Chemistry IA · Structure 3 · Classification of matter · topic 12 of 25
The C=O stretch: why does it shift between carbonyls? · IB Chemistry IA idea: guide, sample graph and step-by-step plan
clean idea: one bond, lots of evidenceOne bond, many wavenumbers, and a simple reason for each. Can you find it, and show it with numbers?
- ✓ you like reading spectra
- ✓ you want one clear bond to explain
- ✓ you are happy to use a database
The big idea
← this is your RQ hook!The C=O stretch always sits in the same region of an infrared spectrum, but not at the same place. Electron-withdrawing groups on the carbonyl carbon tend to shift it up, while groups that donate electrons into the C=O through resonance tend to shift it down. Your job: pull peak positions for several families from one library, compare like with like, and test whether bond strength explains the order. Measure peaks properly, and the explanation becomes a real analysis, not a list.
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 →
Use the harmonic oscillator model (not in the course: introduce it yourself) to convert each peak position into a force constant for the C=O bond.
→ Your values become a measure of bond stiffness. You test a simple model against the families and evaluate where it is only approximate.Compare several ketones, or several esters, of different size.
→ A within-family graph. You check whether the shift between families is bigger than the spread inside one family.Include an aryl ketone or an unsaturated carbonyl alongside the saturated ones.
→ A new factor to explain. Your prediction has to say which way conjugation moves the peak and why.Take the same compound in two phases, for example gas and liquid, from the same library.
→ Turns the main control into the variable. Shows how much hydrogen bonding and phase move the peak.Take the same peaks from two sources and compare.
→ Gives you a real, measurable uncertainty for database data and something concrete for the evaluation.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 functional group, or a property of it) and the dependent variable (C=O peak position in cm⁻¹).
- Names which families and how many compounds from each, with the number chosen so a spread within a family can be seen.
- Fixes the sample phase and the library, and says so, because both move the peak.
- Says how peak positions will be read (peak list or magnified spectrum) and to what precision.
- Is narrow enough to finish in about 10 hours and sharp enough to explain with bonding theory.
The set-up
Your 'apparatus' is a spectrum and a way to read it. Draw one of yours, with the C=O region marked and the details of how you read it.
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.
Your question; why the C=O stretch is a good probe of bonding; the order you predict across the families and why.
How infrared absorption works for a bond; why bond strength and bond polarity change the wavenumber; inductive and resonance effects, with the link to your prediction. Cite sources.
Independent, dependent and controlled variables, with how each control (phase, library, chain length) was enforced.
Which library; which compounds and why; how you chose them before looking; how you read each peak and what precision that gives.
No chemicals. Say so briefly and mention the database terms of use.
Compound, family, phase, library ID, peak position, with units and the precision of reading.
Averages and spread for each family; any force-constant calculation with one worked example; how you treated several peaks near the C=O region.
A graph that shows families side by side with their spread; what the ordering tells you about bond strength; where one family breaks the pattern.
Answer with values and spread; say whether the differences are bigger than the variation inside a family; link back to your prediction.
Limits of the data (phase, resolution, choice of compounds) and of the explanation; what you would add; which limit matters most.
The library with the compound IDs, and every source used.
What your Chemistry IA graph could look like
what does this graph NOT test?Made-up data to show the form of a strong graph, not the answer. Here, each point is a compound, plotted at the force constant calculated from its peak. example only
- The graph is built from peak positions, so the points have to fall on a curve. Say what it does and does not test.
- Error bars show the reading precision. Library peak lists are often rounded, so say to what.
- Units are in the labels: ν̃ in cm⁻¹ and k in N m⁻¹, and the mass in the formula is in kg.
- Colour or label the points by family so the grouping can be seen at once.
- Another graph to consider: peak position by family with every compound shown, so the spread within a family is visible.
Your game plan
the trial run decides how you read peaksTick steps off as you go. We'll remember on this device.
0 of 10 done
- Read and predictRevise infrared absorption, bond strength, inductive and resonance effects. Write down the order of the families you expect, and why, before you open a library.
- Choose and test a libraryPick one spectra library. Check what it gives: a peak list or only an image, the phase, the resolution.
- Trial run (1 lesson)Pull one compound from each family. Can you read the C=O peak reliably? Is it the strongest peak? Are two peaks close to it? Fix your reading method now.
- Decide your sampleChoose several compounds per family, of similar size, in the same phase. Write down your rules before collecting.
- Collect the peak positionsRecord each position with library ID, phase and precision. Use peak lists or a magnified image, not a thumbnail.
- ProcessGroup by family, find means and spread, and decide how you will treat a compound that shows two peaks in the region.
- Explain with evidenceFor each family, state what your theory predicts for the shift, then compare with the numbers. Where the order surprises you, say why.
- EvaluateWhat limits your conclusion most: the choice of compounds, the phase, the reading precision, or the model? Rank them.
- Write, then cutWrite in the order of the layout. Let tables and graphs carry detail.
- Check before your teacher's draftRun the checklist below and get your RQ or draft checked.
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
- Comparing spectra measured in different phases: gas, liquid and solid peaks differ, so the family effect gets mixed with the phase effect.
- Reading peaks by eye from a small image, so your precision is worse than the differences you want to explain.
- One compound per family, so you cannot tell whether a difference is the family or just that molecule.
- Treating two nearby peaks as one, for example from Fermi resonance or an overtone, without checking.
- Explaining the order with one effect only. Several effects act at once and your analysis should weigh them.
- An evaluation that says 'the database may have errors' without saying which and how big.
More Chemistry IA ideas like this
- Ionisation energies: where do the shells show?●●○
- Ethanol and water: do the volumes really add up?●○○
- Hydrogen's spectrum: pulling out the Rydberg constant●●○
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.