Chemistry IA · Reactivity 1 · What drives chemical reactions? · topic 14 of 25

Hess's law with copper(II) sulfate hydration · IB Chemistry IA idea: guide, sample graph and step-by-step plan

a classic that rewards care

You cannot add five waters to a salt inside a calorimeter, but a Hess cycle can do it for you.

Difficulty●●○ Medium
KitBasic: polystyrene cup, thermometer or probe, balance (0.01 g)
TimeAbout 10 hours
FitsSL and HL
  • ✓ you like energy cycles
  • ✓ you want a result you can compare with a literature value
  • ✓ you are careful with a balance

The big idea

← this is your RQ hook!

Adding water to anhydrous copper(II) sulfate to make the blue pentahydrate is hard to measure directly. But dissolving each form is easy to measure, and Hess's law links the three. Subtract the two enthalpies of solution and you get the hydration enthalpy. The catch is that one of the two numbers is small, so its uncertainty dominates. Your job: choose amounts that give a clear temperature change, make sure everything dissolves, and propagate uncertainty through the cycle, not just through each run.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Vary the amount in a planned series

Measure each salt at several amounts and use the slope of temperature change against moles.

→ Replaces a single value with a gradient and its uncertainty, and shows whether the enthalpy depends on how much you use.
Use a data logger

Record temperature every second and extrapolate the cooling line back to the mixing time.

→ Better ΔT values and a clearer uncertainty. The pentahydrate run, with its small cooling, benefits most.
Change the cycle

Use sodium carbonate and its hydrates, or magnesium sulfate, as a second system.

→ Tests whether your method is general and gives a second comparison with literature.
Calibrate the cup

Measure its heat capacity with a known electrical or neutralisation step first.

→ Removes a hidden assumption, and your evaluation has a real number for heat capacity to discuss.
Change the volume of water

Repeat at two volumes for the same amount of salt.

→ Tests whether the enthalpy depends on concentration. It also helps if the larger amounts do not fully dissolve.

Your Chemistry IA research question: run the check

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable (the amount of solute, with range) and the dependent variable (the molar enthalpy of solution, then the hydration enthalpy that follows).
  • Says why those amounts give a temperature change big enough to measure, and that both salts will dissolve completely at that amount.
  • States how the temperature change will be found, and how many repeats for each salt.
  • Names the controls: water volume, starting temperature, stirring, the cup and lid, and how the anhydrous salt is kept dry.
  • States which cycle and which enthalpy you will compare with literature, and the conditions of the accepted value.

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The set-up

One way to set it up. Draw your own version, with your real quantities and equipment, for the Method section.

lid with holes: tip the solidin at once, lid back onprobe, ±0.1 °C:log every 30 s50 cm³ water,measured, samestart temperatureweigh by difference(cap the bottle!)

How your Chemistry IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; why hydration energies matter (hydrated salts, hand-warmers, cold packs); what you predict for the two enthalpies of solution and why.
Background chemistry~450
Enthalpy of solution; Hess's law and your cycle with equations; how the cycle gives the hydration enthalpy; the heat equation. Cite sources.
Variablestable
Independent, dependent and controlled variables, with how each control is set and checked.
Method~400
Apparatus with resolutions; how you weigh the anhydrous salt and keep it dry; how long you record; how you check it all dissolved; repeats.
Safety, ethics, environment~80
Copper(II) sulfate (both forms) is harmful if swallowed, irritates skin and eyes, and is very toxic to aquatic life: eye protection, gloves when weighing, no solution down the sink; collect it for recovery. Name these specifically.
Raw datatable
Masses, temperatures against time for each run, units and resolutions in the headers; observations on dissolving.
Processing~300
Moles, ΔT with extrapolation, heat, molar enthalpy of solution with uncertainty; the cycle calculation with propagated uncertainty. One worked example each.
Graphs and analysis~350
Temperature change against moles with a fit, or the cooling curves; what the gradient and intercept mean.
Conclusion~450
Answer with values and uncertainty for the hydration enthalpy; compare it with literature and say under what conditions that value applies.
Evaluation~600, the biggest part!
Weaknesses ranked by effect on the hydration enthalpy: moisture in the anhydrous salt, heat loss, incomplete dissolving, ΔT uncertainty; linked fixes.
Referencesnot counted
Literature enthalpies with their source and conditions, and every other source.

What your Chemistry IA graph could look like

does the line through zero make sense?

Made-up data to show the form of a strong graph, not the answer. Temperature rise against amount of anhydrous salt in 50 g of water, kept well under the solubility limit, with a line through your data. example only

0.0000.0050.0100.0150.0200.0250.0300.0350.0400.0450.050024681012141618Amount of CuSO₄ / mol (± 0.0001 mol)Temperature rise ΔT / K (± 0.2 K)no heat lost: ΔT = nΔH/(mc)line through your datayour runs, with error bars0.05 mol is 8 g in 50 g of water:check it all dissolves in the trialILLUSTRATIVE · MADE-UP DATA
  • The gradient is proportional to the molar enthalpy, so its uncertainty feeds straight into the result.
  • Points slightly under the theory line are what heat loss looks like. Say whether yours do and by how much.
  • A line through the origin makes sense here: no solute, no heat. Say why, and check the intercept.
  • If the top points bend away, ask whether the salt really dissolved fully before blaming the thermometer.
  • Another graph to consider: the same for the pentahydrate, which cools the water, on the same axes.

Which uncertainty matters most?

Rank by the effect on your result, the hydration enthalpy from the cycle. Subtracting two numbers adds their uncertainties, so the small, cooling run often matters more than you expect.

Heat lost to surroundings (est. 3 %)2 kJMoisture in the anhydrous salt (est. 2 %)1.3 kJΔT, anhydrous run (± 0.2 K on 16 K)0.84 kJΔT, pentahydrate run (± 0.2 K on 2.8 K)0.83 kJBalance (± 0.01 g)0.1 kJbiggest effect: discuss this first!uncertainty in hydration enthalpy / kJ mol⁻¹ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

check it all dissolves in the trial!

Tick steps off as you go. We'll remember on this device.

0 of 10 done

  1. Read and predictWrite the cycle with state symbols. Find literature values for the two solution enthalpies and the hydration enthalpy so you know what to expect, and the conditions they apply to.
  2. Trial run (1 lesson)Dissolve a moderate amount of the pentahydrate and anhydrous salt. Does everything dissolve? How big is each temperature change? How quickly does the anhydrous salt pick up moisture? Fix the amounts now.
  3. Decide amounts and repeatsChoose the amounts so both runs give a clear signal and dissolve fully: in 50 cm³ of water, stay at or below about 0.05 mol (8 g anhydrous, 12.5 g pentahydrate). Decide the number of repeats and the water volume, and write down why.
  4. Plan the weighingDecide how you weigh the anhydrous salt quickly and keep it dry, and how you will know if it took up water.
  5. Collect the dataRecord temperature at regular times before and after adding the solid, with masses and observations.
  6. Process each runExtrapolate to the mixing time, find ΔT, the heat, the moles and the molar enthalpy of solution, with uncertainty.
  7. Apply the cycleCombine the two enthalpies of solution. Propagate the uncertainty through the subtraction.
  8. Compare with literatureIs your hydration enthalpy within its uncertainty of the accepted value? Which run is responsible for the difference?
  9. EvaluateRank the sources of error by effect on the final value. Make each improvement specific.
  10. Write, then checkWrite in the order of the layout, then run the checklist below before your teacher's single read.

Chemistry you'll need

Traps to avoid in this Chemistry IA

  • Weighing the anhydrous salt slowly: it takes up water from the air, so you have fewer moles of CuSO₄ than you think.
  • Amounts that do not fully dissolve at the top of the range, so the last points are lower than they should be.
  • Using a different mass of water in the two runs, then subtracting the enthalpies as if they were comparable.
  • Forgetting that the pentahydrate run cools the solution, so ΔT is negative and the sign of the enthalpy flips.
  • Ignoring the heat capacity of the cup and thermometer, and not saying so.
  • Propagating uncertainty in each run but not through the final subtraction.

More Chemistry IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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