A level chemistry help - pt 2

Q15.
The apparatus in the figure below was set up to measure the time taken for 20.0
cm3 of sodium thiosulfate solution to react with 5.0 cm3 of hydrochloric acid in a
100 cm3 conical flask at 20 °C. The timer was started when the sodium
thiosulfate solution was added to the acid in the flask. The timer was stopped
when it was no longer possible to see the cross on the paper.
AQA Chemistry A-Level - Kinetics QP PhysicsAndMathsTutor.com
Page 19 of 22
What is likely to decrease the accuracy of the experiment?
A Rinsing the flask with acid before each new experiment.
B Stirring the solution throughout each experiment.
C Using the same piece of paper for each experiment.
D Using different measuring cylinders to measure the volumes
of acid and sodium thiosulfate.
(Total 1 mark)
Q16.
The experiment was repeated at 20 °C using a 250 cm3 conical flask.
Which statement is correct about the time taken for the cross to disappear when
using the larger conical flask?
A The time taken will not be affected by using the larger conical flask.
B The time taken will be decreased by using the larger conical flask.
C The time taken will be increased by using the larger conical flask.
D It is impossible to predict how the time taken will be affected by
using the larger conical flask.

The answers are A and C. I don't understand the answers for either. For Q16, why would time change if the volume of solution remains the same?

Reply 1

UtterlyUseless69

Original post by mitostudent

Q15.
The apparatus in the figure below was set up to measure the time taken for 20.0
cm3 of sodium thiosulfate solution to react with 5.0 cm3 of hydrochloric acid in a
100 cm3 conical flask at 20 °C. The timer was started when the sodium
thiosulfate solution was added to the acid in the flask. The timer was stopped
when it was no longer possible to see the cross on the paper.
AQA Chemistry A-Level - Kinetics QP PhysicsAndMathsTutor.com
Page 19 of 22
What is likely to decrease the accuracy of the experiment?
A Rinsing the flask with acid before each new experiment.
B Stirring the solution throughout each experiment.
C Using the same piece of paper for each experiment.
D Using different measuring cylinders to measure the volumes
of acid and sodium thiosulfate.
(Total 1 mark)
Q16.
The experiment was repeated at 20 °C using a 250 cm3 conical flask.
Which statement is correct about the time taken for the cross to disappear when
using the larger conical flask?
A The time taken will not be affected by using the larger conical flask.
B The time taken will be decreased by using the larger conical flask.
C The time taken will be increased by using the larger conical flask.
D It is impossible to predict how the time taken will be affected by
using the larger conical flask.
The answers are A and C. I don't understand the answers for either. For Q16, why would time change if the volume of solution remains the same?

Q15: the reaction taking place each time is the decomposition of thiosulphate by H^+ ions - washing the flask out with an acid leaves a bunch of H^+ ions in the flask and these should increase the speed at which the reaction proceeds.

Q16: this is a guess: a flask with a larger volume probably has a base with a larger area. The reaction should take the same amount of time to reach completion, but the precipitation will be more spread out and so the time taken for a thick enough layer of it to deposit over the base for the cross to disappear will be longer.