The Presence of a Catalyst
A catalyst is a substance that alters the speed of a reaction, without being used up. It works by giving the reacting particles a surface to attach to and collide into one another. The presence of a catalyst will increase the rate of a reaction because of this ability to increase the particle collisions without being changed or wasted.
Surface Area of the Reactants
Increasing the surface area of a solid reactant will speed up a reaction. This is because there will be more area available for molecule collisions.
Original Solid Reactant Crushed Solid Reactant
Available area for collisions Increased available surface area
Colliding Particle
Concentration of the Reactants
Increasing the concentration of the reactants can increase the rate at which a reaction will occur because a highly concentrated solution will contain more particles of the relevant chemical and this will result in more collisions.
Low Concentration High Concentration
Colliding Particles Colliding Particles
I am going to study how temperature affects the rate of a chemical reaction.
I shall be using the reaction between sodium thiosulphate and hydrochloric acid.
Hypothosis
I think that as temperature increases, the rate of the chemical reaction between the sodium thiosulphate and the hydrochloric acid will increase in proportion (therefore reducing the reaction time), as shown by the Kinetic Theory in ‘Background Knowledge’. I think that as the particles in the reactants are heated, their energy will increase, in turn increasing the amount of collisions that break chemical bonds. As this is the action that creates a chemical reaction, it makes sense that it will increase.
Apparatus
25cm³ measuring cylinder, 250ml beaker, 10cm³ measuring cylinder, black pen, white paper, Bunsen burner, thermometer, stop clock, heatproof mat, gauze, metal tongs.
Chemicals: 25cm³ thiosulphate solution, 25cm³ water, 5cm³ hydrochloric acid.
Detailed Plan
25cm³ of thiosulphate solution will be measured using a 25cm³ measuring cylinder and then poured into a 250ml beaker. 25cm³ of cold tap water will be measured in a clean 25cm³-measuring cylinder and added to the 250ml beaker containing the thiosulphate solution. 5cm³ of hydrochloric acid will then be measured using a 10cm³-measuring cylinder. Then, a small black cross will be drawn on a piece of white paper, so that it can clearly be seen. The thiosulphate and cold tap water solution in the 250ml beaker will then be heated by the Bunsen burner, turned to a small, blue flame.
It will be heated until it reaches 50ºC (the solution must be heated to 5ºC below the intended final temperature is reached, to allow for additional increases in temperature once the solution has been removed from the direct heat of the Bunsen burner.) This temperature will be found using the thermometer. The solution will be observed until it reaches the first chosen temperature of 55ºC. The exact temperature will be noted.
Then, the white paper with the black cross will be placed underneath the thiosulphate and tap water solution in the 250ml beaker, with the aid of metal tongs. The 5cm³ hydrochloric acid in the 10cm-measuring cylinder will be added. Immediately, the stop clock will be started and the black cross viewed closely until it is completely obscured by the solution. The stop clock will then be stopped and the time it gives in seconds noted in a results table. The 250ml beaker containing the thiosulphate, hydrochloric acid and tap water solution will then be emptied into a sink and washed thoroughly in cold, clean water.
The experiment will be repeated with the following temperatures:
25º, 30º, 35º, 40º, 45º, 50º, and 55ºC.
The whole experiment will be repeated twice for accuracy.
The results will be recorded in a table as shown:
Results
1st Results
2nd Results
Average results Table completed from the two repeats of the experiment.
I shall now put these results into a graph.
Conclusion
I conclude that as temperature increases, the time taken for the reaction to occur between Sodium Thiosulphate and Hydrochloric acid decreases. This means that the rate of reaction is increasing, because it is taking less time for the same amount of energy to build in the particles, collide and break chemical bonds resulting in the reaction.
These results support my original hypothesis.
In addition, I can see from my graph that temperature and time are roughly proportional to each other. E.g. As time increases, temperature decreases at the same rate.
Evaluation
I think that the evidence presented was reliable because it was repeated twice to ensure that anomalies in the results were eliminated and that the average result wasn’t skewed. I can assume that these results are correct because I used an average of the results to create a graph, and took my subsequent conclusions from that.
I think that the experiment was accurate because the same equipment and chemicals were used for the two repeats. There may have been a slight error in measuring the chemicals using the 25cm³ measuring cylinder or the timer not being stopped quick enough due to human error. This however, can be overlooked as if any discrepancies have occurred; they haven’t affected the final results to a relevant degree.
There could have been improvements to my experiment, such as using timers that are more accurate.
As I repeated the experiment twice, I think that the results are capable of supporting the conclusion. This is because the results have come from a fair and careful experiment and support what was said in ‘Background Knowledge’ – as temperature increased, the rate of reaction did decrease.
Other Experiments that could be completed on this Topic are:
- How surface area affects the rate of a reaction
- How the presence of a catalyst affects the rate of a reaction
- How the concentration of a solution affects the rate of a reaction
