0620 · 4.1Electrolysis

Ions and electrolytic cells

0:00A metal spoon can acquire a copper coating without being dipped into molten copper. Electricity can transfer copper from a solution onto its surface. The same underlying process can produce gases or separate a molten compound into its elements. In this video, we will explain these changes using moving ions and electron transfer. Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

0:26Decomposition means breaking down a substance into simpler substances. Molten means melted, while aqueous means dissolved in water. The electric current drives chemical changes that would not happen in this arrangement without the power supply. The starting point is an ionic compound. It contains both positive and negative ions. In a solid, those particles occupy fixed positions within a giant ionic lattice. They cannot move through the substance to carry charge, so a solid ionic compound does not conduct electricity.

0:54Melting breaks up the fixed arrangement without removing the charges on the ions. Dissolving a suitable ionic compound in water also allows its charged particles to move. These mobile ions make conduction possible. The important change is their freedom of movement, not the creation of electrons inside the liquid. An electrolytic cell has two electrodes connected to a direct current power supply. The anode is the positive electrode, and the cathode is the negative electrode.

1:18The electrolyte is the molten or aqueous substance that undergoes electrolysis. Each electrode provides a surface where a chemical reaction can take place. Opposite charges attract, so positive ions move towards the negative cathode. The positive anode attracts the negative ions. Chemists call the positive ones cations and the negative ones anions. Those names help connect each type of particle with its destination, but the electrical attraction explains the movement.

1:44For many examples, we use inert electrodes made of platinum or carbon, usually graphite. Inert means that the electrode does not react in the process being considered. Its job is to conduct electricity and provide a reaction surface. Later, we will see why using copper instead can change the anode reaction. For the examples in this topic, metals or hydrogen form at the cathode. Non-metals other than hydrogen form at the anode.

2:10This pattern describes products, not everything present beside an electrode. Some ions reach an electrode but remain in solution because a different reaction occurs more readily.

Molten and aqueous electrolysis

2:18Consider molten lead(II) bromide using inert electrodes. It contains lead ions with a 2+ charge and bromide ions with a 1− charge. Since the compound is molten, both types can move. There is no water present, so we only need to consider these two kinds of ion. Lead ions move towards the cathode, where lead metal forms. At the anode, the bromide ions produce bromine. The observations are droplets of lead at the cathode and red-brown bromine vapour at the anode.

2:46Write the products as lead and bromine, rather than the names of their original ions. This gives a straightforward method for predicting products from a molten binary compound. Binary means that the compound contains two elements. Identify the positive and negative ions, then match each with the correct electrode. The metal forms at the cathode, and the non-metal at the anode. For example, molten magnesium chloride contains magnesium ions and chloride ions.

3:12Magnesium ions move to the cathode and become magnesium metal. Chlorine is produced at the anode from the chloride ions. This prediction applies to the molten substance; an aqueous solution introduces water and can produce different results. Molten sodium chloride follows the same pattern, giving sodium metal at the cathode. Chlorine gas forms at the anode. Dissolve the same compound in water, and the cathode product changes from sodium to hydrogen.

3:37Water is the reason, as the next cell shows. Melting and electrolysis are therefore separate changes. Heating makes the ions mobile, but it does not itself turn lead(II) bromide into lead and bromine. Passing an electric current causes the decomposition. A question asking you to explain why the molten compound conducts needs an answer about mobile ions. Aqueous electrolysis needs more care because water can participate in the electrode reactions.

4:02The products do not necessarily come only from the dissolved compound. For concentrated aqueous sodium chloride, hydrogen forms at the cathode and chlorine at the anode. Both products appear as bubbles of gas. Sodium does not form at the cathode in this aqueous example. Producing hydrogen from the water is easier than depositing this very reactive metal. The sodium ions remain in solution instead. This is why predicting aqueous products by simply separating the compound into its elements gives the wrong answer.

4:32At the anode, chloride ions from the concentrated solution produce chlorine. The concentration matters, so the word concentrated is not an unnecessary detail. Changing the concentration can change which gas forms at this electrode. The Extended rules for aqueous solutions, later in this video, explain that difference. With dilute sulfuric acid and inert electrodes, hydrogen forms at the cathode and oxygen at the anode. You observe gas bubbles at both electrodes.

4:57If collected under the same conditions, the hydrogen volume is twice the oxygen volume. This matches the relative amounts of hydrogen and oxygen in water. The overall change in this example is the decomposition of water into hydrogen and oxygen. The dilute acid supplies ions that help the liquid conduct electricity. Sulfur does not appear at the anode simply because sulfuric acid contains it. Identifying all the elements present is not enough to predict an electrode reaction.

5:24Keep observations separate from product names when describing a practical. Hydrogen is the identity of a substance, whereas bubbles are something you can see. For molten lead(II) bromide, red-brown vapour is an observation and bromine is the product. Giving both makes your account chemically clear and practically useful.

Electron transfer and half-equations

5:41This next part, up to electroplating, is for the Extended paper. Attraction explains where ions move, but electron transfer explains how products form. At the cathode, positive ions can gain electrons and become neutral atoms. The negative terminal supplies those electrons through the connecting wire. A lead ion has a 2+ charge because it has two fewer electrons than the neutral atom. Gaining two electrons removes that positive charge. The product is a lead atom, which joins the metal forming at the cathode.

6:08Gaining electrons is called reduction, so reduction occurs at the cathode. We represent this using an ionic half-equation. One lead ion plus two electrons gives one lead atom. The balanced symbol equation is on the board. It shows one lead ion gaining exactly two electrons, so both the number of lead atoms and the total charge are conserved. At the anode, bromide ions lose electrons. Each bromide ion loses one electron to become a neutral bromine atom.

6:36Two of those atoms join to form one bromine molecule. Losing electrons is oxidation, so oxidation occurs at the anode. The half-equation says that two bromide ions give bromine plus two electrons. The balanced symbol equation is on the board. Two bromide ions are needed because bromine exists as molecules containing two atoms. Together they carry a charge of 2−, which matches the electrons released. A half-equation describes the reaction at one electrode rather than the whole cell.

7:05Electrons appear on the left when particles gain them. For electron loss, they appear on the right. Check both the atoms and the total charge, because balancing only the atoms can leave an incorrect equation. Charge moves in different ways through different parts of the apparatus. Electrons move through the metal wires in the external circuit. Inside the electrolyte, moving ions carry charge. Electrons are transferred at the electrode surfaces, connecting the electrical process with the chemical reactions.

7:32Do not draw electrons travelling through the electrolyte from one electrode to the other. Positive and negative ions move through that liquid instead. The power supply delivers electrons to the cathode and removes them from the anode. This keeps reduction and oxidation occurring while the circuit is complete. Electrode signs are also easy to reverse accidentally. In an electrolytic cell, the anode is positive and the cathode is negative. A positive ion gains electrons at the negative electrode because the power supply makes them available there.

8:01Ion movement and electron transfer are related, but they describe different stages.

Aqueous product prediction

8:06In aqueous solutions, several reactions may be possible at an electrode. Product prediction depends on which reaction occurs more readily under the stated conditions. For Cambridge questions, the important clues are the dissolved ions, their concentration, and the electrode material. At the cathode, very reactive metals such as sodium and magnesium are not deposited from their aqueous solutions. Hydrogen forms instead. Less reactive metals such as copper and silver can be deposited.

8:33The reactivity series helps organise this competition, although its wider chemistry belongs in the metals topic. At an inert anode, a concentrated aqueous halide solution produces the corresponding halogen. Concentrated chloride gives chlorine, bromide gives bromine, and iodide gives iodine. For the dilute halide solutions considered here, predict oxygen instead. Concentration changes the competing anode reactions without changing the electrode signs. Use sodium chloride to compare the two cases directly.

9:03The concentrated solution produces hydrogen at the cathode and chlorine at the anode. With a dilute solution, hydrogen still forms at the cathode, but the anode now gives oxygen. Dilution changes the anode product in this comparison. Apply the same rules to aqueous potassium bromide with inert electrodes. Potassium is a very reactive metal, so hydrogen forms at the cathode in both cases. A concentrated solution gives bromine at the anode.

9:29A dilute solution of this salt gives oxygen there instead. Sulfate ions normally remain in solution when an inert anode is used in these examples. Oxygen forms from water or hydroxide ions instead. This explains the oxygen produced during electrolysis of dilute sulfuric acid. Avoid predicting sulfur merely because the electrolyte contains sulfate ions. Consider aqueous copper(II) sulfate with inert graphite electrodes. The solution contains copper ions with a 2+ charge and sulfate ions with a 2− charge.

10:00Water is also present and can take part. We therefore predict each electrode product separately rather than splitting the compound into copper and sulfur. At the cathode, copper ions gain electrons and form copper metal. A copper deposit builds up on the electrode. Oxygen forms at the anode, so bubbles appear there. As copper ions are removed without being replaced, the blue colour of the solution fades. That fading colour connects an observation with a particle explanation.

10:28Copper ions are responsible for the blue solution, and their concentration decreases as metal is deposited. The graphite anode does not supply replacement copper ions. Its role is to conduct electricity while oxygen is produced at its surface. Now replace both graphite electrodes with copper. Copper still forms at the cathode, but the anode reaction changes. Instead of oxygen production, copper atoms in the positive electrode lose electrons and enter the solution as copper ions.

10:55The anode dissolves and loses mass. Meanwhile, the cathode gains mass as copper is deposited. Copper ions entering the solution replace those leaving it. The blue colour therefore remains approximately unchanged during the ideal process. The electrode material matters because copper is participating in the reaction rather than acting as an inert surface. This comparison is a useful worked example of product prediction. First, identify the electrolyte as aqueous copper(II) sulfate.

11:22Next, determine whether the electrodes are inert graphite or copper, which takes part in the reaction. Predict copper at the cathode in both cases, then use the anode material to choose oxygen production or copper dissolution. The copper cathode half-equation says that one copper ion plus two electrons gives one copper atom. At a copper anode, the reverse change occurs. A copper atom gives a copper ion plus two electrons, showing oxidation rather than reduction.

11:49For hydrogen production in an acidic solution, two hydrogen ions gain two electrons to form one hydrogen molecule. Each ion has a 1+ charge, and the two electrons cancel both. The two hydrogen atoms become one molecule. This half-equation shows reduction at the cathode. One common oxygen half-equation uses hydroxide ions. Four hydroxide ions give oxygen plus two water molecules and four electrons. Count four oxygen atoms and four hydrogen atoms on each side.

12:17The total charge is 4− on both sides, and electron loss identifies oxidation. In acidic solution, oxygen formation can instead be written using water molecules. Two water molecules give oxygen, four hydrogen ions and four electrons. Both forms balance atoms and charge correctly. Use the form appropriate to the question, rather than trying to place sulfate ions into the oxygen reaction. A common half-equation mistake is adding electrons to the wrong side.

12:44Decide whether the reacting particle gains or loses them before writing the equation. Another mistake is producing single chlorine or bromine atoms instead of their diatomic molecules. Those molecular products determine how many halide ions and electrons are needed. The chloride half-equation follows the bromide pattern. Two chloride ions give one chlorine molecule plus two electrons. This is the anode reaction for concentrated sodium chloride solution. The electrons appear on the right because the chloride ions lose them, which is oxidation.

Electroplating a metal object

13:14Electroplating is on both papers. It uses the deposition of metal at a cathode to coat another object. Metals are electroplated to improve their appearance and resistance to corrosion. The object being coated must be the cathode. That is where metal ions gain electrons and form a layer of metal. To copper-plate a spoon, clean its surface so the coating can adhere properly. Connect it to the negative terminal of a direct current power supply.

13:37Place a copper anode beside it in a solution containing copper ions. Connect that anode to the positive terminal, ensuring the electrodes do not touch. When current passes, copper forms on the spoon. The copper anode supplies replacement ions as it dissolves. This arrangement transfers the plating metal through the solution onto the object. The electrolyte must contain ions of the metal required for the coating, not simply any conducting solution.

14:02Reversing the connections would not plate the spoon as intended. At the positive electrode, oxidation occurs rather than metal-ion reduction. Equally, using a solution without copper ions cannot supply the desired copper coating. The positions of the object, plating metal and electrolyte all follow from the electrode reactions. The same method works for other metals. To silver-plate a spoon, make the spoon the cathode and use a silver anode. The electrolyte must contain silver ions, for example silver nitrate solution.

14:31Silver then forms on the spoon, while the silver anode slowly dissolves.

Exam answers and recap

14:35An exam might ask you to state the products, describe the observations, and explain a change in the electrolyte. These command words require different kinds of response. State asks for the relevant fact, while describe asks for what happens. Explain requires the chemical reason behind the change. For this graphite-electrode example, a precise answer begins with the products: copper at the cathode and oxygen at the anode. The observations are a copper deposit building up on the cathode and bubbles of gas at the anode.

15:02The blue colour fades because copper ions are removed from the solution. If asked to construct the cathode half-equation, start with a copper ion carrying a 2+ charge. Add two electrons on the reactant side and write neutral copper as the product. Check that one copper atom appears on each side and that the total charge is zero on both. Another question may ask why lead(II) bromide must be molten before electrolysis.

15:27A creditworthy answer says that the ions in the solid cannot move. Once melted, the ions are free to move and carry the current. Name them as lead ions and bromide ions, never as bromine ions. Bromine is the element made at the anode, not the ion present in the melt. Practical papers can also assess apparatus, observations and interpretation of results. Label the electrodes with their signs, name the electrolyte, and describe the visible changes precisely.

15:54Treat lead compounds, bromine and chlorine as hazards in teacher-controlled demonstrations. None of these experiments is suitable for home use. The central idea is that electrolysis links electrical movement with chemical change. Mobile ions carry charge through a molten or aqueous electrolyte. At the cathode, reduction produces a metal or hydrogen. Oxidation at the anode produces a non-metal in inert-electrode examples, or dissolves a copper anode. For molten binary compounds, predict products directly from the two ions present.

16:24In aqueous solutions, consider water, concentration and electrode material as well. Electroplating applies the same cathode reaction to form a useful metal coating. These connections explain the products and observations, rather than leaving them as disconnected facts to memorise.