Chemistry IA · Structure 2 · Models of bonding and structure · topic 5 of 25
The ionic model vs real salts: where does it break? · IB Chemistry IA idea: guide, sample graph and step-by-step plan
HL-ish, but very doable at desk levelThe ionic model says every 'ionic' salt is made of perfect spheres. The energy numbers say otherwise, for some of them.
- ✓ you like energy cycles
- ✓ you enjoy testing a model against data
- ✓ you would rather skip the wet lab
The big idea
← this is your RQ hook!A lattice enthalpy from a Born–Haber cycle is built from experiments. One from the ionic model is built from charges and radii alone, assuming perfect, spherical ions. If the two agree, the model works. If the experimental value is bigger, there is extra bonding, usually described as covalent character. Compare the size of that gap across a series and for two metals, and the gap becomes your evidence about polarisation. The marks come from clean handling of data and conventions, not from equipment.
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 →
Include sodium fluoride and silver fluoride.
→ Check how fluoride sits in your trend; it may not follow the pattern, so you will need a good explanation.Compare group 1 halides with copper(I) or other group 11 halides.
→ More comparisons, so you can plot the gap against ionic radius or polarising power.Use a lattice enthalpy equation (such as Born–Landé or Kapustinskii; not in the course, so introduce it yourself) with ionic radii you choose.
→ Your own calculation, with an uncertainty that comes from the radii set you pick.Plot lattice enthalpy against the reciprocal of the sum of the ionic radii.
→ The ionic model predicts a straight line. The deviation from it is the evidence, and gradients can be compared.Plot the gap against electronegativity difference.
→ Links the gap to the bonding continuum. A good HL extension.Frame it around why silver halides are so much less soluble in water than sodium halides.
→ A real context. You must link lattice enthalpy and hydration enthalpy.Your Chemistry IA research question: run the check
Write your own question first. Then make sure it ticks every box:
- Names the compounds, the two quantities you compare, and which sign convention and units you use.
- Names the source of every number, and checks that sources use the same conditions and convention.
- Says what the model value assumes: which radii set and which equation.
- Says how the 'gap' is measured (difference or per cent difference) and how its uncertainty is estimated.
- Is narrow enough: one series, two metals, one source for each kind of number.
The picture to draw
No apparatus here: the key sketch is the energy cycle and the gap you are measuring. Draw yours for your own compound.
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 ionic model is used and where it is questioned; what you predict about the gap across the series.
Lattice enthalpy and the Born–Haber cycle; what the ionic model assumes; polarisation and covalent character; what trend you predict and why.
Where each value comes from (edition and page), the sign convention, how model values are obtained, and why these compounds. Say where sources disagree.
Independent, dependent and controlled variables, each with how it is chosen or held constant.
No lab hazards, so say so. Data ethics: cite each value, record where sources disagree, and do not pick the value that fits your story.
One clear table: compound, experimental value, model value, source, and sign convention for each.
All values on one sign convention, the gap for each compound, and an uncertainty from the spread between sources. One worked example.
The graph that shows the gap against the ion or the metal, with uncertainty bars; what the trend says about polarisation.
Answer with numbers: how the gap changes, for which compounds the ionic model fits and for which it does not, and how this compares with a published explanation (cite it).
Quality and consistency of data, the radii set, the structure assumed, the small number of compounds; weaknesses ranked, with improvements linked.
Every source for each number, edition and page.
What your Chemistry IA graph could look like
line through the origin = the model ✓Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only
- Axis labels state the quantity, its unit and which radii set you used.
- The model is a straight line through the origin, so the graph shows where it fails at a glance.
- Error bars from the spread between sources, not from the percentage difference with the model.
- Label each point with its compound so the reader can see which are furthest from the line.
- Another graph to consider: the gap against ionic radius or polarising power, to test your explanation directly.
Your game plan
fix your sign convention FIRSTTick steps off as you go. We'll remember on this device.
0 of 11 done
- Read and predictRead about lattice enthalpy, Born–Haber cycles and polarisation. Write down which compounds you expect to match the ionic model and which not, and why.
- Trial run (1 lesson)Collect experimental and model values for one compound from two sources. Do the signs and units agree? Do the values? Settle your convention now.
- Choose compounds and sourcesPick your series and metals. Decide the sources for experimental values and for model values. Check each uses the same conditions.
- CollectRecord each value with its source (edition and page). Where two sources disagree, record both.
- ProcessPut everything on one sign convention. Work out the gap for each compound. Decide how to estimate its uncertainty.
- Graph and fitChoose the graph that shows the gap best. Add uncertainty bars. Decide what pattern would support, or contradict, your prediction.
- InterpretLink the gap to ion size and charge. Does the trend match your prediction? Are the differences bigger than your uncertainty?
- Compare with literatureFind a cited explanation of the same trend. Does your data support it? Where does it not?
- EvaluateRank the weaknesses: sources, radii set, structure assumed, few compounds. Make every improvement specific.
- Write, then cutWrite in the order of the layout above. Tables, calculations and graphs are not counted.
- Check before your teacher's draftRun the checklist, then get your RQ or draft checked. Your teacher reads one draft only.
Chemistry you'll need
Traps to avoid in this Chemistry IA
- Mixing lattice formation and lattice dissociation values. They have the same size and opposite sign. State which you use.
- Taking numbers from books that use different conventions or different data sets.
- Treating the spread between sources as zero. Sources disagree by some kJ mol⁻¹, and that is your uncertainty.
- Using ionic radii for the model without saying which set. Radii depend on coordination number and on the source.
- Calling the gap 'covalent character' with no link to the size and charge of the ions (polarisation).
- Comparing compounds with different crystal structures (silver iodide differs from the other halides) without saying how the model handles that.
- Claiming a strong trend from only six compounds.
- An evaluation that says 'the radii were inaccurate' without saying how much that changes the model value.
More Chemistry IA ideas like this
- Bond length and strength by computer modelling●●○
- Same molar mass, different boiling points●○○
- Chromatography with a changing solvent polarity●○○
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.