Catalysts Theses are chemicals, which are added in a reaction to increase the rate, which it occurs at. Yet they are not actually converted or consumed in the reaction. Specific catalysts are used for each type of reaction. They may often achieve the same product of yield as the normal reactants but with a lower temperature or pressure. Industry finds catalysts very useful and makes good use of them. Lower temperatures and pressure will always be favourable, economical, and safer.
I think the most important factor that affects the rate of the reaction is the concentration because an increase or decrease in the concentration of the reactant will have a huge affect on the rate at which it occurs. I also think that the temperature is another important factor to control.
For a chemical reaction to take place, some bonds in the reactants must be broken. The colliding particles must have enough energy to break these bonds. This minimum amount of energy is called the activation energy. Only the very fastest moving particles have enough energy to break bonds.
In gases, liquids and in a solution, the particles move at a range of speeds. Some are moving very slowly and others are moving very fast. To react, particles must collide with enough energy and in the correct orientation for bonds to be broken.
Prediction
I predict that the higher the concentration of Sodium Thiosulphate solution the faster it will react with the hydrochloric acid. Firstly for a reaction to take place the particles must collide and secondly collide with enough energy to break and make new bonds. When the sodium thiosulphate is concentrated there are more particles in a given volume, therefore there is a greater chance of the particles successfully colliding and reacting with the hydrochloric acid. The more collisions between particles in a given time, the faster the reaction will take place.
Below is the relevant equation: -
Sodium thiosulphate + hydrochloric acid sulphur + sulphur dioxide + sodium
chloride + water.
Na2S2O3 (aq) + 2HCl (aq) S (s) + S2O (g) + 2NaCl (aq) + H2O (L)
Preliminary
I drew a black cross using a black biro on a piece of supplied paper, and collected a stop watch, a conical flask, and three measuring cylinders. Throughout my experiment, I would be having a total concentration of 0.05dm3 , so as to ensure a fair test. In each experiment, there would be 10cm3 of 2 molar of HCl, as this is a fixed variable. I would have different amounts of 20g/dm3 Sodium Thiosulphate, and to make sure the concentration totalled 0.05dm3 , varying amounts of H2O. I mixed the different liquids with different volumes, and placed the conical flask containing them over the black cross. Looking directly above the cross, with my goggles on, I counted the time it took for the cross to disappear due to the opaque solution. My results are on the following page.
Our preliminary results illustrate a definite trend, that as the volume of Sodium Thiosulphate increases, the time take for the X to disappear decreases. This shows that the two are inversely proportional to each other. My graph shows that there is a more dramatic decreasing in time for the X to vanish when you add something between 0 and 20cm3 of Sodium Thiosulphate. When the sodium thiosulphate is concentrated there are more particles in a given volume, therefore there is a greater chance of the particles successfully colliding and reacting with the hydrochloric acid. The more collisions between particles in a given time, the faster the reaction will take place.
Main Investigation
Due to the availabilty of chemicals, I will be halving all the different volumes used in my preliminary, so the total concentration will be 0.025 dm3 Although I would have liked to do experiment repeats, in order to improve accuracy and help my conclusion, this is not possible due to the restricted use of the chemicals on offer.
Factors to control
1) To ensure that reliable results are gained form this experiment there are a number of factors which need to be controlled, these include:
2) Only changing the concentration of the reactant to the required measurements there for I will need to.
3) Keep the temperature of the substances the same. Changing the temperature could effect the rate at which the reaction occurs. Increasing the temperature increases the kinetic energy the molecules have, therefor increasing the chance of collisions between the particles. Decreasing the temperature would have the opposite effect.
4) The volume of the container needs to be kept the same. Increasing or decreasing would effect the rate at which the particles collide and react with each other. This is because the change in the volume of the container results in a change of the area the particles have to move.
5) Use the same marked piece of paper, which is positioned under the beaker because the shading of the cross may effect the rate at which it disappears. Darker crosses may be a lot easier to see through the solution than lighter shaded ones.
6) For each concentration, I will need to take the results three times and get an average of the time taken (in seconds) for the cross to disappear. This will give me more reliable results.
Conical flasks will be rinsed thoroughly before being reused, which we will do as there are limited conical flasks within the laboratory. Finally, we kept constant the person who watched the solution to see when they could no longer see the cross. This ensured that results would not differ due to differences in eyesight, and maintained the quality.
Method
I will place the conical flask containing the sodium thiosulphate and water onto a laminated diagram of a cross. The volumes of Na2S2O3 and H2O will vary depending on which experiment im doing.
My partner will then pour in the 5cm3 hydrochloric acid whilst I activate the stop clock.
My partner will watch the solution to see when the cross is no longer visible, and deactivate the stop clock.
When the reaction is complete, the results must be recorded in a table.
Whilst they are observing, I will prepare the other water and sodium thiosulphate solutions so as to make best use of the time available.
Analysis
Having successfully completed the investigation, I have come to the conclusion that, as stated in my prediction, the rate of the reaction between sodium thiosulphate and hydrochloric acid can be sped up by using a higher concentration of sodium thiosulphate, i.e. the higher the concentration, the quicker the reaction takes place. This is because, as previously illustrated with an annotated diagram, the higher the concentration the more particles there are which are available to react. As previously explained, not all particles have enough energy to be able to react, i.e. not enough activation energy. If there are more particles to begin with, there is more of a chance that a fair number of them will have enough activation energy to break bonds. For example, say 30% of particles in a high concentration of sodium thiosulphate had sufficient energy to react. That 30% is more than the 30% of particles in a lower concentration of sodium thiosulphate that has enough energy to react, as there are less particles in a lower concentration than in a higher concentration. Or, to put it another way, say 20% of particles had sufficient energy to break bonds, in any concentration of sodium thiosulphate. That 20% in a higher concentration of sodium thiosulphate would be made up of more particles than 20% of particles in a lower concentration of sodium thiosulphate, as in a lower concentration there were fewer particles to begin with.
So, a higher concentration of sodium thiosulphate will have more particles with sufficient activation energy in it, and therefore a higher concentration of the aforementioned chemical will have an increased rate of reaction when mixed with hydrochloric acid than a lower concentration of sodium thiosulphate would have in the same environment. My results support these conclusions: at a 20cm3 volume of Sodium Thiosulphate the time taken for X to disappear was 61 seconds, where as at a volume of 2.5cm3 the time taken for X to vanish was 411 seconds, 351 seconds more. I seem to have supported my prediction.
My graph demonstrates that the volume of Sodium Thiosulphate is inversely proportional to the time taken for X to vanish: as the volume increaes, the time taken decreases. The curve gets shallower and shallower but never touches the zero line. This is because the more dilute the acid becomes the longer it will take for the Sodium Thiosulphate Solution to react with the Hydrochloric acid. It showed that the concentration of the Sodium Thiosulphate Solution (when reacted with Hydrochloric acid) is proportional to the inverse square. At a volume of 10cm3 the time taken for X to disappear is 131 seconds, where as at a volume of 15cm3 the time taken for X to disappear is 81 seconds. My graph shows no anomalies, as all the points are remarkbly close to the line of best fit.
Evaluation
My experiment went according to plan but there were flaws in it. I think there is a human error factor involved when you are measuring liquids and looking for an end point in the reaction. Although the reaction I chose had a fairly definite end point it was still hard to tell whether the whole cross had disappeared or not. Instead of using a cross a light beam could be used and when the beam goes out that is the end point. A better standard of measuring cylinders and pipettes could be used.I feel that because my results have a certain amount of inconsistency between them it would be wise to repeat them again if I had time
The biggest factor which affected the rate of reaction in my experiment was the temperature. It was almost impossible to keep it constant, especially as the weather is changeable at the time of year in which the experiment was performed. This could be remedied by using some sort of thermos control, for example; a water bath. The temperature of a body of water can be kept constant, and easily checked. Also, the temperature can be recreated on consecutive days, so that doing the experiment over several days would not be a problem. Alternatively, the experiment could be completed all in one day. This would not only compensate for the changes in weather, and therefore temperature, it would also allow the possibility of any differences in the strength of hydrochloric acid and also sodium thiosulphate solutions that have been prepared for us to be declared irrelevant. The concentrations of these chemicals could differ slightly according to which batch they originated from, and this could affect the rate of the reaction.
Investigating the effect of temperature on reactions would have been something worth doing, as you can be very versatile with the temperatures and would have been less effected by the natural surroundings, which can cause mistakes. I would have used a Bunsen burner, a water bath and ice to get the temperatures. I think that if I had done this experiment then the higher the temperature then the faster the reaction would take place. I feel this would happen as the particles would have more energy and move around more, causing more collisions, therefore the reaction takes place faster.
The accuracy could be improved in a several ways. Wider containers would mean that the amount of solution would be 'thinner', and so it would take a considerable amount of precipitate until the cross was no longer visible, and vice versa. Therefore it is important that the width of the conical flask is maintained. Another way would be to change the method of the experiment completely. A light probe and data logger attached to a computer will give more accurate results than the human eye it would replace. It would act not only as an eye, but also a stop clock, measuring until there is zero light intensity getting through the solution. The downside to this method, however, is that the temperature of the bulb will eventually heat up the solution, and speed up the reaction
If I conducted the test again I would perform several repeats for each concentration, making the results more reliable. I would consider changing the concentration of the Hydrochloric acid and react it with Sodium Thiosulphate Solution, as it would be intriguing to see if the results were the same. I could use similar concentrations but at higher temperatures, and add this information on to the graph that I had already drawn in order to see any contrasts. Further experiments could involve the usage of other reactants and possibly catalysts.