Chemistry IA · Reactivity 2 · How much, how fast, how far? · topic 20 of 25
Iron thiocyanate equilibrium: how does K change with T? · IB Chemistry IA idea: guide, sample graph and step-by-step plan
red solution, clean maths, good choiceA pale orange solution turns blood-red when you add thiocyanate. How red it gets tells you K, and how K moves with temperature tells you whether the reaction gives out or takes in heat. You're going to measure the sign of ΔH with a colorimeter.
- ✓ you like a result you can see (a red solution)
- ✓ you can plan a calibration carefully
- ✓ you want to link an experiment to energetics
The big idea
← your RQ hook is in hereFe³⁺(aq) + SCN⁻(aq) ⇌ [Fe(SCN)]²⁺(aq) is a gift for a colorimeter: only the complex is strongly coloured, so absorbance tracks its concentration. Le Chatelier gives a prediction: if the forward reaction is exothermic, K falls as temperature rises. Your job is to turn absorbances into equilibrium concentrations, calculate K at each temperature, and see whether the trend matches. At HL you can go further and get a value of ΔH from the shape of the trend. A calibration decision sits underneath everything, so plan it well.
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 →
Calculate K at five or more temperatures and use ΔG = −RT ln K together with ΔG = ΔH − TΔS to see what a graph of ln K against 1/T should give.
→ Your graph becomes a straight-line test, with ΔH from the gradient and ΔS from the intercept. You derive the link yourself; it is built from two HL equations, not quoted ready-made.Measure your standard solution at the lowest and highest temperatures too, not only at room temperature.
→ Tests whether the absorbance per mol dm⁻³ changes with temperature, which would bias every K you calculate.Use a different ligand, for example a chloro-complex or a salicylate complex of Fe³⁺.
→ New colour, new wavelength and calibration, but the same logic. Needs an extra check that only one complex forms.Repeat at one temperature with several starting ratios of Fe³⁺ to SCN⁻.
→ Tests whether K stays constant when the concentrations change, which is what constant in equilibrium constant means. A second, smaller study.Use a colorimeter with a thermostatted holder, or work quickly with a pre-warmed cuvette and a stated delay.
→ Reduces the biggest hidden error. The evaluation compares the two ways of controlling temperature.Link to iron in water supplies or to a medical test that uses iron(III) complexes.
→ Gives your introduction a purpose; the chemistry and the graph stay the same.Your Chemistry IA research question: run the check
write it FIRST, then check itWrite your own question first. Then make sure it ticks every box:
- Names the independent variable and its range (which temperatures, in °C, and how many values across 10 to 50 °C).
- Names the dependent variable as a calculated quantity (K from absorbance) and says how absorbance becomes concentration.
- Says which starting concentrations, acidity, wavelength and cuvette are kept the same, and how.
- States the reaction and which direction of change you predict, with the reason.
- Is narrow enough for about 10 hours, with repeats at every temperature.
The set-up
One way to set it up. Draw your own with your real volumes and concentrations 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.
Your question; where iron(III) complexes matter; the equilibrium and what Le Chatelier predicts for temperature.
The expression for Kc; why the complex absorbs and the reactants hardly do; the link between K, ΔG and ΔH if you are HL; what trend your graph should show.
Independent, dependent and controlled variables, each with how you set, measure or hold it, and why it matters (acidity, wavelength, ionic strength, cuvette).
How you made the stock solutions and why you acidified them; the calibration; how the mixtures reach each temperature; how you read the cuvette quickly; repeats. Repeatable by someone else.
Specific hazards at your concentrations: dilute nitric acid (about 0.5 mol dm⁻³) irritates eyes and skin; iron(III) nitrate is an irritant and oxidiser; potassium thiocyanate is harmful and gives a very toxic gas with concentrated acid, so never mix the solids with strong acid. Goggles, water bath under 60 °C, and waste collected, not poured away.
Calibration absorbances and each mixture's absorbance at each temperature, with units and uncertainties in the headers.
How absorbance becomes [complex], then the equilibrium concentrations of the other two species, then K. One worked example for one temperature, with the uncertainty.
K (or ln K) against temperature, with error bars, and what the trend says about the sign of ΔH. Compare with the prediction.
Answer with numbers: how K changed across your range, what that says about ΔH, and how far a literature value agrees.
Strengths, weaknesses ranked by their effect on K, and realistic improvements linked to each.
Your source for K or ΔH, and any method you adapted.
What your Chemistry IA graph could look like
error bars on EVERY point ✓Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only
- The axes make the physics a straight line: temperature on the right is colder, because it is 1/T.
- Error bars on every point. The uncertainty in ln K is mostly the colour calibration and the cuvette temperature, not the thermometer.
- The flat dashed line shows what no effect would look like, so the reader can see the trend is real.
- State how the gradient relates to ΔH and R, and compare with a literature value if you can find one.
- Another graph to consider: K against temperature in °C, to show the trend simply before you linearise.
Which uncertainty matters most?
Rank by the effect on K itself, not on the raw readings. Here the colour calibration and the temperature of the cuvette beat the colorimeter's own resolution. Your numbers will decide.
Your game plan
don't skip the trial run!Tick steps off as you go. We'll remember on this device.
0 of 11 done
- Read and predictWrite the equilibrium expression. Predict whether K rises or falls with temperature, and say which sign of ΔH that implies. Decide what your graph should look like if you are right.
- Plan the calibrationOnly the complex absorbs strongly, so you need a way to convert absorbance into [complex]. Decide which solutions make that possible and why one reactant is in large excess.
- Trial run (1 lesson)Make one mixture at room temperature and one at 50 °C. Does the colour settle? Does it fade as the cuvette cools? Is the solution clear, or does a brown tinge appear?
- Decide range, repeats and controlsChoose at least five temperatures between 10 and 50 °C, the starting concentrations, the filter, and how many repeats. Write the reason for each choice; this becomes your method justification.
- Collect the dataTime how long each cuvette is out of the bath before the reading, and keep it the same. Note anything odd as it happens.
- ProcessTurn each absorbance into [complex], then into the other two equilibrium concentrations, then into K. Carry the uncertainty through one worked example.
- Graph and fitPlot K or ln K against a temperature quantity that gives a clear trend. Add error bars and decide what a straight line would mean.
- Compare with theoryWhat does your trend say about ΔH? Does it agree in sign with Le Chatelier? Does it agree in size with a published value?
- EvaluateWhich source of uncertainty moved K most? Rank them and make every improvement specific to a weakness you found.
- Write, then cutWrite in the order of the layout above. Keep within 3,000 words: tables, graphs, equations and references are not counted, so let them carry detail.
- Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.
Chemistry you'll need
Traps to avoid in this Chemistry IA
- Solutions that are not acidified: Fe³⁺ hydrolyses to brown hydroxide species and the colour no longer comes only from the complex.
- A calibration done only at room temperature, then used at every temperature without checking.
- Reading the cuvette after it has drifted away from the bath temperature, so the K you record belongs to a different temperature.
- Using the starting concentrations as if they were the equilibrium ones: the reactants are used up a little.
- Too few temperatures, or a range too narrow to see a trend beyond your error bars.
- Uncertainty only on the absorbance, none on the temperature, volumes or calibration.
More Chemistry IA ideas like this
- Does lime water get less soluble when warm?●●○
- The vitamin C clock: finding activation energy●●○
- Does the rate really double every 10 °C?●●○
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.