I decide to use these apparatus in my preliminary experiments:
-- 2 Cylinder
-- Flask
-- Stop watch
-- A paper marked with a cross
-- A pair of goggle
Here is my method:
Experiment – changing the concentration
I will pour out 50 cm³of sodium thiosulphate and 5cm³ of hydrochloric acid into two measuring cylinders and then pour them into a flask, which is set on a paper marked with a cross. At the same time, the stop watch will be started. As the clear mixture gradually turns cloudy and eventually the cross can not be seen from above, I will stop the stop watch, then record the time. I will repeat the experiment with different concentrations. (I will reduce the volume of sodium thiosulphate and put water instead of it in order to change the concentration of the reactant.) The whole procedure is then repeated.
Prediction:
I predict that as the concentration of the sodium thiosilphate decreases the rate of reaction will decrease. The graph which I will plot after doing the experiment will have positive correlation, and will be curved (as the decrease in rate of reaction will not be the same as the concentration is increased).
Safety – When I am doing the experiment, I have to wear an apron to protect the skin and clothing and a pair of goggles is also needed all the time. I have to do my experiment in a place where I can take some fair results. I should wash my hands after doing the experiment (because I have touched some chemical things).
These are the results that I took from the experiment. However, I found the results were too close to each other, so that the investigation would be worth less..
So I decide not to use the results in the table above. I change the method to dilute the concentration by subtracting 10 cm³ of sodium thiosulphate each time repeating the experiment.
I round the results of the time to two significant figures and so do the results of the average time. Because it will be easy for me to use these numbers to plot a graph.
I am going to calculate the rate of reaction and use my results which take from the experiment to plot two graphs in order to find out the relationship between the concentration and the rate of reaction.
I prefer to use the collision theory to explain how the reaction takes place. For a reaction to occur, the particles of reactants have to move closer or collide with each other. However, the collision between the particles does not mean the reaction will happen all the time. Because when the particles collide with each other, there would be barrier energy. If the particles have enough energy to overcome the barrier energy, they will react to each other after colliding. On the other hand, if there is not enough energy to overcome the barrier, the reaction won’t be happened after the collision. This minimum barrier energy that the particles have to overcome is called the activation energy (Ea). It is used to break the initial bonds. Different reactions have different activation energy. If the frequency of collisions is increase the rate of reaction will increase. However, the percent of successful collisions remains the same. The frequency of collisions can be changed by changing the concentration, surface area or temperature. Furthermore the impingement angle of colliding is also very important.
The five factors that affect the rate of reaction can be explained by according to the collision theory:
Temperature – [The process of a reaction can be described as: the energy which stores in the substance changes into heat energy (or others) and then releases or the heat energy (or others) changes into the energy which stores in the substance.] Different substances have different amount of energy. If the amount of energy of the reactants is higher than the amount energy of the resultants, some of the energy will change into heat energy and then releases during the reaction – this is called exothermic reaction. If the amount of energy of the reactants is lower than the amount of energy of the resultants, the reactants are needed to absorb some energy in order to turn to resultants – this is called endothermic reaction.
The reaction I am investigating is an exothermic reaction. An exothermic reaction usually needs activation energy-as the initial bonds are broken, the new stronger bonds are formed. So more energy is stored between the particles, these energy will changes into heat energy and gives out. Therefore, the temperature will have an average rise. When I heat up the mixture, the energy level of molecules in the reaction will be rose-so they will move quicker. [The kinetic theory])
When the temperature is increased, the particles all move quicker (they have more energy). They are going to have more collision as they get more energy. When the particles collide with enough energy, the reaction would be happened. At a higher temperature, the particles would store more energy and increase the frequency of the reaction.
Concentration – if the solution is made more concentrated, the particles of the reactants will collide more frequently. Because there would be more particles to react each other per unit volume and this makes the collision more likely to happen.
Pressure – increase the pressure in a gas means that the molecules would move closer to each other (squash up together), this cause the collision easier to occur. Therefore, if the pressure has been increased, the rate of reaction will increase.
Surface area – if a solid is broken into many little pieces (i.e. the surface area of the solid would increase), this means that the particles of the solution will have more particles of the solid to react with per unit volume. So the collision will happen more frequently.
Catalyst – a catalyst can neither increase the rate of collision (of the particles), nor can increase the average kinetic energy of the particles. But it can reduce the activation energy by providing different ways of reaction (the method of absorption: molecules stick onto the surface of a catalyst; and the formation of intermediate compounds). Therefore, the percentage of molecules which can overcome the minimum barrier energy increases, more reaction will be occurred and the rate of reaction would increase. This means that a low temperature can be used. A catalyst can also make the time that takes the reaction to take place shorter, but it cannot change the amount of the resultant of the reaction.
Therefore, the collision theory is useful to predict the rates of chemical reactions. The collision theory is based on the assumption that for a reaction to occur, it is necessary for the reacting particles to come together or collide with each other. So as more collisions occur, the rate of the reaction would increases. The rate of reaction depends on how frequently and how hard the reacting particles collide with each other – for a reaction to occur, the reacting particles have to collide hard enough ( with enough energy).
I predict that as I change the volume of NSO from 50cm³ to 10cm³, the time taken for the cross to vanish will be a lot longer. And the rate of reaction will gradually reduce. One of my graphs which I will plot after doing the experiment will have a smooth curve.
Through the results I have taken, I find that as the concentration of sodium thiosulphate reduces, the rate of reaction reduces.
Then I calculate the rate of each experiment with different concentrations by dividing one by time taken for the reaction to take place.
The rate of reaction is multiplied by 1000 – it is easy for me to use these numbers to plot the graph.
I draw a graph called time for reaction against concentration of sodium thiosulphate, which shows a smooth curve. And the graph proves my conclusion, because I can see that as the concentration increases, the time for the cross to disappear reduces.
Actually, this is because the rate of the reaction depends on the concentration of sodium thiosulphate. If the rate began to decrease, there must be more water adding into the solution instead of sodium thiosulphate. So there will be less sodium thiosulphate molecules in the solution and it will be difficult for sodium thiosulphate molecules and hydrochloric molecules to find each other and have a reaction. This means the reaction will take more time so that the rate would be reduced.
Here is a diagram which I assume what will be happened between molecules when the concentration reduces:
However, I cannot make a specific conclusion between the concentration and the rate of reaction by plotting this graph (which shows a curve). So I then draw a graph which is called volume of sodium thiosulphate against rate of reaction – there is a straight line pass through the origin and all the points on the graph are uniformly spread out around the straight line. I can see that as the volume of sodium thiosulphate increases, the rate of reaction increases. This means that the concentration is proportional to the rate of reaction. In other words, if I doubled the rate of reaction, the concentration would be doubled.
I think my results fit well to my prediction. Because I have predicted that as I dilute the concentration of sodium thiosulphate, the time takes for the cross to vanish gradually increases. And there would be a curve when I connect the points on the graph (time for reaction against concentration of thiosulphate) together. So I can say that all my results do support my prediction by looking at my results on the graph.
There are not any results which are anomalous – the points all fit well on my graph. It is the same as my prediction.
I think my results which took from the experiment were fairly accurate. I washed the flask and the cylinders each time after doing the experiment. I used the same solution each time doing the experiment and I also stopped the stop watch as soon as the cross had been vanished.
However, I realize that the values of the results of the time taken for the reaction in my first table are more than the ones which I took in the second table. This is because I did not do the experiment at the same day and the temperatures are different between the two days that I did the experiment – the temperature of the day which I took the results of the first table is lower than the temperature of the day which I took the results of the second table. If the temperature increases, the particles would collide more frequently, so the rate of reaction would be increases. That is why the same solution of the same concentration would have different time which took for the reaction to take place.
If I can do the experiment again, I will take more results than I did before in order to reduce the inevitable human errors. So when I average them, the results will be more fair and accurate. I think I should use five different cylinders for the five different concentrations – there would not be any liquid left in the cylinder, so that the results of the experiment would be more fair and accurate. And also, it is better for me to take all the results at the same day, so there would be less effect of the temperature.
If I can do other experiments to investigate the rate of reaction, I prefer to do the experiment by changing the temperature. I can change the temperature by heating the solution of sodium thiosulphate and the solution of hydrochloric acid at the same time. And let them react with each other when both of the solutions reach a particular degree. I will take the results of the time takes for the reaction. This time, I will use some more apparatus, such as beaker, thermometer, heatproof mat, Bunsen burner, a pair of tongs and gauze. Refer to the collision theory-as the temperature gradually increases, the rate of reaction would increase, I can predict my conclusion of this experiment will be: temperature is proportional to the rate of reaction. Although I do not actually do this experiment, I can relate it to the experiment that I did before by changing the concentration of the solution. I could find that as the concentration increased the rate of reaction increased by plotting the graphs. This fits the parlance in the collision theory. However, the collision theory tells the same about the temperature – as the temperature gats higher, more particles begin to collide, therefore the rate of reaction increase. So I assume that temperature is directly proportional to the rate of reaction.
