Science 7–10 · Year 10

Warm it up: how temperature changes the rate of the thiosulfate reaction

Reactions

PracticalMedium risk

School laboratory, not for home

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

The idea

Raising the temperature makes the particles move faster, so they collide more often and more energetically, and the reaction goes faster.

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 forms in small amounts and increases at higher temperature
hot water bath
hydrochloric acid 2 mol/L is corrosive to eyes

Controls

good ventilation or fume cupboard; keep the top temperature at 55 °C
eye protection
stop bath for all mixtures

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 40 g/L, 10 mL per run, plus 40 mL of water (mixture about 0.032 mol/L if made from the pentahydrate)
hydrochloric acid 2 mol/L, 5 mL per run
100 mL conical flask, test tube for the acid, paper cross, thermometer, water bath (beaker of warm water) and ice, stop clock, stop bath of sodium carbonate solution

How to do it

Mix 10 mL of thiosulfate with 40 mL of water in the flask and stand it in the water bath, with the 5 mL of acid in a test tube beside it, until both reach the first target temperature (five runs spanning 15 to 55 °C).
Record the temperature, pour in the acid, start the clock, swirl, and stop when the cross disappears; record the final temperature too.
Pour into the stop bath and repeat at the other temperatures.
Plot 1/time against temperature. Read off how much the rate changes for each 10 °C rise.

What you should see

Time falls sharply as temperature rises, and the 1/time plot curves upward rather than rising in a straight line. The learner knows it worked when the time falls at every step, the points lie on one smooth curve, and a repeat at one temperature agrees with the first run.

What changes

What you change: temperature of the mixture
What you measure: time for the cross to disappear
What you keep the same: concentrations and volumes
same flask and cross
same observer
acid at the same temperature as the flask

Common misconceptions

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

Heating only makes particles move a little faster, so the rate should rise a little.
The reaction needs a flame to be "activated".
A doubled rate means the reaction gives twice as much product.

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-03
Australian Curriculum v9AC9S10U07AC9S10I02

Sources

The pages the author read to write this activity.

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