Chemistry 11–12 · Year 11

Temperature changes on dissolving ionic solids and on burning a fuel: classifying reactions as endothermic or exothermic

Module 4: Drivers of 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

Energy released or absorbed by bond changes shows up as a temperature change of the surroundings, measured in an insulated cup and predicted from enthalpy data.

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

sodium hydroxide pellets are corrosive and the solution gets hot
ammonium nitrate is an oxidiser
anhydrous calcium chloride irritant
ethanol flame

Controls

the teacher weighs the sodium hydroxide pellets into stoppered vials; learners tip the vial into the cup and do not handle the pellets
no more than 2.0 g of sodium hydroxide
burner capped, not blown out
eye protection
ammonium nitrate kept away from the burner

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. CSIS Volume 2 Appendix F lists solid sodium hydroxide among the chemicals to be used with caution, largely by science teachers, and asks for eye protection whatever the concentration, so the teacher weighs the 2.0 g portions.

What you need

Polystyrene cups with lids inside 250 mL beakers, four
Thermometer reading to 0.1 degrees Celsius or a temperature probe with logger
Ammonium nitrate 5.0 g, anhydrous calcium chloride 5.0 g and potassium chloride 5.0 g, weighed to 0.01 g; sodium hydroxide pellets 2.0 g, weighed by the teacher into a stoppered vial for each group
Deionised water at room temperature, 50.0 mL portions measured by cylinder
Spirit burner with ethanol, 100 mL conical flask with 100 mL water, retort stand, balance (for the combustion station)

How to do it

Measure 50.0 mL of water into a cup and record its temperature to 0.1 degrees Celsius for one minute to check it is steady.
Add 5.0 g ammonium nitrate, stir gently with the thermometer and record the lowest temperature reached; record the time to reach it.
Repeat with fresh water for the 2.0 g of sodium hydroxide, tipped straight from its vial into the cup (highest temperature), then 5.0 g anhydrous calcium chloride and 5.0 g potassium chloride.
Combustion: weigh the spirit burner, heat 100 mL of water in the flask until it rises by 20 degrees Celsius, cap and reweigh the burner; record the mass of ethanol burned.
For each process calculate q = m c delta T for the water and the enthalpy change per mole of solid or fuel; classify each as endothermic or exothermic and draw an energy-profile sketch.
Enter the same masses in the simulation and compare the predicted temperature changes with the measured ones; account for the difference (heat capacity of the cup and thermometer, heat exchange with the room, incomplete dissolution).

What you should see

Ammonium nitrate cools the water: with 5.0 g in 50.0 g the ideal temperature change is -7.0 degrees Celsius. Sodium hydroxide warms it: 2.0 g in 50.0 g gives ideally +10.2 degrees Celsius. Anhydrous calcium chloride warms it by +16.0 degrees Celsius ideally. Potassium chloride (enthalpy of solution +17.2 kJ/mol) cools it: 5.0 g gives an ideal change of -5.0 degrees Celsius. Measured changes are smaller than these ideal figures because the cup, thermometer and room exchange heat, but the sign of every change holds, and per gram ammonium nitrate cools more than potassium chloride while sodium hydroxide warms more than calcium chloride. Ethanol combustion is strongly exothermic: 0.28 g of ethanol would warm 100 g of water by 20 degrees if all the heat entered the water; an open flask needs more because much of the heat escapes.

What changes

What you change: identity of the solid dissolved (or the fuel burned)
What you measure: temperature change of the water (degrees Celsius) and the enthalpy change per mole
What you keep the same: water volume
starting temperature
mass of solid
stirring
same cup and thermometer

Common misconceptions

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

Cold packs feel cold because they take cold in (they absorb heat from the hand; energy is absorbed by the dissolving ions).
Endothermic reactions cannot happen on their own (ammonium nitrate dissolves spontaneously; entropy drives it).
The thermometer measures the energy of the reaction (it measures the water's temperature; q = m c delta T converts that to energy).

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)CH11-11 CH11/12-1 CH11/12-5 CH11/12-6
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-03 CH-11WS-01 CH-11WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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