CBSE Class 12 chemistry practical, Chemical kinetics · Physical chemistry experiments

Source: CBSE Class 12 chemistry practical syllabus · use your school laboratory manual (NCERT Lab Manual where your school uses it).

Rate of reaction between sodium thiosulphate and hydrochloric acid (Class 12 chemistry practical)

You mix sodium thiosulphate with hydrochloric acid and time how long until a cross on paper disappears.

Concentrations and volumes come from your manual — plot or compare times using your own data only.

Published practice page. Concentrations, masses, and numerical answers come from your school laboratory manual and your own readings — not invented here. Not yet reviewed by a named chemistry reviewer. Version 1.0.0, updated 2026-10-03.

Safety — read before you start

Safety warning. Practise here. Perform the real experiment only in your school lab, with your teacher present.

HCl corrosive.
SO₂ toxic — do not inhale.

Wear: Lab coat, Splash goggles, Gloves, as your teacher directs.

Follow your teacher’s disposal instructions. Do not pour leftovers down a household drain.

Loads only after you tap. Use the lab hands to pick up and use equipment (pointer hidden in the scene). Falls back to a flat diagram if WebGL is unavailable.

Aim

To study the effect of concentration (or temperature, if your manual assigns that variant) on the rate of reaction between sodium thiosulphate and hydrochloric acid using the ‘disappearing cross’ method.

Methods and quantities can differ between schools. Follow your teacher's instructions.

Apparatus and chemicals

Apparatus and chemicals
Item	Note
Conical flask (100 mL or as manual)	Place on a tile marked with a cross.
Measuring cylinder / pipette	For volumes in manual.
Stopwatch	Start at mixing; stop when cross is no longer visible.
Thermometer (if temperature study)	For heated or cooled runs only if assigned.
Water bath (optional)	If manual varies temperature.
Sodium thiosulphate solution (Na2S2O3)	Use the concentration, mass, and volume values printed in your school laboratory manual. This page does not invent a numerical result for you.
Dilute hydrochloric acid (HCl)	Strength and volume from manual.
Distilled water (H2O)	For diluting thiosulphate in concentration runs.

Calorimeter cup with thermometer for acid–base mix — Note T₁ before mixing, then T₂ after. ΔT and ΔH come from your thermometer and your manual’s formula — not from this photo.

Theory and reaction

Open

Thiosulphate reacts with acid to produce sulphur, which colloids and makes the mixture opaque.

Rate ∝ change in concentration / time. Higher reactant concentration usually gives shorter time to the same turbidity.

The balanced equation shows sulphur as a solid and SO₂ as a gas — work in a ventilated lab.

Thiosulphate with acid gives sulphur, sulphur dioxide, and water.

Procedure

Draw a bold cross on paper and place the empty flask over it at fixed height.
Pour the thiosulphate volume your manual gives into the flask. Record concentration label.
Add the HCl volume stated. Start the stopwatch immediately and swirl once.Safety: HCl is corrosive; SO₂ is toxic — avoid inhaling. Work with ventilation.
Stop when the cross is just invisible. Record time. Repeat for other concentrations your manual lists.
Tabulate time versus concentration (or temperature). Discuss trend without inventing a rate constant.

Observation table

Enter the readings from your school lab in the practical file below. This page does not fill them in for you.

Type the readings in your practical file

S. No.	Na₂S₂O₃ concentration / volume (—)	HCl volume (mL)	Time for cross to vanish (s)
1
2
3

Calculation

Compare 1/time as a measure of initial rate for different concentrations (if your teacher asks). Use only your timed readings — do not paste textbook rate constants.

Result

State whether rate increased or decreased with concentration (or temperature) as your table shows.

Precautions

Same viewing angle and lighting for every run.
Same total volume when diluting — add water to thiosulphate before acid if the manual says so.
Rinse flask between runs.

Viva questions

Answer one question at a time. The full answers stay closed until you open them.

Question 1 of 4

In this experiment, rate is tracked indirectly by…

A. Weighing the flaskB. Time for a fixed turbidity (sulphur)C. Titrating with KMnO₄D. Measuring pH to 14 always

Pick an answer first.

Show all questions and answers

In this experiment, rate is tracked indirectly by…
Answer: Time for a fixed turbidity (sulphur). Reason: Sulphur colloid hides the cross; time is inversely related to rate.
Increasing Na₂S₂O₃ concentration at fixed HCl usually…
Answer: Decreases time to turbidity. Reason: More reactant per unit volume gives faster disappearance of the cross.
Sulphur dioxide formed is…
Answer: A toxic gas — ventilate. Reason: SO₂ has a choking odour and needs ventilation.
Molecularity of an elementary step differs from order because order is…
Answer: Determined experimentally from rate law. Reason: Order comes from kinetic experiments; molecularity applies to an elementary step mechanism.

Common mistakes

Starting the stopwatch before mixing.
Changing flask or cross position between trials.
Using undiluted acid stronger than the manual allows.

Frequently asked questions

Can I do this at home?: No. Use this page to practise the order of steps and viva. Do the real experiment only in your school laboratory, with your teacher present.

Your practical file

Name, school, and roll number are optional and are not sent anywhere.

The practical file opens on this device.

Review

Not yet reviewed by a named chemistry reviewer. Published for practice only — follow your teacher and laboratory manual for quantities and safety.

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