RP: Sodium Thiosulfate

What is the purpose of the sodium thiosulfate experiment?

To investigate how concentration affects reaction rate.

Which two chemicals react in the disappearing-cross experiment?

Sodium thiosulfate and hydrochloric acid.

What is the word equation for this reaction?

Sodium thiosulfate + hydrochloric acid → sodium chloride + sulfur dioxide + sulfur + water.

What is the balanced symbol equation?

Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O.

Which product causes the solution to become cloudy?

Solid sulfur.

Why does the cross disappear?

The sulfur precipitate scatters light and obscures the cross.

What is the independent variable?

The concentration of sodium thiosulfate solution.

What is the dependent variable?

The time taken for the cross to disappear.

What variables should be controlled?

Temperature, acid concentration and volume, total solution volume, and viewing conditions.

Why is distilled water added to sodium thiosulfate?

To reduce its concentration.

Why should the total volume of sodium thiosulfate solution and water remain constant?

To keep the depth of liquid consistent.

Why should the same flask be used each time?

To maintain a consistent liquid depth and viewing distance.

Why should the same observer judge when the cross disappears?

To reduce variation in subjective judgement.

Why should the flask be placed on the same cross each time?

To ensure consistent viewing conditions.

Why does increasing sodium thiosulfate concentration shorten the time taken?

More particles are present per unit volume, causing more frequent successful collisions.

What does a shorter time for the cross to disappear indicate?

A faster reaction.

Why is the reciprocal of time sometimes used as a measure of rate?

For a consistent endpoint, 1 ÷ time is proportional to reaction rate.

What are the units of 1/time when time is measured in seconds?

s⁻¹.

What is one limitation of the disappearing-cross method?

The endpoint depends on human judgement.

How can reliability be improved?

Repeat measurements and calculate a mean.

What safety precaution is important in this experiment?

Work in a well-ventilated laboratory and avoid inhaling sulfur dioxide.