Electrochemistry

• Oxidation is loss of electrons, and it happens at the anode. Reduction is gain of electrons, and it happens at the cathode. Say the full words before you say the mnemonic — the exam marks the words.
• Eight Core statements. Five more for Extended. Two routes. Nothing else.
• Core: the anode is positive and the cathode negative in a simple electrolytic cell; and the words molten, concentrated and dilute are never decoration. Supplement: electrons outside, ions inside; cations to the cathode, anions to the anode.
• Say the condition out loud with every product you name. "Chlorine at the anode" is half an answer; "chlorine at the anode, because this is a concentrated aqueous chloride with inert electrodes" is the whole one.
• Two advantages, two limitations, and a condition attached to any claim about carbon. That structure will answer almost any version of this question.
• If you can reconstruct table 3 from memory — three cells, six products, six observations — you have most of the Core marks in this topic already.
• Reconstruct the last table from memory — five cells, ten products, ten half-equations — and you have most of the Extended marks in this topic as well.
• Eight ticks for Core, thirteen for Extended — earned by recall, not by reading. That is the whole of Topic 4.

• I can define electrolysis as the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current. Core 4.1.1
• I can identify, in simple electrolytic cells, (a) the anode as the positive electrode, (b) the cathode as the negative electrode, and (c) the electrolyte as the molten or aqueous substance that undergoes electrolysis. Core 4.1.2
• I can identify the products formed at the electrodes and describe the observations made during the electrolysis of (a) molten lead(II) bromide, (b) concentrated aqueous sodium chloride and (c) dilute sulfuric acid, using inert electrodes made of platinum or carbon / graphite. Core 4.1.3
• I can state that metals or hydrogen are formed at the cathode, and that non-metals other than hydrogen are formed at the anode. Core 4.1.4
• I can predict the identity of the products at each electrode for the electrolysis of a binary compound in the molten state. Core 4.1.5
• I can state that metal objects are electroplated to improve their appearance and their resistance to corrosion. Core 4.1.6
• I can describe how metals are electroplated. Core 4.1.7
• I can describe the transfer of charge during electrolysis, including (a) the movement of electrons in the external circuit, (b) the loss or gain of electrons at the electrodes and (c) the movement of ions in the electrolyte. Supplement 4.1.8
• I can identify the products formed at the electrodes and describe the observations made during the electrolysis of aqueous copper(II) sulfate, using inert carbon / graphite electrodes and using copper electrodes. Supplement 4.1.9
• I can predict the identity of the products at each electrode for the electrolysis of a halide compound in dilute or concentrated aqueous solution. Supplement 4.1.10
• I can construct ionic half-equations for reactions at the anode, to show oxidation, and at the cathode, to show reduction. Supplement 4.1.11
• I can state that a hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as the only chemical product. Core 4.2.1
• I can describe the advantages and disadvantages of using hydrogen-oxygen fuel cells in comparison with gasoline / petrol engines in vehicles. Supplement 4.2.2

Why this map exists. Topic 4 is usually lost not because the chemistry is hard but because it is revised as a dozen unconnected rules. There are not a dozen rules. There is one cell, two electrodes, and a short list of conditions that decide the products. Learn the story and the rules stop needing to be remembered separately.

Electrolysis of dilute sulfuric acid

The decomposition of dilute sulfuric acid by an electric current using inert electrodes, in which hydrogen is released at the cathode and oxygen at the anode. The oxygen comes from the water in the solution rather than from the sulfate ion, which takes no part, and both products are seen only as streams of colourless bubbles.

Molten electrolyte

An ionic compound that has been heated above its melting point so that its ions are free to move, allowing it to conduct electricity and be decomposed by an electric current. Because no water is present, the only ions available are those of the compound itself, so each electrode can release only one possible product.

Ionic half-equation

An equation showing the change at one electrode only, written with the electrons transferred included as \(e^-\). Electrons appear on the left when a species gains them and is reduced, and on the right when a species loses them and is oxidised; a correct half-equation balances both the atoms of each element and the total electrical charge on the two sides.

Electrolyte

The molten or aqueous substance that is undergoing electrolysis. It contains ions that are free to move, so it conducts electricity, and those moving ions carry charge from one electrode to the other inside the cell.

Cathode

The negative electrode of a simple electrolytic cell, joined to the negative terminal of the power supply. The product formed there is always either a metal or hydrogen. For Extended candidates, it is also the electrode at which positive ions arriving from the electrolyte gain electrons, so reduction happens there.

Anode

The positive electrode of a simple electrolytic cell, joined to the positive terminal of the power supply. The product formed there is a non-metal other than hydrogen. For Extended candidates, it is also the electrode at which negative ions arriving from the electrolyte lose electrons, so oxidation happens there, and where an active electrode such as copper is itself oxidised.

Oxidation

The loss of electrons by a species. In electrolysis it happens at the anode, where negative ions arriving from the electrolyte give up electrons to the electrode and are released as neutral atoms or molecules; in a half-equation the electrons are written on the right-hand side.

Inert electrode

An electrode made of a material such as platinum or carbon (graphite) that conducts electricity into and out of the electrolyte but takes no chemical part in the reaction and supplies no ions of its own. Every product therefore comes from the electrolyte, not from the electrode.

Aqueous electrolyte

An ionic compound dissolved in water so that its ions are free to move and the solution can be decomposed by an electric current. Because water is present as well as the dissolved compound, more than one species may be available at each electrode, so the identity of the solution, its concentration and the electrode material can all affect which product is released.

Active electrode

An electrode that takes chemical part in the electrolysis instead of merely conducting. A copper anode in aqueous copper(II) sulfate is the required example: the copper of the electrode itself is oxidised and passes into the solution as copper ions, so the anode loses mass and no gas is released there.

Hydrogen-oxygen fuel cell

A cell that uses hydrogen and oxygen to produce electricity, with water as its only chemical product. It is supplied continuously with its two reactants rather than being charged. Extended candidates also compare it with a petrol engine in vehicles, weighing the water-only output at the point of use against the cost and difficulty of producing, storing, transporting and supplying the hydrogen.

Electroplating

The use of electrolysis to coat a metal object with a thin layer of another metal. The object being plated is made the negative cathode, the coating metal is made the positive anode, and the electrolyte is a solution containing ions of the coating metal, which are deposited on the object. It is done to improve the object's appearance or its resistance to corrosion.

Reduction

The gain of electrons by a species. In electrolysis it happens at the cathode, where positive ions arriving from the electrolyte take electrons from the electrode and are released as neutral atoms; in a half-equation the electrons are written on the left-hand side.

Electrolysis

The decomposition of an ionic compound, either molten or in aqueous solution, brought about by passing an electric current through it. The compound must be ionic, it must be molten or dissolved in water so that its ions are free to move, and a current must be passed through it; the elements of the compound are then released at the two electrodes.

• Aluminium extraction, bauxite, cryolite and carbon-anode consumption. These are examined — but in Topic 9, not here. Statement 9.6 makes bauxite and extraction by electrolysis a Core requirement, and adds cryolite, anode replacement and the electrode half-equations at Supplement. Revise them with the metals chapter; do not bring them into a Topic 4 answer.
• Electrolytic refining of copper, anode slime and purity figures.
• Simple cells and voltaic cells, cell voltage and reactivity-gap voltage predictions.
• Batteries and rechargeable cells, and any comparison of fuel cells with batteries.
• Fuel-cell half-equations, electrode polarity, membrane chemistry and catalysts.
• Charge, current and time calculations, and calculations of plated mass.
• A general discharge algorithm covering every possible aqueous salt.
• Industrial chlor-alkali production and sodium hydroxide manufacture.
• 1. "Electrons travel through the solution from one electrode to the other." Supplement 4.1.8 Repair Electrons move only in the external metallic circuit; inside the electrolyte, charge is carried by moving ions and by nothing else. Taught in Lesson A.
• 2. "Positive ions go to the positive anode." Supplement 4.1.8 Repair Opposite charges attract, so positive cations move to the negative cathode and negative anions move to the positive anode. Taught in Lesson A.
• 3. "The anode is positive, in every cell, everywhere." Core 4.1.2 Repair In a simple electrolytic cell the anode is positive and the cathode is negative because an external power supply makes them so; that sign pattern belongs to this context and should not be carried unexamined into other kinds of cell. Taught in Lesson A.
• 4. "Oxidation is gain of electrons." Supplement 4.1.11 Repair Oxidation is loss of electrons and happens at the anode; reduction is gain of electrons and happens at the cathode — write the full words, then check the side your electrons are on. Taught in Lesson F.
• 5. "Molten and aqueous electrolysis work the same way." Core 4.1.3 Repair A molten compound supplies only its own two ions, but an aqueous solution has water present as well, so the solution, its concentration and the electrode material can all change the answer. Taught in Lesson C.
• 6. "Chlorine is a colourless gas" — or — "concentrated sodium chloride solution gives oxygen at the anode." Core 4.1.3 Repair Chlorine is a pale yellow-green gas with a sharp choking smell, and it is the anode product of the named cell, which uses a concentrated aqueous chloride. Extended candidates add: a dilute chloride solution gives oxygen at the anode instead Supplement 4.1.10 Taught in Lesson D.
• 7. "A copper electrode behaves like a carbon one." Supplement 4.1.9 Repair Carbon and platinum electrodes take no chemical part, but a copper anode is itself oxidised and passes into the solution as copper ions, so the anode product is completely different. Taught in Lesson G.
• 8. "Connect the spoon you want to plate to the positive terminal." Core 4.1.7 Repair The cathode is the electrode where metal from the solution is deposited, so the object being plated is the negative cathode and the coating metal is the positive anode. Connected the other way round, the object is the electrode that dissolves. Extended candidates can add the electron reason: depositing a metal ion is a reduction, and reduction happens at the cathode Supplement 4.1.11 Taught in Lesson I.

• Two checks, every time. First count the atoms of each element on both sides. Then add up the total charge on each side, counting each \(e^-\) as \(-1\). Both must match. In \(2\mathrm{Br^-} \longrightarrow \mathrm{Br_2} + 2e^-\): two Br each side, and \(2 \times (-1) = -2\) on the left against \(0 + 2 \times (-1) = -2\) on the right.
• On weightings. The percentages in the table are weightings of the whole qualification. They say nothing about how much of any paper is Topic 4, and no reliable figure for that exists. Prepare all eight Core statements — and all thirteen if you are entered for Extended.
• The arrow that costs marks. A single electron arrow drawn straight from the anode across to the cathode, ignoring the power supply, describes a cell that does not exist. Electrons in an electrolytic cell are pushed by the supply; draw them leaving the negative terminal on one side and arriving at the positive terminal on the other.
• Why the lead is written \((l)\). Lead melts at a lower temperature than lead(II) bromide does, so at the temperature needed to keep the electrolyte molten, the lead produced is itself a liquid. That is why the observation is a bead of molten metal rather than a grey solid coating.
• Sodium does not appear. The cathode product here is hydrogen, not sodium, even though sodium ions are the majority cation in the solution. This is the reason the general statement in Lesson C reads "a metal or hydrogen" — in an aqueous cell, hydrogen is a genuine possibility at the cathode.
• Where the oxygen atoms come from. Count them in the anode equation: four hydroxide ions bring four oxygen atoms, and those four end up as two in the \(\mathrm{O_2}\) molecule and two in the two water molecules. Every oxygen atom is accounted for, and none of them came from a sulfate ion.
• A careful way to say it. Write "the anode loses mass and the cathode gains mass" — that is what is observed. Do not claim the two changes are exactly equal unless a question gives you data or a calculation that establishes it.
• The two purposes, in the syllabus's words. Electroplating can improve a metal object's appearance, and it can improve its resistance to corrosion. A question asking for the reasons wants those two, and either one alone is half an answer.
• This section is short on purpose. Section 4.2 of the syllabus is two statements: one Core fact, and one Supplement comparison for Extended candidates. Everything that gets added to fuel cells elsewhere — half-equations, electrode polarity, membranes, catalysts, comparisons with batteries — is outside Topic 4 and is not required on either route. Learn the two statements properly instead.
• The order that catches all eight. Product formula → atoms → charge → which electrode → state symbols. Items 4, 6 and 8 all pass an atoms-and-charge check and are still wrong, which is why the other three steps exist.
• Marking yourself. Give the mark only where you wrote the point, not where you "knew it". Under 60% on your own route means going back to the lessons named in the answers rather than doing another mixed set.
• If a session goes badly. Do not repeat the whole chapter. Go to the specific lesson named in the answer you got wrong, spend ten minutes there, and re-test that one point the next day. Spacing works because of the gaps, so keep them.