paper 2 required practicals

Question 1

describe the method for the ‘disappearing cross reaction’ (rates of reaction):

Answer 1

• sodium thiosulfate solution + hydrochloric acid = sulphur (solid)
• the sulphur makes the solution go cloudy - scientists call this cloudiness turbidity. we can use this to see how long the reaction takes to finish.
• use measuring cylinder, put 10cm^3 of sodium thiosulfate solution into a conical flask.
• place conical flask onto a printed black cross.
• add 10cm^3 hydrochloric acid into the conical flask.
• swirl the solution and start a stopwatch.
• look down through the top of the flask. after a certain time, the solution will turn cloudy. stop the clock when you can no longer see the cross.
• repeat the experiment with lower concentrations of sodium thiosulfate solution.
• repeat the entire experiment, calculate mean values for each concentration of sodium thiosulfate solution.

Question 2

what is the problem with the reproducibility of the ‘disappearing cross’ experiment?

Answer 2

• a measurement is reproducible if it can be repeated by another person or by using a different technique or equipment and still get the same result.
• the problem with this experiment is that different people have different eye sights.
• this means that some people can see the cross for longer than others, meaning they may not get the same results.
• however, because all students should use the same size printed cross, this problem shouldn’t be too great.

Question 3

what are the hazards, risks, and solutions for the ‘rates of reactions’ practicals?

Answer 3

• hydrochloric acid - skin and eye irritation. wear goggles. cover skin.

Question 4

what does the ‘rates of reaction’ experiment show us?

Answer 4

shows us that the greater the concentration of a chemical in a reaction, the faster the reaction takes place.
- because this result is shown by two different experiments, the finding is reproducible.