric acid had taken place and sulphur had been created. I realised that as I increased the concentration of the thiosulphate, the reaction was a lot faster and the solution became cloudy quicker.

I can prove this is correct by referring to my graphs which show that when the concentration of thiosulphate is increased the time for the reaction to take place is less. This is due to the particles in the solution becoming crowded together. As the concentration increases, more particles are packed into a given volume and the percentage of successful collisions increases resulting in a faster rate of reaction. I noticed that the shape of the two graphs are like those I had predicted.

I believe the results I have collected are quite reliable and seem to support my prediction. The main prediction I made was that as I increased the concentration of thiosulphate the time taken for the solution to go cloudy would decrease. I can safely say that this is true because my results and graphs confirm this theory and show the effects of increasing the concentration very clearly.

I have realised that temperature is a very important factor of this experiment and that a slight adjustment to the temperature can easily alter the rate of the reaction. Unfortunately I was not able to control the room temperature and make it the same each test due to some tests being on different days to others which possibly quickened or slowed the rate of reaction. However I monitored the temperature as well as I could and my results seemed reliable.

Evaluation

Overall I think my results have proved to be reliable and I have not encountered any major problems which might have caused the experiment to go wrong. I can confirm the reliability of the experiment by referring to the similarity of between each set of results and also the fact that the shape of the graph matches that of my predicted graph. However I did come across a couple of anomalous results. These anomalous results might have occurred because of slight mistakes in small things like timing or measuring. This shows that even minor errors can cause unreliable results. Fortunately I was able to repeat these results and they seemed to fit in better with the line of best fit.

I think my graphs have given me reliable evidence to support my predictions. I think the apparatus I used was adequate for such an experiment although there are some improvements that could be made to guarantee more accurate results. One example would be judging the cloudiness of the solution. It all depends on your eyesight whether or not you think the cross has disappeared and with different people doing the same experiment, the results are going to vary slightly depending on how good or poor your vision is.

One way of getting round this situation is to use a device which detects the cloudiness of the solution. The device can be set up as follows: firstly a beam of light is directed through the solution and on to a photocell at the other side. The photocell is linked to a voltmeter. The solution starts to get cloudy as the reaction takes place. When the solution gets so cloudy that the beam of light no longer reaches the photocell the voltmeter stops reading the voltage and a timer stops automatically. Using such a device enables you to get more accurate and fair results because the elapsed time will be recorded at the same degree of cloudiness for each test.

I think the range of results I had was satisfactory, although maybe a bigger range of measurements would make it more reliable. However I don’t think this was necessary when looking at my graph, because the range of results I obtained clearly indicates the effects of concentration on the rate of reaction.

Apart from two anomalies, which were repeated and confirmed , my experiment was generally accurate. I can confirm this is true because the line of best fit on both my graphs is similar to that on the predicted graph which demonstrates accuracy. I believe it was a fair test. I gathered enough evidence to support my prediction and therefore can say that my experiment went well.