I decided which concentrations to use during my preliminary series of experiments they were:
1 molar of Hydrochloric
25g of sodium thiosulphate (all of these concentrations will be tested in turn going up in steps of 5g)
5 and 40g’s of sodium thiosulphate were available to me but my preliminary work showed that the 5 g/dm3 and 40g/dm3 were too slow and fast respectively in reacting to be worth testing.
Using my preliminary experiments I decided on using the following apparatus:
- 1 thermometer
- 1 beaker
- 2 measuring cylinders
- 1 conical flask
- 1 tripod
- 1 gauze
- 1 heatproof mat
- 1 stopclock
- 1 Bunsen burner
- X board
- 1 pair of tongs
- 1 pair of goggles
- 1 apron
Method
Experiment 1 - Changing the concentration
5 cm3 of HCl (at concentration 1 mol) and 15 cm3 of sodium thiosulphate (at varying concentrations 10 to 35 g/dm3) are poured out into two measuring cylinders and then poured into a conical flask, which is placed on top of a board marked with letter X. The stop clock will now be started. When the mixture has turned sufficiently cloudy so that the letter X can no longer be seen by the human eye the stopwatch will be stopped and the time will be recorded. The experiment is repeated with all the concentrations. The whole procedure is then repeated.
Repeated results and averages will be taken to improve the credibility of the findings, and present solid grounding for the final conclusion. The repeat results will help to iron out any anomalies and the average will give a good summary of the results of the experiment. However if one set of results is entirely different to the other, a third experiment will be performed to replace the anomalous set of results.
Safety
A pair of goggles will be worn during the experiment in order to protect the eyes against any chemicals that may splash. An apron will also be worn to protect the skin and clothing for the chemicals. A gauze and heatproof mat will be used while heating to avoid any damage to the equipment.
Fair Test
In order for my findings to be valid the experiment must be a fair one. I will use the same standard each time for judging when the X has disappeared. I will make sure that the measuring cylinders for the HCl and thiosulphate will not be mixed up. These precautions will make my final results more reliable and keep anomalies at a minimum so thus make the entire investigation more successful.
Prediction
I predict that as the concentration of the sodium thiosulphate increases the rate of reaction will increase. This means that the graphs drawn up in my analysis will have positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the concentration is increased. This can be justified by relating to the collision theory.
The Collision Theory
When the temperature is increased the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully. If solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur. 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. 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 frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.
Concentration - 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.
Pressure - If the pressure is increased the particles in the gas are pushed closer. This increases the concentration and thus the rate of reaction.
Surface Area - If a solid is powdered then there is a greater surface area available for a reaction, compared to the same mass of unpowdered solid. Only particles on the surface of the solid will be able to undergo collisions with the particles in a solution or gas.
Obtaining Evidence
Concen.(g) Time 1 (s) Time 2 (s) Average (s)
10 222.63 224.38 223.505
15 150.90 147.03 148.965
20 105.25 105.97 105.61
25 66.04 68.75 67.395
30 55.63 56.1 55.865
35 27.32 25.96 26.64
Analysis
In this experiment I have found that as the concentration is increased the time taken for the reaction to take place decreases. This means the rate of reaction increasers as it takes less time for a reaction to take place, so more take place. When solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur.
The graph for concentration shows that when the concentrations were relatively low (10, 15, 20 g/dm3), the increase of rate was also fairly small (increasing from 4.47 to 6.71 to 9.47). There was then a gradual increase in the difference, and between 30 and 35 g the rate more than doubled from 17.90 to 37.56s-1. This shows that there are far more collisions at a concentration of 35 g than at 30 g.
For this to fully make sense it is necessary to recap the collision theory briefly: 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 frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.
