Chemistry IA · Reactivity 1 · What drives chemical reactions? · topic 16 of 25

Why do carbonates decompose at different temperatures? · IB Chemistry IA idea: guide, sample graph and step-by-step plan

a model you build yourself

Predict how hot a carbonate must be for ΔG to turn negative, with only a data table. Then see how wrong you are, and why.

Difficulty●●● Stretch
KitComputer only: data tables + spreadsheet
TimeAbout 10 hours
FitsHL (entropy and Gibbs energy)
  • ✓ you like a model you can build yourself
  • ✓ you are happy with a spreadsheet
  • ✓ you want to compare a prediction with real data

The big idea

← this is your RQ hook!

A carbonate decomposes to an oxide and carbon dioxide, and the reaction takes in heat but releases a gas, so entropy rises. Gibbs energy decides: ΔG = ΔH − TΔS is zero at T = ΔH/ΔS, and above that temperature the carbonate decomposes. Down group 2 the ΔH grows, so the temperature climbs. Standard conditions are 1 bar of CO₂, but air has very little, so the real temperature is lower. You calculate both, compare with measured temperatures, and explain the gap.

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 →

Add the pressure of CO₂ as a second variable

Compute the decomposition temperature at several CO₂ pressures, from 1 bar to the level in air.

→ Your analysis has a second curve. Le Chatelier's principle becomes a number you can show on a graph.
Replace ΔH and ΔS constants with temperature-dependent values

Use heat capacities from the same source to correct ΔH and ΔS at the predicted temperature (beyond the course: introduce it yourself).

→ Tests the assumption that they are constant and gives your evaluation a quantified result.
Compare group 1 and group 2

Add sodium and potassium carbonate as a second family.

→ Shows a very different pattern, so your model has to explain why not all carbonates behave the same.
Link to ionic radius or charge density

Plot the predicted temperature against the cation radius.

→ Turns a table of numbers into an explanation based on polarisation, a model you can check.
Use a simulation or database of reaction energies

Take ΔG at several temperatures from a thermodynamic database instead of a single ΔH and ΔS.

→ Your model is checked against a source with more physics in it, so your evaluation is about a better model.

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 metal, and the CO₂ pressure if included) and the dependent variable (the temperature at which ΔG = 0, in K or °C, stated).
  • Names the single data source for every ΔH and ΔS, and the state of every species.
  • States the assumptions: ΔH and ΔS constant, standard state, pressure of CO₂, and how each will be checked.
  • Says what the prediction will be compared with (measured decomposition temperatures) and from which source.
  • Is narrow enough for about 10 hours: one family and one or two pressures, not every carbonate.

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The set-up

No apparatus: the picture is the model. Sketch ΔG against T for your own metals and mark where each line crosses zero.

ΔGTT₂T₁ΔG > 0: no reactionΔG < 0: decomposesΔG = 0 is thedecompositiontemperature1 bar CO₂:ΔG = ΔH − TΔSair (0.0004 bar):steeper, so T₂ < T₁ΔH in kJ, ΔS in J: convert first!

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 decomposition temperatures matter (lime and cement, flame retardants); the trend you predict and why.
Background chemistry~450
Decomposition equation with state symbols; ΔG = ΔH − TΔS and the equation with a pressure term; what each assumption means. Cite sources.
Variablestable
Independent, dependent and controlled variables: source, units, state and pressure.
Data source and sampling~400
Which tables, which metals and why; how you checked units and states; how you chose the pressures.
Safety, ethics, environment~50
No chemicals. Say so briefly, mention data terms of use and the environmental relevance of CO₂ released by lime making.
Raw datatable
ΔHf and S° for each species with source, units and the precision given.
Processing~300
ΔH and ΔS of the reaction, then T = ΔH/ΔS and the pressure-corrected version; one worked example; uncertainty propagation.
Graphs and analysis~400
Predicted temperature against metal or radius for each pressure; a comparison plot of predicted against measured values.
Conclusion~450
Answer with values and uncertainty; compare with measured temperatures; say which assumption explains most of the gap.
Evaluation~600, the biggest part!
Assumptions (constant ΔH and ΔS, ideal gas, pure phases), data limitations and how each shifts the prediction; improvements.
Referencesnot counted
Every data table and text, with edition or date.

What your Chemistry IA graph could look like

add the measured values too

Made-up data to show the form of a strong graph, not the answer. Predicted decomposition temperature against ionic radius of the metal, for two CO₂ pressures. example only

607080901001101201301403005007009001100130015001700Ionic radius of M²⁺ / pmTemperature at which ΔG = 0 / K1 bar CO₂0.0004 bar CO₂ (air)values calculated from your datasame trend, lower temperatures:which term in ΔG changed?ILLUSTRATIVE · MADE-UP DATA
  • Both curves use the same ΔH and ΔS from one source. Only the CO₂ pressure term differs.
  • No error bars here because the table values have none. Say what precision they have and propagate it.
  • Units in the formula: kJ for ΔH and J for ΔS. The factor 1000 is where many calculations go wrong.
  • Add measured decomposition temperatures as a third set of points, with their source, so the gap is visible.
  • Another graph to consider: ΔG against temperature for each metal, so every crossing point is on one picture.

How much does the assumption matter?

Compare the same model under different assumptions. The shift from one pressure to another is bigger than any rounding in the data, which tells you where to look first.

1 bar of CO₂ (standard state)1123 K0.1 bar of CO₂1002 K0.0004 bar of CO₂ (air)797 Kthe standard state overstates itpredicted decomposition temperature / K (made-up)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

do one calculation by hand first

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

0 of 10 done

  1. Read and predictRevise ΔG, ΔH, ΔS and ΔG = ΔH − TΔS. Write down the trend you expect down the group and what you think happens at a low CO₂ pressure.
  2. Trial run (1 lesson)Do the calculation for one metal by hand and then in the spreadsheet. Do the two agree? Is the temperature sensible? Check units.
  3. Choose the data sourcePick one source for every ΔHf and S°. Check the states and the temperature the values apply to.
  4. Decide metals and pressuresChoose the metals and the CO₂ pressures, and write down why, before you see all the results.
  5. Build the spreadsheetKeep raw data on one sheet and calculations on another, with units in every column header.
  6. Calculate and propagateCalculate ΔH, ΔS and the temperature. Decide how you propagate the precision of the data.
  7. Compare with measurementFind measured decomposition temperatures from a cited source and compare. Say what conditions they apply to.
  8. Explain the gapList the assumptions that could shift the prediction, test the biggest, and show its effect with a number.
  9. EvaluateRank the assumptions by their effect on your result and 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

  • Mixing kJ and J: ΔH is usually in kJ mol⁻¹ and ΔS in J K⁻¹ mol⁻¹, so the temperature comes out 1000 times wrong.
  • Assuming ΔH and ΔS stay constant over hundreds of kelvin without saying so.
  • Using standard conditions (1 bar CO₂) and then comparing with measurements made in air.
  • Mixing data from different sources, so differences may be the table and not the chemistry.
  • Comparing the calculated value with the literature without saying what the literature temperature means (onset, 1 bar, in air).
  • An evaluation that says 'data may be inaccurate' without testing any one assumption with a number.

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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