Rates of reaction III - Changing Concentration of reactants
Purpose of the lab: (You complete it)
Materials:
- 10 mL graduated cylinder
- 50 mL graduated cylinder
- 250 mL Beaker labelled “HCl” – 2M
- 250 mL Beaker labelled “Na2S2O3 or Sodium Thiosulfate” – 0.1 M
- Timer or stop watch
- Conical flask (125 mL or 250 mL)
- “I ♥ Chem” paper
- Chemicals:
- 150 mL of Na2S2O3 or Sodium Thiosulfate
- 50 mL of HCl
Procedures:
In each trial you will be mixing different volumes of the chemicals based on the table below.

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.
- measure 10 mL of the acid using the graduated cylinder.
- Measure 50 mL of Thiosulfate.
- 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
- 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

Graphs.
You will be doing two graphs:
- Time vs Concentration of thiosulfate
- 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
- How does the concentration of the thiosulfate affect the rate of the reaction?
- Why do we keep the amount of HCl constant during all trials?
- What is the element produced in the reaction that does not allow us to see through the liquid?
- Research about the reaction that takes place and infer why does it smell bad.
- 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. - How many moles of HCl were present in the volume used in all trials? The concentration of HCl is 2M.
- 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↑ - What does the down arrow next to the Sulfur mean?
- What does the up arrow next to the Sulfur dioxide mean?
- Which is the total volume inside the flask in each trial after the reaction?
EXTRA CREDIT - 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.
