Science 7–10 · Year 10

The disappearing cross: how concentration changes the rate of reaction

Reactions

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

Rate rises with concentration because more particles per volume means more collisions per second; the time for a fixed amount of product to form falls in proportion.

Safety card

Medium riskA teacher supervises

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

Hazards

sulfur dioxide, a toxic gas that can trigger asthma, forms in small amounts
hydrochloric acid 2 mol/L is corrosive to eyes

Controls

good ventilation; a fume cupboard or reduced scale if any learner has asthma
eye protection
every mixture into the sodium carbonate stop bath, not the sink

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

sodium thiosulfate solution 50 g/L (about 0.20 mol/L if made from the pentahydrate), 50 mL per run, plus deionised water for dilutions
hydrochloric acid 2 mol/L, 5 mL per run
250 mL conical flask (as in the RSC method), a paper cross under it, 50 mL and 10 mL measuring cylinders, stop clock
waste beaker of sodium carbonate solution in the fume cupboard, which neutralises the acid and stops further sulfur dioxide forming

How to do it

Make five thiosulfate dilutions totalling 50 mL: 50 + 0, 40 + 10, 30 + 20, 20 + 30 and 10 + 40 mL of thiosulfate and water.
Stand the flask on the cross, add 5 mL of acid, start the clock at once, swirl and look down through the liquid.
Stop the clock when the cross is no longer visible; pour the mixture into the stop bath.
Plot time against concentration, then plot 1/time (a measure of rate) against concentration.

What you should see

A cloudy yellow precipitate of sulfur forms. Time falls as concentration rises; the 1/time plot is close to a straight line through the origin, so rate is proportional to concentration. The 20 + 30 mL mixture, at 0.4 of the full concentration, takes about 2.5 times as long as the undiluted run, and the 10 + 40 mL mixture about five times as long. Absolute times depend on the batch, the room temperature and the observer, so the model is fitted to the class's own undiluted run. The learner knows it worked when the five points fall on one line and a repeat of any run agrees within 10 percent.

What changes

What you change: concentration of sodium thiosulfate
What you measure: time for the cross to disappear
What you keep the same: total volume 55 mL
acid volume and concentration
temperature
same flask and cross
same observer

Common misconceptions

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

Diluting the solution makes the reaction stop rather than slow.
Rate and time are the same quantity.
The cross disappears because the sulfur sinks onto it.

Curriculum references

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

Science 7–10 Syllabus (2023)SC5-RXN-02 SC5-WS-05
Australian Curriculum v9AC9S10U07AC9S10I04

Sources

The pages the author read to write this activity.

All Concept Studio activities