Investigating Science 11–12 · Year 12

Temperature and reaction rate: stopwatch against light sensor

Module 6: Technologies

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

The rate of the thiosulfate–acid reaction rises steeply with temperature, and the technology used to detect the end point, the eye with a stopwatch or a light sensor, decides how reliable the result is.

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 is released, a risk for people with asthma
2 mol/dm³ hydrochloric acid irritates eyes and skin
hot water baths

Controls

good ventilation or a fume cupboard; learners with asthma do not lean over the flasks
safety glasses
keep bath temperatures at or below 55 °C
dispose of mixtures as the school’s CSIS procedures direct

Note

Chemicals are used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1 (general information for all staff, including the CSIS 1.7 risk assessment requirement), read the Safety Data Sheet for each chemical and complete a RiskAssess risk assessment before the lesson.

What you need

sodium thiosulfate solution, 40 g/dm³, 10 cm³ per run; water, 40 cm³ per run; hydrochloric acid, 2 mol/dm³, 5 cm³ per run (RSC quantities)
250 cm³ conical flask (RSC), paper marked with a black cross, thermometer, water baths from 15 to 55 °C
stopwatch, and a light sensor with data logger (or a phone light meter such as phyphox) under the flask

How to do it

Warm the thiosulfate solution and water to the chosen temperature in the flask.
Add the acid, start the stopwatch and start the light logger.
Stop the stopwatch when the cross can no longer be seen; from the logger, read the time at which light falls to a fixed fraction (for example 10 %) of its starting value.
Record the start and end temperatures and use the mean.
Repeat at five temperatures, three runs each, with the same observer for the stopwatch; have three observers time one run to compare.
Plot 1/t against temperature and ln(1/t) against 1/T (T in kelvin) and assess the random and systematic errors of each timing method.

What you should see

Time falls and rate (1/t) rises steeply as temperature rises. The ln(1/t) against 1/T graph is close to a straight line whose gradient is −E_a/R, from which the learner calculates an activation energy. If the rate doubles for each 10 °C rise (an activation energy near 50 kJ/mol gives a factor of 1.97 between 20 and 30 °C), a 60 s time at 20 °C becomes about 30 s at 30 °C and 15 s at 40 °C. The learner compares the spread of the three observers’ stopwatch times for one run with the spread of the repeat sensor times; the cross judgement depends on eyesight and lighting, which the fixed sensor threshold removes.

What changes

What you change: temperature
What you measure: time to the end point
What you keep the same: concentrations and volumes
same flask and cross
same end-point rule
same observer for the stopwatch series

Common misconceptions

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

A digital reading is always more accurate than an analogue one.
Rate doubles for every 10 °C in every reaction.
The temperature stays constant during the reaction.

Curriculum references

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

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS12-13 INS11/12-1 INS11/12-2 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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