Prediction
My prediction for the rate of reaction is
‘The greater the concentration the faster the rate of reaction’.
I hypothesise this because of the collision theory and my preliminary work. The greater the concentration the more particles there will be, causing a greater chance of a collision therefore, increasing the rate of reaction. Two chemicals react when their particles collide with each other with sufficient energy for the reaction to take place. The two particles will only react if they have enough energy. If the concentration is reduced the reaction rate will also be reduced, this is because there will be less particles to collide with each other to a cause a reaction. If the temperature is increased the reaction rate will increase as this will provide an optimum temperature for the catalysts to work therefore causing the reaction rate to increase.
Fair Testing
There are many variables I must consider if I want my investigation to fair. I will make sure I change only one variable and keep the others as controlled as possible. As I am measuring one variable, changing two variables will make it unsure of which of the variables is causing the effect I am seeing. One variable I will keep the same is the concentration of hydrochloric acid. The variable I will change is the concentration of sodium thiosulphate. I will also make sure that:
I will make sure that I use the same flask because if I use a different flask then that flask may not be the same size; shape, volume and it may not have the same transparency. All these factors might change my results, making my investigation to be unfair.
It is always the same person recording the time for the X to disappear because other people have a better or worse eyesight and judgement. This will not give me accurate readings because different people have different reaction times. To keep this variable constant, I will use the same observer.
The cross is the same. For example, if the cross was not the same throughout the investigation then this might affect the person judgement because another cross might be darker or thicker. This will affect the time for the X to disappear.
The flask is washed every time I carry out a new concentration because the flask tends to go cloudy which might affect the person’s judgement who recording the time, which may change the rate of reaction.
I do not stir or touch the flask while the reaction is taking place because that might affect the rate of reaction. If you touch the flask this may cause the particles to collide more, speeding up the reaction.
I will always stay the same distance away from the flask; else it wouldn’t be fair, as standing closer would make it easier to see changing my reaction time.
Temperature is a variable that has to be controlled because if you increase it, the particles will obtain more energy and this will cause them to move faster, therefore increasing the amount of collisions. It also causes there to be more successful collisions due to the sufficient energy. Increasing the temperature increases the chances of the reactants reacting. Here are two diagrams to help understand what happens to the particles when you increase the temperature.
Theory
The Collision theory is a chemical reaction which, can only occur between particles when they collide.
Particles may be atoms, ions or molecules. There is a minimum amount of energy that colliding particles need in order to react with each other. If the colliding particles have less than this minimum energy then they just bounce off each other and no reaction occurs.
This minimum energy is called the activation energy. The faster the particles are going, the more energy they have. Fast moving particles are more likely to react when they collide. You can make particles move more quickly by heating them up (raising the temperature). In order for it to be a fair test, the temperature will have to remain constant, the light will also have to remain constant as the light could affect the readings. The less concentration, the longer it would take in seconds for the X marked paper to be no longer visible. Vice-versa there a no catalysts in this experiment. Catalysts speed up reactions, by increasing the surface area. This could make the test unfair as the catalyst may speed up the reaction in one reading more/less than in another.
Results
Below is a table of my results which I recorded during my experiment. As you can see I did the experiment three times with the same molar to get an accurate result.
Table 1 - 0.25 Molar
Table 2 - 0.50 Molar
Table 3 - 0.75 Molar
Table 4 - 1.00 Molar
Table 5 - 1.25 Molar
Table 6- Average table
Chart 1 – Average Graph
Analysis
My graph shows me that the more concentration of sodium thiosulphate used the faster the reaction time is. It shows that more particles are colliding therefore increasing the reaction time.
The line of best fit shows a negative correlation which shows a decrease in the time it takes to react. This is because of the higher concentration of sodium thiosulphate.
My hypothesis
‘The greater the concentration the faster the rate of reaction’.
This concords with my hypothesis as it shows that the higher concentration of sodium thiosulphate used the faster the rate of reaction is, this is because it is reacting with hydrochloric acid because there are more particles reacting.
I can explain my prediction and results scientifically because I know that the process that causes the reaction rate to be higher is the collision theory which explains the cause why two particles collide with each other. The higher the concentration the more particles there will be involved causing a higher reaction rate through the collisions of these particle. My results illustrate that the temperature remained around the same throughout the experiment however I had no control over this and it could have caused an error but no anomalous results were found during the experiment. There was little difference between the first set of results and the second set of results which shows that both experiments were successful. I conducted two experiments to get accurate results and then worked out the average results. We used a stopwatch to check how long it took until the x marked on the paper was no longer visible, this indicated how long it took for a reaction to take place.
Conclusion
The experiment was very successful. My results seem consistent and accurate as the results table and graph show the effect of concentration on a reaction rate. They show the more concentration used the less time it takes for a reaction to take place. All my results fit a pattern. My results seem to be reliable however there were some anomalies. The stop watch has an error of (+ or -) 0.005 seconds however that was the only way we could record the time.
The steps that I would take to make sure that my practical was not affected by the factors above would be using a different stop watch or undertaking the experiment in a controlled room temperature.
The further work that I would do would be, to add more concentration of sodium thiosulphate to see if the pattern remains constant.