LearnChem · Part IV — What makes a reaction go further, or faster?
An iron grille on a Chennai balcony goes brown in one season. The aluminium window frame beside it stays bright for years. Both stand in the same salty air. This module asks what rust takes from iron. It asks what the weather has to do with it. And it asks what a battery has to do with a grille. It ends by carrying your textbook's own 1.67 V from the page to the wet balcony.
"Why do batteries die — where was the electricity hiding? And why do only some recharge?"
Nothing was hiding. A battery holds a reaction that has not run yet. By the end you can say why a rusting grille and a running battery are one chemistry.
Module 20 stirred a spoon of salt into a glass of water. The crystal came apart into ions, and water held every one of them. That was a salt meeting water on purpose. This module starts with a metal meeting the weather. Nobody pours a glass of water over the grille on the balcony. It gets rain, sea air and sun. The question is what the weather does to the metal, and what the metal does back.
This module sits across three school years. Class 10 covers corrosion and the reactivity series in words. Class 11 gives redox and the oxidation number. Class 12 gives electrode potentials, cell voltage and the Nernst equation. The Nernst equation is in the current syllabus. You are fifteen, so nothing from the later years is assumed. Everything is built from module 20, module 12 and module 6. The class 10 placement was checked against notes and not against the official syllabus. Treat it as the least certain of the three.
Start on the balcony. The iron grille goes brown in one season. The aluminium frame beside it does not. Both stand in the same salty air. This module asks why, and it starts with the grille. Rust is not a coat that lands on the iron. The iron itself changes. Something happens to each iron atom at the surface. Before the chemistry, say what you think that is.
An iron atom at the surface of the grille ends up as rust. What happens to the atom itself?
Answer from your gut. The reveal follows.
The iron atom loses two electrons. Chemists call losing electrons oxidation. The word comes from oxygen, because oxygen was the first known taker. The definition has moved on. Oxidation is loss of electrons, whatever takes them. One iron atom becomes one iron(II) ion. An ion is an atom that has lost or gained electrons, so it carries a charge. The equation is one line.
Fe(s) → Fe2+(aq) + 2e-
Bookkeeping, not a real per-atom charge: iron starts at 0 and leaves as +2. Losing electrons is what "oxidation" means — oxygen is one partner that takes them, not the definition itself.
So every rusting iron atom sends two electrons away. Something must take them. First, count. A count of electrons needs a rule that works for any atom in any compound. Chemists call the rule the oxidation number. An oxidation number is a bookkeeping charge given to each atom in a compound. It is not a real charge sitting on that atom. Its one test is simple. The oxidation numbers in a species must add up to the charge of the species. A species is any one kind of atom, molecule or ion. Oxygen in a compound is usually given −2. Before the bench, apply the test to rust itself.
Rust is mostly Fe2O3. Oxygen in it carries −2. What oxidation number does each iron atom carry?
Predict, then drive the bench.
Now pick the overall rusting reaction. Each of 2 iron atoms goes from 0 to +2, so 4 electrons leave. Each of 2 oxygen atoms goes from 0 to −2, so 4 electrons arrive. The two counts match, and they must. Electrons are not made and not lost. Every electron the iron gives up, something takes. Now pick the zinc and copper reaction. No oxygen appears in it at all, and the count still balances. That is why oxidation means losing electrons, and not meeting oxygen. Rust is one case of a general rule.
Two more come from outside iron chemistry, so the rule reads as general and not as a fact about rust alone. Manganese in permanganate carries +7, and chromium in dichromate carries +6.
There is one more step. The +2 ion is not yet rust. Oxygen takes one more electron from it later, and it becomes +3. That is the iron in Fe2O3. The bench's reaction named Fe2+ → Fe3+ counts that single electron. Here is a fact about readers. One study asked 63 class 10 students. In it, 16.67 per cent defined oxidation by oxygen and not by electrons. And 39.68 per cent had the direction the wrong way round. They said oxidation was gaining electrons. The bench makes the direction a thing you count. It is not a thing you remember.
In hydrogen peroxide, H2O2, each hydrogen is +1. The molecule carries no charge. What oxidation number does each oxygen carry?
— Rusting is iron losing electrons. The bookkeeping says how many.
Section 1 counted the electrons that leave the iron. Something must take them. Now comes the hinge of the whole module. Here are two grilles, alike in every way but one. Say which one rusts before you read on.
Two identical iron grilles stand in ordinary air. One is on a wet Chennai balcony. One is in a room where the air is kept very dry. Which rusts?
Guess before you are shown.
Ask most people, and they say rust is iron plus oxygen. Water, they say, only speeds it up. Dry iron does not rust. Below about 60 per cent relative humidity, clean iron in clean air stays bright. Relative humidity is the water vapour in the air, as a share of the most it can hold. Above that share, a thin film of water forms on the metal. Then two things happen, at two different spots on the surface. At one spot, iron atoms lose electrons and go into the film as ions. At another spot, oxygen takes electrons, together with hydrogen ions from the film. Chemists call each of these a half-reaction, one half of an electron exchange written on its own. Add the two halves, and the water appears on the right.
Fe(s) → Fe2+(aq) + 2e-
O2(g) + 4H+(aq) + 4e- → 2H2O(l)
2Fe(s) + O2(g) + 4H+(aq) → 2Fe2+(aq) + 2H2O(l)
So water does two jobs. It is where the iron ions go. And it carries the hydrogen ions that the oxygen half uses. Your textbook says those hydrogen ions come from carbon dioxide dissolved in the film. Take the film away and both halves stop. Here is one honest gap. No survey has measured how many students believe water is not needed. The nearest finding is about a rusting nail's weight. About a third of 15-year-olds say the nail gets lighter. About a third say it stays the same. About a third say it gets heavier. That is a different question, and this page reports it as a different question. Now go to the coast. Chennai's air carries sea salt. Predict what the salt does.
Chennai's air carries sea salt, and some of it lands on the grille. What does the salt do to the rusting?
Commit, then read what the measurements say.
Sea salt is hygroscopic. Hygroscopic means it pulls water out of the air. A salt crust on the grille keeps a film of water under it, even in dry weather. Under acidified sea-salt deposits, steel has been measured still rusting at 11 per cent relative humidity. The water is still there. The salt is holding it. Natesan and others exposed bare mild steel for one year at ten Indian sites. At Chennai port, about 150 m from the sea, it lost 524.0 micrometres of thickness. A micrometre is a millionth of a metre, written µm, where µ is the sign for a millionth. At Mahendragiri, an inland hill station, it lost 13.57 µm. That is 38.61 times more at the coast, for the same metal in the same year. No wire and no battery did that. Salt and water did.
One correction is made here, in the open. The paper labels Chennai C5 on the ISO 9223 scale. Here are ISO's own first-year bands for steel.
| ISO 9223 category | first-year loss, µm/year |
|---|---|
| C1 | up to 1.3 |
| C2 | 1.3 to 25 |
| C3 | 25 to 50 |
| C4 | 50 to 80 |
| C5 | 80 to 200 |
| CX, added in 2012 | 200 to 700 |
So 524.0 is CX, not C5. The 1992 edition the authors used had no CX band. The measurement is trustworthy. The label is not. Check it against the bands rather than take it from this page.
In a textbook demonstration, an iron nail sits in a sealed tube. The water around it was boiled to drive out its air. What happens over a month?
— Iron with no water film does not rust. The film is where the ions go.
Iron gives its electrons to oxygen. Would zinc? Would gold? Chemists have measured how tightly every common metal holds its electrons. They do not measure it directly. They measure a voltage. The metal stands in a solution of its own ions, and a fixed reference stands beside it. The reference is a hydrogen electrode, set at zero. A coulomb is the unit of electric charge. A volt is one joule of energy for each coulomb that moves. The numbers make a ladder. Your class 12 textbook prints it as Table 2.1. Before you see it, predict where three metals sit.
The ladder puts the metal that gives up electrons most readily at the most negative number. Put zinc, iron and gold in order, most negative first.
Predict before you are told.
Zinc comes first, then iron, then gold. Here is the ladder itself, as your textbook prints it. Every line is written as a reduction. A reduction is the reverse of oxidation. It is an atom or ion gaining electrons. So each line reads as the ion taking electrons to become the metal. A negative number means the ion takes electrons less readily than a hydrogen ion does. Read the line backwards, and the metal gives them up more readily. Chemists call the number the standard electrode potential. Standard means every ion at 1 mol per litre, every gas at 1 bar, and 298 K. A bar is a unit of pressure, close to the air pressure at sea level. Your textbook says a negative value marks a stronger reducing agent than hydrogen. A reducing agent is a substance that gives electrons away.
The basic oxygen couple, O2 + 2H2O + 4e− → 4OH− at +0.401 V, is not in NCERT's table. Its value is from the CRC Handbook's electrochemical series, compiled by Vanýsek. It appears only when the box is unticked. It is marked CRC, so you can see which row your own textbook lacks. Section 6 needs it.
The table has gaps, and the bench names them. It carries no metal couple for manganese, cobalt, cadmium or platinum. It carries no oxygen couple for basic solution. Row 28 is a hydrogen couple in base, not an oxygen one. Two rows share +1.23 V. So a number alone does not name a row. Row 5 is gold at +1.40 V. Other tables give gold +1.498 V or +1.52 V. This page could not settle which. So gold is used here only for which side of oxygen it sits on, and every table agrees on that. NCERT rounds iron to −0.44 V. A fuller table gives −0.447 V. This page uses NCERT's number throughout, so its arithmetic matches your book.
A ladder tells you which of two metals gives up electrons to the other. It does not tell you how fast. Lithium sits at the very bottom, −3.05 V. Sodium sits above it at −2.71 V. Drop each into water, and sodium reacts far more violently. So the order of how far and the order of how fast disagree. Your class 11 textbook puts the slowness down to lithium's small ion and its large hydration enthalpy. Lithium's is −506 kJ/mol against sodium's −406. Hydration enthalpy is the heat given out when water gathers round one mole of ions. That same enthalpy is part of why lithium's number is so negative. So one quantity feeds both, and the two orders still disagree. Module 16 dealt with how fast.
Every table puts gold above oxygen's +1.23 V, whichever value it prints for gold. So oxygen cannot take electrons from gold under standard conditions. The push runs the other way. A gold ring in Chennai's air stays gold, because nothing in the air takes what gold holds. Silver, at +0.80 V, is nearly as safe from oxygen. Yet a silver bangle dulls. Module 10 said why. A trace of sulphur in the air does what oxygen cannot, and the black product is silver sulphide.
A copper coin drops into a silver nitrate solution. A second copper coin drops into a zinc sulphate solution. Where does a grey coat of new metal appear?
— The ladder says which metal gives up electrons to which.
On the grille, electrons leave the iron at one spot and reach oxygen at another. The two spots can be a millimetre apart. So the electrons travel. Now comes a question about the route. Between the two spots there is solid iron. There is also the film of salty water on top of it. Which one carries the electrons? Commit before the reveal.
On a rusting grille, iron dissolves at one spot and oxygen takes electrons at another. How do the electrons get from the first spot to the second?
Commit, then read what follows.
The electrons never enter the water. They move through the metal. Module 9 poured a metal's outer electrons into a sea shared by the whole piece, and that sea is the wire. The water film carries ions instead. Ions moving are a current too, because they carry charge. So there is a full loop. Electrons go through the iron one way. Ions go through the film the other way. Nakiboglu and others tested two senior-school groups, one Turkish and one Indonesian, on this. About 70 per cent said electrons flow through the solution. It is the most common wrong picture in this whole subject. Now stretch the loop.
Picture a crowd leaving a stadium. The people are the electrons. Rusting is the gate right beside the stand. The crowd pours out on the spot, and nothing is gained from it. Now shut that gate and open a long corridor instead. The corridor is a wire. Put a turnstile in the corridor. The turnstile is a load, anything that does work as each person pushes through it. The crowd still leaves, and the leaving now turns a turnstile. One more thing moves. The far end cannot fill up while the stand empties. So stewards walk back across the pitch to keep the two ends level. The stewards are the ions, and the pitch is the water film or the salt bridge. No electron crosses the pitch. The picture breaks in two places. People decide to leave. Electrons decide nothing, because the energy difference between iron and oxygen settles it. And a crowd thins out until the stadium is empty. The iron keeps supplying electrons until the iron is gone.
A magnesium strip stands in a half-bucket of its own ions. An iron strip stands in another. A wire joins the strips and a salt bridge joins the buckets. Which metal is eaten?
Commit, then build the cell yourself.
The magnesium is eaten and the iron is spared. Now set the bench to the module's own cell, and read the arithmetic in the open. Set the anode to row 25, iron, and the cathode to row 8, oxygen in acid. The oxygen line has E° = +1.23 V and the iron line has E° = −0.44 V. E°cell is the cathode's number minus the anode's number, both read as reductions. So E°cell = (+1.23 V) − (−0.44 V), and taking away a negative is adding. That gives +1.23 V + 0.44 V, which comes to +1.67 V. That is your textbook's own number for the rusting cell, to the last digit. A positive E°cell means the pair runs as written, with iron eaten and oxygen used. Swap the two rows on the bench and the sign flips to −1.67 V. A negative sign means you have wired the pair backwards.
Here are two more loop facts, each with a number. The salt bridge carries ions between the two half-buckets. No electron crosses it. A different study, of 360 Ethiopian students, asked about the bridge itself. In it, 68 per cent said electrons flow through the salt bridge. Next, the current is the same at every point of the loop. The load uses up energy. It uses up no charge. One class 11 group of 172 put that wrong at 34 per cent. A university group put it at about 1 per cent. So the figure depends on who is asked. Here is one last fact. In a cell that delivers energy, electrons pour out of the anode into the wire. So a meter marks the anode negative. That is a fact about this cell.
A copper strip is bolted to an iron plate, and seawater covers the join. No wire, no salt bridge, no bucket. Which metal is eaten?
— The same reaction through a wire can do work on the way.
Bench 2 gave the rusting cell +1.67 V. Module 12 measured reactions in kilojoules per mole. Module 14's referee, ΔG, uses the same unit and says which way a change runs. A volt and a kilojoule look like two different worlds. They are not. Before the arithmetic, guess the size of the link.
Bench 2 gave +1.67 V for the rusting cell as written, with four electrons moved. Roughly how much energy is that, per mole of reaction?
Predict, then check the arithmetic.
Several hundred is right. Here is the link. Section 3 said a volt is one joule for every coulomb that moves. One mole of electrons carries 96485.33212 C. That number is the Faraday constant, F. So a cell's voltage, multiplied by the charge that moves, is an energy. Module 14's referee is ΔG, the Gibbs energy change. It says which way a change runs on its own. The two are one number in two units, with a minus sign between them. ΔG° = −nFE°. The minus sign is there for a reason. A positive voltage means the reaction runs forward. The referee marks a forward change negative.
Here is the arithmetic, worked in the open. n is the number of moles of electrons per mole of reaction as written. Two iron atoms give two electrons each, so four electrons leave the iron. One oxygen molecule takes those four electrons, so n is 4. F is 96485.33212 C/mol, from above. E° is +1.67 V, the standard cell voltage from Bench 2. So ΔG° = −(4)(96485.33212 C/mol)(1.67 V), three factors and one minus sign in front. Multiply 4 by 96485.33212 C/mol by 1.67 V, and the product is 644 522 J/mol. The units work because one coulomb multiplied by one volt is one joule. Put the minus sign back, and ΔG° = −644 522 J/mol. Divide by 1000 to turn joules into kilojoules, and ΔG° = −644.52 kJ/mol. A positive E° has given a negative ΔG°, so the referee says rusting runs forward on its own. The sign is the whole point of the calculation, so read it twice. The readout below repeats the product, rounded to one decimal place.
A zinc and copper cell gives +1.10 V, with n equal to 2. Which statement about its ΔG° is true?
— A cell's voltage and a reaction's energy are one number in two units.
Your textbook says the rusting cell gives 1.67 V. That is right. It is also a standard-state number, with hydrogen ions at 1 mol per litre, which is pH 0. This section climbs down three rungs. pH 0 is the textbook. pH 7 is pure water, the reference. pH 5 is the balcony, whose film holds dissolved carbon dioxide.
The tool is the Nernst equation. It uses a logarithm, and module 19 built one. A logarithm is a count of tens. The logarithm of 1000 is 3, because 1000 is three tens multiplied together. pH is that count with its sign flipped. Say which way the voltage moves on the first step down.
Keep the dissolved iron at 1 mol per litre and the oxygen at 1 bar. Move only the acidity, from pH 0 to pH 7. What happens to the cell voltage?
Predict, then run the bench.
It falls, and by a lot. The Nernst equation says how much. E = E° − (0.05916 V ÷ n) × log Q. E° is the standard number and n is the electrons moved, 4 here. The 0.05916 V is a slope built from R, the temperature and F. R is the gas constant, 8.314462618 J/(mol·K). Q is the reaction quotient, which module 17 wrote first. It is the products' concentrations, each raised to its coefficient, divided by the reactants' concentrations, treated the same way. Solid iron and liquid water are pure, so their amounts cannot change, and each counts as 1. So Q = [Fe2+]2 ÷ ([H+]4 × pO2). The bench sweeps it.
Here is the arithmetic, worked in the open. First write log Q in pH terms, because pH is a logarithm already. log Q = 2 log [Fe2+] + 4 pH − log pO2. The 4 pH term comes from [H+]4, whose logarithm is −4 pH. Start at the standard state. That is pH 0, iron(II) at 1 mol per litre and oxygen at 1 bar. All three terms are 0, so log Q is 0 and the correction is 0. So E equals E°, which is 1.67 V. Now step down to pH 7, with the iron(II) and the oxygen still at 1. Then 4 × 7 is 28 and the other two terms are still 0, so log Q is 28. The slope 0.05916 V divided by n, which is 4, is 0.01479 V. Multiply 0.01479 V by 28, and the correction is 0.4141 V. Take 0.4141 V from 1.67 V, and E is 1.2559 V. That is the pure-water rung, with everything else still at standard state.
Now make the pure-water rung real, in two steps. Rain washes iron(II) away, so first take [Fe2+] down to 10−6. Keep the oxygen at 1 bar for this step. Then 2 log [Fe2+] is 2 × (−6), which is −12. So log Q is −12 + 28, which is 16. Multiply 0.01479 V by 16, and the correction is 0.2366 V. Take 0.2366 V from 1.67 V, and E is 1.4334 V. The push has risen by 0.1775 V, from 1.2559 V to 1.4334 V. Iron(II) is a product, and taking away a product made the reaction push harder. Module 18 said a balance with too little product runs forward, and here it is as a voltage. Second, air is only 0.21 bar of oxygen, so take pO2 down to 0.21. The logarithm of 0.21 is −0.6778, and subtracting it adds 0.6778, so log Q is 16.6778. Multiply 0.01479 V by 16.6778, and the correction is 0.2467 V. Take 0.2467 V from 1.67 V, and E is 1.4233 V. So 1.4233 V is the pure-water reference in real air, and it is not the balcony.
The balcony is the third rung, near pH 5, so 4 × 5 is 20 in place of 28. Tier 2 works that rung, and its answer, 1.5417 V, is the balcony's number. So the textbook's 1.67 V, the pure-water 1.4233 V and the balcony's 1.5417 V are three rungs of one ladder.
The balcony film stops draining, and iron(II) builds up in a stagnant puddle. The pH and the oxygen stay where they were. What happens to E?
The pH slider stops at 8.8. Above about pH 9, iron(II) stops staying dissolved, and the bench's assumption fails. Where exactly cannot be pinned. Published solubility products for iron(II) hydroxide run from 4.87 × 10−17 to 7.9 × 10−15, and none is primary. At 10−6 mol per litre of iron(II), those put the onset between pH 8.84 and pH 9.95. So 8.8 is the safe bound, and the spread is real. Next, untick the box on Bench 3. The acid couple, +1.229 V, and the basic one, +0.401 V, are one line through the Nernst equation. They give 0.8149 V and 0.8151 V at pH 7. At pH 0 they give 1.2290 V and 1.2292 V, and at pH 14 they give 0.4008 V and 0.4010 V. The gap is −0.2 mV every time, so it is rounding in two table values, not two chemistries. No corrosion textbook this page could reach says so in words. The slope's temperature flag sits with Tier 2.
— Change what is dissolved and you change the push.
Go back to the balcony. Bench 3 puts aluminium at −1.66 V and iron at −0.44 V. Aluminium gives up electrons far more readily than iron does. So the ladder says the window frame should go before the grille. It does not. The frame outlives the grille by decades, in the same salty air. Say why before the reveal.
Aluminium sits at −1.66 V and iron at −0.44 V, yet the aluminium frame outlives the iron grille by decades. Why is that?
Predict before you are told.
The oxide's volume settles it. When a metal atom becomes oxide, the oxide takes up more room than the atom did. Chemists compare the two volumes as the Pilling–Bedworth ratio. The ratio is the oxide's volume divided by the volume of the metal it replaced. Between 1 and 2, the oxide fits. It covers the whole surface, stays attached, and seals the metal from the air. Aluminium's oxide, Al2O3, has a ratio of 1.28. It is far too thin to see, and it seals. Iron's main rust, Fe2O3, has a ratio of 2.14. It is too bulky for the metal it replaced, so it cracks and flakes. Air and water get back in. How far a reaction goes and what happens next are two different questions.
| oxide | metal | Pilling–Bedworth ratio | fits the protective band, 1–2? |
|---|---|---|---|
| Al2O3 | aluminium | 1.28 | yes |
| Fe2O3 | iron | 2.14 | no — too bulky |
The oxide itself: +3 on every iron atom, same as the ion in solution.
So iron needs help. One common help is a coat of zinc. Chemists call it galvanising. Galvanised sheet is the grey roofing sheet with a crystal pattern on it. The zinc coat keeps air and water off the steel, the way paint does. Then the coat gets scratched through to bare steel. Predict what happens at the scratch.
A galvanised steel sheet is scratched through to bare steel, and rain wets the scratch. What happens there?
Commit, then read what protects what.
The zinc is eaten and the steel is spared. Zinc sits at −0.76 V and iron at −0.44 V. Where the two touch under water, they make the cell of Bench 2. Zinc is the anode, so zinc gives the electrons. The bare steel is the cathode, and oxygen takes the electrons there. Nothing leaves the steel. The protection reaches as far as the water film carries ions. Ships and pipelines use the same trick on purpose. Blocks of zinc or magnesium are bolted to the steel and eaten instead of it. Chemists call this sacrificial protection. Zinc is softer than steel, not tougher. Natesan's table backs this. At Chennai port, galvanised iron lost 11.66 µm in its first year, against bare mild steel's 524.0 µm.
Rust is not one compound. It is a family. The bench's Fe2O3 is red-brown, with every iron at +3. Magnetite, Fe3O4, is black. Its iron averages +8/3, because two of its three irons are +3 and one is +2. The bookkeeping gives a fraction, and that is allowed. The number is bookkeeping, not a charge on one atom. The rust you scrape off a grille is mostly FeOOH, an oxide with hydrogen built in. Its water content varies. So textbooks write rust as Fe2O3·xH2O and leave x open. Module 11 said where a compound's colour comes from. Iron metal is grey because its electrons are free to move, as module 9 said. Bound into an oxide, they are not.
Three iron compounds tell three oxidation-number stories. Magnetite's black crystal averages +8/3, and the rust you actually scrape off a grille carries +3.
A steel can is coated inside with tin, which sits at −0.14 V. The tin is scratched through to the steel, and the can holds watery food. What happens at the scratch?
— An oxide that fits protects. An oxide that flakes does not.
Every cell on this page has run downhill. Iron gave electrons to oxygen, and the referee marked ΔG° negative. The stadium emptied through the corridor. Now comes the module's last question. A phone battery is a cell that has run down. Its reaction sits at the bottom of the hill, and the reverse direction has a positive ΔG°. Commit before the reveal.
A cell has run down. Its reaction sits at the bottom of the hill, and the reverse direction has a positive ΔG°. Can the reverse reaction be made to run?
Predict before you are told.
Yes, it can, with a push from outside. Connect a power supply the right way round. It forces electrons into the electrode they were leaving. It pulls them out of the electrode they were reaching. The reaction runs backwards. Chemists call this electrolysis. Electrolysis is a reaction driven by electricity. The supply must give back at least the energy the referee marked. For the rusting cell at standard state, that is 644.52 kJ per mole of reaction run in reverse. In practice it gives more. Real electrodes need extra voltage before they run at a useful speed. That is a phone battery on charge. It is also how aluminium is won from its ore, and module 22 takes that up. Iron is won another way, with carbon.
A power supply now drives Bench 2's magnesium and iron cell backwards. Which terminal of the supply is the anode joined to?
Commit, then read the definition that survives both directions.
The anode is joined to the positive terminal. Here is the definition that survives both directions. The anode is where oxidation happens. The cathode is where reduction happens. When a cell delivers energy, electrons pour out of the anode, so it is the negative terminal. When a supply drives the cell, oxidation is forced where the supply pulls electrons out. That is its positive terminal. So the sign swaps, and the name stays with the reaction. In the Ethiopian study of 360 students, 63 per cent carried the sign across between the two kinds of cell.
Electrolysis of concentrated salt water, called brine, gives chlorine gas at the anode and hydrogen gas at the cathode. Chlorine forms rather than oxygen, even though oxygen is the easier product on paper. Chlorine needs far less extra voltage to form at a useful rate. Chlorine damages the lungs. NIOSH sets a ceiling of 0.5 ppm, where ppm means parts per million of air. OSHA sets a ceiling of 1 ppm, and 10 ppm is immediately dangerous to life. Hydrogen burns in air at any mix between 4 and 75 per cent. Its flame is hard to see, and it lights at about 574 °C. Mixed, the two gases explode. Industry makes chlorine this way, about 97 million tonnes in 2022, in plants built to keep the gases apart. This reaction belongs in such a plant and nowhere else. This page describes no way of running it.
For chlorine, CDC/NIOSH Pocket Guide to Chemical Hazards, npgd0115, REL ceiling 0.5 ppm, PEL ceiling 1 ppm, IDLH 10 ppm. For hydrogen, NOAA CAMEO Chemicals, LEL 4%, UEL 75%, autoignition about 574 °C. For scale, about 97 million tonnes of chlorine in 2022, chlor-alkali process. The only sourced extra-voltage figures are for platinum electrodes, so none is printed here.
Household bleach is sodium hypochlorite, a compound made from the chlorine above. It is an oxidising agent, a substance that takes electrons. A dye's colour comes from a chain of alternating single and double bonds, which absorbs visible light. Hypochlorite takes electrons from that chain and breaks it. The molecule left behind no longer absorbs visible light, so the shirt goes pale at that spot. It is rusting's idea, electrons taken, with a shirt in place of a grille.
Bench 3 gives hydrogen from acid at 0.00 V, row 21. It gives hydrogen from water in base at −0.83 V, row 28. A supply drives each to make hydrogen. Which needs the larger push?
— Run it backwards and you pay for it in electricity.
This deal wires {anode} as the anode and {cathode} as the cathode, both as the ladder prints them. E°cell, cathode minus anode, is {ecell} V. Take n as {n} for every deal, so only the arithmetic is tested. Find ΔG° in kJ/mol, with its sign. Module 14's referee is ΔG. Module 12 gave its unit, the kilojoule per mole. A negative E°cell gives a positive ΔG°, and that pair does not run as wired. If the deal hands you silver, module 10's bangle is the everyday case. Sulphur dulls it and oxygen leaves it alone. Module 19's logarithm is not needed here. Tier 2 needs it.
1.
2. Section 6 named the balcony's rung and its number. Now get there yourself. Take the film as pH 5. Take iron(II) at 1 × 10−6 mol per litre and oxygen at 0.21 bar. E°cell is 1.67 V and n is 4. Find E in volts, to three decimals, showing every product.
Start with log Q, which is 2 log [Fe2+] plus 4 pH minus log pO2. Two times the logarithm of 10−6 is 2 × (−6), which is −12. Four times the pH of 5 is 4 × 5, which is 20, and it is added. The logarithm of 0.21 is −0.6778, and subtracting it adds 0.6778. So log Q is −12 + 20 + 0.6778, which comes to 8.6778. The slope 0.05916 V divided by n, which is 4, gives 0.01479 V. Multiply 0.01479 V by 8.6778, and the Nernst term is 0.1283 V. Take 0.1283 V from the standard 1.67 V, and E is 1.5417 V. The readout rounds it to 1.542 V, and the sign of every step is above. It sits above the pure-water 1.4233 V, because pH 5 holds more hydrogen ions than pH 7. One flag on the slope, which is a 25 °C number and becomes 0.06015 V at 30 °C. That shift is not why rusting is faster in Chennai's heat. Speed is kinetics, and this equation says nothing about speed. The potentials are not sourced at 30 °C either, so no warm-Chennai voltage is computed on this page.
Iron loses electrons and its oxide flakes. Aluminium loses them and its oxide seals.
Sources for this module, in the order the page uses them. The ladder, the rusting cell and its +1.67 V are from NCERT Chemistry Part I, Class XII, Rationalised 2023–24, Chapter 2. They are in Table 2.1 and section 2.8. The basic oxygen couple, +0.401 V, is from the CRC Handbook electrochemical series, Vanýsek. It is labelled CRC wherever it appears. F = 96485.33212 C/mol and R = 8.314462618 J/(mol·K) are CODATA 2022, via the NIST wallet card. The 0.05916 V slope is derived from them at 298.15 K. The 60 per cent critical humidity is Vernon's, via the NPL atmospheric corrosion guide. The 11 per cent figure is from Sandia National Laboratories, via OSTI. Chennai and Mahendragiri corrosion rates are from Natesan, Venkatachari and Palaniswamy, Corrosion Science 48, 2006, Table 5, doi:10.1016/j.corsci.2006.02.006. ISO bands are from ISO 9223:1992 Table 5 and ISO 9223:2012 Table 2. The CX correction is this page's. Pilling–Bedworth ratios are secondary, from the standard reference tables. Hydration enthalpies are from NCERT Class XI Table 10.1. The two-couples identity is drawn as one line in MIT OpenCourseWare 10.626, Lecture 11. No corrosion textbook this page could reach states it in words. Iron(II) hydroxide solubility products are from four secondary tables, and none is primary. Safety figures are from CDC/NIOSH npgd0115 and NOAA CAMEO Chemicals. The beliefs the gates test come from four studies. One is Nakiboglu and others, 2023. One is an Ethiopian grade 11 and 12 study of 360 students. One is Khouna and others, 2025. One is Rahmiati and others, 2023. The nail-weight finding is from Barker, Beyond Appearances, Royal Society of Chemistry. No survey figure exists for the belief that water is not needed, and this page prints none. Verified 26 September 2026.