Chemistry 11–12 · Year 12

Measuring the enthalpy of neutralisation of a strong acid with a strong base, and comparing a weak acid

Module 6: Acid/Base 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

Neutralisation releases the same heat per mole of water formed for any strong acid with any strong base, because the reaction in every case is hydrogen ion with hydroxide ion.

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

1.0 mol/L sodium hydroxide is corrosive
1.0 mol/L acids irritant
warm solution

Controls

eye protection
measuring cylinders rather than pouring from stock
rinse any splash at once

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

1.0 mol/L hydrochloric acid, 1.0 mol/L nitric acid, 1.0 mol/L ethanoic acid, 1.0 mol/L sodium hydroxide, 50.0 mL portions
Polystyrene cup with lid in a 250 mL beaker, thermometer to 0.1 degrees Celsius or a probe with logger
Measuring cylinders 50 mL, two; stopwatch

How to do it

Measure 50.0 mL of acid into the cup and 50.0 mL of sodium hydroxide into a cylinder; leave both for five minutes and record their temperatures (they should agree within 0.2 degrees Celsius; use the mean).
Add the base all at once, fit the lid and stir with the thermometer; record the temperature every 15 s for four minutes and take the highest reading (or extrapolate the cooling line back to the mixing time).
Calculate q = m c delta T with the solution mass as 100 g, then divide by the moles of water formed (0.050 mol) to get the enthalpy of neutralisation in kJ/mol.
Repeat with nitric acid and with ethanoic acid.
Compare the three values and test the common claim that a weak acid must release less heat.
Use the simulation to predict the temperature rise for other concentrations and volumes.

What you should see

Hydrochloric and nitric acids give the same temperature rise within the thermometer's resolution: the ideal rise for 50.0 mL of 1.0 mol/L acid with 50.0 mL of 1.0 mol/L base is 6.7 degrees Celsius, an enthalpy of neutralisation of -55.8 kJ per mole of water from formation enthalpies; a school cup reads a little lower because of heat loss. Ethanoic acid gives almost the same rise: its ionisation enthalpy from the same tables is only -0.25 kJ/mol, so its neutralisation enthalpy is -56.1 kJ/mol, and any difference the class sees is within its measurement error. Repeat runs that disagree markedly point to unequal starting temperatures or slow mixing.

What changes

What you change: identity of the acid (strong or weak)
What you measure: temperature rise (degrees Celsius) and enthalpy per mole of water formed (kJ/mol)
What you keep the same: volumes and concentrations
starting temperatures equal
same cup and thermometer
stirring

Common misconceptions

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

A weak acid always releases much less heat on neutralisation (ethanoic acid ionises with almost no enthalpy change, so its value matches the strong acids within measurement error).
Doubling both volumes doubles the temperature rise (it doubles the heat and the mass; the rise is unchanged).
The enthalpy of neutralisation is per mole of acid (it is per mole of water formed; sulfuric acid forms two).

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)CH12-13 CH11/12-3 CH11/12-5
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 6, which it allows in any moduleCH11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities