In this required practical, you investigate how changing the concentration of a reactant affects the rate of reaction. The reaction involves sodium thiosulfate and hydrochloric acid.

The reaction

Sodium thiosulfate + hydrochloric acid → sulfur + sulfur dioxide + water

• A cloudy yellow precipitate of sulfur forms
• You measure how long it takes for the solution to go cloudy

This is called a disappearing cross experiment.

Method: Disappearing cross

1. Use a measuring cylinder to add a set volume of sodium thiosulfate to a conical flask
2. Place the flask on a piece of paper with a black cross drawn on it
3. Add a set volume of hydrochloric acid to the flask and start the timer immediately
4. Look down through the solution – stop the timer when the cross is no longer visible
5. Repeat using different concentrations of sodium thiosulfate (by diluting with water)
6. Keep the volume and temperature constant

What is being measured?

• The time taken for the cross to disappear
• As the reaction gets faster, the cross disappears sooner

How to improve accuracy

• Use the same observer each time (subjective judgement)
• Repeat each test and calculate a mean
• Use a light sensor or data logger for more precise detection (if available)

Variables

Expected results

• Higher concentration of sodium thiosulfate = faster reaction
• This supports collision theory – more particles = more frequent collisions