It is important in this core practical to use appropriate apparatus to make and record a range of measurements accurately, including time, temperature and volume. This includes the safe use of apparatus, and monitoring chemical changes.
This outlines one way to carry out the practical. Eye protection must be worn.
To investigate the effect of changing the temperature on the rate of a reaction.
Sodium thiosulfate solution reacts with dilute hydrochloric acid:
sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur
Na2S2O3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)
The sulfur forms a cloudy yellow-white precipitate during the reaction. The time taken for this to achieve a given cloudiness provides a way to measure the reaction time.
Record the results in a table. This table gives some example results.
|Temperature of reaction mixture (°C)||Reaction time (s)||Reaction rate 1000/s|
1. Calculate 1000/time for each temperature. This value is proportional to the rate of reaction.
2. Plot a graph to show:
Describe the effect of increasing the temperature of the reaction mixture on the rate of reaction. Use your graph to help you.
The rate of reaction increases as the temperature increases. The rate increases by a greater amount at higher temperatures.
Suggest a reason why the same person should look at the black cross each time.
Different people may decide that they cannot see the cross at different amounts of cloudiness, leading to errors in deciding when to take the reaction time.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
|Hazard||Possible harm||Possible precaution|
|Hot sodium thiosulfate solution||Burns to the skin||Do not heat above 60°C|
|Sulfur dioxide||Can cause irritation to the eyes and lungs, particularly to people with asthma||Make sure the room is well ventilated, avoid breathing directly over the top of the flask|