In my experiment I will use sodium thiosulphate and hydrochloric acid. When they react, they will create a cloudy liquid. I can use this fact to test the effect of concentration on the rate of reaction.
In the experiment, I have a range of molars of acid available to me. I will test 0.5, 1 and 2 molars and observe which will give me the best result.
I must also decide on an amount of thiosulphate to use. I could either use a range of 50 going down in tens or 25 going down in fives.
I need to conduct a trial experiment in order to decide these two factors:
Method for trials
Take three different molars of acid; 0.5, 1 and 2.
Measure 5cm3 of 0.5 molar hydrochloric acid and 25cm3 of sodium thiosulphate.
Place the conical flask on top of a piece of paper with an 'x' drawn on it.
Pour in the hydrochloric acid and sodium thiosulphate.
Swirl the flask gently and start the timer
Observe the amount of time taken for 'x' to disappear.
Record times in a table of results.
Results
Conclusion
From my results I can see that 0.5 molar acid reacted the slowest. I will use 2 molar acid in my actual experiment because it gave me the best range of results in the fastest time.
I will also use 50cm3 of sodium thiosulphate. This is because if I use above 50 the results will not be affected anyway and so a substantial amount of thiosulphate is not obtainable, due to limitations on the school's resources. This is also the best range to use because in my evaluation I will need to discuss further work, and then I could use 5, 15, 25, 35 and 45 to create further results and make my experiment more accurate.
Method
Take a piece of paper with an "x" clearly marked on it, conical flask, measuring cylinder and timer.
Measure 50cm3 of sodium thiosulphate and 5cm3 of hydrochloric acid.
Add the hydrochloric acid to the sodium thiosulphate in the conical flask and gently swirl.
Place conical flask on the paper with "x" marked and start the timer.
Observe the reaction through the top of the conical flask and stop the timer as soon as the cross can no longer be seen.
Repeat experiment with 40cm3 thiosulphate, 5cm3 hydrochloric acid and 10cm3 water.
Keep repeating, adding 10cm3 less thiosulphate and 10cm3 more water each time.
Record results in a table of results.
Apparatus
Sodium thiosulphate, hydrochloric acid and water
Paper with "x" clearly marked.
Conical flask
Measuring cylinder
Timer
Results
Analysis
From my graph I can see that most of my results were correct, although I did have an anomalous result (see evaluation).
My results agree with my prediction because I predicted that the lower the concentration of sodium thiosulphate, and the higher the amount of water, the longer the reaction would take to occur.
My graph shows this because it curves downwards, indicating that as one factor decreased, so did the other.
This is because before two particles can react they must meet. In a low concentration, the particles will be few and widely spread. This means that the number of reactions will be limited because less particles will meet. At higher concentrations there are more particles and so they probability of them coming into contact with other particles is increased.
However, not all collisions bring about a reaction. A reaction will only occur if the colliding particles possess more than a certain minimum amount of energy. This is known as the activation energy. This activation energy will enable the chemical bonds that already exist between the particles to stretch and eventually break, and atoms, ions and electrons will be rearranged.
Therefore, as I increased the amount of water and decreased the amount of thiosulphate, the reaction will take longer and my graph line will curve down.
Evaluation
For 10cm3 sodium thiosulphate, 5cm3 of hydrochloric acid, and 40cm3 of water, it took 192 seconds for the cross to disappear. In my 1st and 3rd attempts I got 181 and 185. This means that the second attempt was probably incorrect.
This could have been caused by any number of things such as:
Measuring the actual measuring of the chemicals is difficult because of the meniscus of the liquid and the accurateness of the measuring cylinders. Also, the human eye causes errors such as this
The eye could also have made mistakes in judging whether or not the cross had actually completely disappeared
If the same person watching the reaction was working the timer, errors could occur in their co-ordination.
We could remove these errors by:
Making sure the bottom of the meniscus is read so that it is the true reading.
Using a burette to measure. This would remove the measuring errors associated with measuring cylinders, as they are correct to 0.1 cm where measuring cylinders are only 0.4. Burettes are a far more accurate way of measuring the correct amounts.
Use light sensors to detect when the cross is no longer visible. The experiment could be connected to a light sensor, and to a timer. These light sensors will detect when there is no light shining through the substance. This would automatically stop the timer and therefore make the experiment fairer and more accurate.
To further investigate this experiment, I could use a wider range of times. I used 10, 20, 30, 40 and 50 and so I could use the mid points of these times for more accuracy. I could also go up to 100cm3 to obtain more results but this possibly would not affect my experiment.
