VARIABLES
What Factors Are Likely To Affect Your Investigation? Why Are These Factors Important?
Volume of acid
Type of acid
Concentration of acid
Volume of Sodium Thiosulpahte
Concentration of Sodium Thiosulphate
Method of timing
Size of conical flask
These factors are important because they all affect the rate of reaction of the experiment.
What Will You Change?
I will change the concentration of the Sodium Thiosulpahte by diluting the 40g/dm³ Sodium Thiosulphate with distilled water to see how it affects the rate of reaction.
How Are You Going To Control The Other Factors?
I will control the other factors by using the same volume of the hydrochloric acid using a measuring cylinder. I will ensure the temperature is constant by carrying out the experiments at room temperature in a short time space e.g. 80 minutes. I will see use 1M HCL in each amount. I ensure then I use 40cm³ of Sodium Thiosulphate measured using a measuring cylinder. The same flask is used in each repetition, as the depth of the flask would affect the ability to see the cross.
What Observations/And Or Measurements Are You Going To Make And Over What Range? Will You Make Sure These Are As Accurate As Possible?
I will measure how long it takes for the Sodium Thiosulphate to completely cloud over when it reacts with 10cm³ hydrochloric acid, using a stopwatch. I will measure the concentration of the Sodium Thiosulphate using two 50cm³-measuring cylinders
(1 for Na S O and 1 for distilled H O) I will make sure the results are accurate by
² ² ³ ²
repeating the experiment two times and taking an average rate for each concentration. My range will be from 1M of Sodium Thiosulphate to 0.2M of Sodium Thiosulphate with 0.2M intervals.
RESULTS
What Will You Do With Your Results?
I will record the results in a table with an average time taken for the Sodium Thiosulpahte to cloud over. From this I shall work out the rate (1/time) of reaction of each concentration. I will then draw a line graph with concentration on the x axis and average rate on the y axis. I will draw a line of best fit and find the mathematical relationships between the concentration and rate.
PRATICAL PROCEDURE
Apparatus:
Conical Flask
Two 50cm³-measuring cylinders
Piece of paper with an X on it
Stopwatch
Chemicals:
50cm³ 40g/dm³ of Sodium Thiosulphate
10cm³ 1M of Hydrochloric Acid
Safety:
Wear safety glasses
Keep apparatus to side of bench
Clean spills immediately
Diagram:
Method:
Measure out 50cm³ 40g/dm³ of Sodium Thiosulphate using a clean and washed measuring cylinder. Place in a conical flask. Measure out 10cm³ 1M of Hydrochloric Acid using another clean measuring cylinder. Place a piece of paper with an X on it beneath the flask. Place the acid in the flask and start the stopwatch. Time how long it takes until you can no longer see the X on the paper. Record this result in your results table. Then clean out the measuring cylinders and measure out 40cm³ 40g/dm³ of Sodium Thiosulphate and place it in the flask. Use another clean measuring cylinder to measure out 10cm³ of distilled water and place it in flask. Measure out 10cm³ of Hydrochloric Acid and place it in the flask and start the stopwatch and record the time until you can no longer see the X.
Repeat the experiment using:
30cm³ of Sodium Thiosulphate and 20cm³ of distilled water
20cm³ of Sodium Thiosulphate and 30cm³ of distilled water
10³cm of Sodium Thiosulphate and 40cm³ of distilled water
Repeat the whole experiment two times to increase the accuracy of the results and work out the average for each concentration and plot a graph of rate against concentration.
INTERPRETING
How Can You Best Use And Present Your Results?
We can present our results in a results table and use them to calculate the rate (1/time). We can then use our results to draw a concentration against rate graph.
Can You See Any Clear Patterns?
We can see from the concentration against rate graph that as the concentration increases the rate of reaction increases, as there are more particles of reactants to collide and react. As the concentration increases beyond 0.8 moles the reaction increases greatly.
What Have You Found Out?
We have found out that the rate of reaction depends on the concentration of the reactants so the reaction rate decreases as the concentration of Sodium Thiosulphate decreases.
How Does Your Conclusion Fit With Your Prediction?
As I predicted the rate of reaction is directly proportional to the concentration of the Sodium Thiosulphate. The dilution results in less Sodium particles so the number of successful collisions with acid atoms decreases- thus decreasing the rate of reaction.
Try To Explain What Has Happened? (As scientifically as possible)
As the concentration of the sodium thiosulpahte decreased, the rate of reaction of the experiment decreased therefore the concentration is directly proportional to the rate (conc ∞ rate). This happened because in concentrated sodium thiosulpahte there are more sodium particles. This means there is a greater chance of a sodium particle hitting an acid atom with a sufficient amount of energy to force a reaction. The greater the amount of successful collisions the faster the rate of reaction. Therefore in dilute sodium thiosulpahte there are less sodium thiosulphate particles so the chance of a successful collision with an acid atom is smaller, so the rate of reaction of the experiment is slower.
EVALUATING
Do You Think Your Method Was The Best Way Of Doing The Investigation?
The method I used was a reliable method at this level. It was relatively easy to control the variables and to identify the end part of the reaction.
If You Did It Again, What Improvements Would You Make?
If I did the experiment again I would make a few improvements:
- I would use a colorimeter to give an exact measurement of the cloudiness of the solution.
- We would repeat each experiment more times to ensure it was as reliable as possible but to enable us to do this we would need a lot more time to carry out the investigation.
Were There Any Results That Did Not Fit With The Main Pattern? If So, Can You Explain Why?
There was one result at 8g/dm³ of Sodium Thiosulphate when we repeated it the second time the time was a sufficient amount less than the first time. This could have been because there was a fault in the timing or we possible diluted the solution too much.
Do You Think Your Results Are Good Enough To Convince Other People?
I believe that these results are accurate enough to convince other science students that my prediction was correct.
