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.
• If you can hold two sentences in your head for the rest of this chapter, hold these: electrons outside, ions inside; and cations to the cathode, anions to the anode. Six of the eight traps above are a failure of one or the other.
• 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 6 from memory — five cells, ten products, ten half-equations, ten observations — you have most of the marks in this topic already.
• Twenty-two ticks 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. 4.1.1
• I can identify the anode as the positive electrode in a simple electrolytic cell. 4.1.2a
• I can identify the cathode as the negative electrode in a simple electrolytic cell. 4.1.2b
• I can identify the electrolyte as the molten or aqueous substance that is undergoing electrolysis. 4.1.2c
• I can describe how electrons move in the external circuit during electrolysis, including both wire segments and the power supply's terminals. 4.1.3a
• I can describe the loss or gain of electrons at the electrodes. 4.1.3b
• I can describe how ions move in the electrolyte. 4.1.3c
• I can identify the products and describe the observations for molten lead(II) bromide with inert platinum or carbon/graphite electrodes. 4.1.4a
• I can identify the products and describe the observations for concentrated aqueous sodium chloride with inert platinum or carbon/graphite electrodes. 4.1.4b
• I can identify the products and describe the observations for dilute sulfuric acid with inert platinum or carbon/graphite electrodes. 4.1.4c
• I can identify the products and describe the observations for aqueous copper(II) sulfate with inert carbon/graphite electrodes. 4.1.5a
• I can identify the products and describe the observations for aqueous copper(II) sulfate with copper electrodes. 4.1.5b
• I can state that metals or hydrogen form at the cathode, and that non-metals other than hydrogen form at the anode. 4.1.6
• I can predict the products at both electrodes for a molten binary compound. 4.1.7
• I can predict the products at both electrodes for a dilute or concentrated aqueous halide compound. 4.1.8
• I can construct an ionic half-equation at the anode showing oxidation. 4.1.9a
• I can construct an ionic half-equation at the cathode showing reduction. 4.1.9b
• I can state that electroplating can improve a metal object's appearance. 4.1.10a
• I can state that electroplating can improve a metal object's resistance to corrosion. 4.1.10b
• I can describe how metals are electroplated. 4.1.11
• I can state that a hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as its only chemical product. 4.2.1
• I can describe advantages and disadvantages of hydrogen-oxygen fuel cells compared with gasoline/petrol engines in vehicles. 4.2.2

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

Electrolysis

The decomposition of an ionic compound, either molten or in aqueous solution, brought about by passing an electric current through it. Charge is carried through the external circuit by electrons and through the compound itself by mobile ions, which gain or lose electrons at the electrodes and are released there as new substances.

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; those ions are reduced onto the object. It is done to improve the object's appearance or its resistance to corrosion.

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, and is compared with a petrol engine in vehicles by weighing the water-only output at the point of use against the cost and difficulty of producing, storing, transporting and supplying the hydrogen.

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.

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.

Anode

The positive electrode of a simple electrolytic cell, joined to the positive terminal of the power supply. Negative ions move through the electrolyte towards it and lose electrons there, so oxidation happens at the anode and the product is a non-metal other than hydrogen.

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.

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, 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.

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.

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. Positive ions move through the electrolyte towards it and gain electrons there, so reduction happens at the cathode and the product is either a metal or hydrogen.

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.

• 1. "Electrons travel through the solution from one electrode to the other." 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." 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." 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." 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 B.
• 5. "Molten and aqueous electrolysis work the same way." 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 D.
• 6. "Chlorine is a colourless gas" — or — "concentrated sodium chloride solution gives oxygen at the anode." Repair Chlorine is a pale yellow-green gas with a sharp choking smell, and it is the anode product from a concentrated aqueous chloride; oxygen is what a dilute one gives. Taught in Lesson E.
• 7. "A copper electrode behaves like a carbon one." 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." Repair Metal ions are reduced onto the object, and reduction happens at the cathode, so the object being plated is the negative cathode and the coating metal is the positive anode. 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.
• Read the IDs correctly. The lettered suffixes — the a, b, c — are Academiq Edu traceability IDs used to split compound official statements for quality assurance. They are not Cambridge numbering and will not appear on a question paper. The numbers before them are the official statement numbers.
• On weightings. If you have seen the figures Paper 1 = 30%, Paper 2 = 50%, and either Paper 3 or Paper 4 = 20%, those are the 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 twenty-two checks.
• 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 D 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 fact and one comparison. 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. 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 30 out of 48 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.