Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

Rates of reaction III – Thiosulfate -HCl

Rates of reaction III - Changing Concentration of reactants

Purpose of the lab: (You complete it)


  • 10 mL graduated cylinder
  • 50 mL graduated cylinder
  • 250 mL Beaker labelled “HCl” – 2M
  • 250 mL Beaker labelled “Na2S2Oor Sodium Thiosulfate” – 0.1 M
  • Timer or stop watch
  • Conical flask (125 mL or 250 mL)
  • “I ♥ Chem” paper 
  • Chemicals: 
    • 150 mL of Na2S2Oor Sodium Thiosulfate
    • 50 mL of HCl


In each trial you will be mixing different volumes of the chemicals based on the table below. 

Rates II data table

Each time you will be having the same total volume ( 60 mL) , 10 mL of HCl and a combination of Thiosulfate and water in different proportions. 

  1. measure 10 mL of the acid using the graduated cylinder.
  2. Measure 50 mL of Thiosulfate.
  3. Be ready to measure the time of the reaction. You should stop timing when you CANNOT see the letters underneath the liquid FROM THE TOP of the flask
  4. Stop timing, rinse the flask thoroughly and begin again with a different combination of liquids based on the table at the top.

Lab Setup:

In the following graph you have the view before and after the reaction takes place


You will be doing two graphs: 

    1. Time vs Concentration of thiosulfate
    2. 1/Time vs Concentration of thiosulfate

USE GRAPH PAPER AND RULER. The two graphs should be done separately!!!

You can use the volume of the thiosulfate as a measurement of the concentration of thiosulfate. 

Analysis questions

  1. How does the concentration of the thiosulfate affect the rate of the reaction?
  2. Why do we keep the amount of HCl constant during all trials?
  3. What is the element produced in the reaction that does not allow us to see through the liquid?
  4. Research about the reaction that takes place and infer why does it smell bad.
  5. Sketch the graph you obtain in graph #1 again (only the shape).
    -How would this graph look like if we warm up the thiosulfate?
    -Show the result in the same graph with another color. 
  6. How many moles of HCl were present in the volume used in all trials? The concentration of HCl is 2M.
  7. How many moles of Thiosulfate were present in the volume used in trial 1? The concentration of Thiosulfate is 0.1 M

    The reaction in the flask is the following

    Na2S2O3 + 2 HCl ——–> H2O + 2 NaCl + S ↓+ SO2

  8. What does the down arrow next to the Sulfur mean?
  9.  What does the up arrow next to the Sulfur dioxide mean?
  10. Which is the total volume inside the flask in each trial after the reaction?


  11. Supposing that the sulfur can be separated by filtration in the first trial and the sulfur dioxide evaporates (it is a gas), calculate the molarity of the NaCl solution obtained. (hint: you should calculate how many moles of NaCl will be produced in the final volume)……….

Your lab report should have the following parts:

  • Title
  • purpose
  • materials
  • procedures
  • data tables
  • calculations
  • analysis questions

ONLY IF YOU WERE ABSENT OR LATE TO THE LAB you can use the following data to build your graph and answer the questions in the analysis section.