For a simple over-all EXOTHERMIC reaction, the energy graph looks like this:
If the particles collide with less energy than the activation energy, nothing important happens. They bounce apart. You can think of the activation energy as a barrier to the reaction. Only those collisions which have energies equal to or greater than the activation energy result in a reaction.
The energy graph for an ENDOTHERMIC reaction looks like this:
The same principles can be used for the Endothermic reaction. If the particles collide with less energy than the activation energy, again, nothing important happens. Only those collisions which have energies equal to or greater than the activation energy result in a reaction.
Now, back to the factors that affect the rate of reaction:
Concentration
The concentration of the solutions will significantly affect the rate of reaction. This is because, increasing the concentration, increases the probability of collisions between reactant particles because there are more of them. I will choose the concentration of thiosulphate as the variable in my experiment because it is clear to me that concentration is a clear factor to the rate of reaction time and also it is relatively easy to handle and measure etc.
Temperature
It is well known that if you heat up a substance in a reaction, you will increase the speed of the reaction. This is because the heated particles will move around faster, and they will collide more often, therefore causing a speed up in the rate of reaction. Of course the collisions will only occur in a reaction if the particles collide with enough energy to start a reaction. This minimum level of energy required is called the Activation Energy, and has been explained previously.
Surface Area
The effect of surface area is a factor affecting the rate of reaction. If a solid piece of magnesium,that is a perfect cube shape, each side measuring 5mm, and the overall surface area shown is 150mm², was put into an acidic solution, then the reaction would be slower than if a magnesium cube of exactly the same mass was divided exactly in half, as the two new pieces of magnesiums overall surface area would equal 200mm². Therefore, the two smaller pieces have a larger surface area shown to the acid, so the acid will react with them quicker. However, with my experiment, the surface area will not affect my results as the two substances I am using are liquids and not solids.
Pressure
If one or more of the reactants are gaseous, (which neither of mine aren’t), then increasing pressure will effectively increase the concentration of the reactant molecules and therefore speed up the reaction.
Catalyst
The word catalyst means an added substance, in contact with the reactants, that speeds up the rate of reaction without itself being chemically changed in the end. A catalyst works by lowering the activation energy needed to start the reaction. Here are two graphs explaining the use of a catalyst on activation energy in an EXOTHERMIC reaction and an ENDOTHERMIC reaction.
They are widely used in different reactions in industries to increae the speed of reaction and therefore lower the cost of producing the reactions as the high temperatures and pressures needed, are decreased.
To ensure a fairer test I will keep the following possible variables, constant: The volume of hydrochloric acid will always be 5.00cm³. I will try and keep the temperature of the substances the same each time (room temperature). I will try my hardest to stop the watch at the correct time, but due to unavoidable human error, I can not ensure 100% accuracy every time.
Prediction
I predict that the higher the concentration of sodium thiosulphate, the quicker time it will take to react with the hydrochloric acid. This would hopefully be shown by the direct proportionality of the pattern of the graph that I will produce after the experiment. This prediction is justified by the theory below: -
When two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place. This is the collision theory. The two molecules will only react if they have enough energy. This is known as activation energy. Different factors (as stated above) affect the rate of reaction, but I will use the increase of concentration of the reactants. This will increase the frequency of collisions between the two reactants, therefore increasing the rate of reaction.
Apparatus
The apparatus I will use are as follows:
- A conical flask.
- A measuring cylinder.
- A laminated cross on paper.
- A stopwatch.
- 6 different concentrations of sodium thiosulphate solution – (0.01 mols/dm³, 0.03 mols/dm³, 0.06 mols/dm³, 0.09 mols/dm³, 0.12 mols/dm³ and 0.15 mols/dm³.)
- Hydrochloric acid.
Method
- I made sure all apparatus is clean and sterile to avoid contamination.
- I will setup the conical flask with the laminated cross on paper underneath it.
- I will the pour the 5.00cm³ of hydrochloric acid into the flask.
- I will then pour the selected concentration of thiosulphate into the conical flask with the hdrochloric acid and I will start the timer.
- I will look closely down into the flask and when the black cross has been completely obstructed by the cloudy precipitate, I will stop the stopwatch.
- I will record the length of time it took for the cross to become invisible in a results table.
- I will plot the results in a graph
Safety
I will make sure to wear goggles when conducting my experiment incase any substances get into my eyes. I will also wear an apron and gloves because the hydrochloric acid can damage my clothes and skin.
Results
Analysis
By conducting this experiment and by looking at the line of best fit in my first graph, I can clearly see that as the concentration of thiosulphate increases, the time in which it takes for the cloudy precipitate to obscure the cross decreases. I am pleased as the final results correspond to with my prediction. If I conduct two different experiments, one with two solutions that are not concentrated but react with each other, and one experiment with the same two solutions but one solution is concentrated with water, (bearing in mind that the two experiments both use the same volume of solution) I know that if two solutions are reacting, and one solution is diluted with water, then the reaction will take longer as there are less reactive atoms present to collide and cause a reaction.
Evaluation
I am very pleased with my experiment’s final results as they match my prediction. The results I got fitted in the line of best fit of my graph so again, I am pleased with my experiment.
If I were to do my experiment again, there are a few things I would consider. Due to a factor of human error, I cannot be exactly accurate when timing how long it takes for the cross to become invisible, or when measuring solutions. So next time, if I do this experiment again, I would use a burette to measure the solution as this instrument is a lot more accurate then the measuring cylinder. Also I would use a light beam to more accurately decide when the cloudy precipitate has completely covered the cross. To make the results even more accurate I would have used a wider range of concentrated solutions of thiosulphate. E.g. instead of only 0.01 mols/dm³, 0.03 mols/dm³, 0.06 mols/dm³, 0.09 mols/dm³, 0.12 mols/dm³, 0.15 mols/dm³, I could use from 0.01 mols/dm³, going up by .01 each time until 0.15 mols/dm³. This would ultimately give me a more accurate final result.
