Chemistry 11–12 · Year 11

Hess's law: the enthalpy of hydration of copper(II) sulfate from two dissolution measurements

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

An enthalpy change that cannot be measured directly is found by combining changes along another route, because enthalpy change depends only on the start and end states.

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

copper(II) sulfate is harmful and irritant
anhydrous salt is hygroscopic and warms in the hand

Controls

weighing boats and spatula, no handling of the solid
eye protection
copper waste collected

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

Anhydrous copper(II) sulfate, 3.99 g (0.025 mol), kept in a desiccator
Hydrated copper(II) sulfate, 6.24 g (0.025 mol), finely ground
Polystyrene cup in a beaker, lid, thermometer to 0.1 degrees Celsius or probe
Deionised water, 50.0 mL portions, two
Balance to 0.01 g, weighing boats, stirring rod

How to do it

Put 50.0 mL of water in the cup; record the temperature every 30 s for two minutes.
Add the anhydrous copper(II) sulfate all at once, stir continuously and record the temperature every 30 s for five minutes; extrapolate the cooling line back to the time of addition to find the true maximum.
Repeat with fresh water and the hydrated salt; record the minimum temperature in the same way.
Calculate the enthalpy of solution of each salt per mole (q = m c delta T, mass of solution = water plus salt).
Draw the Hess cycle: CuSO4(s) + 5 H2O(l) gives CuSO4.5H2O(s), with both salts dissolving to the same solution; subtract to obtain the enthalpy of hydration.
Compare with the value from formation enthalpies in the simulation and list the sources of error in order of size.

What you should see

Anhydrous copper(II) sulfate dissolving is exothermic: 0.025 mol in 50.0 g of water gives an ideal rise of 8.1 degrees Celsius, so delta H_sol is about -73 kJ/mol (formation-enthalpy value). The hydrated salt dissolving is slightly endothermic: an ideal fall of 0.64 degrees Celsius, delta H_sol about +6 kJ/mol. The cycle gives an enthalpy of hydration near -79 kJ/mol. A school result usually falls short in magnitude because the anhydrous powder dissolves slowly and heat is lost while it does, which is why the cooling line is extrapolated back to the moment of mixing.

What changes

What you change: form of the salt (anhydrous or hydrated)
What you measure: temperature change of the solution (degrees Celsius) and the enthalpy of solution (kJ/mol)
What you keep the same: moles of salt at 0.025
water volume
stirring
same cup and thermometer

Common misconceptions

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

Hess's law only works for reactions that can be done in one step (it is used precisely when the direct step cannot be measured, as here).
The hydrated salt should dissolve exothermically because it contains water (its lattice already holds the water; dissolving it is slightly endothermic).
The temperature change equals the enthalpy change (delta T is converted through q = m c delta T and the moles used).

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 CH11/12-5 CH11/12-6 CH11/12-7
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities