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Chemical Reaction Kinetics: Sodium Thiosulfate and HCl

Experiment video

What you will learn

Investigate the effect of concentration on the rate of reaction between Sodium Thiosulfate (Na2S2O3) and Hydrochloric Acid (HCl) by timing the opacity created by colloidal sulfur.

Materials needed

0.1 M Sodium Thiosulfate (Na2S2O3) solution mL
0.1 M Sodium Thiosulfate (Na2S2O3) solution mL
₹50
What is it?
Standard 0.10 M Sodium Thiosulfate pentahydrate (Na2S2O3·5H2O) AR solution.
Specifications
Concentration
0.100 M ± 0.002 M
Solute
Sodium thiosulfate pentahydrate AR
Preservative
Trace sodium carbonate (0.1 g/L) to prevent bacterial breakdown
Volume
500 mL
1.0 M Hydrochloric Acid (HCl) solution mL
1.0 M Hydrochloric Acid (HCl) solution mL
₹95
What is it?
1.0 M Hydrochloric Acid volumetric reagent solution.
Specifications
Concentration
1.00 M HCl
Purity
Analytical Reagent grade
Specific Gravity
1.016 g/mL at 20°C
Conical Flask (100 mL)
Conical Flask (100 mL)
₹140
What is it?
Precision 250 mL borosilicate glass conical flasks with optical-flat bottom.
Specifications
Quantity
5 matched flasks
Base Flatness
Optically flat base for minimal distortion
Material
Borosilicate glass 3.3
Graduation
White ceramic graduations
Measuring Cylinders (10 mL and 50 mL)
Measuring Cylinders (10 mL and 50 mL)
₹185
What is it?
Standard laboratory apparatus used in Chemical Reaction Kinetics: Sodium Thiosulfate and HCl.
Specifications
Category
Chemistry Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard
White paper tile with a bold black cross mark (X)
White paper tile with a bold black cross mark (X)
₹230
What is it?
Standard laboratory apparatus used in Chemical Reaction Kinetics: Sodium Thiosulfate and HCl.
Specifications
Category
Chemistry Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard
Digital Stopwatch (least count 0.01 s)
Digital Stopwatch (least count 0.01 s)
₹275
What is it?
Standard laboratory apparatus used in Chemical Reaction Kinetics: Sodium Thiosulfate and HCl.
Specifications
Category
Chemistry Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard
Distilled water wash bottle
Distilled water wash bottle
₹320
What is it?
Standard laboratory apparatus used in Chemical Reaction Kinetics: Sodium Thiosulfate and HCl.
Specifications
Category
Chemistry Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard

Tick items as you collect them. Tap + to see the product, specifications and buy options.

Stay safe

Keep eye position directly above the mouth of the flask at a uniform distance for every trial.
Use the same conical flask and black cross throughout the experiment for optical consistency.
Rinse the conical flask thoroughly with distilled water between trials to remove trace sulfur.
Conduct in a well-ventilated lab because sulfur dioxide (SO2) gas is produced.

Steps

0 of 7 steps done

1Draw a bold black cross 'X' on a white sheet of paper and place the conical flask directly over it.
Observation
Draw a bold black cross (+) with waterproof marker on white paper sheet; place under 250 mL conical flask.
Safety
Perform in a well-ventilated laboratory; reaction generates pungent sulfur dioxide (SO2) gas.
2Measure 50 mL of 0.1 M Na2S2O3 into the flask.
Observation
Measure 50 mL of 0.1 M Na2S2O3 into flask; cross is crisply visible looking vertically through solution.
Safety
Use separate graduated cylinders for thiosulfate and hydrochloric acid.
3Measure 5.0 mL of 1.0 M HCl; pour it rapidly into the flask and start the stopwatch immediately.
Observation
Add 10 mL of 1 M HCl and simultaneously start stopwatch; swirl once and place immediately over cross.
Safety
Start stopwatch at the exact moment acid mixes with thiosulfate.
4Swirl the flask once and look down vertically through the liquid at the cross mark.
Observation
Solution turns opalescent milky yellow as colloidal sulfur precipitates; cross disappears completely at t = 20.0 s.
Safety
Stop timer the moment the black cross lines are no longer distinguishable.
5As colloidal sulfur precipitates (Na2S2O3 + 2HCl -> 2NaCl + SO2 + S↓ + H2O), the solution turns milky yellow.
Observation
Repeat with diluted thiosulfate (40, 30, 20, 10 mL + water); cross disappearance time increases systematically up to 100.0 s.
Safety
Immediately dispose of turbid mixture and rinse flask to prevent sulfur coating inside glass.
6Stop the stopwatch the exact moment the black cross mark becomes invisible. Record time (t).
Observation
Plot 1/t vs concentration: straight line confirms reaction rate is directly proportional to concentration.
Safety
Keep flasks covered with clock glass after reaction to contain SO2 vapors.
7Repeat the experiment using 40 mL, 30 mL, 20 mL, and 10 mL of thiosulfate diluted with water to maintain a constant 50 mL volume. Plot 1/t vs concentration.

Tap + on a step to see what you will observe and any safety tips.

Record your observations

Flask No.	Na2S2O3 0.1M (mL)	Water Added (mL)	Conc. Na2S2O3 (M)	Time for Cross to Disappear t (s)	Reciprocal Time 1/t (s⁻¹)
1	50 mL	0 mL	0.100 M	20.0 s	0.0500 s⁻¹
2	40 mL	10 mL	0.080 M	25.0 s	0.0400 s⁻¹
3	30 mL	20 mL	0.060 M	33.3 s	0.0300 s⁻¹
4	20 mL	30 mL	0.040 M	50.0 s	0.0200 s⁻¹
5	10 mL	40 mL	0.020 M	100.0 s	0.0100 s⁻¹

Tap a highlighted cell to write your own observation.

Result and conclusion

Reaction rate (1/t) increases linearly with sodium thiosulfate concentration. The rate versus concentration plot is a straight line, confirming first-order kinetics with respect to thiosulfate.

Na2S2O3 + 2 HCl -> 2 NaCl + SO2↑ + S↓ (Colloidal Sulfur turbidity) + H2O; Rate ∝ 1/t

Observation	What it means
Direct First-Order Linearity	Plot of 1/t versus concentration of Na2S2O3 is a perfect straight line passing through the origin (0,0).
Reaction Rate Law	Rate = k · [Na2S2O3]¹; doubling thiosulfate concentration cuts precipitation time exactly in half (100s -> 50s -> 25s).
Colloidal Turbidity Endpoint	The optical extinction of the black cross mark corresponds to a fixed, constant optical density of precipitated sulfur particles.
Temperature Control Significance	Conducting all 5 runs at identical room temperature (25°C) isolates concentration dependence from Arrhenius activation kinetics.

Concentration of Na2S2O3 vs Reaction Rate (1/t)

Sample readings. Replace with your own results.

Experiment photos

Before the experimentDuring the experimentResult