Prediction
I predict that as the concentration increases, the rate of the reaction will also increase, therefore the time for the cross to disappear will decrease. The collision theory states that the rate of reaction depends on how many collisions there are and how much energy the particles have. When the concentration is increased, the frequency of collisions will increase because there are more particles per unit volume and therefore there is an increased probability that a thiosulphate particle will collide with an acid particle. The more collisions that occur the greater the chance that a product particle will be formed. I will draw a graph to show that as the concentration increases, the quicker the cross will disappear. I have drawn a basic graph below to show how I will draw my real graph.
Things I will keep the same (constant factors)
Temperature
Same concentration of hydrochloric acid (1 M)
Same apparatus
Same cross
Same eye distance from cross (30 cm)
Same volumes of acid and sodium thiosulphatte
I will keep all these factors the same to ensure that my experiment is fair and accurate. I will not swirl the sodium thiosulphate and the acid.
The temperature affects the rate because temperature speeds up the reaction and makes the particles move around more and have more energy therefore more collisions will be successful.
Independent Variables:
Concentration of sodium thiosulphate: I will use 0.03,0.06,0.09,0.12 and 0.15. Molar solutions.
Dependant Variable:
Time taken for cross to disappear. The less time taken for the cross to disappear, the faster the rate.
Diagram
Method
- Set up the apparatus as shown in the diagram above.
- Measure 50 ml of sodium thiosulphate into a measuring cylinder. Start off at the lowest concentration of 0.03(measure the chemicals accurately so the test is accurate)
- Measure 10 ml of hydrochloric acid into a measuring cylinder.
- Draw a cross on a piece of paper and place the conical flask on top of the cross.
- Now add the sodium thiosulphate and the hydrochloric acid to the conical flask and begin to time on the stopwatch until you can no longer see the cross(keep your eye height at 30 cm from the surface.
- Repeat this procedure using different concentrations of sodium thiosulphate: 0.03,0.6,0.09.0.12 and 0.15. This will show how the results vary and I will be able to recognise any anomalies.
- I will repeat the whole experiment twice to check to see if my results are reliable.
OBTAINING EVIDENCE
1
2
3
AVERAGE TIME FOR CROSS TO DISAPPEAR
Analysis
From my results and from my graph, I can see that as the Concentration increases the time taken for the cross to disappear decreases. My results are all constant and there are no anomalies.
I predicted that as the volume of sodium thiosulphate solution increases, the time taken for the cross to disappear decreases. My results agree with what I predicted.
This happens because as the concentration is increased, the frequency of collisions will increase because there are more particles per unit volume and therefore there is an increased probability that a sodium thiosulphate particle will collide with an acid particle.
I think my experiment was successful because my results were as I expected from my prediction in which I based upon scientific knowledge.
Evaluation
From my graph, my results all follow the same pattern, as the concentration increases, the time taken for the cross decreases and there are no points out of place which proves that my experiment was successful. I think my results are very accurate as I was very careful when measuring chemicals and I kept my eye distance at 30 cm from the cross.
Improvements
Instead of investigating how temperature affects the rate of reactions, I could have investigated how temperature affects the rate of reactions. I could have made my experiment more reliable by taking more results.
