Chemistry IA · Reactivity 3 · Mechanisms of chemical change · topic 24 of 25

A cell with a changing concentration · IB Chemistry IA idea: guide, sample graph and step-by-step plan

small voltages, big skill. good choice

Dilute one half-cell by a thousand and the voltage barely moves. A few tens of millivolts, but they follow a logarithmic law you can test.

Difficulty●●○ Medium
KitStandard: high-impedance voltmeter or data logger
TimeAbout 10 hours
FitsSL and HL (E° is HL; Nernst: introduce it yourself)
  • ✓ you like electrochemistry
  • ✓ you enjoy a log-law that makes a straight line
  • ✓ you can measure small voltage changes carefully

The big idea

← your hook is the log law

A zinc–copper cell has a standard cell potential from the data booklet. Change the concentration of Cu²⁺ and the potential shifts, because the position of equilibrium in each half-cell changes. The Nernst equation, which describes that shift, is outside the course: you must introduce it yourself, as a model, and explain where it comes from. Measure the cell potential across several decades of concentration, plot it against the logarithm of concentration, and ask whether a straight line fits and whether its gradient agrees with the model. The effect is small, so careful technique decides your marks.

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 →

Change the Zn²⁺ concentration instead

Hold Cu²⁺ constant and vary Zn²⁺.

→ The sign of the gradient reverses. The method is the same, and the comparison with the model tests both half-cells.
Vary temperature

Hold concentrations constant and measure the cell potential at 10 to 50 °C in a water bath.

→ A different independent variable. The change is only a few millivolts and links to ΔS through ΔG = −nFE (HL), so your uncertainty must be small.
Build a true concentration cell

Use copper electrodes in both half-cells with different Cu²⁺ concentrations.

→ The standard potential cancels, so the potential should be zero at equal concentrations. A cleaner test of the log law, but with a smaller voltage.
Use a different metal pair

Replace zinc–copper with magnesium–copper, or silver–copper only with your teacher's approval (silver nitrate is corrosive and stains skin).

→ A different E° and gradient. You compare the experimental gradient with each model value.
Add half-decade points

Add values between the decades, at about 0.003, 0.03 and 0.3 mol dm⁻³.

→ More points on the graph, which gives a better straight-line fit and a clearer picture of any curvature at high concentration.
Change the context

Link to concentration-based sensors, batteries running flat, or corrosion cells.

→ Gives your introduction a purpose; the method and graph stay the same.

Your Chemistry IA research question: run the check

write it FIRST, then check it

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

  • Names the independent variable and its range (concentrations from 0.001 to 1.0 mol dm⁻³ and how many values).
  • Names the dependent variable (the cell potential, in volts) and says how you read it once it is stable.
  • Says what you keep the same (the other half-cell, electrodes, salt bridge, temperature).
  • Says how you will test a logarithmic model, and where you got the model from.
  • Is narrow enough for about 10 hours, with at least five concentrations and repeats.

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The set-up

One way to set it up. Draw your own with your real solutions and electrodes for the Method section.

Vpolish the Zn each runvary [Cu²⁺] in decadessalt bridge (KNO₃)high-impedancevoltmeter

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; where concentration-dependent potentials matter (sensors, batteries); the prediction you will test.
Background chemistry~450
The two half-equations and E° from the data booklet; the cell reaction; the model you introduce for concentration, with its source; what graph it predicts.
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or keep it constant.
Method~450
How you made each Cu²⁺ solution from the stock and checked the dilution; how you cleaned the electrodes; how you built the cell; when you read the voltage. Repeatable by someone else.
Safety, ethics, environment~60
Copper(II) sulfate (up to 1.0 mol dm⁻³) is harmful if swallowed and irritates eyes and skin; zinc sulfate can cause serious eye damage; both are very toxic to aquatic life. Goggles and gloves; the salt-bridge solution (potassium nitrate) is an oxidiser. All waste collected for heavy metals, never down the drain.
Raw datatable
Concentration, calculated log of concentration, each voltage reading, with units and uncertainties in the headers.
Processing~250
Dilution calculations and the uncertainty in each concentration; mean voltage; one worked example.
Graphs and analysis~350
Potential against log of concentration, error bars, line of best fit, and what the gradient means compared with the model.
Conclusion~400
Answer with numbers: the gradient you found, how it compares with the model value, and where the data stop following the line.
Evaluation~650, the biggest part!
Strengths, weaknesses ranked by their effect on the gradient, and improvements linked to each.
Referencesnot counted
Your data-booklet values, and where you found the model.

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

-3.0-2.5-2.0-1.5-1.0-0.50.00.981.001.021.041.061.081.101.121.14log₁₀ ([Cu²⁺] / mol dm⁻³)Cell potential E / V (± 0.002 V)ideal straight line (made up)if concentration did not matteryour data, with error barsgradient = volts per tenfold change.Does your line match the model?ILLUSTRATIVE · MADE-UP DATA
  • Plot against the logarithm of concentration, not the concentration itself: that is what turns a curve into a line.
  • Error bars on every point. The vertical bars are the ones that matter here: the concentration axis is calculated, not read.
  • Draw the model line and your data line together so the reader can compare the gradients.
  • The scale starts near your data, not at zero. Say so, because the whole effect is under 0.1 V.
  • Another graph to consider: E against the ratio of ion concentrations, to test the model in a way that works for both half-cells.

Which uncertainty matters most?

Rank by the effect on the total change in potential across your range, here about 0.09 V. Notice that the dilution errors barely register: a log scale is forgiving. The electrode surface is not.

Oxide on the zinc (±5 mV)5.6%Voltage still settling (±3 mV)3.4%Voltmeter resolution (±1 mV ×2)2.3%Temperature (± 2 K)0.7%Dilution error (1% in concentration)0.14%biggest effect: discuss this first!% uncertainty in the total potential change (made-up)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

the trial run will show you the drift

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

0 of 11 done

  1. Read and predictWrite the half-equations and the cell reaction, find E° in the data booklet, and decide which way the potential should move when Cu²⁺ is diluted and why.
  2. Find and understand your modelLocate a source for the equation linking potential to concentration, and write in your own words where it comes from. It is beyond the course, so you must explain it.
  3. Trial run (1 lesson)Build the cell with two concentrations. How long does the reading take to settle? Does it drift? Does a freshly cleaned zinc strip behave differently?
  4. Decide range, dilution method and repeatsChoose your concentrations and how you will make them. Decide whether to dilute in one step from stock or by serial dilution, and how you will check the result.
  5. Collect the dataClean the zinc each time, rinse the copper, replace the salt bridge. Wait for a steady reading and note how long you waited.
  6. ProcessCalculate each concentration and its logarithm, with uncertainties. Decide how you will turn the voltmeter's resolution and any drift into an uncertainty.
  7. Graph and fitPlot potential against log concentration with error bars. Compare your line's gradient and intercept with the model's.
  8. Compare with theoryWhere does your data separate from the model? Is the gradient close, and if not, what could be responsible, such as the non-ideal behaviour of concentrated solutions?
  9. EvaluateWhich source of error moved the potential most? Rank them and make every improvement specific.
  10. 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.
  11. 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

  • Plotting E against concentration: the curve hides the log law. Use the logarithm.
  • A zinc strip with an oxide layer, or one that was not cleaned between runs, so the potential differs from run to run.
  • A voltmeter that draws current, which lowers the reading. Use a high-impedance meter or a data logger.
  • Reading the voltage before it has settled, or after it has drifted.
  • Quoting the Nernst equation without saying what it is or where it comes from. It is outside the course, so you must introduce it.
  • Assuming the solutions behave ideally at 1.0 mol dm⁻³ without a comment on the gradient that results.

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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