Chemistry 11–12 · Year 11

Sodium thiosulfate and hydrochloric acid (the disappearing cross): rate against temperature and concentration

Module 3: Reactive Chemistry

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Reaction rate rises with concentration because collisions are more frequent, and with temperature because a larger fraction of collisions carry the activation energy.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

sulfur dioxide gas is toxic and triggers asthma
hydrochloric acid 2 mol/L irritant
hot water baths

Controls

well-ventilated room; learners with asthma observe from a distance or use the light sensor
no mixing above 55 degrees Celsius
dispose in the fume-cupboard sink with running water
eye protection

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

What you need

Sodium thiosulfate pentahydrate solution 40 g/L (0.161 mol/L), 600 mL (the three concentration series take 450 mL and the temperature series 50 mL)
Hydrochloric acid 2 mol/L, 120 mL (the 20 runs take 100 mL)
100 mL conical flasks, five; 50 mL and 10 mL measuring cylinders
Paper with a bold cross, stopwatch or the Concept Studio timer
Thermometer or temperature probe, water baths (kettle, ice) for 15 to 55 degrees Celsius
Light sensor and data logger under the flask, if available, for an objective end point

How to do it

Concentration: make five mixtures of thiosulfate and water totalling 50 mL (50:0, 40:10, 30:20, 20:30, 10:40 mL); place each flask on the cross.
Add 5 mL of acid, start timing at once, swirl once and look down through the liquid; stop when the cross vanishes. Repeat each concentration twice more.
Temperature: use 10 mL thiosulfate plus 40 mL water; warm or cool each flask to a target (about 15, 25, 35, 45, 55 degrees Celsius), record the temperature, add 5 mL acid and time as before; record the temperature again at the end and use the mean.
Calculate rate as 1/time for every run; plot rate against thiosulfate concentration and rate against temperature.
Fit the temperature data in the simulation by adjusting the activation energy until the model curve passes through the points; read off the doubling temperature.
Pour all mixtures into the fume-cupboard sink with running water.

What you should see

1/time rises with thiosulfate concentration; if the rate is first order in thiosulfate, as the model assumes, the plot is a straight line through the origin and halving the concentration doubles the time (a 40 s run at full strength would become 80 s at half strength; absolute times depend on the cross and the lighting). Rate against temperature curves upward: a 10 degree rise from 20 to 30 degrees Celsius shortens the time by a factor of 1.97 if the activation energy is 50 kJ/mol, and 20 to 50 degrees Celsius by a factor of 6.72 (Arrhenius model); the learner's fitted activation energy sets the factor for this reaction. The sulfur precipitate is pale yellow and the flask smells faintly of sulfur dioxide.

What changes

What you change: thiosulfate concentration, or temperature
What you measure: time for the cross to disappear (s), converted to rate (1/s)
What you keep the same: total volume 55 mL
acid volume and concentration
same cross, same observer or light sensor
temperature (in the concentration series) or concentration (in the temperature series)

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Rate doubles for every 10 degrees for all reactions (that rule of thumb holds only near room temperature for activation energies near 50 kJ/mol).
Higher temperature makes collisions harder, so more bonds break (the main effect is the larger fraction of collisions with energy above the activation energy).
A catalyst or heat changes how much product forms (it changes only how fast the same amount forms).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH11-10 CH11/12-2 CH11/12-3 CH11/12-4
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 3, which it allows in any moduleCH11/12-5
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-03 CH-11WS-02 CH-11WS-03 CH-11WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities