Electrolysis quiz
This interactive quiz is no longer available. The topic notes and past-paper questions remain available.
Electrolysis – Tyler de Witt video
Although quite long, this Tyler de Witt video is a good summary of Electrolysis.
2:24 (Triple only) be able to comment on a metal extraction process, given appropriate information
Extraction of a metal from its ore typically involves removing oxygen from metal oxides.
If the ore contains a metal which is below carbon in the reactivity series then the metal is extracted by reaction with carbon in a displacement reaction.
If the ore contains a metal which is above carbon in the reactivity series then electrolysis (or reaction with a more reactive metal) is used to extract the metal.
2:23 (Triple only) explain how the method of extraction of a metal is related to its position in the reactivity series, illustrated by carbon extraction for iron and electrolysis for aluminium
Extraction of a metal from its ore typically involves removing oxygen from metal oxides.
If the ore contains a metal which is below carbon in the reactivity series then the metal is extracted by reaction with carbon in a displacement reaction.
If the ore contains a metal which is above carbon in the reactivity series then electrolysis (or reaction with a more reactive metal) is used to extract the metal.
2:22 (Triple only) know that most metals are extracted from ores found in the Earth’s crust and that unreactive metals are often found as the uncombined element
Most metals are found in the Earth’s crust combined with other elements. Such compounds are found in rocks called ore, rocks from which it is worthwhile to extract a metal.
A few very unreactive metals, such as gold, are found native which means they are found in the Earth’s crust as the uncombined element.
2:20 in terms of gain or loss of oxygen and loss or gain of electrons, understand the terms: oxidation, reduction, redox, oxidising agent, reducing agent, in terms of gain or loss of oxygen and loss or gain of electrons
Oxidation
- Oxidation is the loss of electrons. For example a sodium atom (Na) loses an electron to become a sodium ion (Na⁺). Another example is a chloride ion (Cl⁻) losing an electron to become a chlorine atom (Cl).
- Another definition of oxidation is the gain of oxygen. For example if carbon combines with oxygen to form carbon dioxide, the carbon is being oxidised.
Reduction
- Reduction is the gain of electrons. For example a sodium ion (Na⁺) gains an electron to become a sodium atom (Na). Another example is a chlorine atom (Cl) gaining an electron to become a chloride ion (Cl⁻).
- Another definition of reduction is the loss of oxygen. For example when aluminium oxide is broken down to produce aluminium and oxygen, the aluminium is being reduced.
Redox: A reaction involving oxidation and reduction.
A good way to remember the definitions of oxidation and reduction in terms of electrons is:
- OILRIG : Oxidation Is the Loss of electrons and Reduction Is the Gain of electrons
Oxidising agent: A substance that gives oxygen or removes electrons (it is itself reduced).
Reducing agent: A substance that takes oxygen or gives electrons (it is itself oxidised).
2:16 understand how metals can be arranged in a reactivity series based on their displacement reactions between: metals and metal oxides, metals and aqueous solutions of metal salts
A metal will displace another metal from its oxide that is lower in the reactivity series. For example, a reaction with magnesium and copper (II) oxide will result in the magnesium displacing the copper from its oxide:
A metal will also displace another metal from its salt that is lower in the reactivity series. For example, the reaction between zinc and copper (II) sulfate solution will result in zinc displacing the copper from its salt:
The blue colour of the copper (II) sulfate solution fades as colourless zinc sulfate solution is formed.
1:60 (Triple only) practical: investigate the electrolysis of aqueous solutions
The diagram shows an electrolytic cell.
The electrolyte is an aqueous solution. For example it might be concentrated sodium chloride, NaCl (aq).
The test tubes over the electrodes must not completely cover them to make sure the ions are free to move throughout the solution.
In the case of NaCl (aq) bubbles of gas will be seen forming at the electrodes. These float up and collect in the test tubes when each gas can be tested to assess its identity.
1:59 (Triple only) write ionic half-equations representing the reactions at the electrodes during electrolysis and understand why these reactions are classified as oxidation or reduction
Oxidation: the loss of electrons or the gain of oxygen
Reduction: the gain of electrons or the loss of oxygen
Example: The electrolysis of lead (II) bromide, PbBr2
At the cathode (negative electrode): Pb2+ (l) + 2e– → Pb (l) (reduction)
At the anode (positive electrode): 2Br– (l) → Br2 (g) + 2e– (oxidation)
Example: The electrolysis of aluminium oxide, Al2O3
At the cathode: Al3+ + 3e– → Al (reduction)
At the anode: 2O2- → O2 + 4e– (oxidation)
Example: The electrolysis of sodium chloride solution (NaCl (aq))
At the cathode: 2H+ (aq) + 2e– → H2 (g) (reduction)
At the anode: 2Cl– (aq) → Cl2 (g) + 2e– (oxidation)
Example: The electrolysis of copper sulfate solution (CuSO4 (aq))
At the cathode: Cu2+ (aq) + 2e– → Cu (s) (reduction)
At the anode: 4OH– (aq) → O2 (g) + 2H2O (l) + 4e– (oxidation)
1:58 (Triple only) describe experiments to investigate electrolysis, using inert electrodes, of molten compounds (including lead(II) bromide) and aqueous solutions (including sodium chloride, dilute sulfuric acid and copper(II) sulfate) and to predict the products
Electrolysis: The breaking down of a substance caused by passing an electric current through an ionic compound which is molten or in solution. New substances are formed.
ELECTROLYSIS OF MOLTEN IONIC COMPOUNDS
Example: The electrolysis of molten lead bromide (PbBr2)
-
Solid lead bromide is heated and becomes molten. Explanation: ions become free to move.
- Electrodes attached to a power source are placed in the molten lead bromide. Explanation: these electrodes are made of either graphite or platinum because both conduct electricity and are fairly unreactive.
- From the diagram, the left-hand electrode becomes positively charged, this is called the anode. The right-hand becomes negatively charged, this is called the cathode. Explanation: delocalised electrons flow from the anode to the cathode.
- At the anode a brown gas is given off. This is bromine gas (Br2(g)). Explanation: Negatively charged bromide ions are attracted to the anode (positive electrode). At the anode, bromide ions lose electrons (oxidation) and become bromine molecules.
- At the cathode a shiny substance is formed. This is molten lead (Pb(l) ). Explanation: Positively charged lead ions are attracted to the cathode (negative electrode). At the cathode, lead ions gain electrons (reduction) and become lead atoms.
Overall Reaction
word equation: lead bromide –> lead + bromine
chemical equation: PbBr2(l) –> Pb(l) + Br2(g)
Remember
OILRIG : Oxidation Is the Loss of electrons and Reduction Is the Gain of electrons
PANCAKE : Positive Anode, Negative Cathode
ELECTROLYSIS OF IONIC SOLUTIONS
Rules for working out elements formed from electrolysis of solutions
Follow these rules to decide which ions in solution will react at the electrodes:
At the cathode
Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series:
- The metal will be produced if it is less reactive than hydrogen
- Hydrogen gas (H2) will be produced if the metal is more reactive than hydrogen
At the anode
At the anode, the product of electrolysis is always oxygen gas (O2) unless the solution contains a high concentration of Cl–, Br- or I– ions, in which case a halogen is produced, e.g. chlorine gas (Cl2), bromine gas (Br2), and iodine gas (I2).
The electrolysis of sodium chloride solution (NaCl(aq))
-
Solid sodium chloride is dissolved in water. Explanation: The sodium ions and chloride ions become free to move.
- The solution also contains hydrogen ions (H+) and hydroxide ions (OH–). Explanation: Water is a very weak electrolyte. It ionises very slightly to give hydrogen ions and hydroxide ions:
H2O(l) ⇋ H+(aq) + OH–(aq)
- Chloride ions (Cl–) and hydroxide ions (OH–) are attracted to the anode.
- Sodium ions (Na+) and hydrogen ions (H+) are attracted to the cathode.
- At the anode a green gas is given off. This is chlorine gas (Cl2(g)). Explanation: chloride ions lose electrons (oxidation) and form molecules of chlorine. The chloride ions react at the anode instead of the hydroxide ions because the chloride ions are in higher concentration. The amount of chlorine gas produced might be lower than expected because chlorine is slightly soluble in water.
Electron half equation: 2Cl–(aq) –> Cl2 (g) + 2e–
- At the cathode a colourless gas is given off. This is hydrogen gas (H2(g)). Explanation: hydrogen ions gain electrons (reduction) and form molecules of hydrogen. The hydrogen ions react at the cathode because hydrogen is below sodium in the reactivity series.
Electron half equation: 2H+(aq) + 2e– –> H2 (g)
- The solution at the end is sodium hydroxide (NaOH(aq)).
The electrolysis of copper sulfate solution (CuSO4(aq))
- Copper sulfate solution is composed of copper ions (Cu2+), sulfate ions (SO42-), hydrogen ions (H+) and hydroxide ions (OH–).
- At the cathode a brown layer is formed. This is copper. Explanation: copper ions gain electrons (reduction) and form atoms of copper. The copper ions react at the cathode instead of hydrogen ions because copper is below hydrogen in the reactivity series.
Electron half-equation: Cu2+(aq) + 2e– –> Cu (s)
- At the anode, bubbles of gas are given off. This is oxygen gas (O2(g)). Explanation: hydroxide ions lose electrons (oxidation) and form molecules of oxygen and water. The hydroxide ions react at the anode instead of the sulfate ions because the hydroxide ions are less stable.
Electron half-equation: 4OH–(aq) –> O2 (g) + 2H2O(l) + 4e–
The electrolysis of sulfuric acid (H2SO4(aq))
- Sulfuric acid is composed of sulfate ions (SO42-), hydrogen ions (H+) and hydroxide ions (OH–).
- At the cathode bubbles of gas are formed. This is hydrogen gas (H2(g)). Explanation: hydrogen ions gain electrons (reduction) and form molecules of hydrogen.
Electron half-equation: 2H+(aq) + 2e– –> H2(g)
- At the anode, bubbles of gas are given off. This is oxygen gas (O2(g)). Explanation: hydroxide ions lose electrons (oxidation) and form molecules of oxygen and water. The hydroxide ions react at the anode instead of the sulfate ions because the hydroxide ions are less stable.
Electron half-equation: 4OH–(aq) –> O2 (g) + 2H2O(l) + 4e–
- Twice the volume of hydrogen gas is produce compared to oxygen gas. Explanation: from the two half equations, O2 needs 4e– but H2 only needs 2e– as can be seen from the equation
2H2O(l) –> 2H2(g) + O2(g)
There are twice the amount (in moles) of H2 compared to O2
1:57 (Triple only) know that anion and cation are terms used to refer to negative and positive ions respectively
A negative ion is called an anion. Examples are the bromide ion (Br⁻) and the oxide ion (O²⁻).
A positive ion is called a cation. Examples are the sodium ion (Na⁺) and the aluminium ion (Al³⁺).
A trick to remember this is to write the ‘t’ as a ‘+’ in the word cation: ca+ion
1:56 (Triple only) understand why ionic compounds conduct electricity only when molten or in aqueous solution
Ionic compounds only conduct electricity only when molten or in solution.
When solid the ions are not free to move.
When molten or in solution the ions are free to move.
1:55 (Triple only) understand why covalent compounds do not conduct electricity
Electrical conductivity is the movement of charged particles.
In this case, charged particles means either delocalised electrons or ions.
These particles need to be free to move in a substance for that substance to be conductive.
Covalent compounds do not conduct electricity because there are no charged particles that are free to move.
1:54 (Triple only) explain typical physical properties of metals, including electrical conductivity and malleability
Metals are good conductors because they have delocalised electrons which are free to move.
Metals are malleable (can be hammered into shape) because they have layers of ions that can slide over each other.
1:53 (Triple only) understand metallic bonding in terms of electrostatic attractions
Metallic bonding is the strong electrostatic attraction between positive metal ions and a sea of delocalised electrons.
1:52 (Triple only) know how to represent a metallic lattice by a 2-D diagram
When metal atoms join together the outer electrons become ‘delocalised’ which means they are free to move throughout the whole structure.
Metals have a giant regular arrangement of layers of positive ions surrounded by a sea of delocalised electrons.
1:51 know that covalent compounds do not usually conduct electricity
Electric current is a flow of charged particles that can move.
Covalent compounds do not conduct electricity.
1:43 Know that ionic compounds do not conduct electricity when solid, but do conduct electricity when molten and in aqueous solution
Ionic compounds do not conduct electricity when solid.
However, ionic compounds do conduct electricity if molten or in solution.
1:37 understand how ions are formed by electron loss or gain
Ions are electrically charged particles formed when atoms lose or gain electrons.
They have the same electronic structures as noble gases.
Metal atoms form positive ions (cations).
Non-metal atoms form negative ions (anions).