Below is a typical energy diagram, displaying the activation energy:
Changing Certain Factors
Temperature – The Collision Theory states that as temperature increases, so does the rate of reaction. This is because the reactant particles have more energy, which causes them to move faster. This causes more collisions with energy greater than the activation energy, causing the rate to increase.
Surface Area/Particle Size – As surface area increases/particle size decreases, the rate increases. This is because there are more solid particles or total surface area exposed, so there will be more frequent collisions, thus the rate increases.
Catalysts – These are chemicals which provide a different route for the reaction, with a lower activation energy. Therefore, more particles have sufficient energy to react, causing the rate to increase. The mass and chemical form of the catalyst remains unchanged at the end of the reaction.
Concentration – As the concentration of an acid increases, the rate also increases. This is because there are more reactant particles, so there will be more collisions between the reactants. Therefore, more reactions will occur, causing the rate to increase. This is the factor I shall be investigating.
Below are diagrams explaining how changing the concentration affects the rate of reaction:
Method
The experiments involved in this investigation are very simple. The solutions I will be using are 0.25M sodium thiosulphate (Na2S2O3) and 0.25M hydrochloric acid (HCl). I will draw a cross on a piece of paper, and place it underneath a conical flask containing the HCl. I will then add the Na2S2O3 to the conical flask and swirl the flask to mix the solutions. This will produce a pale yellow precipitate of sulphur. I will also start to time how long it takes for the solution to turn cloudy enough to obscure the cross, using a stop-clock. I will use five different solutions, each with a total volume of 60cm3. I will keep the volume of HCl constant, at 10cm3. Therefore I will increase the Na2S2O3 by 10cm3 each time. The temperature must remain the same. Below is a diagram of the apparatus:
Preliminary Experiment
I decided to carry out a preliminary experiment to help me choose the best variables for my main investigation. I did four tests in all.
For the first three tests, I used a total volume of 50cm3, however I changed this volume to 60cm3, because the rate was slightly faster than I wanted it to be. The concentration of HCl was at a constant of 10cm3. The variables that I would change therefore, are the concentration of Na2S2O3 and the amount of water. 60cm3 was overall a better total volume than 50cm3, because it allows me to increase the volume of Na2S2O3 by 10cm3 each time.
I wanted the reactions to last between thirty and three hundred seconds, as this would enable me to take sensible measurements and make more accurate results. The cross must remain the same throughout the investigation, and the person watching the cross obscure must remain the same also, as eyesight varies from person to person. Here are the results of the preliminary experiment:
Prediction
I predict that as I increase the concentration of sodium thiosulphate, the solution will become cloudy, faster (as it reacts faster). I am expecting this to happen from my preliminary results and the Collision Theory. My preliminary results were fair, as there was no temperature increase in any of the tests. These results also indicate that my results from my main investigation will be between thirty and three hundred seconds.
Safety
Safety goggles must be worn, as hydrochloric acid is harmful if it comes into contact with your eyes. Any splashes of this acid must be washed off the skin, as it is also an irritant.
Obtaining Evidence
I conducted the main investigation, taking repeats to improve their reliability and avoid anomalies. I obtained these results:
To calculate the rate and relative rate of reaction, I have used the following methods:
Concentration of HCl = concentration x volume
= 0.25 x 10/1000 = 0.0025 moles
Concentration of acid at the start of the experiment =
Moles of acid / total volume = 0.0025/0.06 = 0.042
Concentration of Na2S2O3 = initial volume/total volume x 0.25
e.g. 20/60 x 0.25 = 0.83
Rate = 1/time
e.g. 1/99 = 0.010
Relative Rate = Rate/slowest rate
e.g. 0.004/0.004 = 1
I have tabulated the results from these calculations:
I have plotted a graph showing the relative rate as the concentration of sodium thiosulphate increases. From the line of best fit, I can see that they are directly proportional. Therefore my prediction appears to be correct.
Extension Experiment
For my extension experiment, I decided to test how absorbing