Experiment about reaction times & result analysis

[Barclay]

1. Homework Statement

A student was investigating the reaction between sodium thiosulfate solution and dilute hydrochloric acid. As the reaction takes place a precipitate of sulfur forms in the solution and makes the solution change from colourless (and clear) to pale yellow (and opaque). The time it takes for a certain amount of sulfur to form can be used as a measure of the rate of the reaction.
The student used 1.0 mol/dm³ sodium thiosulfate solution and made up different concentrations of the solution by using the quantities of the solution and water shown in the table.

He then drew a mark in pencil on a piece of paper.
He then added 5 cm³ of dilute hydrochloric acid to the solution in one of the flasks, the clock was started, the mixture was quickly stirred or swirled and then the conical flask was put on top of the pencilled cross.

The student looked down through the conical flask to the mark and stopped the clock as soon the mark could no longer be seen.

He then repeated the process with the other four solutions.

Q1
Why the clock was started when the acid was added and not when the flask was put on the pencilled mark?Q2
What do you think are the main sources of error in this experiment? (Pick the two which you think would have the greatest effect on the accuracy of the results.)
2. Relevant results

Volume of sodium thiosulfate solution (cm3) 50 40 30 20 10

Volume of water (cm3) ......0 10 20 30 40

Volume of hydrochloric acid (cm3) .... 5 5 ... 5 ... 5 5

Time for the mark to be obscured (s) ...14 18 23 36 67

The Attempt at a Solution

My answer to Q1
The reaction starts (so timing must begin) as soon as the acid is added. By placing the flask on the X is just a method to check when the reaction has ended (when the X is obscured).My answer to Q2
The size of the pencil mark must be kept equal.

The experiment should be carried out at least twice with different observers. They may notice the X disappearing at a slightly different time depending on their eyesight.Solution must be mixed an equal number of rotations & speed for each experiment.

Could you look at my answers please and give me alternatives if possible. My answers seem wishy-washy to me. This is high school level physics. Thanks

 

[sk1105]

Your answer to Q1 seems fine to me, and is what I would have suggested. Your answers to Q2 have exactly the right idea, although I would suggest that a more specific word (or words) than 'size' could be used when describing the pencil mark. To be really comprehensive with your other answers, bear in mind that errors are likely to be introduced whenever human judgement is involved in a measurement, so ideally an experiment will require very little human intervention beyond setting up the equipment and reading the results.

 

[Borek]

Your answer to Q2 says what to do to get better results, not what are the main sources of error.