In order for KNO3 to be dissolved, it must come into contact with water molecules. The collision between the water molecules and KNO3 generates energy which, if strong enough allows the reaction to take place causing the KNO3 to dissolve.
What follows is the Maxwell-Boltzman distribution for molecular energies.
(See Diagram) On the x-axis, we have the ‘energy of molecules’ and on the y-axis we have the ‘number of particles’. The activation energy is marked from the point ‘Ea’. The area under the curve to the right of ‘Ea’ represents the number of particles with enough activation energy to dissolve in the water. With a lower temperature, the curve is shifted to the left. As a result, there are very few particles with enough activation energy to dissolve should the water and the KNO3 particles collide. At a higher temperature, the curve is shifted to the right. As a result, there are a greater number of particles with enough activation energy to dissolve in the occasion of the water and KNO3 particles colliding. Therefore we can see that increasing the temperature, increases the general energy of each particle in the form of kinetic energy; the
Cold Water Hot Water
Independent Variables
- The volume of water used
-
The time taken to allow the KNO3 to dissolve
- The duration of which I spend shaking the test tube
-
The mass of crystalline KNO3 used.
In this experiment, I must use an excess of KNO3 salt. An excess is required to ensure that all of the salt dissolves to form a solution at the saturation point.
To ensure that fair testing is maintained, I must ensure that only one independent variable is tested at any one time. In this experiment, I will be altering the independent variable of the temperature of the solution; all other variables must be kept constant to the highest degree of accuracy available in the lab.
Apparatus
- Test tube rack
- Five test tubes
- Five Plunges
- Potassium Nitrate
- Spatula
- Thermometer
- Stop Watch
- Scale (Electric)
-
Water Bath (at 40 oC, 60 oC and 80 oC)
-
Ice (for 4 oC)
-
Room at 20 oC
- Water
Method
- Collect all the required Apparatus.
- Fill up the five test tubes with 10ml of water, which are at five different temperatures.
- Weigh the Potassium Nitrate without the containers lid on.
-
Add one spoonful of potassium nitrate to the fist test tube, which has the lowest water temperature (4oC).
- Place a plunge on the test tube and shake for 1 minute.
- Check if the solution has become saturated this is where no more salt can be dissolved in the water and crystals form at the bottom of the test tube. If no crystals appear repeat steps 4 to 5until the solution becomes solute.
- Once the solution is saturated, re-weigh the potassium nitrate and work out the difference. By working out the difference this shows how much potassium nitrate was dissolved (Starting weight take away end weight equals difference).
Results Table
My Results
Class Results
Analysis
My Graph and table shows that the temperature of water does affect the solubility of potassium nitrate, as I said in my prediction, which was higher, the temperature, the more potassium nitrate will be dissolved. The main pattern in my graph was that the higher the water temperature is the more potassium nitrate is dissolved, At 80 oC more potassium nitrate dissolves because the warm water particles spread out letting the potassium ions getting into the gaps, then it did at 4 oC, when the KNO3 atoms came into contact with each other, there wasn’t enough thermal energy, which could be generated into kinetic energy to provide enough Activation energy for the KNO3 to be dissolved. In my experiment there were two sets of anomalous results, I do not know the reasons for these results they could be caused by human error or when the KNO3 was added to the water it decreased the water temperature giving me incorrect results.
Evaluation
The results that I acquired from this experiment are the kind of results, which I expected. In my opinion I think that the results, which I got where quite accurate, but there were some inaccurate, and I have highlighted these in my results table. The results which were anomalous which were compared to others have many reasons behind them. One of these reasons is that it is a scientific fact that potassium nitrate reduces the temperature of water when added so to solve this problem we could have kept an eye on the temperature more. Also we could have taken the temperature of the water while we were adding the potassium and when we were shaking the tubes we could have used a machine so the rigorousness of the shakes is almost the same.
Another reason for this was that as we would be adding potassium nitrate to the water the temperature would be decreasing, the reason for this is as I have written before, the reaction needs Activation energy and this energy is taken away from the warm water particles as heat energy and then, when the particles collide with KNO3 ions this is then converted into kinetic energy so if the energy is strong enough allows the reaction to take place causing the KNO3 to dissolve.
I think I should have repeated this experiment at least three times making sure that the results I gather are accurate or I could have worked out an average, or I could have repeated the experiment the results which were anomalous. I think I did get a suitable range because these are the results I predicted and to prove this I could plot them on my scatter diagram and they would fit in with the correlation of the points. There is a particular pattern in my graph because as I increase the water temperature the more potassium nitrate is being dissolved, so the line of best fit in my graph is a curve. There is only one of a possible many variables of the original question that I could investigate, if given time I would do the experiment the other way round, not testing the solubility of potassium nitrate in water, but how water evaporates in a solution of potassium nitrate and water.
