Chemistry IA · Reactivity 1 · What drives chemical reactions? · topic 13 of 25

Do weak acids give off less heat when neutralised? · IB Chemistry IA idea: guide, sample graph and step-by-step plan

Neutralisation is not always the same textbook value. Weak acids add an extra step, and the difference is small, so your technique has to be good.

• ✓ you like calorimetry
• ✓ you want a small effect to measure well
• ✓ you enjoy chasing uncertainty

The big idea

When a strong acid meets a strong base, the reaction is just H⁺ + OH⁻ → H₂O, with an enthalpy change close to the same value every time. A weak acid is mostly un-ionised, so the measured heat includes its ionisation step as well. That extra term can be small and is not the same sign for every acid. The differences between acids may be only a few kJ mol⁻¹, so the real investigation is whether your method can see them: cooling-curve extrapolation, repeats and a clear uncertainty decide your result.

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 →

Your Chemistry IA research question: run the check

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

• Names the independent variable (which acid, and its pKa) and the dependent variable (enthalpy of neutralisation per mole of water formed, in kJ mol⁻¹).
• States the concentrations and volumes, and says why they give a temperature rise big enough to measure.
• Says how the temperature change is found (extrapolation of the cooling curve) and how often you will read it.
• Names what stays fixed: volumes, concentrations, starting temperatures, stirring, the cup and lid.
• Says what you will compare with, and for which conditions that accepted value applies.
• Is narrow enough for about 10 hours and has enough repeats to make a comparison between acids meaningful.

Get your RQ checked · S$29

The set-up

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

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.

What your Chemistry IA graph could look like

Made-up data to show the form of a strong graph, not the answer. This is one run: the extrapolation of the cooling line back to the mixing time. example only

• Both axes carry units and the reading uncertainty. Time is in seconds and temperature in °C.
• The extrapolation line is drawn on the graph, so the reader sees how ΔT was taken.
• Points from the first minute after mixing are excluded if they are still rising. Say why.
• Error bars on every point show the probe resolution; the scatter shows whether it is the real limit.
• Another graph to consider: enthalpy of neutralisation against pKa for all acids, each with its own uncertainty bar.

Which uncertainty matters most?

Rank by the effect on your result, the enthalpy per mole, not on the raw readings. Differences between acids are a few percent, so the biggest bars decide whether you can say anything.

Your game plan

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

0 of 10 done

1. Read and predictFind the enthalpy of neutralisation for a strong acid and the ionisation enthalpies of your weak acids. Predict the order, and write down roughly how large you expect the differences to be.
2. Trial run (1 lesson)Do one strong-acid run. Does the temperature peak and then cool slowly? How long must you record before and after mixing? Is the rise big enough? Fix the volumes and concentrations now.
3. Decide your acids and amountsChoose the acids, the concentrations and the volumes. Write down why, including why the rise gives a measurable signal.
4. Plan repeatsDecide the number of repeats per acid, enough to show the spread between runs. Randomise the order of the acids.
5. Collect the dataRecord temperature every 30 seconds (or logger) before mixing, at mixing, and after. Note start temperatures and anything odd.
6. ProcessFor each run, extrapolate the cooling line to the mixing time, calculate ΔT, the heat, the moles of water, and the enthalpy per mole, with uncertainty.
7. Graph and comparePlot each cooling curve with its extrapolation, then a summary of the enthalpy for each acid with error bars.
8. Compare with literatureWhere do your values sit against the accepted ones, and are the differences between acids larger than your uncertainty?
9. EvaluateWhich uncertainty or systematic error mattered most? Rank them and make each improvement specific to a weakness you found.
10. Write, then cutWrite in the order of the layout, then run the checklist and get a draft checked before your teacher's single read.

Chemistry you'll need

• IB Chemistry data booklet →
• Uncertainties and error bars (physics examples, same rules) →
• IB command terms explained →

Traps to avoid in this Chemistry IA

• Using the highest temperature as ΔT: the cup is already losing heat by then, so ΔT comes out too small.
• Comparing acids whose differences are smaller than your uncertainty and still claiming a ranking.
• Giving the result per mole of acid or base instead of per mole of water formed, which differ if the amounts are not equal.
• Mixing without both solutions at the same starting temperature, so the starting point is a guess.
• Assuming the solution has the heat capacity of pure water without saying so.
• Safety copied from a worksheet, with nothing on the actual concentration of your sodium hydroxide.

More Chemistry IA ideas like this

• Hess's law with copper(II) sulfate hydration●●○
• Dissolving uphill: entropy and potassium nitrate●●●
• Why do carbonates decompose at different temperatures?●●●