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Welcome to GCSE Edexcel Science revision.

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Unit S C 10: Electrolytic processes.

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Electrolysis uses electrical energy from a direct-current (DC) supply to break down (decompose) an electrolyte.

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It is a chemical change that forms new substances.

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Electrolytes are ionic compounds when molten or dissolved in water: their ions are free to move.

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A solid ionic compound does not conduct because its ions are fixed.

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An electrode is a conductor contacting the electrolyte.

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In an electrolytic cell the cathode is connected to the negative terminal and the anode to the positive terminal.

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Positive ions (cations) move to the negative cathode.

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Negative ions (anions) move to the positive anode.

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Electrode signs are for electrolysis driven by a DC supply.

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Current in the external metal wires is carried by electrons; in the electrolyte it is carried by moving ions.

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Do not draw electrons travelling through the solution as the main charge carrier.

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Inert electrodes, such as suitable graphite or platinum electrodes, do not react in the GCSE model.

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Reactive metal electrodes can take part in the reaction and change the products.

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OIL RIG means Oxidation Is Loss, Reduction Is Gain of electrons.

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A half-equation shows the electron transfer for one electrode reaction.

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At the cathode, cations can gain electrons and are reduced.

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Example: C U, charge 2 plus, plus 2 electrons produces C U.

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At the anode, anions can lose electrons and are oxidised.

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Example: 2 B R, charge minus, produces B R 2 plus 2 electrons.

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Balance both atoms and total electric charge.

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In C U, charge 2 plus, plus 2 electrons produces C U, the left charge plus 2 minus 2 equals 0 equals the neutral right side.

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Reduction gains electrons at the cathode.

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Electrons appear on the left for reduction and on the right for oxidation.

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A half-equation is not complete if atoms balance but charge does not.

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Half-equation writing is Higher-tier material.

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State symbols may be included when needed: an ion dissolved in water is aqueous, not liquid.

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A molten binary ionic compound contains just two elements and no water.

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Its own ions form the products: the metal at the cathode and the non-metal at the anode.

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Molten lead bromide contains P B, charge 2 plus, and B R, charge minus,.

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Lead forms at the negative cathode: P B, charge 2 plus, plus 2 electrons produces P B.

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Bromine forms at the positive anode: 2 B R, charge minus, produces B R 2 plus 2 electrons.

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Overall: P B B R 2 produces P B plus B R 2.

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Molten P B B R 2 contains P B, charge 2 plus, and B R, charge minus,; water is absent.

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Molten sodium chloride gives sodium at the cathode and chlorine at the anode: N A, charge plus, plus an electron produces N A; 2 C L, charge minus, produces C L 2 plus 2 electrons.

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These are predictions or supervised demonstrations, not home experiments: hot melts, reactive metals and harmful halogens require controlled specialist handling.

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Do not use the aqueous hydrogen rule for a molten salt; there is no water supplying hydrogen-containing ions.

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Water contributes additional possible electrode reactants in aqueous electrolysis, so the products can differ from a molten salt with the same name.

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At the cathode, metals less reactive than hydrogen, such as copper, are commonly deposited.

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If the metal is more reactive than hydrogen, hydrogen is produced instead in the standard GCSE model.

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At the anode, concentrated chloride, bromide or iodide solutions commonly produce the corresponding halogen; otherwise oxygen is commonly produced.

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Conditions, particularly concentration, can affect competition.

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Copper chloride solution with sufficient chloride concentration produces copper at the cathode and chlorine at the anode;

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copper ions gain electrons and chloride ions lose electrons.

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Concentrated sodium chloride solution produces hydrogen at the cathode and chlorine at the anode, not sodium metal.

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Sodium hydroxide remains in the solution.

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Sodium sulfate solution with inert electrodes produces hydrogen and oxygen.

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Water acidified with sulfuric acid also gives hydrogen at the cathode and oxygen at the anode, in a two to one gas-volume ratio at the same conditions.

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The diagram’s chloride examples assume sufficient chloride concentration.

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In sulfate solution or acidified water, the hydrogen to oxygen volume ratio is two to one.

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Useful half-equations include 2 H,

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charge plus,

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plus 2 electrons produces H 2 and 4 O H,

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charge minus,

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produces O 2 plus 2 H 2 O plus 4 electrons,

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using the ions appropriate to the electrolyte.

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Set up copper sulfate solution with inert electrodes connected to a suitable low-voltage DC supply.

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Keep electrodes separate and use the controlled school procedure with eye protection.

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Copper coats the negative cathode: C U, charge 2 plus, plus 2 electrons produces C U.

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Oxygen bubbles at the positive anode rather than sulfate depositing as a solid.

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With inert electrodes, copper ions are removed from solution without a copper anode replacing them.

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The blue colour becomes paler as C U, charge 2 plus, concentration falls.

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With inert electrodes, copper removed from solution is not replenished at the anode.

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Identify gases using the teacher's safe method: hydrogen gives a squeaky pop, oxygen relights a glowing splint, and chlorine bleaches damp litmus paper.

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Record electrode changes and gas observations.

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Do not infer product identity from bubbles alone; use the electrode polarity and electrolyte composition.

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Electrolysis depends on electrode material as well as solution.

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Changing from inert to copper electrodes changes what happens at the anode.

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With copper electrodes in copper sulfate, copper dissolves from the positive anode: C U produces C U, charge 2 plus, plus 2 electrons.

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The anode loses mass.

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Copper ions gain electrons and plate onto the negative cathode: C U, charge 2 plus, plus 2 electrons produces C U.

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The cathode gains mass.

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In the ideal case, anode production of copper ions replaces those removed at the cathode, so copper-ion concentration and blue colour remain approximately constant.

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To purify copper, use impure copper as the anode and pure copper as the cathode.

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Copper transfers through solution onto the cathode; some insoluble impurities collect below the anode as sludge.

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Copper ions move through the solution; copper transfers from impure anode to pure cathode.

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Compare cleaned, dry electrode masses before and after electrolysis.

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Control current and time when comparing runs; rinse carefully without removing the deposit and dry consistently.

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Plot cathode mass gain against electrolysis time or compare mass changes.

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Product loss during handling, incomplete drying and side reactions can affect measured results.

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That completes Electrolytic processes.

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Revisit the notes and test yourself on the revision website.